How to Lower Compression?
 

Hey guys/gals
I have a 588 NA motor (Dart Big M tall deck, 4.56 bore) with AFR 335 heads that is at 700hp on the dyno, compression is 10.5 to 1. I would like to see the motor more around 800-850hp, so I figured best bet would be to put a Whipple 3.3 on her, but compression is obviously to high. So, what would be the best method to get it down and what would be the best number to get it down to? Motor only has a few hours on her so prefer not to do a total rebuild. Am I screwed? Your thoughts?

Thanks
Frank

I'm no engine builder, but. You could lower the compression somewhat (not nearly enough I don't think) with a thicker head gasket. Probably would have to change pistons to get compression down around 8-8.5. I think it is way more money for you to do that. It would seem that combination could make 800-850hp with some bolt on mods at way less money.....way less!

1. Have chambers opened up
2. Sell heads and buy ones with larger chambers
3. Replace pistons
4. Thicker head gasket, won’t get you far.

what are the specs on all of the above you have currently? Chamber cc, piston compression height, piston cc, bore and stroke, rod length and head gasket thickness?

Need more info. Assume chambers are 121 if stock and likely .041 head gasket +/-. Need to know at the very least what current head gaskets are and if chambers are 121. You have a small dome or flat tops pistons? If you get into swapping out pistons you'll end up rebalancing.

At 700 hp 588 with 10.5:1 CR seems low. Also what cam you running?

I purchased boat/engine all together so I am working off of build sheets.
Bore - 4.56
Callies H beam rods - 6.7
Callies forged Crankshaft Compstar (2 numbers 4.75 and 4.5)
JE SRP forged pistons 1.245ch, 4.560bore 257962
Comp cam part#11-000-13
Gross valve lift - Intake .673 Exhaust - .680
Duration @.006 Tappet lift 301, 309
I assume 121 for stock chambers -can't be sure
No Info on type of head gaskets

Had a few dyno sheets - best was 716.7 POWER CHP, Torque FT-LBS 649.1 RPM-5800
Most pulls around 699
Inconel valves
ProMarine 1200 carb

11-000-13 is a generic number/code for a special/custom order cam from comp cams. The closest lobe I could find in the master, with your .006 numbers and lift numbers, are in the "HUC" lobe family. Intake lobe 13862, which has 302 @ .006, and 247* @ .050 with .673 lift with a 1.7 rocker. Exhaust looks like lobe 13064, which has 310 @ .006 , and 255 @ .050 with .680 lift with 1.7 rocker. Those are published numbers, but once they are ground on certain core applications, the duration measurments may change a degree or so. Sounds like you have a 246/254 @ .050 , with .673/.680 lift. LSA and ICL, are unknown without more info on the grind. Grind number, cam card, etc.

That piston part number, comes up as a flat top. A flat top, in a 4.5 stroke, 4.56 bore, is in the 9.8:1 neighborhood with a zero decked block and .040 gasket. Going to a .066 gasket, brings it down to 9.6:1 . If piston is in the hole .010, down to 9.25:1. Opening the chambers up with some work to 125cc, now down to 9:1.

Hope my internet expertise was of some assistance . :)

you will need to change to a dished blower piston.

My head chamber port brought me from 121cc to 129cc and a performance increase.

IF all you want is 850 HP you could do that easily with just a cam change. With 588 cubes at 10.5 to 1 with the proper cam you should easily make 850 HP.
I have a 598 at 8.5 to 1 with the intentions of adding a blower later and I am at 800 HP now. With the M4 Procharger it dynoes at 1425 HP...
But I am running a pretty stout cam. Idles nice and smooth with EFI at 750 RPM.
I am running AFR 357 CNC'd heads.

I find it hard to believe he will find 100+ hp with a cam change.