Formula to shave pistons to reduce compression ??

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Brooks James

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Is there a formula? 4.030 pistons 308 heads. It’s almost 11:1 want to go to 9:1. Use: pump gas and nitrous.
 
Sell the race pistons & buy the correct pistons for your application. I'm sure there are people here who are building race engines that would buy the .030 high compression units.
 
Sell the race pistons & buy the correct pistons for your application. I'm sure there are people here who are building race engines that would buy the .030 high compression units.
Not an option. $$$$$$. I got screwed on these ones.
 
Demonracer is correct. If you mill them down they will have no valve reliefs and if you mill them below deck you will have no quench (not that open chamber 308's would have any without the right piston). Thick head gaskets are not a fix either. It sucks you got screwed but you'd be putting good money after bad wither either of these directions.
 
Another option: Switch to e85 and leave it alone, enjoy the extra power.
 
I don't think you can reduce compression 2 points with head gaskets unless they are a inch thick. LOL
 
Thanks guys flat top srp. Pics Friday. What would it cost to shave them and valve reliefs.
I don’t want to run hi cr with nitrous.
 
Then it is simple. Measure the OD of the piston head above the top ring and mill only what you need. Pi x r squared x d. Valve reliefs should be uneffected. Or you can get fancy and pocket mill them to try and save quench. Break out your math book. Did you cc the chambers yet?
 
Thanks guys flat top srp. Pics Friday. What would it cost to shave them and valve reliefs.
I don’t want to run hi cr with nitrous.
Probably $200-$300. Aftermarket pistons have pretty deep valve reliefs and thick tops so I'm guessing it could be just milling and you're good to go with no valve relief work if you're going with a smaller cam for the 9to1 deal.
 
Tbere are three ways (at least) to reduce compression ratio. Mill the pistons to reduce deck height or dome, enlarge the combustion chamber (minimally effective) or a thicker head gasket.
If you have open chamber heads, and dont have a quench dome piston, you dont have any quench now to lose. A set of thick head gaskets can lower your compression quite a bit. Cheapest, easiest ,simplest solution.
If you post your calculations and engine specs that say you are at 11 to 1, we can better advise what we would do.
 
Also, if it were me, I'd go to the compression ratio calculator at wallace, enter my specs, and play with the numbers. You can alter deck heights, head gasket thickness, etc (by the way, et ceteta is abbreviated etc, NOT ECT! [NOT referring to the op]), for a lot less money than in real life.
 
Tbere are three ways (at least) to reduce compression ratio. Mill the pistons to reduce deck height or dome, enlarge the combustion chamber (minimally effective) or a thicker head gasket.
If you have open chamber heads, and dont have a quench dome piston, you dont have any quench now to lose. A set of thick head gaskets can lower your compression quite a bit. Cheapest, easiest ,simplest solution.
If you post your calculations and engine specs that say you are at 11 to 1, we can better advise what we would do.
Needed info post 1
I’ll want to go as cheap as possible as there will be a compromise between cr and nitrou efficiency. Depending on where you want the sweet spot, with high duration and compression with the high duration cam I have will put power in mid to top end. With nitrous, you need to retard the timing. So with what I just described I would need to pull even more timing out of the equation. Thus reducing power. So I’m hoping that I can find a head gasket thick enough to get down to 10:1
 
Use a compression calculator where you can vary parameters to attain the compression you want. I like the one on the Diamond piston site. Covers just about everything!!
 
Have a shop mill a "D" shaped dish into the pistons, doing some very crude math, I guessed .250 in deep and dish dia of 3.28 in, I did not calc filet radius , I came up with a 17cc dish, between that and the appropriate gasket should get you where you want to go ........people with sharper pencils may do better

After exploring this though, you may find new pistons just might be easier
 
Needed info post 1
I’ll want to go as cheap as possible as there will be a compromise between cr and nitrou efficiency. Depending on where you want the sweet spot, with high duration and compression with the high duration cam I have will put power in mid to top end. With nitrous, you need to retard the timing. So with what I just described I would need to pull even more timing out of the equation. Thus reducing power. So I’m hoping that I can find a head gasket thick enough to get down to 10:1
Have you ever used nitrous? Just asking, because having more compression is really not a problem. To safely run nitrous you will need to retard your timing AND use high octane fuel (like race gas). The compression is a non-issue in my opinion...and I ran nitrous for quite a few years on motors that I built specifically for nitrous use.
 
Tbere are three ways (at least) to reduce compression ratio. Mill the pistons to reduce deck height or dome, enlarge the combustion chamber (minimally effective) or a thicker head gasket.
If you have open chamber heads, and dont have a quench dome piston, you dont have any quench now to lose. A set of thick head gaskets can lower your compression quite a bit. Cheapest, easiest ,simplest solution.
If you post your calculations and engine specs that say you are at 11 to 1, we can better advise what we would do.
Aren't there ways to reduce compression by changing the cam specs??????
 
Is there a formula? 4.030 pistons 308 heads. It’s almost 11:1 want to go to 9:1. Use: pump gas and nitrous.


If you are going to use a power adder milling the piston is a bad idea. Especially that much. You can’t make the ring land that thin. It will eat itself.

Are you 100% sure of the compression ratio? Unless you CC the heads and PIR the cylinder you are only guessing.
 
Is there a formula? 4.030 pistons 308 heads. It’s almost 11:1 want to go to 9:1. Use: pump gas and nitrous.
Take them to a machine shop & have them milled to a dish... I haven't done the math on your engine but probably a 12cc or more dish will get you there. Problem maybe if they're milled & you're using nitrous you'll most like make a very large gas port about 3 inches around because they were to thin.
Sometimes we have to do things we don't want to, Lord knows I have had to buy something else more than once. Good luck.
You could get away with a .030 over factory 5.9 magnum piston (depending on a few things)
 
Aren't there ways to reduce compression by changing the cam specs??????
Yes & no, you can change the dynamic compression but cannot change the static (which is the accumulation of the mechanical parts)
 
If you are planning on running nitrous, I would not cut those pistons.
Thicker good head gasket,use a cam that will bleed off some compression.
 
If you're up for a challenge... separate fuel system and storage for the nitrous enrichment with e-98 or Methanol and let it eat... Just remember it takes a greater volume of alcohol for enrichment than gasoline and you still need to retard the timing when spraying but it makes more power for a given amount of N2O.
 
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