Short Pistons - how short can you go?!!
Discussion
Hey all,
Kind of following on from my rod/stroke theory thread I've been making some slow but steady progress in my engine build. I got hold of a complete 2.8 engine and have also managed to get a stroker crank for it - somewhat of a holy grail in the Z world as they're from a Nissan Maxima diesel engine not available in the UK.
So here we go then, 3029cc engine, 88mm bore, 83mm stroke. Nice, should be good for oodles of torque.
Relating back to the rod/stroke theory to get the supposed ideal of 1.75:1 I'll need 145mm con rods. OK, not too much of a prob, I can get a set of forged ones from the states for around $1200. Downside is that leaves me with a piston with a compression height of only 21mm which isn't going to leave a lot of room for 3 rings.
So should I sacrifice piston height or rod length? e.g. a 140mm rod would give me a ratio of 1.69 with a 26mm piston which seems a little more sensible to me.
What do you think?
Cheers,
Rob
Kind of following on from my rod/stroke theory thread I've been making some slow but steady progress in my engine build. I got hold of a complete 2.8 engine and have also managed to get a stroker crank for it - somewhat of a holy grail in the Z world as they're from a Nissan Maxima diesel engine not available in the UK.
So here we go then, 3029cc engine, 88mm bore, 83mm stroke. Nice, should be good for oodles of torque.
Relating back to the rod/stroke theory to get the supposed ideal of 1.75:1 I'll need 145mm con rods. OK, not too much of a prob, I can get a set of forged ones from the states for around $1200. Downside is that leaves me with a piston with a compression height of only 21mm which isn't going to leave a lot of room for 3 rings.
So should I sacrifice piston height or rod length? e.g. a 140mm rod would give me a ratio of 1.69 with a 26mm piston which seems a little more sensible to me.
What do you think?
Cheers,
Rob
Well I'm asking, is it feasible to build an engine with pistons that have comp height of 21mm?
Being a happy and innocent type bloke this seems like a perfectly reasonable question
Whatever pistons I go with they will be custom forged jobbies so it's just a matter of getting the dimensions workable.
Good point about the gudgeon pin diameter. Stock size of that is 21mm, so that would leave a vast 10.5mm of piston above the pin to fit 3 rings into. I have heard about some oil rings that can go over the pin hole but that's just getting flash really!
Perhaps another way of asking this is what would be the recommended minimum height from the top of the piston to the top of the first ring, and from there what would be the minimum distances between each ring. All added up that should give me the minimum height above the gudgeon pin and hence I get the minimum comp height and maximum rod length I could use.
Cheers,
Rob
Being a happy and innocent type bloke this seems like a perfectly reasonable question
Whatever pistons I go with they will be custom forged jobbies so it's just a matter of getting the dimensions workable. Good point about the gudgeon pin diameter. Stock size of that is 21mm, so that would leave a vast 10.5mm of piston above the pin to fit 3 rings into. I have heard about some oil rings that can go over the pin hole but that's just getting flash really!
Perhaps another way of asking this is what would be the recommended minimum height from the top of the piston to the top of the first ring, and from there what would be the minimum distances between each ring. All added up that should give me the minimum height above the gudgeon pin and hence I get the minimum comp height and maximum rod length I could use.
Cheers,
Rob
Crown thickness is usually very safe at 10% of bore diameter so would you use an 8.8mm top ring land or less? You can't cram the other 2 rings into 2mm of piston. Me thinks you have a problem. I'd go for the oil ring with a suport rail and give yourself some space for the other two rings. Why not ask a piston company? After all, you're going to buy from them right?
Even if you used a 5mm top ring land you're still stuffed.
Boosted.
>> Edited by Boosted LS1 on Thursday 4th November 21:37
Even if you used a 5mm top ring land you're still stuffed.
Boosted.
>> Edited by Boosted LS1 on Thursday 4th November 21:37
Hi Rob, I think we've covered this already, while I don't adhere to this 1.75 "ideal" (lots of engines such as package compromised Porsche 911s, racing engines or more significantly –finely optimised machinery like E46 M3s don't follow this)- however I agree that the long rod offers less angularity into the bores and overall are a better idea.
I've been looking into the compression height issue in some depth, perhaps you can learn from my example-as I'm stumbling across similar problems:
One of my potential avenues for a REVVY large capacity stroker (yes they appear contradictory until you hear the ingredients)- is stretching my 2 valve BMW M20 to the limit.
The current recipe is boring out the standard 84mm bore block out to 87mm (I'm looking into block thicknesses already)
Using the 85.8 mm throw crank of the euro E36 3 litre S50 M3 engine and offset grinding (0.5mm under size) to get a 86.8 mm throw. This gives a capacity of 3.096 litres The Euro M3 crank uses larger 50 mm con rod journals as opposed to the standard M20/M50 45 mm big ends.
I was hoping to use 139 or 138 centre to centre length con rods from either the E36 M3 3 litre of E36 M3 evo engine. This combination of rods and crank will be able to withstand considerable revs, without resorting to expensive custom build stuff.
The rocker arms have been polished and individually checked for porosity in the fork area (where they usually fail).
Unfortunately the later M20 block has a deck height of 206.2mm- and it is this later block I need as it has redesigned bottom end oil ways and can use common bearings with either the M50 or the high revving S50 "M" engines. Unfortunately this leaves me with a potential compression height as low as 23.8 mm. THIS is low, 21mm is unprecedented. From my production engine design experience I've found that using a 22 mm gudgeon pin , assuming a ultra small 3 mm top ring land, two 2mm intermediate ring lands, two 1.2 mm thick compression rings, a 2.54 mm thick oil controller ring- will leave you with an absolute MINIMUM requirement of 25 mm. In fact the smallest Compression height I've seen in a production engine has been 27 mm- and that was on a Honda. I don't know if it is possible that the after market can custom build and come up with something that pushes the boundaries more.
In the end I thought that a 3mm top ring land was not a good idea. Production engines go to this solely for Tree hugging reasons. It is absolutely no good from a durability point of view, the pressure on the top ring becomes quite high and also without good ignition control (I'm sticking to old style dizzy!) it is not a good idea. I prefer the margin of safety afforded by a beefy top land.
So I then pursued two new avenues:
A spacer on the block between the head and sandwiched between two gaskets
OR
using an oldtstyle non-slipper skirt design piston that allows an oil control ring BELOW the gudgeon pin!
The problem with the oil control ring below the gudgeon pin for me is that it means the piston must still be cylindrical below the gudgeon pin boss- this will increase contact friction (marginally) and the gudgeon pin mass will increase as it must be full width.
So I'm starting to pursue the latter method. After a talk to Chessmen engineering- a place near here, they've talked me into using a custom made head gasket- which is the thickness I want and need-rather then the more precarious practice of a spacer and two gaskets! I'm only looking for about 4-5 mm- and I'll aim for a nice healthy thick ring land that is bigger then this on my piston.
The next problem to tackle (isn't there always one?) is the new found height and whether the cam belt/tensioner layout can accommodate this. It's a tight enough fit as standard with the tensioner fully retracted. Me being the obsessive BMW engine nut however realised that the 324 tds diesel engine (M21) which uses a very similar block- has a 6 mm taller block already- so I could use a belt from this application!
If you're interested in my other engine proposed specs so far, I'll post these:
I'm designing my own cam, and targeting an 80 Bhp/litre ( the holy grail among road going two valve engines)- I'll be using a new method which zips quickly through zero entrainment velocity point ( you need entrainment velocity for a decent oil film) which should ensure low friction, while maximising lift for the duration. I'm going to try to squeeze in 45mm diameter valves. The large capacity of the engine should allow me to aim for a rather peaky torque curve. Current prediction software is zeroing in on a 290-295 period- but using a late IVC and restricting valve overlap, while also using port throttles should effect idle minimally . The late IVC and the resulting low dynamic compression ratio should allow me to use an 11.2:1 geometric Compression ratio and continue to use 95 octane MON. The high expansion ratio afforded by 11.2:1 , combined with, port throttles ( allows quick "pressure recovery" for lowering pumping loop work) and the late IVC should actually INCREASE fuel economy at part load.
I've been looking into the compression height issue in some depth, perhaps you can learn from my example-as I'm stumbling across similar problems:
One of my potential avenues for a REVVY large capacity stroker (yes they appear contradictory until you hear the ingredients)- is stretching my 2 valve BMW M20 to the limit.
The current recipe is boring out the standard 84mm bore block out to 87mm (I'm looking into block thicknesses already)
Using the 85.8 mm throw crank of the euro E36 3 litre S50 M3 engine and offset grinding (0.5mm under size) to get a 86.8 mm throw. This gives a capacity of 3.096 litres The Euro M3 crank uses larger 50 mm con rod journals as opposed to the standard M20/M50 45 mm big ends.
I was hoping to use 139 or 138 centre to centre length con rods from either the E36 M3 3 litre of E36 M3 evo engine. This combination of rods and crank will be able to withstand considerable revs, without resorting to expensive custom build stuff.
The rocker arms have been polished and individually checked for porosity in the fork area (where they usually fail).
Unfortunately the later M20 block has a deck height of 206.2mm- and it is this later block I need as it has redesigned bottom end oil ways and can use common bearings with either the M50 or the high revving S50 "M" engines. Unfortunately this leaves me with a potential compression height as low as 23.8 mm. THIS is low, 21mm is unprecedented. From my production engine design experience I've found that using a 22 mm gudgeon pin , assuming a ultra small 3 mm top ring land, two 2mm intermediate ring lands, two 1.2 mm thick compression rings, a 2.54 mm thick oil controller ring- will leave you with an absolute MINIMUM requirement of 25 mm. In fact the smallest Compression height I've seen in a production engine has been 27 mm- and that was on a Honda. I don't know if it is possible that the after market can custom build and come up with something that pushes the boundaries more.
In the end I thought that a 3mm top ring land was not a good idea. Production engines go to this solely for Tree hugging reasons. It is absolutely no good from a durability point of view, the pressure on the top ring becomes quite high and also without good ignition control (I'm sticking to old style dizzy!) it is not a good idea. I prefer the margin of safety afforded by a beefy top land.
So I then pursued two new avenues:
A spacer on the block between the head and sandwiched between two gaskets
OR
using an oldtstyle non-slipper skirt design piston that allows an oil control ring BELOW the gudgeon pin!
The problem with the oil control ring below the gudgeon pin for me is that it means the piston must still be cylindrical below the gudgeon pin boss- this will increase contact friction (marginally) and the gudgeon pin mass will increase as it must be full width.
So I'm starting to pursue the latter method. After a talk to Chessmen engineering- a place near here, they've talked me into using a custom made head gasket- which is the thickness I want and need-rather then the more precarious practice of a spacer and two gaskets! I'm only looking for about 4-5 mm- and I'll aim for a nice healthy thick ring land that is bigger then this on my piston.
The next problem to tackle (isn't there always one?) is the new found height and whether the cam belt/tensioner layout can accommodate this. It's a tight enough fit as standard with the tensioner fully retracted. Me being the obsessive BMW engine nut however realised that the 324 tds diesel engine (M21) which uses a very similar block- has a 6 mm taller block already- so I could use a belt from this application!
If you're interested in my other engine proposed specs so far, I'll post these:
I'm designing my own cam, and targeting an 80 Bhp/litre ( the holy grail among road going two valve engines)- I'll be using a new method which zips quickly through zero entrainment velocity point ( you need entrainment velocity for a decent oil film) which should ensure low friction, while maximising lift for the duration. I'm going to try to squeeze in 45mm diameter valves. The large capacity of the engine should allow me to aim for a rather peaky torque curve. Current prediction software is zeroing in on a 290-295 period- but using a late IVC and restricting valve overlap, while also using port throttles should effect idle minimally . The late IVC and the resulting low dynamic compression ratio should allow me to use an 11.2:1 geometric Compression ratio and continue to use 95 octane MON. The high expansion ratio afforded by 11.2:1 , combined with, port throttles ( allows quick "pressure recovery" for lowering pumping loop work) and the late IVC should actually INCREASE fuel economy at part load.
OK, fantastic info as ever, thanks mate!
Right, I'll settle on 140mm rods then. This gives me a 26.37mm compression height with a 0mm deck height. Using your ring measurements and taking the 21mm gudgeon pin diameter into account then that leaves me with a 6.93mm max top ring land, not quite the 8.8mm Boosted recommends but getting close. Close enough I think that if I specify an oil ring with a support rail I'll be able to get there.
If I really wanted to push the compression height then I could go to a positive deck height. I'll be using a 1mm HKS metal gasket and the pistons with have valve relief pockets cut but I think that's probably pushing things a little.
For general piston info I've been using the Accralite piston search:
www.accralite.com/accralite/Search/Pistons.asp
Which made it all sound very straight forward but of course I wasn't aware of the 10% of bore idea Boosted brought up.
I'll be getting the pistons from JE in the states, there's a company out there that are doing a cracking deal on an engine rebuild package and have a lot of experience with the Z engine.
Cheers,
Rob
Right, I'll settle on 140mm rods then. This gives me a 26.37mm compression height with a 0mm deck height. Using your ring measurements and taking the 21mm gudgeon pin diameter into account then that leaves me with a 6.93mm max top ring land, not quite the 8.8mm Boosted recommends but getting close. Close enough I think that if I specify an oil ring with a support rail I'll be able to get there.
If I really wanted to push the compression height then I could go to a positive deck height. I'll be using a 1mm HKS metal gasket and the pistons with have valve relief pockets cut but I think that's probably pushing things a little.
For general piston info I've been using the Accralite piston search:
www.accralite.com/accralite/Search/Pistons.asp
Which made it all sound very straight forward but of course I wasn't aware of the 10% of bore idea Boosted brought up.
I'll be getting the pistons from JE in the states, there's a company out there that are doing a cracking deal on an engine rebuild package and have a lot of experience with the Z engine.
Cheers,
Rob
Yep, 1mm compressed. Rods are forged steel, for 140mm rods I can get nissan stock items from their FJ20 turbo engine which are rock solid.
How come you think it will be a bit tight though with pistons at zero deck height? They'll be manufactured with vale relief pockets cut to an appropriate depth to suit the head thickness and cam/valve lift.
Cheers,
Rob
How come you think it will be a bit tight though with pistons at zero deck height? They'll be manufactured with vale relief pockets cut to an appropriate depth to suit the head thickness and cam/valve lift.
Cheers,
Rob
After reading this thread, I just measured one of the Accralites from my 1.8K...
they run on an 18mm pin, and from pin centre to top are 24mm, this is made up with:
5.5mm top land
1mm top ring
3mm middle land
1mm second ring
2mm bottom land
2mm oil ring
this leaves only .5mm front pin to oil ring, however this does not apprear to be a problem.
they run on an 18mm pin, and from pin centre to top are 24mm, this is made up with:
5.5mm top land
1mm top ring
3mm middle land
1mm second ring
2mm bottom land
2mm oil ring
this leaves only .5mm front pin to oil ring, however this does not apprear to be a problem.
B19GRR said:
.....How come you think it will be a bit tight though with pistons at zero deck height? They'll be manufactured with vale relief pockets cut to an appropriate depth to suit the head thickness and cam/valve lift.....
When I built my Chevy engine the recommendation was not to go below 0.040" of 'Quench'. This strange term refers to the area where the piston approaches a flat surface of the cylinder head face. It is important as it causes turbulence and improved fuel/air mixing.
The problem is that the 0.040" will be reduced as the rods expand. This expansion is even greater with alloy rods so there would be the possibility of the piston hitting the head. Your 1mm is 0.039".
It may be that your combustion chamber does not have a quench area in which case all of the above is a load of b****cks and I’ll go get my coat.
Steve
steve_D said:
B19GRR said:
.....How come you think it will be a bit tight though with pistons at zero deck height? They'll be manufactured with vale relief pockets cut to an appropriate depth to suit the head thickness and cam/valve lift.....
When I built my Chevy engine the recommendation was not to go below 0.040" of 'Quench'. This strange term refers to the area where the piston approaches a flat surface of the cylinder head face. It is important as it causes turbulence and improved fuel/air mixing.
The problem is that the 0.040" will be reduced as the rods expand. This expansion is even greater with alloy rods so there would be the possibility of the piston hitting the head. Your 1mm is 0.039".
It may be that your combustion chamber does not have a quench area in which case all of the above is a load of b****cks and I’ll go get my coat.
Steve
Ah, I get you Steve. I think I'm pretty safe with the chamber design in the head I'll be using. It's pretty old school. I believe some of Nissans later heads had a much better defined quench area than this one.
Simon, I found various K-series piston specs on the Accralite site, all with pretty low top ring lands. Maybe they can get away with it more as they have fairly large intruders? The pistons I'll have will be basically flat tops.
They guys in the States seem pretty clued up though, saying they'll design the piston to my specs but making sure all it's details will work correctly for my engine. Guess it's time to dust off a credit card
Cheers,
Rob
>> Edited by B19GRR on Tuesday 9th November 18:48
Marquis_Rex said:
One of my potential avenues for a REVVY large capacity stroker (yes they appear contradictory until you hear the ingredients)- is stretching my 2 valve BMW M20 to the limit.
The current recipe is boring out the standard 84mm bore block out to 87mm (I'm looking into block thicknesses already)
Using the 85.8 mm throw crank of the euro E36 3 litre S50 M3 engine and offset grinding (0.5mm under size) to get a 86.8 mm throw. This gives a capacity of 3.096 litres The Euro M3 crank uses larger 50 mm con rod journals as opposed to the standard M20/M50 45 mm big ends.
I was hoping to use 139 or 138 centre to centre length con rods from either the E36 M3 3 litre of E36 M3 evo engine. This combination of rods and crank will be able to withstand considerable revs, without resorting to expensive custom build stuff.
The rocker arms have been polished and individually checked for porosity in the fork area (where they usually fail).
For research on my V12 project I purchased a M20 engine rescently and stripped a piston from it today... the piston and rod are almost identical at first inspection to the V12 items, and I think the rods may just be a modified M20 item so they can run together on the crank.
This means that I will be very interested in any info you may have on adapting M3 rods to your M20 project, as I may well do the same myself.
Are you planning to mod the heads yourself or are you sending them somewhere?
Cheers
Andy
Hi Andy, I don't recall whether your V12 is an early single cam affair or a later twin cam pent roof job?
The Euro M3 engine is the same size main bearings as the M20/M70 but the big end bearings are 50 mm diameter rather then the usual 45mm. I also think the small end of the M3 rods is only 21mm rather then 22mm-which means you can't interchange then for your application . The early V12 shared a lot of components with the M40 4 cylinder (those pesky 318is are good for something then!) - it has a similar combustion chamber shape and port shape and uses the same valves too. You're right that M20 bits can be interchanged though as both engine share the same diameter gudgeon pin and big end/bearings. The con rods can be interchanged too. You can also therefore use con rods from the M50 too. The M20 has con rod lengths ranging from 130 to 135mm while the M50 used rods from 135 to 145 mm I think. If your engine is a single cam V12 then the pistons from 316is and 318i are a better bet. If it's a later engine then I'd hazard a guess that it might have more commonality with M50s or the M42 4 valve 318 is. What you need is a list of pistons dimentions, including compression heights, con rod lengths bore, etc of the M50, M20, M40 and M70s. I've been trying to get hold of this information from a piston supplier for a while now.
I'll be doing all of the head work myself, based on what I've learned from my port flow rig work of production road cars and getting them through emissions etc etc. It's amazing when doing your own stuff, because all of the constraints of cost, manufacturer and emissions are removed! Infact I wouldn't trust anyone else again. I've already in the past made the mistake of giving my cylinder head to people like Griffin motorsport. I've also seen some laughable practices with other places, such as machining away squish in all the wrong places! You need some who either understands the philosophy of your engine- or who can hypothesize it and extend it to cover the mods you're doing!
The Euro M3 engine is the same size main bearings as the M20/M70 but the big end bearings are 50 mm diameter rather then the usual 45mm. I also think the small end of the M3 rods is only 21mm rather then 22mm-which means you can't interchange then for your application . The early V12 shared a lot of components with the M40 4 cylinder (those pesky 318is are good for something then!) - it has a similar combustion chamber shape and port shape and uses the same valves too. You're right that M20 bits can be interchanged though as both engine share the same diameter gudgeon pin and big end/bearings. The con rods can be interchanged too. You can also therefore use con rods from the M50 too. The M20 has con rod lengths ranging from 130 to 135mm while the M50 used rods from 135 to 145 mm I think. If your engine is a single cam V12 then the pistons from 316is and 318i are a better bet. If it's a later engine then I'd hazard a guess that it might have more commonality with M50s or the M42 4 valve 318 is. What you need is a list of pistons dimentions, including compression heights, con rod lengths bore, etc of the M50, M20, M40 and M70s. I've been trying to get hold of this information from a piston supplier for a while now.
I'll be doing all of the head work myself, based on what I've learned from my port flow rig work of production road cars and getting them through emissions etc etc. It's amazing when doing your own stuff, because all of the constraints of cost, manufacturer and emissions are removed! Infact I wouldn't trust anyone else again. I've already in the past made the mistake of giving my cylinder head to people like Griffin motorsport. I've also seen some laughable practices with other places, such as machining away squish in all the wrong places! You need some who either understands the philosophy of your engine- or who can hypothesize it and extend it to cover the mods you're doing!
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff