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CR of most Turbocharged cars?

Ok in an effort to kick start this forum my question is.

What is found to be most desirable for a low boost turbo application CR?

I see hot 930 motors run at 8-1. some the 962 ran higher then that. 935 were like 6.5-1 so where is the happy medium and why??

I was 9.3-1 and now shooting for 8-1.. Any thoughts?/
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Old 10-30-2006, 04:30 AM
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IMO. 7:1 - 7.5:1 with 0.9 - 1.0 bar of boost is about ideal for a street turbo. That strikes the balance between really usable power (on the street) and engine longevity, further, you can dial in the correct fuel and timing without significant modifications (other than euro CIS) Much higher than that and you are committed to an engine with a shorter lifespan.

8:1 or higher usually requires some mixture adjustment as well as a timing adjustment. Some sort of crank-fire ignition system where the timing is much more adjustable is recommended.

The race cars: 935, 962, etc are not really good benchmarks. They ran much higher boost, much higher octane fuel, and the engines needed to last only one race before overhaul.

AFJ
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Old 10-30-2006, 04:50 AM
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keep in mind I am using MS and will be using EDIS controlled by MSII next go around and boosting .5 bar
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Old 10-30-2006, 04:53 AM
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FWIW, my 3.4L running .8 BAR of boost has a CR of 7.3:1
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Old 10-30-2006, 06:03 AM
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I think 7.5 to 8 is opitimum for an air cooled EFI car that will run at 1 bar.
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Old 10-30-2006, 06:16 AM
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Quote:
Originally posted by 125shifter
I think 7.5 to 8 is opitimum for an air cooled EFI car that will run at 1 bar.
wow so I could run a lot higher boost then I was intending with my 8-1 setup
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Old 10-30-2006, 06:26 AM
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If I remeber correctly, each pound of boost adds something like 0.3 points to the effective compression ratio.
So if you start with say 7.0:1 (stock 930) and boost it 1.0 bar you end up with an effective comression ratio of 11.2:1 which is at the limit given the combustion technology of the 930 engine.
This may seem a bit simplistic, that the relationship is liear, but I would use that mathmatical logic if building an engine myself.
Corky Bell goes into this very well in his book Maximum Boost.
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Old 10-30-2006, 09:18 AM
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Quote:
Originally posted by RarlyL8
... given the combustion technology of the 930 engine.
Does twin plugging help this?
Old 10-30-2006, 09:31 AM
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From Graham Bell's book:



This is probably for a watercooled engine, but I figure our engines are pretty robust so it should be reasonable for our's.

EDIT: that's weird, are you all seeing a picture or a link above? I see a link.
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Last edited by David; 10-30-2006 at 11:16 AM..
Old 10-30-2006, 11:13 AM
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thats pretty cool really
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Old 10-30-2006, 11:52 AM
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How about Juan's and Mike's EFI'd Carrera's that run 22psi on pump fuel at the normal 7.0:1 C/R?

I suspect that EFI and full bay intercooler conversions bumps up that "limit of 1.0bar" quite substantially

And yes, twin plugging improves the combustion efficiency as well, providing even more safety margin, or allowing even more boost, since flame travel is halved
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Last edited by WydRyd; 10-31-2006 at 04:57 PM..
Old 10-31-2006, 01:52 PM
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This guy who was the chief mechanic on this:



Is a huge fan of 7.5 CR and higher boost vs. 8.0 and lower boost
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Old 11-01-2006, 11:25 AM
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Yup, I vote for lower C/R and more boost, than higher C/R and lower boost

Mike Rombotis was able to run his TT'd Carrera at mid 600HP @ 22psi on pump grade fuel, on a mediocre tune
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Old 11-01-2006, 04:24 PM
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My company car has C/R of 9.35:1 and runs up to 1.3 bar of boost, straight out of factory It's a SAAB.

You can run heaps of boost even on hi-comp engine as long as you retard the ignition enough. You won't be making any power though but it's seems that most important thing nowadays is to boost as much as possible, not to make power

Imagine car A and car B, same C/R, same turbocharger (let's say it uses K27), but different boost.

Car A boosts something like 0.7 bar and has ignition mapped for optimal BMEP troughout range (BMEP is all that matters). It will have good torque, good off-boost response, it will be frugal with fuel and intercooler/turbo won't need to work as hard as turbo efficiency will be on-spot and density-ratio would be low.

Car B runs 1.3 of boost. In order to allow that kind of boost ignition has to be dialed back. Turbo will be out of it's efficiencyy range, pumping hot air around. Intercooler will need to work harder and you will be feeding hot air into engine, further aleviating need for less ignition advance. You will need to pump more fuel into it to compensate all that air and it won't neary make as much power as 1.3 bar of boost indicate, use lot's of fuel and will fail more often.

It's always a trade-off. If you want lot's of power trough lot's of boost you need to build it with low C/R and turbo that is made to work efficiently with high pressure ratios. Such engine will be laggy.
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Old 11-02-2006, 12:04 AM
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Quote:
Originally posted by beepbeep
My company car has C/R of 9.35:1 and runs up to 1.3 bar of boost, straight out of factory It's a SAAB.

You can run heaps of boost even on hi-comp engine as long as you retard the ignition enough. You won't be making any power though but it's seems that most important thing nowadays is to boost as much as possible, not to make power

Imagine car A and car B, same C/R, same turbocharger (let's say it uses K27), but different boost.

Car A boosts something like 0.7 bar and has ignition mapped for optimal BMEP troughout range (BMEP is all that matters). It will have good torque, good off-boost response, it will be frugal with fuel and intercooler/turbo won't need to work as hard as turbo efficiency will be on-spot and density-ratio would be low.

Car B runs 1.3 of boost. In order to allow that kind of boost ignition has to be dialed back. Turbo will be out of it's efficiencyy range, pumping hot air around. Intercooler will need to work harder and you will be feeding hot air into engine, further aleviating need for less ignition advance. You will need to pump more fuel into it to compensate all that air and it won't neary make as much power as 1.3 bar of boost indicate, use lot's of fuel and will fail more often.

It's always a trade-off. If you want lot's of power trough lot's of boost you need to build it with low C/R and turbo that is made to work efficiently with high pressure ratios. Such engine will be laggy.
thats is what some th HP drag racers are doing these days for optimal performance and effeincy
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Old 11-02-2006, 03:05 AM
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