Help me understand C/R, Detonation, Etc.

[jcc911]

Hey guys,
What I am trying to understand is, Is there an equation which correlates Boost to increased C/R?
What I am also trying to understand is what is too high of a C/R before detonation sets in or is it more complicated to that?

As many of you know I am trying to Turbocharge my SC engine again... but I want to make sure I do it safely.
The plan is to use a Euro 930 CIS, The 993 Y and HEs, along with factory IC/ Airbox etc.

The plan is to keep the boost between 0.5-0.7 bar, will the factory 930 IC be sufficient to prevent detonation, or again is it more than that?

If the Turbo Carerra had 7:1 C/R, and did not use an IC, what was the CR underboost?

I saw an engine for sale which was Turbocharged and had a 10.5:1 CR how is that operarted safely, when there is such concern for later SC engines and even my 78 with a C/R of 8.5:1 (I beleive).

I know it is is a lot of questions, but TIA for educating me.

[David]

Here's a chart from A. Graham Bell's book "Forced Induction Performance Tuning"

It's for a water cooled 4 valve engine, but shows the relationship between CR and boost.

[Dynamohum]

Here are a couple of links to calculators, that I find handy in calculating dynamic compression ratios, octane relationships, fuel needs, IC and W/A injection gains when looking at your what if scenarios.

http://www.turbofast.com.au/javacalc.html

and this one is very handy as you can adjust for Elevation, IC, W/A and turbo efficiency and a host of other inputs.

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

[JMA-RUF]

Originally Posted by jcc911

If the Turbo Carerra had 7:1 C/R, and did not use an IC, what was the CR underboost?

FYI, the non-intercooled, 3.0L Turbo Carrera ran a 6.5:1 C/R. Compression was increased to 7.0:1 on the 3.3L intercooled motors from 1978 onwards.

A lot of tings come into play here, such as fuel quality (octane rating), intercooler efficiency and proper tuning.

A 3.2 Carrera can run a safe 10psi/0.7bar of boost at the stock 9.5:1 C/R with appropriate fuel delivery, ignition timing and intercooling.

[Craig 930 RS]

mb911 would know exactly your answer -

[mb911]

jeremy I would error on the safe side for now until you get it up and running and feel comfortable in the tuning area. maybe start with .4 bar (where I started) then slowly go upward in timing and boost levels. your stock dizzy will work fine up to approx .5 maybe .6 bar then time for a recurve. a stock IC will work fine up to .5 bar(a guess but feel it is safe) as long as you don't track it and do long bursts of boost. you may want to look at snowperformance.com for meth injection instead of an IC. thats what that 930 powered 356 does with great luck. Its all about baby steps and go from there

[jcc911]

Thanks for the info Ben, I guess I will go little by little.
I will have to look in the MeOH injection. It sounds interesting and besides I can get all the MeOH I could ever want at work...

[jcc911]

Ben, are you sure about the link? Seems to be a site dedicate to Downhill skiing.

[mb911]

Originally Posted by jcc911

Ben, are you sure about the link? Seems to be a site dedicate to Downhill skiing.

No but google it you will find it.

[midlife]

It is a metaphor for the slippery slope of adding boost.

[mb911]

jeremy here you go

http://www.snowperformance.net/

[wanabe911 owner]

Ben,

Which kit from the snowperformance site should we use?

Alex

[mb911]

Originally Posted by wanabe911 owner

Ben,

Which kit from the snowperformance site should we use?

Alex

I will look when I get a chance but I remember a thread recently with the 930 powered 356 guy talking about his kit and how well it worked.

[911st]

There is info in Bruce Anderson's book on modifying Porsche motors on a method to compute the effective CR.