Can anyone figure this out? No one seems to know.
 

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.

Thank you very much for any help with this.

Chris

Are you planning to run race gas exclusively?

It will NOT like any kind of pump gas if you plan on running much boost (~7psi+). E85 would help if it's available in your area, but be sure to test the ethanol content.

Decreasing the static compression ratio (and running more boost) would be the safer way to do it.

Following


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My question would be why would you keep 11to1 pistons in there you wouldn't be able to run much more than a few pounds of boost before shattering some rings.I am not running a Super charger but running a single turbo charger with 7.5 to 1 compression and boosting 1 bar on low boost.To me it sounds like you are doing it right with your build all the components are the best and pretty much what I have in my 930 including using Motecs M130,I think I would go with a different compression piston and cylinder and also call Chris or Mike at TurboKraft there the best out there.



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This all calculatable using one of Boyles laws.

You can calculate the cylinder pressure of your engine now in NA form and then change the value of pressure above atmo in your new calculation. Keep lowering the CR number until you obtain the same cylinder pressure you have now. Pretty simple.

This is all theoretical should be checked against what the Supercharger supplier also recommends. If they are selling a bolt on system they should be advising any changes required. I hope.

Remember, in your calculations you have 2 variables to consider. CR and manifold pressure.

You can vary either to obtain a safe CR and or manifold pressure. I guess the Supercharger company will tell you what manifold pressure you will use with whatever size pulley they sell with the kit.

Then you use that manifold pressure and adjust the CR number to get the cylinder pressure you are comfortable using. Best way to start is to calculate what you will have now and adjust the CR to get close to that number. But take into account other factors. Thermal changes etc all have to be factored into the final number.

Good luck.

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.

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


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

????? Not sure why you asked the original question.

I want to get other opinions. Some say it's a horrible idea to keep the same CR. Some say it's fine. I just don't want to crater my engine.

OK, riddle me this, what compression ratios did Porsche use on their factory boosted air cooled cars? Did they use the exact same pistons as in the normally aspirated cars? Did they take any other safety measures to keep the engines from grenading or melting themselves down to scrap?

Not trying to confuse more than you already appear to be. Unless I'm reading this wrong, you seem to be comparing two different engines.

You are building a 3.2L but are discussing a 3.6L 964 engine. I think the best is to go back to the supercharger company and ask them.

You can do some very simple math and calculate the BMEP for both engines and compare the results. You will need to know the Torque your 3.2L engine will make.

To give you some idea here, a stock 964 3.6L engine will produce an average cylinder pressure of approx 10.8b. Its peak pressure will be up in the region of 30b.

If you take that same engine and add in the 0.7b positive pressure that same engine will make an average pressure around 16b. Its peak pressure will be upwards of 51b.

Best to get the suppliers advice and ask them to give some sort of way forward. If it was me, I'd go real slow and make some logical decisions. The engine has no idea what its got connected to it. Turbo or Supercharger. It only knows manifold pressure.

Keep in mind that the 3.6 engines had Knock control from the factory, the 3.2 did not. While you might be able to add Knock control with the Motec system it will require tunning and you do risk detonating the engine while setting up Knock control. In general compression ration is lowered for boosted engines. Porsche only ran 9.4 to 1 in the 996/997 turbo motors and these had sophisticated engine controls, were water cooled with 4 valve heads.

john

I tried to talk to the supplier but since I purchased it 2nd hand they're not much help.

The Nickies I'm getting have knock sensors. With the MoTec M130 ECU I'm hoping mapping knock won't be a huge issue. I was thinking I could get the SC ported so it gives a bit more boost then lower the compression slightly and figure out the pully size while tuning? Sorry, that might be a dumb idea.

If we use a thicker cylinder base gasket , will that lower the compression ??

Some of the bolt on supercharger kits were traditionally meant to be installed on engines as-is without rebuilding or changing them internally. Since it sounds like you are doing a ground up build, you're right to ask about compression ratio. You have an opportunity to put custom pistons in your engine. Most people building boosted cars use much lower CR than a stock NA engine. Stock SC compression ratios are in the 9:1 range. Not sure you'd want to raise it to 964 or 993 levels, that would not make sense to me.

That's what I was thinking. I could have the SC bored out a little. I could run 9:5:1 and find what size pully makes it all work. I'm sure that's simplifying it.

Newbie: You should look at the CR's used by major manufacturers when they build an engine with a supercharger. Those are in the range of 8:1, sometimes lower. There are strong reasons for that. If you put in pistons with 9:1 or higher, you will be disappointed in the results, because you will have to severely limit your boost on street gasoline, as others have noted. Whereas, if you go lower, you can turn up the boost when you run race gas.

Dannobee gave you a good hint: Porsche already did a lot of development work for you when they built the 930 turbo engines, so I would plagiarize shamelessly, and use the same CR as the turbo engines. As Neil noted, the engine just sees manifold pressure. Stick to what you already know works. By the way, the supercharger sucks a LOT of power from the engine crank to compress air, whereas a turbo imposes a lot less parasitic drag on the engine. Remember that too.

I know you are enamored with the supercharger, but if I were contemplating this project, I would go with a turbo. And that's coming from someone who owns a Ford GT.

I already have the SC. SC are low boost so the CR is different. I'm going to make it work. Thank you for your post.