The program I use is similar to this one:
http://kb-silvolite.com/calc.php?action=comp2

To find the intake closing point, take the duration at .050" and divide by 2. This number is added to the intake centerline. Then you subtract 180 from this number and you have the intake closing point at .050". The Silvolite program says to add 15 degrees, with a 911 cam 20 degrees is a better number.
Example is an SC cam
228/2=114, 114+113= 227 , 227 - 180 = 47.
47 degrees abdc is the intake closing point at .050", then add 20 degrees and the advertised closing point on the intake valve is 67 degrees.

1976 3.0 934 Turbo was CIS engine.
Was it easy to drive? Hell no, it was an absolute terror.

Did the 934 run 1.4 bar?
How much boost could that engine in factory trim take?

thanks john,
I came up with a dynamic of 6.310 vs a factory 6.12 with 8-1 with SC cams.. that sound right?

what is indicated by this?

Betwwen to 1.2-1.4 bar if I remember correctly.

Slightly higher cylinder pressure. The cranking compression will be higher than factory. Percentage wise the increase in minimal, but with the better breathing camshafts the VE is also increased.

I've spoken with a racer who ran 9.5 CO with 1.5 bar. Yes it was fast- and sounded very nice- safe for the dead silence of DNF.

So yes you can. Will it let go, of course. But if you are serious, you will find EMS, EFI, twin plug, exhaust, plus machine work, parts, and expert labor capacity, if you are ready, and can wait from few weeks to several months. And, there are a few turbo motors on the market right now that have all/most of the right components to do what you are going to do, AND MORE :).

Regards,

I have 8.0:1 JE Pistons and JB Racing cylinders. I also mitigate against detonation by conservative iginition tuning, 11.5:1 AFR (on boost) and very large IC (1 bar max boost).

In addition, it was my intent to have a car that has good all around drivability rather than high HP. If I was going to have a track car running >6000 rpm continuously, then I would have opted for the low CR pistons.

No doubt the Porsche engineers win every time, however I have a slightly different application and have tuned with pump gas.

Good info, Don. Just curious, have you dyno'ed yet?
Mike

Mike, I think Don is making about 500 horses

Thanks to all for the great info...

John, based on the program you refer to, it sounds like SC cam/8.0:1 CR will result in similar dynamic CR as factory set up, so should work quite reliably even if other parameters (boost, ignition, etc.) are kept the same?

Don, that is my dilemma. The car is used really only on track and even with CIS and AFR of 12:1, I don't run more than .9 bar boost. In going EFI, twin plug/bigger intercooler, I'm debating if I can run "higher than norm" CR (8.0:1) and still have the headroom to go up to 1.0 bar boost if need be without blowing things up.

If you have the same boost with the SC cams that you had with the stock turbo cams, you will be making more HP. This means the VE is higher and so is the cylinder pressure. The predicted dynamic compression ratio will be close to stock until the engine comes on boost.
You might have to pull some timing out of it. You should also run the highest octane fuel available. I wouldnt tell a customer in California with our crappy 91 octane fuel to try 8-1 CR/SC cams in a turbo unless he upgrades the intercooler or twin plugs etc.

I'm somewhat surprised to see this is the first real mention of the intercooler in this thread... I would have expected intake charge temperature to be the primary concern.

Although "In theory there is no difference between theory and practice. But, in practice, there is", it's so much easier to theorize :) . From a completely theoretical perspective, here's what I would expect to face:

1. compression-induced detonation : the intake charge is just too hot : this is where the intercooler helps

2. hotspot induced detonation : combustion chamber surfaces too hot in points, too much fuel, not enough cooling air : this is where better cooling technology (e.g. nickies, polishing chamber) helps (does overdriving the cooling fan help?)

3. optimal fuel, timing : getting the right AFR, spark : engine management, right?

4. flame propagation : getting even pressure : twin-plugging

5. pressure : not blowing the head off the cylinder : grooves and rings

Am I way off base?

What the the practical pitfalls?

I have to admit I have no practical experience with overboosting a 930 engine (maybe if I park my 951 very close to my 911S under the full moon some magic will occur...) but plenty of people crank the boost on their 930 up to 1bar, why is increasing static compression significantly different?

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Joe

The intercooler is a big factor. Its amazing the difference changing the intercooler will make. I saw a rear wheel dyno sheet from a Ford Mustang, he had a water to air intercooler, 347 cu in and EFI and printed 2100 hp. I think the car has run in the 6's in the 1/4 mile.

In my mind, its down to controlling heat, and fuel/ignition plays a big part in this.

As Henry asked, why didn't Porsche used higher CR in 934/935 - my guess is because of the non-precise CIS that was used? And both cars ran at 1.2-1.4 bar (yes with a complete engine rebuild after each race).

My question should probably be: if everything else is static, will 8.0CR at say .8 bar generates as much heat as 7.5CR at 1 bar boost?

I'm adding EFI, full bar intercooler, twin plug and ported head. My assumption is that I can control fuel/ignition needs and lowers intake temp as much as I can, so that I can at least reliably run the seemingly standard setup of 7.5CR with 1 bar boost using SC/964 cams.

If I were building a turbo track car, I would use a 6.5 or 7.0:1 CR piston and use the cam to deliver power. John can make a custom grind for you that will give you the best VE in the RPM range you will use. At high RPM, there is no need for high compression on an air cooled, turbo motor with correct VE.

Mike - around 500 rwhp at 1 bar.

How would one go about getting over 8:1 compression? Custom pistons? Weld up the head and remachine?

My engine's already built with 7.7:1 compression using 98mm JE pistons. So I'm not looking to change, just curious.

The PO of my car shaved the heads to get .5 more CR.

Interesting question... I couldn't resist plugging it into a thermo model... so I dusted off something I used on a previous project (see: http://forums.pelicanparts.com/911-engine-rebuilding-forum/261739-engine-model-spreadsheet.html ) and here's the numbers I came up with:

Which would imply that in fact you would expect ~25C greater temp at the end of the compression stroke with 8.5/0.85 versus 7.5/1.0. In order to get the equivalent power you would need about 0.9 bar and you would be somewhere around 30C hotter. The only way the 8.5CR wins is in fuel efficiency. It should use about 10% less fuel for the same HP.

Of course this assumes you trust my thermodynamics. I'd be very interested to hear of practical experience to compare to the model...

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Joe

Great old thread, bumped to the top.

There are some new(er) variables that need to be considered here. Given the ability of a progressive, ECU controlled methanol injection system to functionally boost premium fuel's octane to ~108 at full blast, the ability to blow water mist into the post-IC airstream (also sensor controlled) to further cool the intake charge, and J&S now having perfected their per-cylinder knock retard for the 911/930, my guess is that the old "max" numbers, even for a full-tilt, twin plug EFI don't apply.

Is it unreasonable to expect 1.2 bar from a 9.5:1 static CR, twin plug, big IC, full option EFI motor w/ knock control and all of the above bits? Maybe. This does result in an (at rest, boost corrected) dynamic CR of 16.3:1.

That said, those little watercooled Mitsubishi Evos, running the same spec engine control (EFI/knock control/big FMIC/H2O&meth injection) as above are regularly running near 30 pounds (2 BAR!!!) of boost at 9.5:1 static CR, and pumping a streetable, reliable, 650HP from a 2 liter fourbanger. Discounting the advantage of water cooling, 1.2 BAR (maybe even 1.5 BAR) seem downright accessible to an air-cooled motor if properly designed and managed. If that motor is a turbo 2.2 liter (with much smaller pistons than any production Porsche turbo), it seems even more likely.

That much boost might blow the heads off of a 911, ripping those Supertec head studs right out of their case savers, but the question begs to be answered.

Is this crazy talk or are we just taking convention for granted in light of refined technology?