luckyman

I'm sorry, perhaps a stupid question. When I rebuilt my motor, I failed to properly torque one of the rod caps. You can imagine what happened. Anyway, I'm doing it again, and have to purchase pistons and rods. My question is simply whether I can get more power out of a higher compression ratio? I'm staying with the stock-length rods. Can I use a domed piston with stock heads?

Thanks, fellows and ladies, as always.

Stephen
Plano, Texas

1996man

I am very curious about the specs on this as well. i have heard that simply putting Impala head gasketson our motors raises the compression .3 points in itself. Is there a rule of thumb as to how much percentage .1 point of compression adds?

65Z01

Going to domed pistons will increase CR but won't necessarily yield more HP due to flame propegation changes.

If you currently have dished pistons going to flat top, with recesses, will likely up the CR and HP.

Consider decking the block to reduce quench volume as well as increase CR for a HP boost and less octane sensitivity.

What year Vette & what engine??

CFI-EFI

It is generally considered that a point (1.00) of compression is worth approximately 4%-5% power increase. There are tons of variables. This is not a hard and fast rule.

RACE ON!!!

VetNutJim

Generally, yes.

Can you use domed pistons with your stock heads?

I don't think any of the aluminum GM heads will accomadate domed pistons because of that stupid square boss above the spark plug hole.
Don't know bout the iron heads.

vader86

I dont think you can use the domed pistons, but an increase in CR should up the power a little, but as theyve said there are other variables that would determine how much you got or lost.

Strike3

You don't say what year your car is but generalising about CR could mislead you. The short answer to your question is "yes".

Having said that, lets get practical. You also don't say, but I assume your car is a daily driver rather than one reserved for the race track. If that's the case, the next question you need to get comfortable with is do you want to buy nothing but the hi priced fuel from now on or not. Some people buy the hi-octain stuff all the time anyway. My stock Vette runs fine, to and from work, on 87 octain fuel. The knock sensor and ecm handle that just fine, but there is a limit to what they can compensete for.

The higher the compression, the more susceptable your engine is to pre ignition (knock). Knock is bad. Your ECM will cut back engine timing and, thereby, power. Counter-productive to say the least. Also, the head material(iron or aluminum makes a difference in pre-ignition). Aluminum heads run cooler, all else being equal, and will allow higher compression (about 1 point) before pre-ignition occurs. So if you have an 84 thru early 86 you probably have Iron heads. Later models have aluminum heads. The mid year cars with aluminum heads have about a 54 cc combustion chamber. Not much room for a dome there.

If you decide you are going to go strickly with hi-octain fuel you can take your compression to between the 10 or (closer to) 11 to 1 combression ratio.
Much higher than that and you are entering the area of aviation fuels and or additives. If you set your target within that 10, 10 1/2, 11 range and look at available pistons you will find, you'll be lucky to not exceed that range with a flat top piston, let alone going to a rased dome piston. Of course there are "coatings" and other hi tech stuff you can get into but we're staying real-world here.

Another important factor for pre-ignition seems to be the quench area of the head. This is the space created by the head gasket, adding voluum between the top of the piston and the roof of the combustion chamber. Most reports agree that the smaller the quench area, the higher compression the engine can handle before pre-ignition becomes prevelent.

How do you effect quench area?? Several ways, from decking the block, milling the heads, or using a thinner head gasket. Eliminating the first two, find a set of head gaskets that torque to a compressed thickness of about .029"rather than a typical .047".

Hope my 2Cents helps.

CFI-EFI

The quench area is space from the top, of the flat part of the piston to the flat (deck surface) of the head. Not from any other part of the piston to the "roof of the combustion chamber". The quench distance is the piston deck height plus (or minus) the head gasket thickness.


Milling the heads have no effect on the quench area. Compression ratio? Yes! Quench? No!

RACE ON!!!

1996man

that means if i had a compression ratio of 10.1:1 making 100 HP and raised it to 11.1:1 i would only have about 104-105 HP? i would think a full point compression raise would give you more power than that. or did you mean that going up .1 would give you a 4-5% increase? im still a little confused

CFI-EFI

You read it correctly. That is why I put the "1.00", with the words, "one point", to help avoid exactly, that misinterpretation. Surprisingly little, isn't it?

RACE ON!!!

tythor

300 HP would be 12 -15 HP increase.....Not too bad..

ZD1

just go with a 2 cc forgred piston for higher compression ratios. I wouldn't go over 11.0 though

Corvette Kid

And since it was mentioned, you NEVER want a domed piston on a street engine. They are only for extremely high output race engines running on very high octane fuels. They are much more prone to detonation, etc. and are totally unnecessary since as much or more compression than you could want is obtainable with flat tops.

JAKE

I posted on the question a few months back and included all the HP and Torque numbers that were listed in a mag article.

One of the mags ran a series of tests on an engine changing the CR from 11.0:1 down to 8.0:1 in 1.00 increments. 11:1 to 10:1, then 10:1 to 9:1 then 9:1 to 8:1.

The percentage change was on the order of 4%, give or take.

The big difference was in going from 9:1 to 8:1; that's where power took the biggest hit. If I remember correctly, comparing 11:1 to 8:1, the engine lost around 60 HP.

Jake

mean8t

That is on the order of what I have read. If I can remember from my Internal combustion engine design class way back in college the power increase tappers off as you raise compression, with the biggest increase coming from 8:1-9:1, and then tappering off as you go up. That doesn't take into consideration cam specs and design which also plays into it. If the engine had a cam that is designed for higher compression (read more Duration) then it would show more power gained from the higher compression. Therefore the power may not be totaly derived from the compression alone but more from the engine taking advantage of its cam.

kruizinkareem

There are many factors involved, it's a tricky subject and there are a lot of different calculations in play, but here's a good video that discussed high compression and what it does to power in different engines.

auburn2

I would not go higher than 10:1 with a stock cam and 93 octane.

If you get a cam with more duration you can go a bit higher.

Raising the CR will raise power if you keep it out of detonation. If you have to retard timing then you lose any extra power you would have made.

Higher CR will also improve throttle response.

cv67

% increase doesnt sound like much but it will be more efficient
Throttle response idle quality tq..youll feel it
If you have a head wiht a modern combustino chamber shape, properly cammed absolutely do it. Run a little over 11.5 (shoulda gone 12.5).
Depends on what you want though as said dont get it so high with a small cam that you have to pull timing. Map your plan out well & stick to it.

ctmccloskey

On a 1968 C3 with it's 427 I rebuilt the engine using 12.25-1 compression ratio and copied the L88 builds closely but used a GM cast iron cylinder head. My cast iron head has pockets in it allowing the piston to fit. The piston has a huge dome on it and matches the cylinder head exactly. If you look up the heads I used they end with 063 and were used on 396 and 427 engines from the factory.
When my pistons are at top dead center the piston crown rises approximately 1/2" above the Engine Block. I have over 10,000 miles on this engine and have not had any trouble with detonation or pinging unless the weather is very hot and I run the car hard.

The reason I can use this compression and still run pump gas is because they are Closed Chamber heads which have been replaced with Open Chamber heads in an effort to get better emissions. The problem is that when GM did this they started having detonation and pinging problems associated with this cylinder design. I have knock sensors on my block and I don't get any indication of any problems.

After using Aviation fuel (100 Low Lead) I started to see deposits building on my spark plugs and valve heads. I removed the heads to verify that everything was working okay and there were lots of lead deposits that had to be cleaned off mechanically. I now use plain old Sunoco 93 octane gasoline without any issues. Race gas worked fine but cost way too much for a street driven Corvette. I have since then installed a Snow Performance Water/Methanol injection system in hopes of cooling down the combustion chambers during ignition. The plus side of the system is that when my carburetor gets under a load the system automatically turns on and adds the methanol to the fuel which gives the fuel an equivalent of 115 octane gasoline.

Raising the compression ratio does help make power, look at todays cars and you will see that everybody seems to be using more compression and still running pump gas, some motorcycles are running 13-1 and higher compression ratios without any problems and lots of power. I love the compression in a 427 like mine, it easily pushes the hp numbers up into the 500+ range if your engine can handle it. My 427 was built for tri-power and has all the needed parts to handle the compression and have longevity along with it. I did have to get the best quality forged rods and spent a long time trying to get a perfect Forged crank for my engine. (I went through 11 crankshafts to find one without any tiny fractures in it) With the right design and quality parts you can use a higher compression ratio and make even more power but do understand that most of today's cylinder heads are made with a open chamber type design. There is one manufacturer who makes aftermarket heads that use the closed chamber, I just don't know which one it is.

When testing my compression ratio I see numbers in the 245 psi range cranking and this seems a bit high. The best part is when I tried to drag race (at a track) I literally spun my two rear tires for 1/8th mile on a 1/4 mile track and had a heck of a time trying to get traction. There are pluses and minuses with every design. I am looking at installing a J&S safeguard on my engine this summer as with it I could run more timing and have protection from detonation. The Safeguard is a great idea for anybody trying to get maximum HP from their engine.

Best Regards,
Chris