10.3:1 on pump gas?

[Steve in SLO]

I am putting together a 2.8L with 10.3:1 pistons, s cams, MFI, twin-plugged, and probably crankfire ignition. I had assumed after talking with some of my buddies that I would be able to run this setup on 91 octane pump gas for street driving, however, I called Andial and mentioned the CR to them and the response was "We never use over 9.8:1 for street engines". This got me thinking...Is 10.3:1 too high for our CA pi##water gas, or am I OK?
Thanks in advance for any sage advice.

[kstar]

When you mentioned the compression ratio to Andial did you also mention that you were twin-plugging the heads?

Some folks say 9.5:1 is the limit for street gas and other say 9.8:1 or a bit higher (single-plug). With twin-plugs you should be able to go a point higher with street gas. And I would probably go a point higher from the more conservative single-plug compression "limits".

Good luck!

Regards,

Kurt

P.S.: My opinion is from a couple of years of research, not from much practical experience. I did run a 2.7 at 10:1 with single plugs for around 5,000 miles, but that is another story . . .

[dtw]

Bone stock 3.2l Euro motor (10.3:1) was last seen at 90k on pump gas and running strong as new. 20k of those miles were put on by yours truly.

[Tyson Schmidt]

It's dependant on more things than just the compression ratio.

The larger the bore, the more critical the twin plugging is. The 2.8 L has a pretty big bore, which means the combustion chamber is very wide. This means it takes longer to completely burn the mixture, requiring more timing advance to get the job done. It also means the piston dome has to be more steeply crowned to keep the compression ratio high enough, due to the larger combustion chamber. This makes for an even longer path for the flame to travel.

Smaller bore engines are less prone to pinging when running high compression ratios. My single-plug 2.4 with 10/1 CR very rarely pings on 92 octane. (Only if I drive it wrong, does it ping.)

My whole point here is that you just can't say "anything over 'X' compression ratio can't run on pump gas." It depends on many other factors. However, the guys who build these hot-rod engines for a living have a lot of practical experience that far outweighs any "theory". If they don't feel comfortable doing something, I'm sure they have good reason. Just make sure you explain *everything* you've done to the motor so they can make the most accurate recommendations.

[Steve in SLO]

Originally posted by kurtstarnes
When you mentioned the compression ratio to Andial did you also mention that you were twin-plugging the heads?

I did mention it, Kurt, and he was skeptical I could run that compression unless I ran the MFI "fat".
DTW: I am glad to hear somebody is running this compression on street gas...I don't look forward to driving around with a bottle of toluene in my trunk. Do you run 91 octane, or are you lucky enough to have 93?

Steve

The other thing to consider is better gearing in the trans.
Reduce the load on the engine and you will have less issues with detonation.
Or pick a cam that produces less cylinder pressure.
Also depending on your choice of ignition systems you can use a driver adjustable timing controll.
www.msdignition.com has a whole slew of different options.
Or pick up a drum of trick or vp fuel.
Chris

[Formerly Steve Wilkinson]

I've got 10.5:1 on a twin-plugged (MFIs) '83 SC with 100mm pistons and cylinders, S cams, headers, which brings it up to almost 3.4 liters, and it was done with the advice, consent and headwork of Steve Weiner. We have 93 octane in New York, and Steve said that would run it just fine.

Stephan

[ChrisBennet]

I twin plugged my 3.4 and learned a couple of things:
1. The factory stated compression ratios are quite a bit higher than what you get when you actually measure them. For instance, the factory "10.3:1" is more like 9.8:1 if I remember.
2. The JE pistons made for EBS seem to run high. Be sure to measure everything. Examples:
98mm/3.4L "10.5:1" = 11.5:1 actual.
95mm/3.0L "9.5:1" = 9.75:1 actual.
92mm/2.8L "9.5:1" = 10.2 actual
-Chris