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Question 8.0/8.5 CR for 930 ???

The general wisdom seems to say 7.5:1 CR is the highest one wants to run on a turbo engine.

But, if I'm willing to lower the max boost on the engine, why not increase the CR to 8.0 or 8.5? Anyone has any experience to how much less boost I can run if the CR is increased to 8.5:1?

Any comments, thoughts?

TIA...

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Old 01-11-2006, 10:04 AM
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i think with good fuel/more fuel you could run .8 twin plug would give you more protection
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Andy

1980 SC soon to be big hp 3.3t powered 73RSR Replica (well, I'm keeping the engine but everything else is going )
Old 01-11-2006, 11:44 AM
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Well, the engine will be twin plug & I'm going EFI.
Just deciding on what compression I can run safely. For background, the car is used mainly on track.
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Old 01-11-2006, 11:59 AM
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then i think you could safely use .8

im sure others will chime in with the real juicy stuff though!
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1980 SC soon to be big hp 3.3t powered 73RSR Replica (well, I'm keeping the engine but everything else is going )
Old 01-11-2006, 12:07 PM
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I've done a few 8.0:1's and even 8.5:1, all on twin-plug engines, but then again they were using them with Nickies.
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Old 01-11-2006, 01:47 PM
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Im also gathering info for my turbo set up.

Since recently I have spoken to a few VERY HIGH hp tuners for turbo cars I get the impression that with a modern tunable EMS you can get away with some pretty high compression. Which is a HUGE bonus for off boost power.

The Tuner I spoke with his personel sled is a Nissan Skyline. He is running HALTECH EMS with 10.5 compression and putting out 900HP with no detonating, but this is with knock sensors.

Since with the 911 air cooled motors knock sensors are not that reliable. I would say with a well tuned EMS you could easily get away with 9.0 compression. which would make a huge off boost difference in HP.

Its all in the air/fuel ratio and timing, and with a good EMS and 21st century technollogy its pretty incredible the difference in performance.

By the sounds of it, it does not sound like you are after max HP. If it were me I would definately be looking into higher compression set up.

I think 7.0- 7.5 is ridiculous with a modern EMS. Unless you are after crazy Hp 700-900hp
Old 01-11-2006, 02:40 PM
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I have 8-1 with SC cams in my otherwise stock 3.3L
i have had no problems so far with 4000miles on it.

Im running stock boost and single plug. With the SC cams and 8.1 it feels like my old SC down low.
if you keep the boost relatively low... i see no problems. I plan on using 1\2 race gas mix when i head to the track....



people can turn up the psi to 1bar on a stock 3.3. why not turn up the comp ratio and keep the pressure the same? I haven't crunched the numbers but how close is the final dynamic compression ratio with those different combinations?





again... i have no background to back this up .. Im just a guy who LOVES his 3.3 with 8-1
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Old 01-12-2006, 05:18 AM
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You can safely go to 8.5:1 with proper EFI tuning and a max of 1 or 1.1 bar. The most important issue is heat management and bad things happen real fast in an air cooled motor.
Old 01-12-2006, 07:45 PM
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Don, what CR are you running with your setup? I think you have 3.4 Nikasil cylinder with JE pistons?
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Old 01-13-2006, 06:42 AM
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Here are a couple of quick numbers to think about when increasing turbo compression ratios.

Parameters
74.4 stroke
97 mm bore
89 cc head
40 deck
Flat top piston
.8 bar of boost (11.2lb)
compression at boost
7.0 cr + boost = 12.62 to 1
7.5 cr + boost = 13.25 to 1
8.0 cr + boost = 14.10 to 1
8.5 cr + boost = 14.98 to 1

Safe octane requirements at 15 to 1 without knock control is 137.
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Old 01-13-2006, 07:36 AM
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Henry, your undoubtly more knowledgeable in Porsches than most. You certainly aren't suggesting that the numbers above (dynamic) are isolated from other features in respect to octane need.

We all know race cars commonly run 14.5 to 15 all day with C12 to C14. I do with no top end retard and total 36BTD, but combustions chamber, fuel management and the CAM shaft is going to determine realized compression.

Keeping to topic and not a debate since again I don't doubt your math, Hobieboy...I would think about cam timing when raising the compression. Talk to a cam grinder....there are ways to run 12:1 water cooled engines on the street if you vent a little pressure. You'll need more overlap than a CIS cam is probably to offer.

Shoot I'll likely send my MFI to Henry when the TT engine is done for a once over so I am NOT flaming.
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Old 01-13-2006, 08:40 AM
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Many factor contribute to detonation control. Ignition timing, dome shape, twin plugging and combustion chamber to mention a few.

Chevy, Ford and other engine configuration is far less likely to detonated with higher CR than Porsche.
Hemi or semi hemi heads like Porsche runs are susceptible to detonation issues. As for We all know race cars commonly run 14.5 to 15 all day with C12 to C14.
I know of no one running 15 to 1 compression ratio in an air cooled Porsche without some sort of active ignition control on any octane.

Think back in the days of unlimited CR in NASCAR. The restrictor plate engines ran 17 to 1 + Cr and if they were accelerated hard through Max torque (4900rpm ?) like on a restart the engines would commonly explode. Detonation run a muck.
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Old 01-13-2006, 09:06 AM
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Quote:
[i]
Safe octane requirements at 15 to 1 without knock control is 137. [/B]
Henry, thanks for the useful feedback. So what is the safe CR with street (93/94) gas?

thanks...
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Old 01-13-2006, 10:49 AM
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Safe CR is a function of driving habits.
With CIS, single plug and .8 bar of boost I would say 7.5 to 1.
Larger intercooler, enrichment device or custom ignitions help.
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Old 01-13-2006, 11:50 AM
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Volumetric efficiency is the unknown variable.
I can calculate a dynamic compression ratio based on the rod length, stroke length, cylinder bore diameter, static compression ratio and intake valve closing point. For example,
Stock 930 3.3 turbo with 7-1 compression and stock cam = 6.12-1 dynamic compression ratio.
Swapping to an SC profile cam lowers the dynamic compression to 5.55-1. Going further and swapping to a 964 profile reduces the dynamic compression to 5.19-1.
Off boost the power will be down with the 964 profile. But on boost you will see a jump in HP. Now if you bump the static compression up to 8-1, the dynamic compression will also jump, up to 5.89-1. This new dynamic C/R is slightly lower than the original turbo engine.

When you increase the engines HP, the VE is also increased. The cylinder is filled with a higher volume. The higher the VE the lower the dynamic compression needs to be. A cooler intake charge will allow you to bump the dynamic compression back up.

The restrictor plate engines, or engines using small carbs will see low VE numbers. These engines will benefit from higher than normal dynamic compression ratios. Back before the 12-1 rule in Nascar, some did run static C/R of 18-1. The plates are very restrictive and the engines have such low VE numbers at 7000 rpms, that they ran fine. At lower rpms, 3000 or so, the engine would see closer to normal VE numbers. If the driver tried to exit pit road in the wrong gear at low RPM the engine would severely detonate and burn a piston.
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Old 01-13-2006, 10:38 PM
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Quote:
Originally posted by camgrinder
Back before the 12-1 rule in Nascar, some did run static C/R of 18-1. The plates are very restrictive and the engines have such low VE numbers at 7000 rpms, that they ran fine. At lower rpms, 3000 or so, the engine would see closer to normal VE numbers. If the driver tried to exit pit road in the wrong gear at low RPM the engine would severely detonate and burn a piston.

Thats great info.
Im still pretty new to the Flat-6 motor and its limits , but with other motors it really comes down to Air/Fuel ratio's and spark timing to have a reliable high HP High compression turbo motor.

Corect me if I am wrong, but dont nascars use Carbs and fixed timing??

I think that would put them at a huge disadvantage VS a EMS system where you almost have infinte adj, low and top end. When I will adjusting my EMS on my motor I will be adj the Air/Fuel ratio's and spark timing every 100RPM if not more.

I think not many people are familiar with EMS systems and how incredible they are for engine tuning and for reliabilty.

Heres a quote from Corky Bell-
"Aftermarket EMS systems indeed offers the opportunity to create the 1000HP daily commuter automobile . The singular aspect of EMS that permits this is its fine degree of tuning available over huge intake manifold pressure ranges. By comparison, the finest carb in the world has four fuel- flow circuits that can be tuned over the range in which it is asked to operate. Over this same range EMS offers literally hundreds of fuel flow circuits- one virtually every hundred- rpm band and every inch of manifold pressure. Its equivalent to having 500 MAIN JET CIRCUITS IN A CARB" ect....ect..

Im pretty sure if that nascar car had a EMS system to run its 18:1 compression it would be interesting to see the results.

Again I am not a pro on p-cars far from it, I just really believe in 21st century electronics. Its pretty incredible what we have available to us.

Im temted to go with 8.5 and even considerin 9.0, 9.5.
I will see first hand if the motor nukes or pings with 9.0, 9.5.
I guess I like to learn the hard way. Even though it might destroy my bank account.

Oh and I feel the porsche turbo piston rings are not as strong as they should be( to thin they need to be thicker) so another week point to address.
Old 01-14-2006, 07:18 AM
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Sure, Henry, I was speaking of my Ford with the 14.5 to 1 compression and not my little 2.7. But to be fair you didn't specify the type of coolant in your above statement. I just felt that the numbers you provided of "137" was a little pesimistic.

Camgrinder addressed my issue of the fact that you cam timing will effect your dynamic realized compression much the same as "boost" and static compression. I felt that was something he should consider in his build up. So just to say "don't raise compression" may be better addressed by "don't just raise the compression". If I swapped out my 270 degree .820" cam for an OEM stick, I'm sure it wouldn't run too well either and just might detonate at idle.

I would think a mild bump in compression to 8.0:1 with 964 cams and adjusted boost might yield the trade off he desires. Use a nice WB O2, EGT's and if funds are there detonation add on features. Big IC, free flowing exhaust and this should allow a responsive engine that won't melt anything.
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Old 01-14-2006, 09:09 AM
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Quote:
Originally posted by herman maire
Corect me if I am wrong, but dont nascars use Carbs and fixed timing??
Yes they use carbs, but should have an advance curve. Some people fix the timing to eliminate a possible failure of the advance mechanism.


Another thing to consider is valve and port sizes etc. If the engine has small ports and is restrictive, the turbo will make boost easier. If you open up the restriction, it is harder for the turbo to create boost. A longer duration cam can also reduce boost. a 300 hp engine with 1 bar of boost has lower VE than a 500 hp engine at 1 bar.
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Old 01-14-2006, 09:33 AM
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I rarely speak in the hypothetical and never give advise about Chevys. I speak to what I know.
The 137 octane figure came straight from a program I have that predicts compression ratios.
The number was not meant be be a definitive required number but more to make a point.
The three liter Turbo (you remember) the original engine in question was run by Porsche for racing purposes and they chose 6.5 to 1 and made 500+ hp. My question, if they could use any compression ratio and any fuel ( within reason) they wanted why 6.5? Why not 8.0 or more.
I am an engine builder not an engineer. Would it be possible for you guys [who know better that Porsche race engineers]
what they were thinking.
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Last edited by Henry Schmidt; 01-14-2006 at 10:47 AM..
Old 01-14-2006, 10:44 AM
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Quote:
Now if you bump the static compression up to 8-1, the dynamic compression will also jump, up to 5.89-1. This new dynamic C/R is slightly lower than the original turbo engine.
thanks... could you post the math behind that? i would like to play with it a little.


Quote:
any fuel ( within reason) they wanted why 6.5?
did that motor have EFI? what year was that? how easy was that to drive?

thanks

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Old 01-14-2006, 01:02 PM
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