Compression Ratio

[ChrisCairns]

I'm putting together a secret project so don't tell anyone.

I'm using a production '84 Buick 4.1 block with stage heads. I just got everything back from the machinest and started the engine assembly. I had the stage heads rebuilt locally and had him do the chamber volume so I could calculate the compression ratio. With all the necessary figures I put them into a calculator at .... Engine Compression Ratio (CR) Calculator

With what I "thought" were the correct figures it comes up with a compression ratio of 10.29. Ouch if that's correct.

So can anyone tell me the following.....

I got Dan's stroker kit and simply assumed the stroke was 3.625. Being the weekend I can't phone Dan...does anyone know if that's correct? Second, the machinest said the chamber volume was 49 cc's. Does that sound right?

I've got a 4" bore, said 3.625 stroke, head gasket bore 4.020, compressed gasket thickness of .060, chamber volume of 49cc's, piston dish negative 14 according to my machinest, and piston deck clearance of .010.

Can anyone shed some light on whether my figures COULD be wrong or are my stage head's chamber volume too small for a turbo engine?

Thanks.

 

[summit]

sounds about right. Typical Stage II heads are small. on mine we opened the chambers in the head a little and used .090 Cometics to keep from having to buy new pistons went I switched from Champions to the Stage II heads. 10 to 1 doesn't scare me, you just need to know where it is at and run boost and timing to match. A little higher compression helps on the bottom end anyway.

 

[Hot Air]

The nChampion web site indicates they are all 46cc. That would yield a higher CR. what gives?
Conrad

 

[ChrisCairns]

sounds about right. Typical Stage II heads are small. on mine we opened the chambers in the head a little and used .090 Cometics to keep from having to buy new pistons went I switched from Champions to the Stage II heads. 10 to 1 doesn't scare me, you just need to know where it is at and run boost and timing to match. A little higher compression helps on the bottom end anyway.

How did you open up the chambers?

And we had planned on a 9 to 1 CR for the bottom end. I even fudged a little since I was going to use O ring gaskets and the compressed gasket thickness is about .050 which would result in a higher CR if that figure were used in the calculator instead of the .060 I used. If I go to the thicker Cometics I'd have to have the .010 O ring groove in the heads machined off. Which would put me back in the same boat.

 

[ChrisCairns]

I just went back to that Compression Ratio calculator page and put in the figures for a stock engine.

3.8 bore, 3.4 stroke, 3.830 gasket bore, .050 compressed gasket, 42 cc combustion chamber (I THINK that's right for stock heads) negative 14 dish and .010 piston deck height. That comes up with 10.28 CR.

I wonder if the calculator is wrong.

 

[wbrophy]

I come up with 10.62 using your figures and that cal.

 

[ChrisCairns]

I come up with 10.62 using your figures and that cal.

Yep, me too now. I must have mistyped something originally. But thanks for making it worse....lol.

 

[wbrophy]

I use one that has a few more details like ring depth, piston dia. but it is with in a hair of yours. example= 10.32 on yours and 10.31 on mine.

PS: An 8 to 1 motor @ 20 psi= 18.88 to 1 compression where a 9/1 motor @ 16 psi= 18.79 final compression ratio. So just run less boost to get the same power. This is called: Effective Compression Ratio

 

[GARY HARVEY]

Fast and dirty ol' method...

should tell the truth..

I've run it a couple of times and ended up a little under 9 and my dia's are about the same as posted.

Gary

 

[Alky V6]

Stock buicks run the piston deep in the hole to get an 8 to one. I prefer to run the piston .025 in the hole with a .040" gasket and order the dish volume I'll need to get my target CR. I prefer to leave chambers small and thick. Of course, my target will never be under 9.25 to one with alcohol fuel so my dish volume is very reasonable for the deck clearance I use.

In a gasoline application, if your target is 8 to one, run a dish of reasonable volume and adjust your deck clearance to achieve the target CR. That may mean having to order custom pistons with a special compression height and/or special length rods. You'll miss out on squish with this setup, but some theorize that squish can actually be a problem in some supercharged applications, accelerating the combustion process too quickly in a highly pressurized cylinder and possibly leading to detonation.

Opening up chambers will give you more surface area for heat to act on which is contrary to one of the anti-detonation characteristics of a small compact combustion chamber.

 

[ChrisCairns]

I use one that has a few more details like ring depth, piston dia. but it is with in a hair of yours. example= 10.32 on yours and 10.31 on mine.

PS: An 8 to 1 motor @ 20 psi= 18.88 to 1 compression where a 9/1 motor @ 16 psi= 18.79 final compression ratio. So just run less boost to get the same power. This is called: Effective Compression Ratio

I disagree with your conclusion that you'll get the same power. All else being equal, if you're running 2 psi less you'll have less power.

Using your theory that you'll have the same power just because the effective
CR is the same would lead one to believe that you can do away with the turbo and just run 18 to 1 compression normally aspirated.

Anyway, thanks guys....I'll phone Dan in the morning to see what he suggests.

 

[Alky V6]

I disagree with your conclusion that you'll get the same power. All else being equal, if you're running 2 psi less you'll have less power.

Using your theory that you'll have the same power just because the effective
CR is the same would lead one to believe that you can do away with the turbo and just run 18 to 1 compression normally aspirated.

Anyway, thanks guys....I'll phone Dan in the morning to see what he suggests.

He's right Chris. My circumstance is much more extreme, but it very clearly showed me that I could make more power with a higher CR and less boost. The problem with the high CR is that it limits your max boost level.

I made more power with an 11.4 to one CR and 22 psi boost than I have so far with this new motor with a 9.25 to one CR and 29 psi boost.

 

[bison]

The stock dish is more than 14 cc's. I think its around 22 cc's. The stock heads are around 48 cc chambers. The pistons are in the hole as Don said which gives you just about 8.0:1. I like the pistons at zero deck and a thin head gasket (under .040 compressed). This reduces the wasted clearance volume. I usually have the piston dish around 27 cc's. I shoot for around 9.0:1 or slightly over. 30 psi on top of 9.0:1 is no problem if there is enough octane. Cometic will make just about any gasket thickness you want or you could have the dish in the pistons increased and re-balance. The later would be the preferred method. You will have a lot more power with extra compression and lower boost if you can reduce the exhaust pressures between the engine and turbo.

 

[ChrisCairns]

The stock dish is more than 14 cc's. I think its around 22 cc's. The stock heads are around 48 cc chambers. The pistons are in the hole as Don said which gives you just about 8.0:1. I like the pistons at zero deck and a thin head gasket (under .040 compressed). This reduces the wasted clearance volume. I usually have the piston dish around 27 cc's. I shoot for around 9.0:1 or slightly over. 30 psi on top of 9.0:1 is no problem if there is enough octane. Cometic will make just about any gasket thickness you want or you could have the dish in the pistons increased and re-balance. The later would be the preferred method. You will have a lot more power with extra compression and lower boost if you can reduce the exhaust pressures between the engine and turbo.

Right Mr. Bison. First I phoned my machinist and he said we could machine the pistons and rebalance if necessary. Then I phoned Dan and he says the JE's I got from him were 28 cc's. So my machinist made an error in his dish measurements. I'll check them at home tonight. (Work right now.) After putting in the new dish volume (and using .050 as compressed gasket thickness for the ones I'm going to use) it comes out to an acceptable 9.34 to 1.

 

[jasjamz]

The stock dish is more than 14 cc's. I think its around 22 cc's. The stock heads are around 48 cc chambers. The pistons are in the hole as Don said which gives you just about 8.0:1. I like the pistons at zero deck and a thin head gasket (under .040 compressed). This reduces the wasted clearance volume. I usually have the piston dish around 27 cc's. I shoot for around 9.0:1 or slightly over. 30 psi on top of 9.0:1 is no problem if there is enough octane. Cometic will make just about any gasket thickness you want or you could have the dish in the pistons increased and re-balance. The later would be the preferred method. You will have a lot more power with extra compression and lower boost if you can reduce the exhaust pressures between the engine and turbo.[/QUOTE]

Do you mean like having headers vs. stock manifolds?

 

[JDEstill]

Do you mean like having headers vs. stock manifolds?

While headers instead of manifolds might lower the exhaust backpressure seen by the cylinders some, it is not as big a player as some other things.

To get the power to drive the compressor side, the turbine needs a certain ratio of inlet to outlet pressure. Suppose for arguments sake that ratio is 2:1, that is the inlet pressure has to be 2x that of the outlet. That means for every 1 psi improvement in the outlet piping (get rid of the cat converter, better downpipe, better mufflers, etc) you drop the pressure between the engine and the turbine by *2* psi.

The other big player is the turbine itself. Some turbines use more flow and a lower pressure ratio to make the power to drive the compressor side. These will tend to be laggier but will make more power (since they impose less backpressure on the engine). Some turbines use less flow and more pressure ratio to power the compressor. These will spool quicker, but cost some top end hp. So same compressor side, but different turbine side, or different a/r turbine housing, can make a big difference.

Other details, like the mechanical details of the wastegate installation, will have some effect too.

John

 

[Hot Air]

John,
Are you saying that it is better (more HP/Torque/etc) to reduce the pressure drop between the air cleaner and the turbo than it is to drop pressure drop between the turbo and the tailpipes? Or, must we maintain a certain ratio between the two?
Conrad

 

[JDEstill]

not really saying that...

Some people are familiar with the pressure ratio on the compressor side, when you use the (discharge pressure / suction pressure) vs. the air flow to look at an operating point on a compressor map. Similarly the turbine side also has a pressure ratio, but it is flipped: (inlet pressure / outlet pressure). Pressure ratio is an important variable for turbines just like it is for compressors.

On the compressor side reducing pressure losses between the air filter and the compressor inlet helps, since it reduces the pressure ratio (for a given boost level) which then requires less power from the turbine which then reduces exhaust backpressure required which then gives you a few hp due to a higher VE. I wasn't talking about that at all

I was just saying that you can reduce the exhaust backpressure that the engine sees by
a. reducing pressure losses on the turbine outlet side, where improvements are multiplied due to the pressure ratio effect, and
b. going to a higher flow turbine wheel/housing, at the possible expense of increased lag, and
c. headers and some other mechanical details will also help, but probably not as much as the other two items, depending on various factors.

Not saying that any old piece of crap header is going to work, of course. Just that if you have something half way decent, then there's other spots to look at first when trying to make improvements.

Reduced backpressure is a good thing, since it improves VE by getting more exhaust out of the cylinders, letting you move more air and fuel in (and make more power) at a given boost level.

John

 

[bison]

Getting the ratio closer to 1:1 the intake pressure to exhaust pressure will make more power than reducing the compressor inlet vacuum or pressure drop in the intake tract form compressor to intake. The size of the engine, wheel design, turbine housing and several other factors play into the exhaust pressure ratio. Getting the turbo spooled and having the lowest possible pressure in the exhaust between the turbo and engine is the most potent combo. Some have achieved lower exhaust pressures than intake pressure on some small ,high revving, high boost engines.

 

[bison]

Right Mr. Bison. First I phoned my machinist and he said we could machine the pistons and rebalance if necessary. Then I phoned Dan and he says the JE's I got from him were 28 cc's. So my machinist made an error in his dish measurements. I'll check them at home tonight. (Work right now.) After putting in the new dish volume (and using .050 as compressed gasket thickness for the ones I'm going to use) it comes out to an acceptable 9.34 to 1.

How could he have made an error in measuring the dish on those? It seems pretty foolproof. I have done it with a burette and a 60 cc syringe with the same result. Maybe he was thinking about someone elses pistons? Your looking good if at 9.34:1 though.