Help me understand C/R, Detonation, Etc.

jcc911 · 02-21-2009, 05:12 AM

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.

cnavarro · 02-21-2009, 07:31 AM

jcc911 · 02-21-2009, 07:37 AM

Thanks Charles,
So If I interpret the data correctly, IF my SC engine has a factory 8.5:1 and I exert 0.6 bar of boost on the engine the resulting CR is 12:1?

That seem like a lot! Is that safe?

cnavarro · 02-21-2009, 07:46 AM

The above chart is for non-intercooled static cr vs boost & effective cr.

Rarely do I see anyone running a turbo at a static CR of 8.5:1, and if they are, they have a later engine with knock sensors, a huge intercooler, twin plugs, coated forged pistons, and some sort of water/alcohol injection on top of that to help keep things from coming apart.

The common safe combos are 7.5:1, single plug, up to about 1 bar of boost, 93 octane, and a large & efficient intercooler. Twin plug gets you to 8.0:1. Both are figuring a set of new head studs. At this point, flame-ringing becomes important to keeping things sealed up. This is pretty much the edge. Some choose to run this compression with a bit less boost, say .7-.8 with occassional peak 1.0 bar boost.

Sure, water-cooled cars with modern advanced engine management can muster more compression and boost, but they are running more efficient four valve, pent-roof configurations, with rediculously tight decks - a modern 997 engine is running a .027" total deck, which is key to allowing compression ratios as high as 12.5:1 on 93 octane normally aspirated. Their turbo equivalents are running 9.5 to 10:1 with similar engine designs and management systems.

Engines are expensive and it's best to build things conservatively and the internals of an SC engine (namely the cast pistons and alusil cylinders) are not up to the task. Alusils don't like heat and the pistons can't handle much, if any detonation. They are even more expensive to rebuild once you grenade one.

Throwing the intercooler out of the equation and assuming sufficient octane, you probably would only be able to run .2 bar before you're at the effective compression ratio typically run with pump premium, single plug engines.

jcc911 · 02-21-2009, 08:17 AM

Thanks for the info.
The engine has had a recent top end and while I was in there I did have the heads ported for TP just in case. I am glad that I did. I also had all steel head studs put in.
The plan is to run an IC.
How efficient are the stock 930 ones if I only run 0.5 Bar?

cnavarro · 02-21-2009, 08:23 AM

I would suggest a post over on the forced induction forum.

IMHO, the correct way to do this would be to put in set of lower compression pistons (which would require new cylinders too) and maybe even twin plugging to allow for more boost in the future reliably.

Steve@Rennsport · 10:31 AM

Totally concur with Charles,...

Turbocharged 911 motors are much more prone to detonation using pump gas on single ignition, especially with higher static compresion ratios. These become more durable with twin-ignition unless you use good Engine Management to accurately control fueling and timing.

That factory intercooler is marginal for operation in hot weather but if you stick to street use, you'll be just fine.

Although you'll hate me for mentioning this, I would not use steel head studs in such an engine. 993TT Dilavars are much preferred due to more consistent clamping forces when hot.

patkeefe · 02-22-2009, 04:04 AM

I am just today putting my turbo 3.0 back together. It had a second compression and oil ring failure due to low grade detonation (that is what the consensus is, anyway). Top ring was OK. I am putting in new JE 8.5 pistons, replacing the factory 8.5 CIS Mahles. I am boost limited at 5 PSI. I run highly modified CIS, with water/alcohol injection and no intercooler. I am going to put in a knock sensor, and get my distributor modded for boost retard this time around. Live and learn. I also run the SC at 6500RPM at the track.

I think the asymmetrical design of the factory Mahles (yes, these are forged Mahles in Nikasil) leads to a weird flame front/pressure wave on the short side of the hump...so, I'm trying these symmetrical JE's, which JE beefed up in the land area for my turbo application.

Please note that I am a just a hack mechanical engineer, and use myself as a guinea pig. I am also glad to take any advice from the collected wisdom of Charles and Steve; that is a ton of experience speaking there.

cnavarro · 02-22-2009, 04:47 AM

Knock sensor is a great idea - the unit from J&S is awesome. I used one years ago when they first came out and it worked as intended. I had a customer put one on a drag race engine and they were able to max out the timing and just let the unit do the retarding and they went significantly faster! Not to say that I'd do that myself, but knock sensing is key to running higher compression safely.

304065 · 02-22-2009, 11:20 AM

Quote:

Originally Posted by cnavarro

Sure, water-cooled cars with modern advanced engine management can muster more compression and boost, but they are running more efficient four valve, pent-roof configurations, with rediculously tight decks - a modern 997 engine is running a .027" total deck, which is key to allowing compression ratios as high as 12.5:1 on 93 octane normally aspirated. Their turbo equivalents are running 9.5 to 10:1 with similar engine designs and management systems.

hmm. . . .027" is .685mm to you and me! I assume that the water cooling allows not only a tighter cylinder wall clearance but a tigher deck height as well- along with more of the combustion taking place in the center of the chamber vs. out at the edges. Only the Factory and Bosch know for sure-- I would love to see the DFI piston tops, I can imagine they have a bowl shaped depression. . .

cnavarro · 02-22-2009, 11:35 AM

I honestly think 12.5:1 is possible on an m96 engine, even without direct injection. We're building a 2.5 boxster to a 2.9 for our test mule and that's our target. The factory pistons now have hard anodized top ring lands on the newest engines, just like GM is doing on the ZR-1 to help prevent micro-welding. Must be expecting some serious beatings on the pistons! We have similar technology available to us, but cost is the biggest problem.