Is 10.3:1 too much for a single plug motor?

[jim72911t]

Hi,

Quick question: Would I be pushing it with JE 10.3:1 pistons in my '72 T with MFI, E cams, and single plugs? Remember that I'm in CA and the highest octane gas we can get (without going to the track) is 91 octane. I was originally going to go with 9.5:1 JE's but thought I'd see if anyone here has any experience/advice.

Thanks,
Jim

[Matt Holcomb]

Jim,

My old man played basketball for the Chico State Wildcats in the late 60s. Great school. Great town.

Eighteen months ago I had my 911/83 engine rebuilt with 11.3:1 JEs and GE-60 cams. I only have one spark plug per cylinder head.



Even with 98-octane gas in Australia, we had to reduce the distributor advance to 28 (down from, I think, 33 or 34) to vitiate the mild detonation we were getting at peak revs.

In reducing the distributor advance, we lost about 10bhp. I'm not exactly sure how much we gained by adding the (very) high compression JEs, but the engine should be good for around 230-240bhp.

Now, 91-octane gas in the U.S. is probably equivalent to our old 95-octane 'Premium' gas, in which case 10.3:1 pistons might be okay for an MFI engine with single-plug ignition, providing you reduce the distributor advance a little.

Hope this helps.

Go Wildcats!


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
Pelican Gallery Profile

[rs911t]

I have this issue with my 2.8 which should be ~10.3. It's borderline. The previous owner ran pump gas for years without any obvious issues. I mix 2 parts Sunoco 94 with 1 part Sunoco 110. Insurance...

[cstreit]

I wouldn't run 10.3 on 91. Heck, 93 would be pushing it...

You might be able to retard the timing enough to avoid majot damage, but would likely lose any gains through that than you gained with the bump in compression.

Steve Weiner at Rennsport also contends, and probably rightly so, that todays gas formulation is so inconsistent, that even if you did find a pump that worked for you car, another pump might not, and if you are at the limits of setup, you have no margin for error.

THe different between 9.8 and 10.3 in terms of HP and torque are probably less that 15, so why risk it?

Take the extra few hundred (or thousand) per year on race gas and build a nice gearbox. In the end this will give you more bang for the buck!

[Formerly Steve Wilkinson]

Your question: Would you be pushing it?

My answer: Absolutely, which is why, courtesy of The Weiner, I have 12 plugs with roughly the same squeeze.

Stephan

[Doug Zielke]

Steve Weiner (Rennsport Systems) has taken the time to address your questions on his excellent website:

http://portal.rennsportsystems.com/2a.html

Steve is one (modest) guy who knows what he talks about.

[Wayne 962]

The answer is yes, and is also discussed in detail in the Engine Rebuild book (http://www.101projects.com)...

-Wayne

[Steve@Rennsport]

Hi Jim:

IMHO, 10.3 is marginal for twin-plug motors using California 91 octane fuels. I would not do it. Folks who have 93-94 octane premium at their disposal will have no problems with that CR.

You may blend 91 & 100 octane Unocal unleaded race gas for a decent margin against detonation; sub-audible or the kind you can hear,... Both shatter rings pretty quickly.

Now then,....its not that difficult to machine the heads a bit to reduce that to 10:1 for some insurance against the variations in all pump gasolines. The HP difference is quite modest.

*********

LOL,...Thanks for the kind words gents and the vote of confidence. I just try to offer something constructive if, and when its appropriate.

[steveW]

Hi Jim, as a 71 T owner it seems like an interesting project. Why go for E cams and are you using cast iron cylinders?

Why not go for an 'S' spec motor?

Cheers

[jim72911t]

Thanks guys. That's what I thought. I'll go with 85mm 9.5:1 JE's then.

Yes, I'm using rebored cast iron cylinders. I didn't go with an S spec motor primarily because my motor is apart for P/C's (the heads were done a year ago, and yes, I'm an idiot for not doing P/C's then ). I'd like to keep the T heads, which don't flow enough for the S cams. Besides, I picked up a set of E cams for cheap and have heard nothing but good things about the E motor for street use.

And yup, Chico is a great town. When I wrote a check today for a down payment on a house here , all I could think about what that $$$ could buy me Porsche-wise. It better be worth it!

Thanks,
Jim

[campbellcj]

Heck, our gas is so 'marginal' here in SoCal that I run a 91-100 blend, or sometimes even straight 100, in my 9.8:1 engine. For the amount I drive the car (basically track-only), the $2/gal difference does not actually add up to much, in the scheme of things. Lot cheaper than fixing a farked-up engine!

[Superman]

Chris. The house is worth it.

[Early_S_Man]

Jim,

Those 9.5:1 JE's & 'E' cams will make a great running engine ...

Another option will be to get your 'T' cams reground to 'S' or GE-60 specs for a trial/experiment in the future. 'T' heads have been used with 'S' cams by the factory to build 'Rallye' engines, and only lost 10 hp at the top end ... your 1.0 point gain in compression and slight displacement gain should offset the flow loss, netting a 190 hp DIN engine ... if you get tired of the 'E' power level and torque curve!

[steveW]

Warren (or Hutch!)

I didn't think 'T' cams could be reground beyond 'E' spec, and Jim, is it reallyworth using the cast iron cylinders?

My motor needs attention for case studs and I hope (finances permitting) doing some worthwhile mods while it is apart.

[jluetjen]

is it reallyworth using the cast iron cylinders?

Why not? Aside from a few ounces of weight, I'm not aware of any performance issues with the cast iron cylinders. Did I miss something?