3.8 rebuild compression choices

[STROSEK]

I'm rebuilding my 3.6 to 3.8 using JE and LNE P/C's. What would be the best choice for compression? 11.5 or 12.5? I will be using only premium fuel with a stand alone EFI but it will after all be a daily driver so reliability is also a concern. Any info regarding the pros and cons and recommendations would be appreciated.


Thanks!

[CBRacerX]

Signing on for the responses....

[Porschekid962]

When you say premium, what octance are you talking about?

[STROSEK]

94 octane. Hope this info gets me more replys!



Thanks

[cnavarro]

Will your aftermarket EFI have knock sensing?

I was hoping Steve might make a post, but I grabbed this off rennsport's web site:

"Although Porsche claims a compression ratio of 11.3:1, we have measured many of these engines around 10.3 to 10.7:1. The only pistons that came closer to 11.3:1 compression ratio were the 101.1mm pistons from the 350 HP 3.8 RSR engines."

If that is the case, going to 11.5:1 with the JE's will be an upgrade over what you have. There is also something to be said that the dome will be a tad shorter with the 11.5s vs the 12.5s, which may provide better flame front propagation.

Although out of context, since it's in reference to earlier engines with smaller bore sizes (requiring larger domes to get the same CRs), I also found this on Rennsport's site:

"The tall piston domes necessary with these heads shrouds the valves quite badly and creates a detonation-prone atmosphere without twin-ignition. This is why these engines do not benefit very much from compression ratios over 11.5:1."

I would further defer compression ratio choice to what cam you plan to use, since the cam grind will in effect change the dynamic compression. Maybe camgrinder can chime in and suggest a cam for your rebuild and optimal CR?

[Carrera3.5L]

Originally posted by STROSEK
but it will after all be a daily driver so reliability is also a concern.

In my opinion you've already answered your own question...

Ralph

[STROSEK]

I still don't have the info I was looking for. Performance, reliability pros and cons etc. More feedback would be appriciated.

Thanks

[Steve@Rennsport]

You need to use no more than 10.5:1 (using twin-ignition) for pump gasolines unless you have active knock-sensing.

Higher compression ratios require race gas and significantly decrease engine longevity.

[STROSEK]

So your saying no even to 11.5:1 compression? I will be using the knock sensors provided with the stand alone ECU. In case I was not clear this is a 964 motor.

Thanks

[Steve@Rennsport]

LOL,..Its your dime of course, but 11.5:1 is too much CR for any pump gasolines, especially in an air-cooled engine where cylinder head temps are a huge variable.

Stick with 10.5:1 for street engines where durability is a top priority and leave the high CR's for competition engines.

[STROSEK]

Thanks for you input, I do value it. Let me ask you one last thing.... Like everyone I'm just trying to get the best bang for the buck and maintain reliability. My last question to you is if 11. to 1 is worth thinking about. I'm not sure if it make a difference but I will be using CMW billet heads and I know that they do help in keeping temps down.

Feedback?

[Steve@Rennsport]

I've not used CMW heads, but I assure you that head material has little to do with cylinder head running temps in air-cooled engines.

Compression ratio, ignition timing, air-fuel ratios, cooling fan airflow and engine loads play a MUCH bigger role.

Using 11:1 CR on pump gasolines really, truly restricts your flexibility with ignition mapping. You are better off with less CR and more aggressive (to a point) timing maps that will yield better throttle response and acceleration.

From much experience, broken rings & damaged pistons are the result of too much CR & timing when using pump gas.

"Less is More".

[STROSEK]

Exactly the answer I was looking for!

Thanks for taking the time to reply. I will be taking your advice and going 10.5 to 1.