rpm vs cylinder pressure

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rpm vs cylinder pressure

Post  slurm on September 5th 2015, 6:25 pm

Running 347 combos in a imca mod with hockey puck tires. They have a crate rule now that is a 350 chevy engine part number 604. The horsepower of these is around 400 to 450. These engines have become the be all to every driver. Running a chevy is not an option. We have built three different combo to compete with, first all out open engine, dome probe pistons with N heads whistles 13.8 comp , roller crower cam hippo lifters 268 in 270ex @ 50 lift 638 106 , funnel web intake, 850 e85 carb. The thing blows the tires off at any rpm and feels good to 9200. Second engine ,older proline heads whistles 10.5 comp , same intake carb, flat top wiseco, solid flat tappet camshaft cam research 238 240 @ 50 lift 540 540 108, pulls strong but have to really babysit the throttle. Third engine srp flat top comp 10.5, same proline heads, crane cam hyd flat tappet 216 216 @ 50 .484 lift 110. The thing drives like a dump truck but seems really easy to hook up and drive tho it does get hot. I know all make different amount of power and at a different rate. Would an engine with more cylinder pressure make power slower and not be so hard to hook and if so can a engine be built to make good numbers but at a slow rate? Guess what i am asking like on an engine dyno can two engines make the same power but one makes rpm faster.
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How does RPM affect cylinder pressure

Post  gmsmkr on September 7th 2015, 10:41 am

Look at a VE chart some time and pay attention to where it peaks. Should be at or near the torque peak and if you can also find some intake velocity information, you'd see that it increases right with VE.
Air has mass and you are correct in your 1st assumption... if the velocity of the charge is high enough, it wil continue to fill the cylinder even as the piston rises. It's not about "pulling"... it's about inertia.

Also....

MEP (mean effective pressure) can actually be calculated roughly, if a dyno chart is handy. HP=PLAN/33,000 P being cylinder pressure in psi, L being length of stroke in feet, A being total area of the pistons in sq.in, and N being the number of power strokes per minute, which would be half the RPMs.
(When the equation is expressed in inches, it becomes HP=PLAN/396000.)
(Even simpler P*D*RPM/792000: P=PSI, D=displacement)

if you were to be able to look at a MEP chart overlayed on a dyno graph, MEP would hang right around with the torque curve all the way through.

Torque can even be calculated if the MEP is known, or backwards, Tq=(MEP*D)/151
MEP is kindof a measurable value, although not for the ordinary man. You would need very high tech ($$$$$$) equipment that can actually measure the extraordinarily fast pressure changes in the cylinder under operation.

But to get back on the topic of your question...knowing that the weight of air is about .0076 lbs/ft^3, if it is moving at a known speed, you would know what kind of acceleration (decel) would be required to stop the air, and go from there, seems complicated now???
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Re: rpm vs cylinder pressure

Post  supervel45 on September 7th 2015, 1:45 pm

I would tend to say NO. The question reminds me a lot of the cam LSA discussion and might be what your after, to shift your power band higher and kill some bottom end?

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Re: rpm vs cylinder pressure

Post  slurm on September 7th 2015, 8:15 pm

Would carb size then be a quick adjustment. To just neglected the engine air ? And bolting on a 650 cfm carb do any harm?
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Re: rpm vs cylinder pressure

Post  whatbumper on September 7th 2015, 9:23 pm

Gearing, trans

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Re: rpm vs cylinder pressure

Post  DILLIGASDAVE on September 8th 2015, 4:18 am

Do the rules allow the use of the MSD stuff/using a timing retard ramp at the hit? Or crazy idea, what about using a hand/squeeze throttle so you could smoothly roll into the power as the launch progresses.
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Re: rpm vs cylinder pressure

Post  supervel45 on September 8th 2015, 8:06 am

slurm wrote:Would carb size then be a quick adjustment. To just neglected the engine air ?  And bolting on a 650 cfm carb do any harm?
I think that would have the opposite affect of what you wanted, and fall off on top end. I was thinking along the line of Dave, when I read your post, and thought the timing ramp up would be good also. Like Wright said you can take some Bite out of the lowend with stall and gearing. One of the oldest tricks around is to run a vacuum secondary carb. and tune the opening rate with springs and port ( hole diameter to diaphram ) adjustments.

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