Please help, Stage II motor with 9.75 compression

[wickedv6]

I have buddy with a stage II motor with stage heads, decent cam, 88 turbo and all the good stuff. What he wants to know is; as the motor sits, the compression ratio is 9.75 to 1. Is this motor safe to run? The car will always be on race gas. Does this mean he can only run 10-15 psi boost? I have heard things from no big deal to he is going grenade that motor with just slight glitch. What are the ratios you guys running? Should we go a thicker head gasket to bring it down? The motor is all assembled and ready to run and my buddy doesn't want to tear it down if he doesn't have to.

T I A
prasad

[turbodave231]

I personally wouldn't build a turbo V6 engine with that much compression. I'd rather make it with boost. Consider pulling the pistons out and flycutting the tops to bring the compression down. If that leaves you too thin on the top of the pistons, I'd replace them or enlarge the combustion chambers.

The standard S2 head has 44cc chanbers I beleive. Some engine builders cut that down to around 38cc for BGN. ASA engines were 9:1 engines and they usually had much bigger chambers. ASA heads are the ones to get if you can find them. The Bush/ASA engines are getting harder to find.

There are guys running more compression, perhaps we can get some other opinions??

[wickedv6]

Originally Posted by turbodave231

I personally wouldn't build a turbo V6 engine with that much compression. I'd rather make it with boost. Consider pulling the pistons out and flycutting the tops to bring the compression down. If that leaves you too thin on the top of the pistons, I'd replace them or enlarge the combustion chambers.

The standard S2 head has 44cc chanbers I beleive. Some engine builders cut that down to around 38cc for BGN. ASA engines were 9:1 engines and they usually had much bigger chambers. ASA heads are the ones to get if you can find them. The Bush/ASA engines are getting harder to find.

There are guys running more compression, perhaps we can get some other opinions??

Thanks Dave for your input. The heads were opened up to 45CC from 38cc. I am getting a lot of conflicting answers from a lot of people. I talk to one of the top Buick engine builders in the nation, his exact words were "all the motors that leaves my shop are 9.5 to 1". I had my stage motor done at 9 to 1. I am pretty sure there are guys here that run higher compression. I hope they chime in.

Thanks again
Prasad

[Nojug]

My SII is almost 10 to 1 and I have ran near 30psi

[BAGN]

I don't think you will have a problem with that compression ratio and 20+ lbs of boost. My 2 cents

[ijames]

Billy Anderson posted a couple of times that he had worked his way up to 10:1 in his 8.2-8.4 sec GN (before building his rocket :-)) to get better spooling, and I'm sure he was running 25+ psi boost. Only thing you might watch for is detonation at lower rpms (3000-5000) as the boost comes up. You might need to use a controller to keep it down until the rpms are up in first gear.

[JayC]

Ive run way more compression and way more boost

Ride out

[chevyII]

I asked dan at DLS about the squeeze factor and he like higher compression where as kenny D likes low compression and alot of boost. I have mine at 9:1 but would actually like it more toward 9.5:1. Either way 5hp per inch is very doable.

[Eticket]

I am sure that cometic, and other major gasket makers... have different thicknesses for head gaskets.

I think you'll find that 9:1 - 9.5 compression is actually more common than seems in stage motored cars. Mine is only an 8.25 to 1 motor, but it is only a street car.

if you can get it down to 9.5 to 1 with just gaskets, you'll probably like it that way to bring the torque/spool up up with the big heads.

[STP]

Keep the timing low and it should be fine. Personally, I like comp vs. boost to a point. I run a 9:1 at 25psi with 24deg timing on pump gas and dual nozzle alky with my 265" stage 2. Unported GN1R's and an old school 76 from John Craig. Spools very quickly with the little extra compression...If it were mine, I'd be looking for headgaskets to lower the comp ratio a touch...Keep in mind a thicker gasket is easier to push out.

I don't understand the prob with running more compression...Isn't cylinder pressure, cylinder pressure...No matter how you go about it? Or am I wrong?

[wickedv6]

Wow, thanks guys for chiming in and sharing your inputs. Looks like my buddy will be running his setup as is.

Thanks again

Prasad

[lazaris]

Cyl pressure is cyl pressure but.... When you add boost pressure, to increase cyl pressure you get the benifit of more air being pushed into the cyl......

Originally Posted by STP

Keep the timing low and it should be fine. Personally, I like comp vs. boost to a point. I run a 9:1 at 25psi with 24deg timing on pump gas and dual nozzle alky with my 265" stage 2. Unported GN1R's and an old school 76 from John Craig. Spools very quickly with the little extra compression...If it were mine, I'd be looking for headgaskets to lower the comp ratio a touch...Keep in mind a thicker gasket is easier to push out.

I don't understand the prob with running more compression...Isn't cylinder pressure, cylinder pressure...No matter how you go about it? Or am I wrong?

[Reginald West]

I run 9 to 1 but as bill says detonation at lower RPMS can be a problem.I have not had the problem using 116 on the stage car but on my street car which is also 9 to 1 using pump gas and methonal ,detonation is a real problem down in the low RPM range.

REG

[Dominic]

9.5 sounds totally fine to me, i've run that compression with 30+ and did not have any problems. Detonation is only going to happen if you have too much timing or too little octane. I like to VP Import, but it's expensive, next step down is VP N02 and it's like 120 motor octane. If you run one of those fuels over C-16 you should be able to run your desired timing curve and still keep detonation away.

[Nasty Wendy]

Originally Posted by Reginald West

I run 9 to 1 but as bill says detonation at lower RPMS can be a problem.I have not had the problem using 116 on the stage car but on my street car which is also 9 to 1 using pump gas and methonal ,detonation is a real problem down in the low RPM range.

REG

Why would detonation be a problem only at low RPMs? Something to do with increased dwell time?

[ITSNOTAGN]

9.5:1 is nothing. Look at the LS1 community. I have the lowest compression LS1 you can get, 9.4:1, and i run 12 psi on mid grade 89 octane. hell the new LS2's have 11:1, and many run aobut 8psi on pump. what hes talking about is that most engines will knock at peak torque, that is where the load is the highest. i run about 10 degrees of advance there, and around 12-13 up top. But ls1's love boost more than timing.
Gary

[ijames]

Detonation is all about time - the octane rating gives an indication of how long it will take for heat to decompose the fuel into something that will spontaneously ignite, and the engine characteristics such as ignition timing, bore, plug location, chamber shape, temperature, and rpm determine how much time is available for that decomposition to occur after the spark plug fires until the flame front reaches the farthest wall of the chamber and all the fuel is consumed. For a given set of circumstances let's say that the fuel is consumed just as the last bit is just starting to spontaneously ignite, so the engine is right at the threshold of detonation. The combustion process is heavily influenced by pressure so it tends to take about the same number of degrees of crankshaft rotation, because the crank rotation determines the piston location and thus the combustion chamber pressure ignoring the combustion process (I know that sounds a little circular but it accounts for the major changes in pressure versus crank position). Now look at it in terms of time - double the rpm and you halve the time needed for the same number of crankshaft degrees. Take that situation where the engine is right at the detonation threshold, and cut the rpm in half, now there is twice as much time for fuel ahead of the flame front to decompose into something that autoignites, and voila!, lots of detonation. Everyone knows about turning the boost down and backing off the ignition timing to reduce detonation (both of which reduce the chamber temperature and peak pressure which slow down the decomposition reactions), but you can also "outrun" it by turning up the rpm. That's one reason motorcycles 20-30 years ago could run 11 and 12:1 compression way before cars could: they operated at double the rpm (and had small bores). In "our" case with the higher compression ratio and high boost and high timing the engine may not detonate at 8000 rpm but may well experience detonation at 4000 rpm unless you keep the boost down or retard the timing until the rpms are up. A second factor is that peak torque happens at whatever rpm it happens at because that is the rpm where the cylinders are filled most efficiently which means they get the biggest charge of air and fuel and thus will have the highest combustion pressure, compared to other rpms, and thus will be more likely to detonate if everything else is the same. And, that peak torque usually happens down between 3500 and 5000 rpms, which is another reason it's more likely to get detonation down there.

About the LSx's, yes, they are high compression and can run some pretty impressive boost numbers, but they have somewhat smaller bores than the typical 4"+ of a 4.1L-based Stage II motor, and heads that give much more swirl and turbulence to the intake charge than Buick heads do (think 70's small block Chevy heads, not 2000's, when you picture a Buick head :-)). If you ran 12-13 deg of timing with a Buick a good chunk of the combustion would still be going on in the exhaust, and you wouldn't make much power and you would kill the exhaust valves pretty quickly. To get the same "completeness" of the combustion process at the same point in the exhaust valve opening cycle (to make up a phrase) the Buick heads need 5-15 deg more advance than the current LSx heads. It's not that it's good that we CAN run more advance, it's great that the LSx's DON'T NEED TO.

Oh, while I'm typing, someone said something about "cylinder pressure is cylinder pressure, right?" Well, not exactly. If you think of the turbo and cylinder as two separate compressors, it turns out that you make more power by letting the turbo provide most of the compression, since it is a more efficient compressor than the piston is and you can do intercooling to cool the air down after the first stage of compression in the turbo before doing the second stage in the cylinder. At any given chamber pressure you will have a cooler, denser charge if the turbo did more of the compression. If all you care about is the peak hp number you want the lowest compression ratio that will barely let the engine start and run, and then a huge turbo. Problem is that turbo will take days to spool so you can get great test-n-tune et's where you wait and wait for it to spool, but you will never win a race because your reaction time will be pathetic. That's why people go to higher compression ratios, to get faster spoolup but at the sacrifice of some peak hp.

[Nasty Wendy]

Originally Posted by ijames

All that stuff that Carl said.

Thanks a million for that thorough explaination. It is much appreciated.