rod/stroke ratio theory
Discussion
Hi all,
I'm starting to spec up a new engine for my 240Z now. Head wise I'm all sorted but for the bottom end I recently came across this whole rod/stroke ratio thang.
Now a little online research seems to be suggesting that getting the rod/stroke right is a good thing, the optimum being 1.75 apparently.
Originally I was just going for a stroker motor, 88mm bore, 83mm stroke and 133mm rod, giving a rod/stroke of 1.6. However if I go down the optimised route I could go with an 88mm bore, 79mm stroke and a 137mm rod giving a 1.73 ratio.
For both option I'll have to have custom pistons. For the stroker though I'll need to source the very rare LD28 crank. For the optimised approach I can get off the shelf crank and rods or even go with custom rods of 138mm which would give the ideal 1.75 ratio.
The stroker motor will be 3029cc and the optimised 2883cc. I'll be aiming for around a 10.5:1 compression ratio for both solutions.
So this is the crux of the argument. Would the 'no substitute for cubes' win out or would the 'rod/stroke' win out in the all important torque stakes?
The head is nicely ported. It'll be running a 270 to 290 degree cam depending on final decision. It will be fuel injection through indiviual 45mm TBs and the exhaust will have a full on 6-3-2 manifold.
What do you think?
Cheers,
Rob
I'm starting to spec up a new engine for my 240Z now. Head wise I'm all sorted but for the bottom end I recently came across this whole rod/stroke ratio thang.
Now a little online research seems to be suggesting that getting the rod/stroke right is a good thing, the optimum being 1.75 apparently.
Originally I was just going for a stroker motor, 88mm bore, 83mm stroke and 133mm rod, giving a rod/stroke of 1.6. However if I go down the optimised route I could go with an 88mm bore, 79mm stroke and a 137mm rod giving a 1.73 ratio.
For both option I'll have to have custom pistons. For the stroker though I'll need to source the very rare LD28 crank. For the optimised approach I can get off the shelf crank and rods or even go with custom rods of 138mm which would give the ideal 1.75 ratio.
The stroker motor will be 3029cc and the optimised 2883cc. I'll be aiming for around a 10.5:1 compression ratio for both solutions.
So this is the crux of the argument. Would the 'no substitute for cubes' win out or would the 'rod/stroke' win out in the all important torque stakes?
The head is nicely ported. It'll be running a 270 to 290 degree cam depending on final decision. It will be fuel injection through indiviual 45mm TBs and the exhaust will have a full on 6-3-2 manifold.
What do you think?
Cheers,
Rob
B19GRR said:
(Snip)... Now a little online research seems to be suggesting that getting the rod/stroke right is a good thing, the optimum being 1.75 apparently.
Rob,
"Optimum" is a little subjective, and it depends upon what you are trying to achieve... a high-rev screamer or a low-grunt puller.
Big strokes and under-square bore/stroke ratios are not consistent with a revy engine. Stroking the 2.4 to 3.0 sorta gives up on the idea of a high-rpm screamer.
But at any given stroke, within it's range of possibilities, a longer rod will nudge the engine's performance toward higher rpm horsepower and a shorter rod will nudge it toward low end torque. Optimum is only relavent to where you want the power to come in. Coordinate the rod length with cam selection, intake/exhaust tuning and driveline gearing so they all work together.
With a really big stroker crank, the rod angles will become a problem. At extremes, the practical choice will be to go with the longest rod you can squeeze in just to keep rod angles, piston thrust and intertia loads realistic. The power curve becomes less important that keeping all the busy parts inside the block.
Good luck,
Tim
As I understood it, the longer rod length will allow the piston to act more directly on the crank and will also produce less bore wear. It will also stop the whole arrangement from being as stressed but I am unable to give you any advice on critical data. You must use your best judgement to decide just how much power you need to help you decide on cams and stroke. 270-290 leaves it kind of open but, by the sounds of things non of your options should really cause too much conflict though anything past a 285 cam will likely put a large dent in low end tractability. The shorter stroke is also more likely to cause valve to piston contact- but in real terms, will probably not make much difference. If I were you though, I'd go for reliability over power in this case.
Ok, let me tackle the rod-to-stroke length ratio one first. Alot has been written on this subject- by people like Smokey Yunick and Vizard etc etc.
I'm not one to boast and parade my credentials if I can help it, but I'm sick to death of people contradicting me online on technical engine design issues- something I do for a living- perhaps quoting vague articles by people like the above.
I've spent years devloping the performance and analysing the gas dynamics of lots of engines, V8s, V6s and even inline 4s- from a clean sheet perspective. From an OEM manufacturers point of view- we have alot more resources at our disposal then some back street bloke with a dyno-jet.
This is what I wrote on another forum.
Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –the piston being driven less into the bore (thrust). If you’re a clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
When I see performance gains I split them up into the following areas
Increases in engine breathing or charge efficiency-commonly known as Volumetric efficiency, often due to less restrictions-improvements in outright flow,or tuning changes of some kind, whether they be simple ram induction tuning or perhaps resonance tuning
Then there are combustion improvements- perhaps from a faster burn, higher CR, better-more thermally efficient chamber. I measure alot of this using a useful parameter called BSAC or Brake Specific Air Consumption- basically a measure of how well an engine uses it's own breathing. Just because an engine has a high VE, it doesn't automatically mean the engine can utilise this breathing well!
Then there is friction and improvements associated with all the various components and ancilliaries
Now my illustrated numerical example shows that there's very little differences in instantaneous peak piston speed-both in magnitude and position, even when you go up in con rod length by 10 mm. From this I would doubt I'd see a benefit in cylinder charge efficiency. 1 dimentional "quasi 3 -d" Cycle simulation codes have corroborated this and I haven't seen benefits on our test beds/dynos.
I can't see how they would effect combustion if the compression height is kept the same.
Most likely the biggest of a longer rod engine is down to engine friction due to the reduced angularity of the con rod, and the favourable geometry-less thrust into the bore. Perhaps the benfit is more apparent at higher engine speeds
If there is a contributory effect from combustion I have found on several test bed runs that the difference is within dyno to dyno variation, however I don't discount it's effect totally, only that's it tends to be exagerrated by people, and that it certainly isn't a phenomenon that exhibits itself on airflow/Volumetric efficiency of an engine- due to the minimal effect on instantaneous piston velocity I've just outlined.
There is a limit however, and my collegues/contacts at BMW agree that going for a ratio smaller then 1.5 on an engine that has a typical working rev range is ill advised.
I'm not one to boast and parade my credentials if I can help it, but I'm sick to death of people contradicting me online on technical engine design issues- something I do for a living- perhaps quoting vague articles by people like the above.
I've spent years devloping the performance and analysing the gas dynamics of lots of engines, V8s, V6s and even inline 4s- from a clean sheet perspective. From an OEM manufacturers point of view- we have alot more resources at our disposal then some back street bloke with a dyno-jet.
This is what I wrote on another forum.
Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –the piston being driven less into the bore (thrust). If you’re a clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
When I see performance gains I split them up into the following areas
Increases in engine breathing or charge efficiency-commonly known as Volumetric efficiency, often due to less restrictions-improvements in outright flow,or tuning changes of some kind, whether they be simple ram induction tuning or perhaps resonance tuning
Then there are combustion improvements- perhaps from a faster burn, higher CR, better-more thermally efficient chamber. I measure alot of this using a useful parameter called BSAC or Brake Specific Air Consumption- basically a measure of how well an engine uses it's own breathing. Just because an engine has a high VE, it doesn't automatically mean the engine can utilise this breathing well!
Then there is friction and improvements associated with all the various components and ancilliaries
Now my illustrated numerical example shows that there's very little differences in instantaneous peak piston speed-both in magnitude and position, even when you go up in con rod length by 10 mm. From this I would doubt I'd see a benefit in cylinder charge efficiency. 1 dimentional "quasi 3 -d" Cycle simulation codes have corroborated this and I haven't seen benefits on our test beds/dynos.
I can't see how they would effect combustion if the compression height is kept the same.
Most likely the biggest of a longer rod engine is down to engine friction due to the reduced angularity of the con rod, and the favourable geometry-less thrust into the bore. Perhaps the benfit is more apparent at higher engine speeds
If there is a contributory effect from combustion I have found on several test bed runs that the difference is within dyno to dyno variation, however I don't discount it's effect totally, only that's it tends to be exagerrated by people, and that it certainly isn't a phenomenon that exhibits itself on airflow/Volumetric efficiency of an engine- due to the minimal effect on instantaneous piston velocity I've just outlined.
There is a limit however, and my collegues/contacts at BMW agree that going for a ratio smaller then 1.5 on an engine that has a typical working rev range is ill advised.
B19GRR said:
Hi all,
I'm starting to spec up a new engine for my 240Z now. Head wise I'm all sorted but for the bottom end I recently came across this whole rod/stroke ratio thang.
Now a little online research seems to be suggesting that getting the rod/stroke right is a good thing, the optimum being 1.75 apparently.
Originally I was just going for a stroker motor, 88mm bore, 83mm stroke and 133mm rod, giving a rod/stroke of 1.6. However if I go down the optimised route I could go with an 88mm bore, 79mm stroke and a 137mm rod giving a 1.73 ratio.
For both option I'll have to have custom pistons. For the stroker though I'll need to source the very rare LD28 crank. For the optimised approach I can get off the shelf crank and rods or even go with custom rods of 138mm which would give the ideal 1.75 ratio.
The stroker motor will be 3029cc and the optimised 2883cc. I'll be aiming for around a 10.5:1 compression ratio for both solutions.
So this is the crux of the argument. Would the 'no substitute for cubes' win out or would the 'rod/stroke' win out in the all important torque stakes?
The head is nicely ported. It'll be running a 270 to 290 degree cam depending on final decision. It will be fuel injection through indiviual 45mm TBs and the exhaust will have a full on 6-3-2 manifold.
What do you think?
Cheers,
Rob
Your project does sound very interesting, especially as I'm in the process of doing my own BMW m20 2 valve revver of the same capacity as yours. I'm designing my own cams though and redesigning the ports and combustion chambers (although they're already very good in stock form).
The comments I'll make on your design are
Using either the 270-290 cam, if your Intake valve closing event is late- you could venture up to an even higher CR then 10.5 to one- depending on how knock resistant your engine is. Is it a cast iron head? The effective compression ratio is key here rather then the geometric CR.
Definately go for the biggest capcacity your budget and durability/cost constraints allow, it will have the biggest effect. Beware of "Rover V8" syndrome, if you go too big on teh capacity and don't accompany it with either larger valve sizing/porting improvements/wilder cams. If you don't you could end up with an increasingly "dieselesque" torque curve with very little top end.
If you're interested, these are the specs I'm designing my straight six engine project to:
2 valve BMW M20 block.
boring out to 87 mm - (which leaves only a 4 mm bridge distance- close but possible, with attention to head gaskets).
Using the E36 85.8 mm throw crank (very strong!)- but offset grinding it to achieve an 86.8 mm stroke!
Gives a 3096cc capacity.
Using the 138 mm long rods from the M3 due to their incredible strength. My biggest problem is that height to the deck is now 206.8-206.2 mm, and that needs a piston with a compression height of 23.8mm, very TIGHT to try and fit a ring pack into!
Using specially treated rocker arms that cam withstand 7500 rpm plus that have had the pads micro polished.
My own design of cam- valve events biassed toward good top end- but still achieving "2.8 litres worth" of low speed torque. Currently aiming for a 296 period, but bring this down as I evolve and develop the cam design. I'm undertaking a new practice in this cam design of
1) using a multipoloynomial method which was never used even 15 years ago
2)and a rapid entrainment velocity method- that pushes the boundarie and uses Dowson (of Leeds) theory - by making sure the entrainment velocity for lubrication only passes through the "zero point" for a minimal amount of time.
Because I'm using port throttles and not lifting the inlet valve more then 2.2 mm at TDC I don't envisage too many idle problems.
The very late intake valve closing event allows me to get the compression ratio up to 11.2:1 and I still envisage using 95 octane fuel. Although the dynamic or effective CR is lowered, the increase in expansion ratio will boost fuel economy and general thermodynamic efficiecny immensely.
The cams hopefully will be Chromium nitrided.
A notable Japanese manufacturer has shown that chromium nitride hardened cams used in conjunction with micro polished pads has similar frictional properties to using roller rockers!
I'm going to then try to squeeze in 45 mm intake valves (hollow)- (up from the usual 42mm). I have a special flow rig developed port shape I intend to use (closely guarded)- all I will say is that i'll be paying close attention to the valve guide protrusion and the shroud area there!
And that's about the size of it!
Marquis_Rex said:
[If you're interested, these are the specs I'm designing my straight six engine project to:
2 valve BMW M20 block.
boring out to 87 mm - (which leaves only a 4 mm bridge distance- close but possible, with attention to head gaskets).
You aren't worried about bore flex? 4mm is pretty darn small...
Mr2Mike said:
[If you're interested, these are the specs I'm designing my straight six engine project to:
2 valve BMW M20 block.
boring out to 87 mm - (which leaves only a 4 mm bridge distance- close but possible, with attention to head gaskets).
You aren't worried about bore flex? 4mm is pretty darn small...
Beat me to it! My thoughts exactly.
There will be a degree of bore distorsion- but the block is cast iron.It IS true that a straight six is more prone to these effects due to the extra heat sink from the cylinders.
The E46 S54 3.2 engine has the same bore spacing and bore sizes. There are no water ways between the bores on this engine, and Metric mechanic (in the USA-where else?) have successfully raced this config for a while now, they are of the opinion that the M20 block is meatear then then M50/S50.
Examples of ALLOY engines with a 4 mm bridge distance are
The V8 M5, and the now defunkt BMW/Rover NG V6 engine. Cast iron should give me a margin of safety over these applications. I'm willing to take the risk in terms of slight oil consumption increase for this one off application but I wouldn't reccommend anyone else try it.
The E46 S54 3.2 engine has the same bore spacing and bore sizes. There are no water ways between the bores on this engine, and Metric mechanic (in the USA-where else?) have successfully raced this config for a while now, they are of the opinion that the M20 block is meatear then then M50/S50.
Examples of ALLOY engines with a 4 mm bridge distance are
The V8 M5, and the now defunkt BMW/Rover NG V6 engine. Cast iron should give me a margin of safety over these applications. I'm willing to take the risk in terms of slight oil consumption increase for this one off application but I wouldn't reccommend anyone else try it.
Wow! Fantastic info there Marquis_Rex!
Little info on the engine. It's a Nissan L-series. Cast iron block, aluminium head, SOHC side flow. Inlet valves are 44mm, exhaust valves are 35mm. Not very fancy valves, 3 angle cut. Head has been ported and the valves unshrouded.
Realistic cam options are:
274 duration, 0.480 lift
270/280 split duration, 0.460 lift
290 duration, 0.495 lift
It's interesting that you mention knock resistance, one of the forums I've been looking at is claiming that detonation is greatly reduced by having a so called optimum rod/stroke ratio. Hmmmm, I guess it's possible but I really have no idea about that.
Overall my aim is to get to 300bhp NA while still having a smooth(ish), reliable(ish) engine that can be used everyday. I think 300 will be pushing it with the 2883cc engine, shouldn't be out of the question with the stroker. Good point about the Rover V8, I want the engine to be able to pull to the redline which I'm thinking will be set at 7500rpm. I'll have a soft cut off at 7000rpm though.
There's just something appealing about exploring this rod/stroke ratio approach. Using the optimum 138mm rods (interesting that that's the same as the M3 rods, I wonder...) the piston compression height is 30mm, so pretty good. Handily that's the same spec and the pistons in the Nissan RB26 (Skyline) engine. To achieve the same with the stoker crank (83mm stroke) I'd need 145mm rods which result in a 21.37mm CH to give a 0 deck height which as you know is a bit of a problem!
One other little idea is to supercharge down the line but that's a whole different issue!
Cheers,
Rob
Little info on the engine. It's a Nissan L-series. Cast iron block, aluminium head, SOHC side flow. Inlet valves are 44mm, exhaust valves are 35mm. Not very fancy valves, 3 angle cut. Head has been ported and the valves unshrouded.
Realistic cam options are:
274 duration, 0.480 lift
270/280 split duration, 0.460 lift
290 duration, 0.495 lift
It's interesting that you mention knock resistance, one of the forums I've been looking at is claiming that detonation is greatly reduced by having a so called optimum rod/stroke ratio. Hmmmm, I guess it's possible but I really have no idea about that.
Overall my aim is to get to 300bhp NA while still having a smooth(ish), reliable(ish) engine that can be used everyday. I think 300 will be pushing it with the 2883cc engine, shouldn't be out of the question with the stroker. Good point about the Rover V8, I want the engine to be able to pull to the redline which I'm thinking will be set at 7500rpm. I'll have a soft cut off at 7000rpm though.
There's just something appealing about exploring this rod/stroke ratio approach. Using the optimum 138mm rods (interesting that that's the same as the M3 rods, I wonder...) the piston compression height is 30mm, so pretty good. Handily that's the same spec and the pistons in the Nissan RB26 (Skyline) engine. To achieve the same with the stoker crank (83mm stroke) I'd need 145mm rods which result in a 21.37mm CH to give a 0 deck height which as you know is a bit of a problem!
One other little idea is to supercharge down the line but that's a whole different issue!
Cheers,
Rob
Hello Rob, this tech talk is addictive!
I think 300 Bhp out of a 2.8 litre is extremely ambitious!
Even for the 3 litre plus- it is POSSIBLE but ambitious.
With my spec I'm aiming for 240-250 Bhp, but with "only" 45 mm valves all I'm getting using engine cycle sim code is about 245 Bhp, but only about 60% volumetric efficiency at low speed- this is REALLY poor!
Just for back ground cycle simulation code is very accurate- if used correctly- I'd certainly rate it over badly calibrated back street dynojets- and not just on consistency! My cycle sim model correlates for torque and power on stock 320i, the 323i,the 525eta and the 325i with good accuracy so I have alot of faith in it!
The highest 2 valver power per litre I've seen has been the Porsche 911 RS (993) which is pushing 80 Bhp/litre. It is more over square then either of our engines and hence can fit 50mm valves. With 45 mm valves in an 88mm bore, I'd hazard a guess that the Datsun/Nissan engine is a Hemi engine-which is good, except there typically isn't alot of squish-squish hand in hand with a longish stroke can mininise knock by getting rid of flame front quench zones- if yoou make sure you get the clearances right.
Your target power MAY be possible, JUST,with yours, if you accept even poorer VE then even I am likely to get. My "gut" is that the 290 period you quote may not be enough to achieve your figures, you'll really need to get peak power WAY into the 7000s!
On to knock- knock is caused when the pressue and temperature conditions ahead of the smoothley progressing flame front (ideally) are such that the end gas spontaneously combusts ahead of this progression in an uncontrolled manner. I don't see how a longer rod for a given stroke can effect this. Some claim that the piston will be in a different position when combustion peak pressure occurs, but this is negligible, when you factor in spark timing and other things into the mix. The position of peak pressure during combustion at full load, occurs at about 10-14 degrees after top dead centre- for ALL engines-assuming you've achieved ignition timing that gets best torque (called MBT spark timing). It is probably still worth pursuing the longest rod you can- for favourable geomentry and friction reasons, but don't break the bank over it!
Good luck!
I think 300 Bhp out of a 2.8 litre is extremely ambitious!
Even for the 3 litre plus- it is POSSIBLE but ambitious.
With my spec I'm aiming for 240-250 Bhp, but with "only" 45 mm valves all I'm getting using engine cycle sim code is about 245 Bhp, but only about 60% volumetric efficiency at low speed- this is REALLY poor!
Just for back ground cycle simulation code is very accurate- if used correctly- I'd certainly rate it over badly calibrated back street dynojets- and not just on consistency! My cycle sim model correlates for torque and power on stock 320i, the 323i,the 525eta and the 325i with good accuracy so I have alot of faith in it!
The highest 2 valver power per litre I've seen has been the Porsche 911 RS (993) which is pushing 80 Bhp/litre. It is more over square then either of our engines and hence can fit 50mm valves. With 45 mm valves in an 88mm bore, I'd hazard a guess that the Datsun/Nissan engine is a Hemi engine-which is good, except there typically isn't alot of squish-squish hand in hand with a longish stroke can mininise knock by getting rid of flame front quench zones- if yoou make sure you get the clearances right.
Your target power MAY be possible, JUST,with yours, if you accept even poorer VE then even I am likely to get. My "gut" is that the 290 period you quote may not be enough to achieve your figures, you'll really need to get peak power WAY into the 7000s!
On to knock- knock is caused when the pressue and temperature conditions ahead of the smoothley progressing flame front (ideally) are such that the end gas spontaneously combusts ahead of this progression in an uncontrolled manner. I don't see how a longer rod for a given stroke can effect this. Some claim that the piston will be in a different position when combustion peak pressure occurs, but this is negligible, when you factor in spark timing and other things into the mix. The position of peak pressure during combustion at full load, occurs at about 10-14 degrees after top dead centre- for ALL engines-assuming you've achieved ignition timing that gets best torque (called MBT spark timing). It is probably still worth pursuing the longest rod you can- for favourable geomentry and friction reasons, but don't break the bank over it!
Good luck!
Here's some pics of the head, still a work in progress. Needs to be skiimed, new valve guides and new inlet seats so I can burn unleaded safely.
As you can see from the old gasket marks there's still plenty of material that can be removed to increase the bowl size/shape if desired. Think I'll leave them as they are though for now.
On the cam front, ther are naturally more extreme options I could go with:
300 duration, 0.560 lift
304/318 split duaration, 0.620/0.624 split lift
Both are quotes as having power between 4000 and 8500rpm, I suspect their idle characteristics are questionable though!
It's very interesting that we're both working on similar projects, although you have a slight technical advantage over me
I'm amazed no one seems to have put a BMW six into an Z yet, they're bloody fantastic. I've even thought about putting in a diesel engine, I love my 325TDS touring! Still probably should keep quiet about that idea!
Looking forward to reading about how your engine progresses.
Cheers,
Rob
As you can see from the old gasket marks there's still plenty of material that can be removed to increase the bowl size/shape if desired. Think I'll leave them as they are though for now.
On the cam front, ther are naturally more extreme options I could go with:
300 duration, 0.560 lift
304/318 split duaration, 0.620/0.624 split lift
Both are quotes as having power between 4000 and 8500rpm, I suspect their idle characteristics are questionable though!
It's very interesting that we're both working on similar projects, although you have a slight technical advantage over me
I'm amazed no one seems to have put a BMW six into an Z yet, they're bloody fantastic. I've even thought about putting in a diesel engine, I love my 325TDS touring! Still probably should keep quiet about that idea! Looking forward to reading about how your engine progresses.
Cheers,
Rob
I'm a complete duffer with this tech-talk, but I'd have thought that the conrod length was related to the capacity of the cylinder and the stroke of the crank?
You squeeze in a load of fuel and air, you ignite it and as it expands it pushes the piston down. I presume that there is an ideal point at which you say that no further useful effort is being applied to the piston, and this is when you open the exhaust valve. So the piston may have reached the limit of its (useful) potential stroke or gone past it, if you change the conrod length...?
Hypothetically, you want the crank stroke to be calculated from the above, and the pistons connected directly to the crank journals..
Open to a slagging...
You squeeze in a load of fuel and air, you ignite it and as it expands it pushes the piston down. I presume that there is an ideal point at which you say that no further useful effort is being applied to the piston, and this is when you open the exhaust valve. So the piston may have reached the limit of its (useful) potential stroke or gone past it, if you change the conrod length...?
Hypothetically, you want the crank stroke to be calculated from the above, and the pistons connected directly to the crank journals..
Open to a slagging...
lol thats funny idea Wedg1e!
The rod length has various effects on the duration of the piston at tdc and bdc, but not as far as im aware anything to do with the volume of the cylinder it serves.
The piston only moves af far as the crank throws it.
I think youre mixing up total cylinder volume as opposed to swept volume, ie the volume displaced or swept by the piston between tdc/bdc.
Total volume is the combined volume of the cylinder when the piston is at bdc plus the combustion chamber volume multiplied by the number of cylinders to find total engine volume, rather than "displacement".....(i think )
IIRC, total engine volume-displacement=combustion chamber volume divided by number of cylinders......wth am i on about?
The rod length has various effects on the duration of the piston at tdc and bdc, but not as far as im aware anything to do with the volume of the cylinder it serves.
The piston only moves af far as the crank throws it.
I think youre mixing up total cylinder volume as opposed to swept volume, ie the volume displaced or swept by the piston between tdc/bdc.
Total volume is the combined volume of the cylinder when the piston is at bdc plus the combustion chamber volume multiplied by the number of cylinders to find total engine volume, rather than "displacement".....(i think
) IIRC, total engine volume-displacement=combustion chamber volume divided by number of cylinders......wth am i on about?
For info and kind of backing up what Marquis Rex was saying I once asked one of the conrod designers in Jag engine design as to what the length should be. He said one and two thirds of the stroke. I know of people developing the aluminum 2.0 Mazda engine for race applications using a 6 inch c/c conrod so a bit closer to 2.
deltaf said:
The piston only moves af far as the crank throws it.
I know that
deltaf said:
I think youre mixing up total cylinder volume as opposed to swept volume, ie the volume displaced or swept by the piston between tdc/bdc.
Total volume is the combined volume of the cylinder when the piston is at bdc plus the combustion chamber volume multiplied by the number of cylinders to find total engine volume, rather than "displacement".....(i think
IIRC, total engine volume-displacement=combustion chamber volume divided by number of cylinders......wth am i on about?
No, you misunderstand! I was pondering on how the length of the conrod is arrived at. Obviously for practical reasons the engine should be a compact size, but you could in theory have a piston travelling a certain distance (as determined by crank throw), but connected to the crank by a 6" conrod or a 6 foot one!
What determines optimal conrod length?
Stroke must surely be determined by the distance that the expanding gases can push the piston USEFULLY, ie you couldn't get a 6 foot stroke out of a 50cc cylinder (unless the crank and rod were made from fairy wings and spun in pure Teflon bearings, in a vacuum...
) Ian
B19GRR said:
......As you can see from the old gasket marks there's still plenty of material that can be removed to increase the bowl size/shape if desired. Think I'll leave them as they are though for now.
This may not apply to your engine but here goes.
In a Chevy engine that area of the cylinder is known as 'quench' and is very important.
When the piston is at TDC the distance between it and that quench area is the thickness of the head gasket plus the distance the piston sits below the top of the block. The ideal is about 0.045" in old money.
The reason it's so important is that when coming to full compression any gases in that area are squirted out into the rest of the combustion chamber and cause turbulence that better mixes the gases.
I think you would be wise to leave it there.
If you were going to do any further work I would concentrate on removing as much material as possible from the side of the chamber closest to the edge of the valve. What you have there is valve shrouding where despite the valve being open the gases can’t get past the valve because it is too close to the wall of the chamber. Ideally the gap would be the same amount as the valve lifts but that is never possible. If you could get close to that then you would at the same time be thinking of putting in bigger valves.
Steve
>> Edited by steve_D on Thursday 12th August 22:00
wedg1e said:
No, you misunderstand! I was pondering on how the length of the conrod is arrived at. Obviously for practical reasons the engine should be a compact size, but you could in theory have a piston travelling a certain distance (as determined by crank throw), but connected to the crank by a 6" conrod or a 6 foot one!
What determines optimal conrod length?
I think practicality is a large part of it - the range of conrod lengths which end up being "sensible" isn't really very wide.
On a two-stroke there is some advantage to having the rod as short as possible, to increase crankcase compression (though you can have too much of a good thing here) and to increase the proportion of time the piston spends around the bottom end of the stroke, where the gas exchange processes take place.
wedg1e said:
Stroke must surely be determined by the distance that the expanding gases can push the piston USEFULLY, ie you couldn't get a 6 foot stroke out of a 50cc cylinder (unless the crank and rod were made from fairy wings and spun in pure Teflon bearings, in a vacuum...
Ian
That distance is surprisingly short - though perhaps less surprising when you consider cranking an engine by hand, and how little of the rotation you experience any real resistance over - it's the same with the hot gases expanding.
A hot two-stroke will open the exhaust port as soon as 90 degrees after TDC. The power you lose by not letting the exhaust gases expand fully to BDC is more than made up for by the extra time available for gas exchange allowing you to get more charge in there in the first place.
wedg1e said:
No, you misunderstand! I was pondering on how the length of the conrod is arrived at. Obviously for practical reasons the engine should be a compact size, but you could in theory have a piston travelling a certain distance (as determined by crank throw), but connected to the crank by a 6" conrod or a 6 foot one!
What determines optimal conrod length?
In theory you want as long as conrod as possible to reduce piston side loading. This benefits friction, piston scuff and piston slap NVH.
However apart form the obvious package constraints of engine height, conrod length is limited by buckling stress a function of rod length and cross-sectional area. Conrod mass has an effect on rotational balance, typically 2/3 of the rod mass (with crankpin) has to be counterbalanced by the crank weights. The other 1/3 mass (with piston mass) has an effect on reciprocating forces and moments. Rod mass also effects inertial forces and limits the safe mechanical rev-range of the forces i.e. bearing loads and conrod whip (see H section rods on racing engines).
These things are all a compromise, which is why all engines have conrods that are typically between 1.5 and 2.0 rod/stroke ratio.
Hope that helps
Phil
wheeljack888 said:
No, you misunderstand! I was pondering on how the length of the conrod is arrived at. Obviously for practical reasons the engine should be a compact size, but you could in theory have a piston travelling a certain distance (as determined by crank throw), but connected to the crank by a 6" conrod or a 6 foot one!
What determines optimal conrod length?
In theory you want as long as conrod as possible to reduce piston side loading. This benefits friction, piston scuff and piston slap NVH.
However apart form the obvious package constraints of engine height, conrod length is limited by buckling stress a function of rod length and cross-sectional area. Conrod mass has an effect on rotational balance, typically 2/3 of the rod mass (with crankpin) has to be counterbalanced by the crank weights. The other 1/3 mass (with piston mass) has an effect on reciprocating forces and moments. Rod mass also effects inertial forces and limits the safe mechanical rev-range of the forces i.e. bearing loads and conrod whip (see H section rods on racing engines).
These things are all a compromise, which is why all engines have conrods that are typically between 1.5 and 2.0 rod/stroke ratio.
Hope that helps
Phil
It does, cheers! Given that the internal combustin engine has been around for a century or so, I'm fairly sure that there aren't many areas that haven't been optimised by calculation, trial and error and more than a few failures - whether an engine exists that is the ultimate collection of these optimals, though... I suppose F1 motors must come close (although perhaps there are a few compromises there too...
)Marquis_Rex your engine interests me as I am using an M70 BMW engine which is very hard to find info on let alone ways of upgrading it...... But I understand its loosly based on two M20 engines joined at the crank, I'd be very interested in any technical info you could share.
The plan for mine is to pretty much ustilise the engine as is (with a refresh) but only modify the heads and fit new cams as I am adding twin turbos to get the power.
All the electrics will be new (Motec M800).
The plan for mine is to pretty much ustilise the engine as is (with a refresh) but only modify the heads and fit new cams as I am adding twin turbos to get the power.
All the electrics will be new (Motec M800).
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