Whats the best compression ratio for a 3.2 short stroke with a Eaton M90 Kompressor

[Newbie911]

Hi again, it's Newbie 911.

I'm figuring this out little by little. So I'm going to keep my original engine. I'm turning my 3.0 into a 3.2 short stroke. I purchased a TPC supercharger with a water-to-air intercooler. I'm switching the ignition to coil on plug MoTec EFI. I'm getting Extreme Heads to help deal with the heat build-up at low rpm with the supercharger. They say those heads are the best at handling heat issues. The heads have Aasco springs with titanium retainers.

The TPC supercharger is a bolt-on for the 964 or 993 engine. The stock compression ratio on a 964/993 from what I hear is 10:4:1. Should I keep the same ratio or decrease to a lower ratio? I've heard it both ways and wanted to hopefully get a consensus.

Even if I'm willing to pay it's hard to get the right info and I get limited replies from people I've contacted. I guess I'm looking for someone to help me design my engine around this supercharger.

I'm adding a photo of my dogs and me. lol

Thank you very much for any help with this.

Chris

[Dr J]

I have researched this topic quite a bit, and here is a summary of what I found. If you lower the compression ratio you can use more boost, for an overall more powerful engine. With a high compression ratio, you need low boost so as to avoid detonation issues, resulting in a less powerful engine.

The 930 was made with a compression ratio of something like 6.5 for the 3.0L and 7.0 for the 3.3L which makes it is slow off boost. The low compression was also due to the use of CIS which is not as controllable as EFI. EFI will allow you to have higher compression ratios. With a supercharger like the Eaton (roots drive) or a twin screw (like a Lysholm), you do not need to build much rpm to have boost so having a lower compression ratio is not as much of a problem as it is in a turbo.

Having said that, the issue is more complex if cams are changed as it gets into the difference between the dynamic compression ratio vs static compression ratio (what people normally refer to when they measure compression).

[Bucketlist]

I am a fan of twin plug heads. I feel they give you more headroom at any boost level or compression ratio. Ask Extreme what he thinks.



My experience says lower compression and higher boost gives the best all around performance. I have never heard many positive results from air cooled motors running a supercharger instead of a turbo.

[bleucamaro]

It depends.

It depends on boost pressure, charge temperature, spark control, and fuel delivery.

I ran an M90 on a 944S2 at 10.9:1 compression. It ran great.

I also sized the pulley so it only made 4.5psi boost.

If this were my project, I'd build the bottom end to a tuned 930 spec, and focus on reducing charge air temps (bigger intercooler / water injection). Then you can keep the boost higher.

[Ollies930]

Realistically the best way to approach this is to figure out what hp you are looking for and what your max rpm will be. What fuel will you be running (E85 for instance) and will you want to run different pulleys for a race tune?

[kamaro]

Boosted engines needs 3 main things, controlled spark timing, cool air charge temperature and high octane fuel, if you have strong control on these things you can have a good amount of boost in a high CR engine. My 3.2 is a Euro engine with 10.9 CR and I'm pushing about 7 PSI with no issues, thanks to a full bay intercooler and a good spark control using my efi conversion (microsquirt), beside using 95 octane fuel.

[Newbie911]

Originally Posted by bleucamaro

It depends.

It depends on boost pressure, charge temperature, spark control, and fuel delivery.

I ran an M90 on a 944S2 at 10.9:1 compression. It ran great.

I also sized the pulley so it only made 4.5psi boost.

If this were my project, I'd build the bottom end to a tuned 930 spec, and focus on reducing charge air temps (bigger intercooler / water injection). Then you can keep the boost higher.

Thanks for your response! I'm going to have my crank cross-drilled and install a GT3 oil pump. I also have 964 camshafts. I'll be using all ARP hardware and racing bearings.

[Newbie911]

Originally Posted by kamaro

Boosted engines needs 3 main things, controlled spark timing, cool air charge temperature and high octane fuel, if you have strong control on these things you can have a good amount of boost in a high CR engine. My 3.2 is a Euro engine with 10.9 CR and I'm pushing about 7 PSI with no issues, thanks to a full bay intercooler and a good spark control using my efi conversion (microsquirt), beside using 95 octane fuel.

I believe the TPC system does keep the regular CR with the 964 and 993 engines. The system I have has an air-to-water intercooler. They also have a system without an intercooler and it supposedly runs fine. I've read that the M90 puts out 10psi which is just a little less than 0.7 bar. I'm not sure what size pully they're using. I'm assuming they gauged the pully to the correct CR for the engine to handle. I'm leaning towards keeping the regular CR like the 964/993 systems this SC was designed for. I mean nowhere in the description does it mention changing the CR.

[Newbie911]

Here's a link to the TPC supercharger system for reference.


https://www.tpcracing.com/product/964-993-supercharger-kit/

[RarlyL8]

The TPC system bolts onto a stock C/R engine which is 11.1:1 for a 993. That is a really really bad idea in my opinion. I've toyed with the idea of bolting one of these systems on my 930 over the years. The 930 engine is designed for forced induction so it would seem to be an excellent experiment. Back in the day a friend had an SC with the SOC Whipplecharger kit. That car was stupid fast with the torque curve of a small block V8. One weak spot was the 915 trans which could not take the instant torque and had to be modified. He blew up the engine twice at 0.7bar with stock SC C/R so they lowered it to 0.5bar.
To directly answer your question, I'd use 930 C/R of 7:1 with a supercharger, same as the 930, and run 0.8bar boost with an intercooler. SC's produce heat differently than a turbo due to being "on" all the time, thus the conservative boost level. With instant boost there should not be any issue with the low C/R, no turbo lag.

[gsxrken]

Sounds like a cool build!
How do you safely tune something like this? Do you have an EFI system that someone local can dyno tune for you?

[Newbie911]

Originally Posted by RarlyL8

The TPC system bolts onto a stock C/R engine which is 11.1:1 for a 993. That is a really really bad idea in my opinion. I've toyed with the idea of bolting one of these systems on my 930 over the years. The 930 engine is designed for forced induction so it would seem to be an excellent experiment. Back in the day a friend had an SC with the SOC Whipplecharger kit. That car was stupid fast with the torque curve of a small block V8. One weak spot was the 915 trans which could not take the instant torque and had to be modified. He blew up the engine twice at 0.7bar with stock SC C/R so they lowered it to 0.5bar.
To directly answer your question, I'd use 930 C/R of 7:1 with a supercharger, same as the 930, and run 0.8bar boost with an intercooler. SC's produce heat differently than a turbo due to being "on" all the time, thus the conservative boost level. With instant boost there should not be any issue with the low C/R, no turbo lag.

From what I've read Porsche says the CR rate is 11:1:1 but the tested rate is 10:4:1. From what the Eaton site says that the SC puts out 10 lbs of boost. That's just under .7bar. I was thinking about getting it ported out and then lowering the CR to 9:5:1 then playing with pully size.

[bleucamaro]

Originally Posted by Newbie911

From what I've read Porsche says the CR rate is 11:1:1 but the tested rate is 10:4:1. From what the Eaton site says that the SC puts out 10 lbs of boost. That's just under .7bar. I was thinking about getting it ported out and then lowering the CR to 9:5:1 then playing with pully size.

Also, you might want to read up on pressure ratios. (google: Supercharger Pressure Ratio Calculator)

that 10psi on a 3.6L will be more on a 3.2L

Remember Boyle's Law from thermal dynamics, P1V1 = P2V2.