Gollum Posted September 24, 2011 Share Posted September 24, 2011 It's bedtime, but I'll put in a few words. Applications like F1 do sacrifice torque, it's not complete bollocks. In order to rev so high (~20,000 rpm), the stroke must be kept very short, thus the mechanical advantage on the crank is less. This equals less torque, but those engines spin so fast that the torque pulses to the crank are happening at a very high rate (more power). Really, when I see discussions like this there's always talk about torque does this, power does that, blah blah blah. Torque and power are related to each other by rpm. Whether you talk about torque or power, both of these things get a car moving. What we should really be talking about is BMEP - brake mean effective pressure. This is a true measure of the effectiveness of an engine no matter the displacement and allows for a valid comparison. It shows how much potential (cylinder pressure) is being extracted out of an engine. Actually you're very correct regarding BMEP. It's a much "truer" measurement for efficiency than torque is. Maybe since our OP wants to talk hard numbers, maybe he should tell us what BMEP he's shooting for. Regarding stroke and torque relation, I can agree to a point. I think it has an effect but not like people seem to think. What you loose in leverage you gain in flow potential for the given displacement, and if you increase the flow for a given displacement then BMEP will go up and subsiquently torque as well. I know this might not be "scientific" enough for most people, but using a semi-accurate program we all know, dyno2003, I just built a motor to compare this will. I've got the valve size set to "auto" so that as I change things around the valve size would be approximately the same ratio to the bore and as valve size changes so will estimated head flow. So in this example we don't have a static head and flow numbers, which isn't "real world". I say that as a precursor to isolate the stroke/torque discussion. In the real world stroke is good because our head is static generally, and we get more displacement, torque, and power generally by doing so. That said, peak Torque at 4" bore and 3.5" stoke (351 ci) is 451 at 4500rpm (79 lbs per liter, this isn't a mild motor). Switching nothing but bore to 4.32" and stroke to 3.5" (351ci) the torque peak is 467 at 5500rpm (81 lbs per liter). I know many people bag on this program, and I admit it's not the most advanced program out there. But it's also generally recognized that when used properly it can produce accurate information, showing that it's math formulas are fairly solid. I've replicated many dyno graphs fairly accurately by accurately entering data, but I can also see that it's just a tool in my box, not a law to live by. So people, please realize I'm just saying all this to bring up a point, which is that peak torque numbers have a lot more going into them than just displacement, or stroke, or both. I've seen people blindly say that stroke is the main factor in torque (which I don't believe is what Leon is saying) and I've seen people say that displacement is the only factor in torque (which is not what I'm saying either). Though both of those play a crucial role, there's many things that make up a good 10-20% of your torque figures. For reference, the motor example given had a 10:1CR, 800cfm rated induction system, sequential fire injection, large tube open headers, cam with .614 valve lift and 286 duration and 110 lobe seperation. I believe a similarly built 351 would be close in numbers, and I've seen it done. Reason we don't see many actually put out 450+ torque and 470+hp like this simulation did I believe is largely due to lower compression and poor intake design with a carburetor that's not great at flowing evenly to each cylinder. This simulation is assuming that each cylinder is getting exactly the same air and fuel, which doesn't happen without very well designed induction and fuel systems. By comparison these numbers perfectly line up with what a LS1 should make with a similar cam, which is basically what this motor would be. The biggest difference would be the exhaust at that point which is about 25HP difference according to dyno2003. That said we have people ON THIS VERY FORUM putting 400+ to the wheels in LS1 motors... some even still get 30mpg on the highway, those bastards. Quote Link to comment Share on other sites More sharing options...
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