SPOautos

I have a question for someone. If you had a n/a engine say a 13B and you increased the displacement to a 20B BUT you kept the same amount of air and fuel going in to it and all other things being equal except the displacement would it make more power or the same power?

Say with the 13B it was running 1000cc's for fuel at WOT with the right amount of air to make a 12.5 a/f ratio. Then say in the 20B you still ran 1000cc's worth of fuel with the same air to make 12.5 a/f ratio.

If the 20B would make more power with the same air and fuel than the 13B can someone explain how? If it wont can someone explain why? I think I know the answer but want to see what others thing.

Also, would this still hold true for a piston engine? Say a 350 stroked to 383 BUT still only getting the same amount of air and fuel.

STEPHEN

peejay

Same power.

Might be slightly less for thermodynamic reasons (more internal surface area for the heat to get lost to)

In a basic nutshell, how much air and fuel you move through the engine is how much power you're making, give or take a few % to engine efficiency (thermal *and* mechanical). How large the engine is determines how fast the engine is turning to move that much air (larger engine = less RPM to move same amount of air therefore more torque). Just a way of looking at the numbers in a "reverse engineering" fashion!

Let's look at your example numbers... 1000cc's (per minute I assume) of fuel is *roughly* 100lb/hr of fuel. Which would make, roughly, 250hp. (These numbers are not near exact but this is just a thought excercise) How much RPM would the 13B have to be turning to make 250hp? For ***** and giggles and making the numbers happy sake let's say 8100rpm. Now let's increase the displacement by 50%, meaning the engine can flow 50% more air per revolution, meaning the engine (theoretically) makes 50% more torque, all else equal. To move that same amount of air to require that amount of fuel will still make about 250hp however it will require 33% lower RPM to do it - it'd make that 250hp at 5400rpm. More math: 250hp at 8100rpm is 162lb-ft of torque. Multiply that torque by 1.5 and you get 243. 243lb-ft at 5400rpm is - surprise - 250hp.

It is true for all engines. You know how they say "the cylinder head defines the engine?" That's why. Used to be that making a stroker big block Chrysler was a waste of time because while it'd make more torque due to being larger, the available heads SUCKED so it wouldn't make any more actual power.

SPOautos

But why was the tq multiplied by 1.5? I realize the displacement is 1.5 times larger but with the same air and fuel wouldnt they make the same amount of tq. That is the energy that the combistion is really creating so wouldnt they create the same tq but that the larger engine would start to create it lower in the rpm band do to effeceincy reasons???

I'm understanding you about 1/2 way haha

STEPHEN

peejay

On a rotation by rotation basis, it is NOT the same amount of air. The larger engine is pulling 50% more air for a given revolution! Which is why it has 50% more torque.

Torque is how much force the engine can produce. This is independent of time. Power (HP) is how much work you can do with that force. The larger engine can put out more force than the smaller one, however if they are both limited to the same amount of air/fuel, the smaller engine can rotate faster to use the airflow and it all ends up being a wash. They make the same power for a given airflow.


Note: I am ignoring MANY other factors like compression ratio, number of cylinders, etc - just looking at the very basic "X engine" and "1.5X engine" scenario to keep the scope down.

SPOautos

Ahhhh, yea I gotcha. I understood what you were saying till I got to the bottom part. Its pretty much what i figured.

So if what your saying backs up what I was thinking it would be something along the lines of this...

They are going to peak out with the same hp with the larger displacement engine is going to end up with a more broad powerband. What would happen with the larger engine if you kept going iwth the rpms? Would the tq start to fall off with lower VE? I guess it would have to for the hp to level out but continue to run higher rpms.

Also, let me ask you something. In the 350 versus 383 stroker example. If you stroke a 350 to 383 and retain the same pistons will it keep the same compression ratio or will it automatically go up?

For some reason I figured you'd be the one discussing most of this. You always seem to be good with this kind of engine talk.

Thanks,
STEPHEN

peejay

The spread of the power curve is dependent on lots of factors. You can't automatically say a 250hp engine has an X wide power curve for a Y displacement.

I mean, you could have a situation where the smaller 250hp engine was just beginning to get into the meat of its powerband when it was making 250hp and you could also have a situation where the larger engine was running out of breath at 250hp.

The important thing when figuring out how wide your powerband is is to look at the ratio of minimum to maximum RPM. I remember a while ago someone was saying his FD had the same powerband width as a certain car (C5?) because his car had a 4000-8000 powerband and the other car was 2000-6000. That is not true, the lower revving car in this case had a powerband twice as large. 8000 is a 100% speed increase from 4000, while 6000 is a 200% increase from 2000. (A 3000-6000, or a 2000-4000 powerband would be the same width as a 4000-8000)

Arguably a lower powerband is more advantageous because rotational inertia goes up with the square of RPM. But then you have to make more torque to get the same HP, which means more displacement (possibly heavier engine) or forced induction (need beef).

If you had the same combustion chamber size and piston deck height/piston dish volume, yeah compression ratio would go up. Same as if you were able to increase the bore a lot without changing the TDC volume.

Jeff20B

iTrader: (13)

Gotta love 5252RPM.

SPOautos

Hmmm, ok I understand where your getting the numbers like 8000 is 100% faster than 4000 while 6000 is 200% faster than 2000 BUT I'm not sure i understand the importance of that.

If both cars were racing and one had engine A (the lower rpm engine) and the other had engine B (the higher rpm) and they made thier peak hp across that rpm range and they can both go all the way down the track never leaving that range what would the difference be. As long as they are making the same hp why would it matter?

Or maybe your not saying that it matter for speed. I guess your just pointing out that if the Vette redlines at 6K then its making peak power for 66% of its total rpm range while the other is making peak hp for only 50% of its total rpm range.

Course in real life if you looked at thier hp the vette only makes peak power for about 700rpms, or so. I think its about 5700-6500. Anyway, thats a different topic.

BTW - was I the one talking comparing my curve to the Vette curve? Seems like I did that a long time ago. But I would have been comparing power (HP) not tq so maybe it wasnt me. Those ranges you were talking about sound like they were comparing tq

STEPHEN

peejay

It was on the RX-7 mailing list a few years back.

The engine which could accelerate the easiest would be faster. Notice I didn't say more powerful!