But surely this reduces engine life since it has to work harder to produce similar effects as the stroker (in a street driven scenario)?
I guess for racing that wouldn't be so much of an issue.
I have an engine....
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Dave-R
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Decreases engine life and uses more fuel etc etc. I did say performance car!
You don't get something for nothing.
Mind you the piston rings wear out faster on a longer stroke. But hell. in this case it is only a little bit of a longer stroke. I was just talking in general terms.
I did also say I like stroker small blocks and B blocks.
I would build a stroker 400 b-block rather than a 440 any day.
However look at Formula 1 engines.
They have a very small strokes. They make their torque at high rpms. They take advantage of low gearing.
You going to tell me they have that wrong?
So it is easy to make an engine perform on a drag strip or attain high speeds isn?t it? You just make a huge amount of torque at a high rpm and away you go!
Ah but nothing in life is that easy. There are forces at work to stop you doing that!
Have you noticed when you look at engine torque curves that torque seems to drop off very quickly at high rpms?
One of the reasons is Volumetric Efficiency (VE). The higher the rpm the less time the cylinders have to draw in air/fuel. The amount of air/fuel you burn is what makes the torque. If the cylinder only fills 75% of its total capacity (VE is 75%) that is the same as reducing the displacement of the engine by 25%. Which means a proportional loss in torque.
Let me explain that another way. Airflow into the engine usually increases with rpm (or more accurately ? piston speed) until it reaches the maximum flow capacity of the carb, or intake runners, or ports, or valves (whichever is smallest) and then it levels off and becomes constant despite increases in rpm. The engine is then flow limited and VE (and therefore torque) drops off. The higher the rpm goes after this point - the faster torque drops off.
Obviously a stroker crank makes this worse because of the already high piston speed.
The fix is to fit huge flowing heads/carbs/valves etc. However that then makes drivability at low rpm a problem.
You don't get something for nothing.
Mind you the piston rings wear out faster on a longer stroke. But hell. in this case it is only a little bit of a longer stroke. I was just talking in general terms.
I did also say I like stroker small blocks and B blocks.
I would build a stroker 400 b-block rather than a 440 any day.However look at Formula 1 engines.
They have a very small strokes. They make their torque at high rpms. They take advantage of low gearing.
You going to tell me they have that wrong?
So it is easy to make an engine perform on a drag strip or attain high speeds isn?t it? You just make a huge amount of torque at a high rpm and away you go!
Ah but nothing in life is that easy. There are forces at work to stop you doing that!
Have you noticed when you look at engine torque curves that torque seems to drop off very quickly at high rpms?
One of the reasons is Volumetric Efficiency (VE). The higher the rpm the less time the cylinders have to draw in air/fuel. The amount of air/fuel you burn is what makes the torque. If the cylinder only fills 75% of its total capacity (VE is 75%) that is the same as reducing the displacement of the engine by 25%. Which means a proportional loss in torque.
Let me explain that another way. Airflow into the engine usually increases with rpm (or more accurately ? piston speed) until it reaches the maximum flow capacity of the carb, or intake runners, or ports, or valves (whichever is smallest) and then it levels off and becomes constant despite increases in rpm. The engine is then flow limited and VE (and therefore torque) drops off. The higher the rpm goes after this point - the faster torque drops off.
Obviously a stroker crank makes this worse because of the already high piston speed.
The fix is to fit huge flowing heads/carbs/valves etc. However that then makes drivability at low rpm a problem.
It's been a hectic week! I'm sneaking 5 mins in me luchtime to get back to this thread!.....and I haven't read Daves last post yet so there might be some overlap?
Anyway, absolutely agree with you Dave. Put very simply - as we know power is torque times rpm. To get max power you need max rpm, and to be making max torque at that rpm. The best way to get max rpm is a short stroke, light components etc. Thats why F1 engines rev to 19K - more rpm means you can make more power. And to get torque in order to make that high rpm power, you need to optimise everything else in the engine around that too - camshaft, breathing, fuel. ignition etc.
Result - lots of power in a very narrow band. To use the power properly you need to stay in the narrow rpm range where it's made (i.e. up high). For a race application use a loose convertor, tall gearing etc. If you have a circuit racer you need close ratios and lots of gears.
On the street it's a different story. Not only because you want longevity. That is a real wear/risk factor but with the amount of use our cars typically see, and the care the engines are built with, high rpm is not as significant as it could be.
But a key difference between the track and the street is predictability!
On the track you know exactly what you need and when you will need it. At the drags you get the chance to wind up your peaky rpm motor against your high stall convertor, or hold at high rpm and dump the clutch. On the circuit you know what kind of track you're on, when you are going to have to change gear etc. Everything can be optimised.
On the street you might want lots of power at any rpm, without knowing in advance. You could be minding you own business at the lights, and when they go green some dork flips you off and starts to speed away! Obviously any sane person would let it go
, but theoretically if you didn't, you need instant response from idle. Or maybe you just didn't want to tip your hat by sitting there at 4000rpm!
Or maybe more relalistically you're following someone who's slowing you down and suddenly an unexpected opportunity to overtake presents itself. Or you're approaching the traffic lights as they go amber! Instant response required in the real world.
Having said all of that..... all of this is relative and to some extent we're splitting hairs. With engines the size we have, even if you have a narrow power band you are still going to have plenty of torque available. When all is said and done, my overwhelming motive for going to a stroker was displacement!! That works at every level. I like rpm's, don't get me wrong, and this engine will rpm just fine. But it won't always have to!
I want to be able to cruise at 90mph, so I'm going with 3.55 gears (3.23 at the moment). and I don't want a radical convertor either. I had a lot of people tell me I needed steeper gears and converter to optimise my combo for ET's. I know!
What it comes down to is that engines/powertrains are a massive comprimise. That's why the auto industry is seeing a huge proliferation in variable technoliges - variable valve/cam timing, variable geometry turbos, variable intake manifolds, variable displacement, variable compression ratio, variable transmissions etc. You can always be optimised, in the 'sweet spot' without comprimise. you don't have to pick high rpm power over low rpm torque - you can have both. Power when you want it, economy when you don't. I'm looking forward to the day when we can buy aftermarket bolt on active valvetrains (no cam!). Mild to wild in one package.
(My company actually makes one but I don't think they're going to let me get my hands on it!)
Anyway, practically you're right Dave - best solution is an overdrive or a 5/6 speed trans. One day I'd love to do that!!
$$:( Until then I'd better concentrate on getting plan A on the road......
Anyway, absolutely agree with you Dave. Put very simply - as we know power is torque times rpm. To get max power you need max rpm, and to be making max torque at that rpm. The best way to get max rpm is a short stroke, light components etc. Thats why F1 engines rev to 19K - more rpm means you can make more power. And to get torque in order to make that high rpm power, you need to optimise everything else in the engine around that too - camshaft, breathing, fuel. ignition etc.
Result - lots of power in a very narrow band. To use the power properly you need to stay in the narrow rpm range where it's made (i.e. up high). For a race application use a loose convertor, tall gearing etc. If you have a circuit racer you need close ratios and lots of gears.
On the street it's a different story. Not only because you want longevity. That is a real wear/risk factor but with the amount of use our cars typically see, and the care the engines are built with, high rpm is not as significant as it could be.
But a key difference between the track and the street is predictability!
On the track you know exactly what you need and when you will need it. At the drags you get the chance to wind up your peaky rpm motor against your high stall convertor, or hold at high rpm and dump the clutch. On the circuit you know what kind of track you're on, when you are going to have to change gear etc. Everything can be optimised.
On the street you might want lots of power at any rpm, without knowing in advance. You could be minding you own business at the lights, and when they go green some dork flips you off and starts to speed away! Obviously any sane person would let it go
, but theoretically if you didn't, you need instant response from idle. Or maybe you just didn't want to tip your hat by sitting there at 4000rpm!Or maybe more relalistically you're following someone who's slowing you down and suddenly an unexpected opportunity to overtake presents itself. Or you're approaching the traffic lights as they go amber! Instant response required in the real world.
Having said all of that..... all of this is relative and to some extent we're splitting hairs. With engines the size we have, even if you have a narrow power band you are still going to have plenty of torque available. When all is said and done, my overwhelming motive for going to a stroker was displacement!! That works at every level. I like rpm's, don't get me wrong, and this engine will rpm just fine. But it won't always have to!
I want to be able to cruise at 90mph, so I'm going with 3.55 gears (3.23 at the moment). and I don't want a radical convertor either. I had a lot of people tell me I needed steeper gears and converter to optimise my combo for ET's. I know!
What it comes down to is that engines/powertrains are a massive comprimise. That's why the auto industry is seeing a huge proliferation in variable technoliges - variable valve/cam timing, variable geometry turbos, variable intake manifolds, variable displacement, variable compression ratio, variable transmissions etc. You can always be optimised, in the 'sweet spot' without comprimise. you don't have to pick high rpm power over low rpm torque - you can have both. Power when you want it, economy when you don't. I'm looking forward to the day when we can buy aftermarket bolt on active valvetrains (no cam!). Mild to wild in one package.
(My company actually makes one but I don't think they're going to let me get my hands on it!)
Anyway, practically you're right Dave - best solution is an overdrive or a 5/6 speed trans. One day I'd love to do that!!
$$:( Until then I'd better concentrate on getting plan A on the road......-
Dave-R
- Posts: 24751
- Joined: Sun Apr 18, 04 11:23 pm
- Location: Dave Robson lives in Geordieland
- Contact:
I think I saw your engine when it was sitting in a New York barn? If that is the one it was a very nice bit of kit.
I say again. I like stroker motors and would have one myself. The reason being more or less what you have just said. They feel geat when you drive them. They may not be the best way for low E.T.s but they sure make a good street engine. As long as the heads (and carb and exhaust) flow enough to stop the piston speed effecting V.E. (and I am sure yours will be very fine indeed) then at this level of performance they are just the ticket! 8)
My comments were really directed to those reading this that might not be aware or how engines make their power. Some people have some VERY strange ideas indeed!
I say again. I like stroker motors and would have one myself. The reason being more or less what you have just said. They feel geat when you drive them. They may not be the best way for low E.T.s but they sure make a good street engine. As long as the heads (and carb and exhaust) flow enough to stop the piston speed effecting V.E. (and I am sure yours will be very fine indeed) then at this level of performance they are just the ticket! 8)
My comments were really directed to those reading this that might not be aware or how engines make their power. Some people have some VERY strange ideas indeed!
Yeah Dave that was the one in NY. As everything else on this car I had really had to make my money go as far as possible! So I was very careful to select what I thought were the right components cos I can't do it again! I'm very happy with what I got - but at the moment it's nothing more than lumps of metal on shelves! The proof of the pudding will come, then we'll see whether I screwed up or have a nice engine.......
Anyway, I think we are all in violent agreement. I agree with what you say, all makes sense, and also agree that many people have strange ideas. Always good to have a discussion about the 'whys'. Course it don't count for nothing until it's put into practice. You've certainly done that in spades with your car, as have many other clued up members!
I look forward to getting the opportunity to do the same!
Anyway, I think we are all in violent agreement
. I agree with what you say, all makes sense, and also agree that many people have strange ideas. Always good to have a discussion about the 'whys'. Course it don't count for nothing until it's put into practice. You've certainly done that in spades with your car, as have many other clued up members! I look forward to getting the opportunity to do the same!
