Quick forced induction theory question.
Discussion
If you were building a blown engine and you had the opportunity to add say, 10% more capacity, would you whack it on in bore OR stroke? Why?
I have read my fair share of forced indution theory and noticed that a few people are opting for bigger bores but not strokes, this goes against my logic.
For the sake of argument, let's say the boost is a maximum of 12 PSi.
What do you reckon?
I have read my fair share of forced indution theory and noticed that a few people are opting for bigger bores but not strokes, this goes against my logic.
For the sake of argument, let's say the boost is a maximum of 12 PSi.
What do you reckon?
In general, increasing the bore seems preferable. Increasing the stroke increases the piston speeds (and hence acceleration loads) and means you run higher rod angles. Increasing the bore potentially gives you more space for bigger valves. I guess that there will usually be practical restrictions which mean you don't have a free choice though.
Obviously running a bigger bore allows you to deshroud the valves a tad.
I would have thought that stroking produced a lot more torque though. Since not many are screamers, I would have thought stroking was a viable solution. I must be missing something as no-one is doing it.
Essentially, I have been mucking around with my old smoky 1275 engine and am looking at a DIY build, so torque is the main criteria as the blower doesn't produce more than 12 PSI (intercooled with 8:1CR). I'm running a very long legged final drive and so need every ft/lb to give good low down accelleration.
Going from an 81 to 84 stroke crank is worth about 40cc but I wonder if it repays more than its worth in torque. (ie:- about £300 worth)
The information about this (and I have read a fair bit of stuff seems to be stuff you have to derive yourself).
I just thought I would see what you chaps reckon.
BTW, I'm using a decompression plate and so chamber size is not a problem.
Will have a think about that GreenV8s, thanks
>> Edited by love machine on Sunday 6th March 21:46
I would have thought that stroking produced a lot more torque though. Since not many are screamers, I would have thought stroking was a viable solution. I must be missing something as no-one is doing it.
Essentially, I have been mucking around with my old smoky 1275 engine and am looking at a DIY build, so torque is the main criteria as the blower doesn't produce more than 12 PSI (intercooled with 8:1CR). I'm running a very long legged final drive and so need every ft/lb to give good low down accelleration.
Going from an 81 to 84 stroke crank is worth about 40cc but I wonder if it repays more than its worth in torque. (ie:- about £300 worth)
The information about this (and I have read a fair bit of stuff seems to be stuff you have to derive yourself).
I just thought I would see what you chaps reckon.
BTW, I'm using a decompression plate and so chamber size is not a problem.
Will have a think about that GreenV8s, thanks
>> Edited by love machine on Sunday 6th March 21:46
How are you getting the extra stroke ? Is it just reprofiling the existing crank or a completly new one ?
If its just reprofiling and you are going for 12 psi boost then expect problems.
If its a new crank (and rods) ask if they can take the extra 12psi. If the answer is "no problem" bore it out as well.
Also think about why the long final drive - can you get a lower final drive with a higher top gear, then you get the better acceleration in the lower gears but still retain the relaxed top gear.
If its just reprofiling and you are going for 12 psi boost then expect problems.
If its a new crank (and rods) ask if they can take the extra 12psi. If the answer is "no problem" bore it out as well.
Also think about why the long final drive - can you get a lower final drive with a higher top gear, then you get the better acceleration in the lower gears but still retain the relaxed top gear.
People tend to go for the overbore bore first if the block has a bit of meat because it's cheaper. A rebore and a set of pistons compares very favourably to a solution involving the same plus new crank and rods, the latter often being necessary due to the increased throw (and hence forces) on a stroked engine.
Isn't the limit on A-series engines around 1440, bored and stroked? Seem to recall that that was around the maximum reliable capacity reached, using a metro crank with bigger journals (but a lower material spec. unfortunately). Seriously recommend the bible on A-series tuning (Vizard I think) - he discusses overall engine geometry with respect to the limits of the different types of block.
Best of luck.
Isn't the limit on A-series engines around 1440, bored and stroked? Seem to recall that that was around the maximum reliable capacity reached, using a metro crank with bigger journals (but a lower material spec. unfortunately). Seriously recommend the bible on A-series tuning (Vizard I think) - he discusses overall engine geometry with respect to the limits of the different types of block.
Best of luck.
Well, lost the job today and so it's likely to stay at the overbore!
The usual method is this, take a 1275 engine (81.3mm stroke, the capacity is usually increased by +20 on the bore to 1293, as this is the first overbore and a popular size for deep-dished pistons, a +20 can hack large boosts and is favoured for strength by the turbo boys). Recently people have started blowing engines with +120 (73.5mm) bores, this gives 1380 with the standard stroke. Offset boring minimises bore flex between the centre pistons. This works well with smaller boosts. One guy runs 8psi boost (S/C) and develops 165lb/ft at about 4000 Rpm, which is a good effort. The next step by N/A tuners is to take an A+ crank and offset grind the journals to Cooper S Size (smaller) and run S Rods. Assuming you have some S rods, it is quite cheap to get an offset ground crank. For the revs which we are considering, (Sub 6000) the setup is reliable. I think you can get a 1480 out of a bare forging. It's getting a bit big to get the CR down though.
As I've said, a lot of forced induction research has not been done as "Twisting the bleed valve" gives more power, no need to really investigate the depths of cam design/valves/bores/strokes. Just more power like that (clicks fingers). As it is my mission to produce as much torque as possible, I figure it would be viable.
From what I gather, across the board, people are using decompression plates as they are seen to be working and not altering the squish characteristics if made properly. This allows much bigger chambers/CR drop for bigger capacities.
My attitude is "in for a penny, in for a pound", I'm spending a fortune, so I might as well spend a real fortune and produce something amazing.
Sidethrusts are a worry in the gearbox and the associated bearing reliability, (as the gearbox is like a swiss watch anyway) and so I will be using a straight cut gears, this is accentuated by having super low down grunt and a tall final drive. The lowest ratio first I can get in this configuration is 2.54.
I suppose the answer is to try it and see if it works.
The usual method is this, take a 1275 engine (81.3mm stroke, the capacity is usually increased by +20 on the bore to 1293, as this is the first overbore and a popular size for deep-dished pistons, a +20 can hack large boosts and is favoured for strength by the turbo boys). Recently people have started blowing engines with +120 (73.5mm) bores, this gives 1380 with the standard stroke. Offset boring minimises bore flex between the centre pistons. This works well with smaller boosts. One guy runs 8psi boost (S/C) and develops 165lb/ft at about 4000 Rpm, which is a good effort. The next step by N/A tuners is to take an A+ crank and offset grind the journals to Cooper S Size (smaller) and run S Rods. Assuming you have some S rods, it is quite cheap to get an offset ground crank. For the revs which we are considering, (Sub 6000) the setup is reliable. I think you can get a 1480 out of a bare forging. It's getting a bit big to get the CR down though.
As I've said, a lot of forced induction research has not been done as "Twisting the bleed valve" gives more power, no need to really investigate the depths of cam design/valves/bores/strokes. Just more power like that (clicks fingers). As it is my mission to produce as much torque as possible, I figure it would be viable.
From what I gather, across the board, people are using decompression plates as they are seen to be working and not altering the squish characteristics if made properly. This allows much bigger chambers/CR drop for bigger capacities.
My attitude is "in for a penny, in for a pound", I'm spending a fortune, so I might as well spend a real fortune and produce something amazing.
Sidethrusts are a worry in the gearbox and the associated bearing reliability, (as the gearbox is like a swiss watch anyway) and so I will be using a straight cut gears, this is accentuated by having super low down grunt and a tall final drive. The lowest ratio first I can get in this configuration is 2.54.
I suppose the answer is to try it and see if it works.
i think it depneds on the engine. for example, if you want to increase capacity in the EVOs 4G63t engine you HAVE to go for a strocker as there is little extra material between the cylinders. the chevy guys also seem to go for strockers, but i think it more down to strocker cranks being of similar price to standard throw units.
i would look at what has been done and what has remanined reliable. you never know, you maybe able to do both!
thanks Chris.
i would look at what has been done and what has remanined reliable. you never know, you maybe able to do both!
thanks Chris.
For simplicitys sake, just bore it.
I ran my 1293 mini turbo with totally std bottom end, pistons machined to lower to CR to about 7.5:1, and ran 20psi, and it was driven daily for about 3 years until I realised minis arent that nice to drive. I did occasionally see a bit more boost than that, but not often.
While there were head gasket and gearbox related problems from time to time, engine reliability was never an issue, and was regularly reved to 7500rpm, and lasted about 40k before I finally parked it up. That same engine is still in it today, and still runs fine, even though I rarely drive it.
Its simple, cheap and it worked.
I ran my 1293 mini turbo with totally std bottom end, pistons machined to lower to CR to about 7.5:1, and ran 20psi, and it was driven daily for about 3 years until I realised minis arent that nice to drive. I did occasionally see a bit more boost than that, but not often.
While there were head gasket and gearbox related problems from time to time, engine reliability was never an issue, and was regularly reved to 7500rpm, and lasted about 40k before I finally parked it up. That same engine is still in it today, and still runs fine, even though I rarely drive it.
Its simple, cheap and it worked.
Stevie, point is that my blower starts heating things up after 10 PSI, I can keep it down with an intercooler to say about 12. The idea is to shift as much air/fuel through it with that as a maximum residual boost. Going up a notch would produce more torque. Bearing in mind I can't afford a billet crank, I will be staying below about 6500.
With the whole setup, I should be able to advance the ignition curve at about 30 points to +/- 0.5 deg which should give good results.
If I can see a cheapish 1430 crank/rods setup, I might go for it. Probably stick with the std stroke though and risk revving it more.
With the whole setup, I should be able to advance the ignition curve at about 30 points to +/- 0.5 deg which should give good results.
If I can see a cheapish 1430 crank/rods setup, I might go for it. Probably stick with the std stroke though and risk revving it more.
My point is, that when it comes to tuning, that it isnt always necessary to believe the hype, and spend loads of dosh on certain things..
The hype suggested I needed steel this and that if I wanted to rev, or go above xxxbhp.
My engine was abused regularly, and ran std crank, std rods, mains ( no straps, or steel caps ), and std turbo pistons, albeit modified for lower CR.
For a while, I even had a stupid camshaft in it that allowed me to rev easily to 8000rpm, although it was soon removed, as it made the car much slower than it was a milder cam, and absolutely horrible to drive..
Same with my rover engine....The hype said you cant rev over 5500rpm, or the crank will break. I was pushing well over twice the factory power output through it, and revved regularly to 6000rpm. It never broke in 2 years use.
The hype suggested I needed steel this and that if I wanted to rev, or go above xxxbhp.
My engine was abused regularly, and ran std crank, std rods, mains ( no straps, or steel caps ), and std turbo pistons, albeit modified for lower CR.
For a while, I even had a stupid camshaft in it that allowed me to rev easily to 8000rpm, although it was soon removed, as it made the car much slower than it was a milder cam, and absolutely horrible to drive..
Same with my rover engine....The hype said you cant rev over 5500rpm, or the crank will break. I was pushing well over twice the factory power output through it, and revved regularly to 6000rpm. It never broke in 2 years use.
I can't see any reason for running forged pistons (apart from they don't break up when they go, which is virtually never-and they come with bigger dishes)
As long as it doesn't detonate, the cast ones are perfectly good. I'm also going to run an Avonbar turbo cam (264/288) for the incresed exhaust duration. That should be the optimum for the range.
I have no reason to believe that a standard engine is not good for 6500. If I find a 1430 set up, I will go with it, but for now I will be sticking with std stroke. It would produce a bag more torque though.
As long as it doesn't detonate, the cast ones are perfectly good. I'm also going to run an Avonbar turbo cam (264/288) for the incresed exhaust duration. That should be the optimum for the range.
I have no reason to believe that a standard engine is not good for 6500. If I find a 1430 set up, I will go with it, but for now I will be sticking with std stroke. It would produce a bag more torque though.
I would make sure the engine breathes as well as possible before you super/turbo charge it, especially if you have spare cash, I think that's of more benefit than increasing the capacity.
If you look at the Hayabusa engine, it's got an 81mm bore and 63mm stroke, the big bore allows the use of relatively large valves, in fact massive valve area compared to most similar capacity car engines. The cams are relatively mild in comparison but the ports are large, the throttle bodies are large and if it does one thing well, it's breathe.
Good breathing also cuts down the amount of heat trapped in the engine - it's exactly what you want as a starting point for turbocharging. This is part of the reason the Hayabusa gives so much power when turbo charged.
I'm sure the same would be true of a car engine and as it's all head work and a cam, not too expensive compared to crank/block and pistons.
If you look at the Hayabusa engine, it's got an 81mm bore and 63mm stroke, the big bore allows the use of relatively large valves, in fact massive valve area compared to most similar capacity car engines. The cams are relatively mild in comparison but the ports are large, the throttle bodies are large and if it does one thing well, it's breathe.
Good breathing also cuts down the amount of heat trapped in the engine - it's exactly what you want as a starting point for turbocharging. This is part of the reason the Hayabusa gives so much power when turbo charged.
I'm sure the same would be true of a car engine and as it's all head work and a cam, not too expensive compared to crank/block and pistons.
Well, I imagine the Hayabusa is a 5 bearing engine, has rods and pistons which weigh nothing and the whole thing is of proper material spec. Also the head is about as good as you can get and designed with performance in mind from scratch. Not powering an A30.
I will suggest that the overvalved nature of a short stroke (read not inadequately valved) nature allows it to breathe properly when N/A and obviously has the reserve exhaust capacity for more. With an A series, it is necessary to use more exhaust duration and lift.
I have one thought about bike engines with turbos, what's the deal with clutches and gearboxes? If you are using them in a car setup, it must add a serious load.......weak point or not?
Back to the stroke/bore thing, optimising a crude head is easy assuming you know the nature of the beast and have metal to play with, you just fine tune it with cam timing/lift. Obviously these characteristics start either constricting the top end (not enough flow/duration/lift) or killing it by too much at low range. Since a blower makes an engine less overlap sensitive, this can be retained a bit. So, with a superduper head, which is more likely to be found on a short stroke multivalver, a better powerband will be realised but it will only be realised if the bottom end can take the revs, of which 99% of normal engines can't.
After reading a bit more, it seems the theme with skinnying big ends down is that the crank flexes more and the resonant frequency goes down. Obviously with my crappy 3 bearing engine, it is likely to explode and undo the good my damper/lightened flywheel has done. Adding boost increases the loads and this flexture. So, in a nutshell, it probably isn't a good idea. Since it will take about £1200 worth of components with it when it fails.
I'm not sure about bike engines and I don't like turbos. (everyone should use screw compressors!) I will leave my final opinion until I've driven one. Mate has Fury Fireblade in the process.#
What do you think?
I will suggest that the overvalved nature of a short stroke (read not inadequately valved) nature allows it to breathe properly when N/A and obviously has the reserve exhaust capacity for more. With an A series, it is necessary to use more exhaust duration and lift.
I have one thought about bike engines with turbos, what's the deal with clutches and gearboxes? If you are using them in a car setup, it must add a serious load.......weak point or not?
Back to the stroke/bore thing, optimising a crude head is easy assuming you know the nature of the beast and have metal to play with, you just fine tune it with cam timing/lift. Obviously these characteristics start either constricting the top end (not enough flow/duration/lift) or killing it by too much at low range. Since a blower makes an engine less overlap sensitive, this can be retained a bit. So, with a superduper head, which is more likely to be found on a short stroke multivalver, a better powerband will be realised but it will only be realised if the bottom end can take the revs, of which 99% of normal engines can't.
After reading a bit more, it seems the theme with skinnying big ends down is that the crank flexes more and the resonant frequency goes down. Obviously with my crappy 3 bearing engine, it is likely to explode and undo the good my damper/lightened flywheel has done. Adding boost increases the loads and this flexture. So, in a nutshell, it probably isn't a good idea. Since it will take about £1200 worth of components with it when it fails.
I'm not sure about bike engines and I don't like turbos. (everyone should use screw compressors!) I will leave my final opinion until I've driven one. Mate has Fury Fireblade in the process.#
What do you think?
I don't know anything about the older car engines, I didn't realise it was a 3 bearing crank, not sure I'd want to put anything more through that. How strong is the crank ?
Bike engines are usually very strong, the larger engines like the Hayabusa and the Blackbird which were designed more for high speed touring rather than out and out track racing have larger clutches and the gearboxes are pretty strong. I have heard of problems with fireblade gearboxes but again it's often suggested this is more down to oil quality (or lack of) and possible abuse. Several people I know have fireblade powered cars and love them and haven't had any problems other than a bit of clutch slip but many others don't even have this problem, probably down to oil again.
One of the best engines at the moment appears to be the Yamaha R1, 998cc, 150bhp in stock form and with a turbo they have gone up to 400 bhp, although 300 bhp would be a more realistic figure for road use. Less than 100Kg including the turbo, engine, gearbox and starter etc. £1400 for a good base engine with the loom and injection system - cheaper than a good head and valve job on a car engine.
Bike engines are usually very strong, the larger engines like the Hayabusa and the Blackbird which were designed more for high speed touring rather than out and out track racing have larger clutches and the gearboxes are pretty strong. I have heard of problems with fireblade gearboxes but again it's often suggested this is more down to oil quality (or lack of) and possible abuse. Several people I know have fireblade powered cars and love them and haven't had any problems other than a bit of clutch slip but many others don't even have this problem, probably down to oil again.
One of the best engines at the moment appears to be the Yamaha R1, 998cc, 150bhp in stock form and with a turbo they have gone up to 400 bhp, although 300 bhp would be a more realistic figure for road use. Less than 100Kg including the turbo, engine, gearbox and starter etc. £1400 for a good base engine with the loom and injection system - cheaper than a good head and valve job on a car engine.
The A series is one of the most developed engines possible. Lasted a good 50 years as well. I think people have run about 200hp through the 3 bearing crank.
Here is the snag, minis have micro space under the bonnet. Hence fitting other engines usually results in having ridiculously unequal length driveshafts, so you're stuck with that gearbox, unless you want enormous torque steer. People have used 5 bearing Lotus TC engines mounted on the mini gearbox but it makes the car understeer ridiculously. BMW K1100 Bike 16V heads have been put on the block, etc, etc, the list is endless. Going back to performance tuning, you are looking at a 4 speed gearbox, a decent strong set of SCCR gears is about £450, the cheapest 5 speed (has to be a dogbox for a strong 1:1 5th) is about £2000. For that price, you might as well have a kitcar with a bike engine!
I think about 8000 can be had out of the crank safely, allthough revs and vibration are the killer. The top torque figure I have heard is 190ft/lb which is pretty much 3L essex territory! (wide band as well). Obviously, that engine had to be rebuilt after 10 minutes.
Minis are the original essence of vehicle tuning and a lot of people lose sight of it being a mini. My perogative is to produce a huge torque engine myself, using all the tricks in the book. I don't want to spend over £2K on the engine as if I do, I will be merely wasting money.
Obviously forced induction is the only real way to go as with a small tuned power spike, you need more gears. So, you either have to do spectacular wheelspin starts/stall lots or suffer a top speed of about 70 (which was what dogged my last engine)
>> Edited by love machine on Wednesday 9th March 12:10
Here is the snag, minis have micro space under the bonnet. Hence fitting other engines usually results in having ridiculously unequal length driveshafts, so you're stuck with that gearbox, unless you want enormous torque steer. People have used 5 bearing Lotus TC engines mounted on the mini gearbox but it makes the car understeer ridiculously. BMW K1100 Bike 16V heads have been put on the block, etc, etc, the list is endless. Going back to performance tuning, you are looking at a 4 speed gearbox, a decent strong set of SCCR gears is about £450, the cheapest 5 speed (has to be a dogbox for a strong 1:1 5th) is about £2000. For that price, you might as well have a kitcar with a bike engine!
I think about 8000 can be had out of the crank safely, allthough revs and vibration are the killer. The top torque figure I have heard is 190ft/lb which is pretty much 3L essex territory! (wide band as well). Obviously, that engine had to be rebuilt after 10 minutes.
Minis are the original essence of vehicle tuning and a lot of people lose sight of it being a mini. My perogative is to produce a huge torque engine myself, using all the tricks in the book. I don't want to spend over £2K on the engine as if I do, I will be merely wasting money.
Obviously forced induction is the only real way to go as with a small tuned power spike, you need more gears. So, you either have to do spectacular wheelspin starts/stall lots or suffer a top speed of about 70 (which was what dogged my last engine)
>> Edited by love machine on Wednesday 9th March 12:10
eliot said:
Here is the snag, minis have micro space under the bonnet. Hence fitting other engines usually results in having ridiculously unequal length driveshafts
What's the issue of different length driveshafts?
Unless you get them totally flat, the different angles cause different amounts of power to be transferred to each wheel, this causes the car to transfer it's weight to the gripping wheel and veer off in a big curve. If you have lots and lots of power, it can cause very exciting high speed weaving or lane changes on the throttle. Very disturbing when it's bad. It causes your steering wheel to fight you if you are running large offset wheels. My blue one used to weave like billyo and it could get a bit unnerving. Also running a large offset makes the weight transfer more marked and the rate of weave quicker, more snappy and generally more unpredictable. It can be a really bad thing. You can get around it by anticipation or running less KPO.
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