C/Ratio vs H Power

[Garys1969RR]

I have 2 69 Roadrunners now. The first one has a 451, 13 to 1 C/Ratio, Holley street dominator with Holley 850 dbl pumper, Auto trans, 509 cam. 3.91 gears. Runs consistent 12.50 at 110 MPH. (5600 feet) Cranking compression is around 220 PSI. Runs on 98 octane fuel. The second one is the same except, 11 to 1 C/Ratio, 440, Performer RPM intake, Holley 3310, 780 CFM, 4.11 gears, Solid lifter cam. 180PSI cranking compression. Runs fine on 91 pump gas, Runs consistent 13.70 at 100 MPH. Not sure what the cam specs are, but it has a smoother idle than the 509 cam has. The Hi Compression 451 has a much stronger sound to it than the 440 does. So very similar built cars/motors except for C/Ratio. I have found that the higher compression motors produce WAY more power than the ten and eleven to one C/R motors do. I always like the higher C/R motors.

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The 2 points higher C/R motor is producing around 100 more Horsepower. Granted there are other variables in the 2 builds, but I had a 13 to 1 C/R 413 that would run 12.80s at 108, Where as a 440 in the same car 65 Belvedere, EXACT SAME HEADS, (915) and intake, headers, with 10.5 to 1 C/R, would only run low 14s.

 

[fratzog lover]

Im surprised your 440 car isnt lower 13's. On paper it sure looks it!

 

[Badvert65]

Have you seen this?
http://wallaceracing.com/hp-cr-chg.php
Not certain of the accuracy.
Back in the day, a point was worth about 5% increase (as a rule of thumb).

 

[HT413]

This is a great question!

 

[Garys1969RR]

Im surprised your 440 car isnt lower 13's. On paper it sure looks it!

Im sure the 440 car would run high 12s at sea level. But the thin air up here in Colo really cuts down our horsepower.

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Using the calculator, I gain 12 horsepower by raising C/R by 2 points. But in the real world, I am seeing closer to 100 horse increase. This has been borne out in 2 separate cases. The most dramatic one was the 65 Belvedere, first 440 at ten to one, flat top pistons, zero deck, ran 14s. Took those heads, intake, headers and put them on a 413 bottom end that had dome pistons, and the car ran 12s. Only other difference (besides dome pistons) was the 440 had a Racer Brown cam around 300 duration, and the 413 had a Crane cam at 292 duration. The 413 had so much cranking compression that it broke the compression gauge.

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I later put another Racer Brown cam in the 413, and it then ran low 12s.

 

[Garys1969RR]

Im thinking that higher C Ratios enhance more complete ex scavenging, as well as drawing the intake charge in more quickly due to a more rapid pressure drop as the piston starts down.

 

[TetanusShot]

Come on now...

That 451 has a single plane compared to the 440's dual plane, has a 100 more cfm carb, and bigger cam. I don't think the higher compression has nearly as much effect by itself.

Also consider that the 451 has pistons nearly 3/4 inch shorter than the 440. That's a much lighter rotating assembly even though the bore is bigger.

I bet the 440 would be better suited to those 3.91 gears as well.

I'm no expert though, I just call 'em like I see 'em...

 

[moper]

Compression ratio very little effect on horsepower output. However, it definately relates that power to a certain rpm range. the two packages you have are drastically different. They make power in different ways, and at different rpm points. That comparison's not apples to apples, it's not even apples to watermelon. It's more like apples to chesseburgers.
Compression does not affect scavenging at all.
There is very little "suction" during any engine's intake stroke that is caused directly by the piston's motion. The pressure drop is primarilly caused by the exhaust gasses exiting the exh valve. The "suction" affect is very small, and because of the elasticity of the incoming charge, occurs well past the point of maximum acceleration away from TDC. Stroke (well, rod to stroke ratio anyways) affects that suction. Not compression ratio.

 

[Cranky]

Going from 9-1 to 10-1 affects horsepower more than say going from 10-1 to 11-1. I would rather take an 8-1 motor to 10-1 than go from 11-1 to 13-1. Bumping from 10 to 13 may increase HP 25 points but it really depends on what your HP level is at. A 400 engine won't see as big of an increase as say a 600 HP engine will. Of course this all depends on if the combo is already optimized. If you're pulling 600 HP on an optimized engine, you might see 35-40 more ponies going from 10-13. I've dealt with several 11-1 engines and just one 13-1 and I'd rather have the lower CR engines over the higher ones. Also, playing around with different cams can also change your cranking pressures quite a bit too.

 

[Garys1969RR]

Compression ratio very little effect on horsepower output. However, it definately relates that power to a certain rpm range. the two packages you have are drastically different. They make power in different ways, and at different rpm points. That comparison's not apples to apples, it's not even apples to watermelon. It's more like apples to chesseburgers.
Compression does not affect scavenging at all.
There is very little "suction" during any engine's intake stroke that is caused directly by the piston's motion. The pressure drop is primarilly caused by the exhaust gasses exiting the exh valve. The "suction" affect is very small, and because of the elasticity of the incoming charge, occurs well past the point of maximum acceleration away from TDC. Stroke (well, rod to stroke ratio anyways) affects that suction. Not compression ratio.

Downward Piston motion has every thing to do with creating a vacumn to draw in the incoming charge.

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Come on now...

That 451 has a single plane compared to the 440's dual plane, has a 100 more cfm carb, and bigger cam. I don't think the higher compression has nearly as much effect by itself.

Also consider that the 451 has pistons nearly 3/4 inch shorter than the 440. That's a much lighter rotating assembly even though the bore is bigger.

I bet the 440 would be better suited to those 3.91 gears as well.

I'm no expert though, I just call 'em like I see 'em...

I used to have the 3310 Holley 780 on this 451, and it still ran 12.60s. So the carb had little effect on the performance.

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I would like to see ANYONE bring their Mopar to the Mile High City, and get it to run well with 10 to 1 C/Ratio. It is quite difficult to get into the 12s at this altitude, with a 3600 pound car.

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And what about the 413 that ran 12.80s with 13 to 1 C/R, compared to the 440 with the EXACT same heads, intake, headers, in the same car that only ran 14s with 10 to 1 C/R?

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Where did the power come from? Only thing different was WAY MORE C/R on the 413, and 8 degrees less duration on the cam. Oh yea 27 fewer cubic inches on the 413, and it ran superbly.