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.

E85. :)

and a couple extra fuel tanks,lol.

This has nothing to do with lowering your C/R but may figure in on the overall cost of the build. What exhaust are you using at this point?

Also curious if engine was dynoed marinized with marine exhaust or bare bones with dyno headers. Wonder if rings weren't seated. Something doesn't seem right with those numbers but regardless I get that's not what he's looking for and I for some reason can't get much below 10.3:1 based on info and figuring .040 head gasket. If piston are .005 or .010 in then it only gets further from 10.5.

I could have "or not" have figured something right. I would imagine a new NA build would aim for 38 to 44 quench but either way I'm coming up with different SC although many unknowns. For all we know heads could be decked also.

what is to be considered ideal SC for whippels. I'm just trying to figure if opening chambers 8 or 9 CC's and thicker gasket even gonna get in the ball park without swapping out to dish like mike mentioned.

I assume engine was dynoed with shop headers since the engine was installed by a performance marine center, not the engine builder. Both, the engine builder and the marine engine installation center are top notch. The engine build/price sheet shows custom work to block, heads, cam, almost everything

Imco risers to short pipes

With some chamber porting, and thicker head gaskets , he could be down in the 8.9:1 range. I would have no issues putting a blower on a 9:1 engine, as I have and had no issues doing so.

Compression Ratio Calculator | Tools | Diamond Racing Pistons

That's what I'm not understanding if the builder and installation are a top notch facility which I'm certainly not trying to imply any differently but how is it the said engine is a 10.5:1 comp. just saying. I'll have to use the diamond calculator and come up with 10.5. Actual and what was told are off to much.

Is this gonnna be intercooled? I did ck on diamonds calculator and glad I did. Only way he could be at 10.5:1 is if the heads got whacked down to around 111. To be honest I'm glad you referred to diamonds calculator as it's much more build specific than Wallace. It change my piston order from dish to flat which I prefer anyway.

I looked around and noticed many don't ask for rod length, block height nor top ring to top of piston. In fact after a quick search diamonds calculator is superior to any others I looked at. I can understand why some do the old fashion way. After entering same info in other calculator I'd get two different ratios. Not good.

"""""""""As it turns out for those who is compression calculators after playing with some different rod lengths and block heights it makes no difference. Top ring to piston top thickness of course does. Just an FYI"""""""""

I'll reach out to builder tomorrow and see if he keeps records.

Engine is in a 22' Talon and does 105-108 on GPS every time, just wanted to put her in the upper teens.

Outboards are so much EASIER & CHEAPER (LOL)

WetteVette's 23'WPM went mid/upper 120's with a pump gas 598" . Give him a PM. Hopefully he answers. Really nice guy !

Be careful.... I think if you get with engine builder and result in some head and intake work and possible timing etc you'll be able to achieve close to your goals. Maybe cam change but your cam seems like it would be within reason if your wanting to stay at or close to 5,500. Cam may be just fine with some head work. You have a nice build there. I'm jealous. Lot of good parts. I just think there's power left on the table. If you can post a cam card it would be interesting to see your timing events.

MiId Thunder - the fact that he isn't putting out much HP with that many cubes tells me he either has real bad heads or a very mild cam.
So with the proper cam I figured he should get it into the HP range he wanted without spending all of the $ on blowers.
I would cam it for at least 6000 RPM because cats like RPM.

He's got a nice conservative build that would likely run many trouble free hours. Tinkerer like you mentioned cam would definately help like something in the 260/270-714-714. Existing cam would be fine with the whipple I think anyway. Just depends on what direction he goes. I think once he finds out what he has for head gasket, stock chambers or not, pistons at tdc etc then he can at least see if it's remotely possible to get near 9.0:1. I have a feeling it's gonna be difficult if it's actually 10.5 currently. That's possible an oversite given flat tops and if stock 121 chambers so might get lucky. In my opinion I'd run it hard and spray it til its ready for rebuild then get comp down. Meanwhile NOS it. That alone will probably expedite his future whipple. Lol.

after a little thought i think a 3.3 a little on the small side for an engine his size so 9 : 1 would work well in this case,jmo.if he had a larger supercharger in my opinion he would make more power and the engine would live longer with less compression and more boost.if he was going pro charger i would say raise the compression to close to 10:1.

i see two problems spraying it,one he will need a barge to haul the nitrous that can,t be had anyway since the plant in florida blew up and two,if the nitrous tune up get,s a little lean it will be over before he can back off the sticks.

I see your mother had obviously dropped you on the coconut much harder than her sister did to me. Leave it be, it's so much more relaxing drinking and listening to the radio than working on things that will break more often and they do break