RS Compression Ratio (in 3.0 form)

[Eric_Shea]

Gang,

I'm building a 3.0 (930/07) to RS spec for my 911. I have the MFI done (Harry) and the pump (Gus). I have S-Cams and I'm having the heads bored out to 36mm (this engine had the 34's).

If you're keen on numbers you might be getting the impression that I'm building a different type of motor, which is true so, before we launch into the bigger is better etc., allow me to explain:

I want to build a very nice, street engine. I want torque. Pump gas. No twin plug. I want to open the lid and see an RS motor basically built on a 3.0 case. I'm not interested in 20 more hp at higher RPM's. I will not be going over 150mph on a daily basis (sad... I know)

I realize I can open up the ports more but, I don't want to. I realize I could go 3.2 right now but it really doesn't interest me. I realize I could bump the CR but... again, I want to turn the key and drive... reliably, for years to come.

That said and, understanding my desires for the motor herein lies my question (went around the world to tell the story... sorry):

Do I get the off the shelf JE's with 9.5:1 or, pay a couple hundred more to get the stock RS ratio of 8.5:1? Money isn't the issue here.

Thanks all for your insight.

[RatBox]

I do not know off hand what the factory compression ratio was on an 1974 MY RS 3.0 motor. But you might get some info. from Rick Cabel about what works well. As I know he set up a Carrera 3.0 (I think 1976) with MFI.

He might have done others

[Eric_Shea]

In the Northeast? I believe I have a couple S-Caliper orders in-house for Rick (if it's the same Rick). Thanks for the info.

[RatBox]

Yes , he's in the Northeast .

Here's the car . I was wrong , it's a 1977 C3.

[kiwiokie]

You may find this thread useful:
Ultimate 3.0l CIS to MFI conversion thread

[Eric_Shea]

Thanks Kiwi... search is your friend.

My initial question is really about the 9.5 vs. 8.5. I seems the factory 3.0 is a 9.5 (9.8) motor to begin with. After talking with Jeff it seems 9.5 might be a better number. I've been contemplating this engine for some time now so, it's nice to see someone else has already built it.

We have two pair of Rick's S-Calipers in house so, I think I'll consult with him as well.

I was basically trying to find out if there were any advantages (low-end torque, drivability, useable fuels?) to going with a 8.5 set of pistons.

[Flieger]

Lower compression ratio has no performance advantages. Only less NOx emissions and less propensity for detonation/pre-ignition.

[Schmierung]

Agree with Herr Flieger.
The 9.5:1 will be sweet with "s" cams and MFI pump with "s" space cam. Just a few clicks richer than a 2.7s.
I predict more usable torque and HP than a hotter set up intended for track use.
Sweet performance with nostalgic look.....nice.

[Flieger]

You actually get better efficiency along with more power and torque with a higher compression ratio provided you can run a reasonably small level of advance (twin plugs or a central spark plug plus good octane fuel). This is why all the little 1.6 liter 4 cylinder turbos have high compression ratios.

[Eric_Shea]

Understood. Which is why I have the 2.5L 11:1 engine for my 914-6/GT twin plugged.

But... I don't want to go there on my 911. I basically like what Rick did with the engine above. Very simple 911 motor that should live forever with turn key reliability and around 240-250hp.

I have dreams of driving to LA and kicking some Max butt on Mullholand!

[Eric_Shea]

The real problem is... I can't afford another dizzy from AARON!!! LOL

[jpnovak]

I would target 9:1 for a long running street engine. Today's crappy ethanol laced fuels do not burn as efficiently and the higher compression ratio puts you at a risk of detonation on pump gas. It only takes one bad tank to give you a bad day.

You will see more performance by running less compression and optimal timing than you will by running higher compression and having to run retarded timing to prevent detonation.

I think this is most important on a relatively large cylinder like a 95mm on a 3.0. Granted you will not have the shielding issues of a CIS dome that can allow for un-even combustion burn. The offset plug on the 911 head does have flame front travel issues across this size piston.

FYI, My 3.2SS street/track engine runs 10:1 twin plug for the same reason. Here in TX the summer heat is brutal on an air-cooled motor.

[r_towle]

I would think that with fuel heading the way it is with Ethanol ratios increasing, you may want to consider the lower CR ratio pistons if longevity is your key factor.

Rich

[Flieger]

I would actually tune for E100 and use a higher compression ratio. It is more stable (higher anti-knock rating). The downside is you would need twin plugs with the high dome, though you can run even less advance for more power, and you need to pour much more fuel into the chamber, and the MFI pump can only deliver so much. You might need a 3.0 RSR fuel map on a 2.7 RSR engine.

[Eric_Shea]

Max... just like my wife, you're not listening.

[Eric_Shea]

Also... rumor is that the $5 BILLION ethanol subsidies are done next year so... most of it will be pulled (with the exception of States that mandate it).

[Flieger]

Originally Posted by Eric_Shea

Max... just like my wife, you're not listening.

It was in response to the thought above that ethanol needs lower compression ratio. I hate the whole ethanol deal, too, but if they are going to shove it down our throats, we might as well adapt rather than stop driving on the street.

[Eric_Shea]

9.5:1 it is... now let's see if I can build an engine. LOL

[LakeCleElum]

I just switched engines in my 73.5 that has the following mods by the PO:

1981 3.0 engine
Max Mortiz 3.2 P & C's with about 9.7.
964 cams
MSD
Early Exhaust
Has CIS - Dyno'ed at 209 RWHP........timing 30 degrees....Runs just fine on 92 octane......Couldn't be happier......Very fun in a light long hood with lower gearing.

From my motorcycle racing days, always go higher compression till it donates.....G'luck.