Which CIS pistons for a 3.0 running 91 pump gas?

[JP911]

This is an engine that will be used in a spec 911 race series. The 3.0 must have CIS pistons and cylinders. I have large port (early) SC heads, an early SC distributor, and carbs. I'm trying to decide if I should go with the higher compression pistons from a later SC, instead of the early (8.5:1) pistons I have. Would the later pistons, at 9.5:1 compression, with 91 pump gas, 35 degrees of total advance, and 46mm PMOs be safe? Or would I have to dial back the ignition advance, negating the benefit of the extra compression?

[Steve@Rennsport]

Jon,

I'm glad you raised this very important subject as I get asked this one (in various forms) all the time.

Your prime consideration is octane. Oftentimes, one must choose between higher compression and limited advance or lower compression and more advance and the latter choice wins hands-down, every time.

The only way you can have BOTH is by using better fuel.

[Flieger]

I always have trouble visualizing this... Please help

Higher octane rating means that "X" amount of a given air/fuel mixture needs more activation energy to combust than if a lower octane fuel was used.

Therefore, with higher temperatures resulting from higher compression ratios, the low octane air/fuel mixture will ignite earlier than the higher octane air/fuel mixture.

To prevent rough running and uncontrolled ignition events with low octane fuel, the timing is advanced (spark plug fires earlier in the compression stroke) so that the cylinders fire harmoniously but the engine looses power because some of the energy is being wasted in pushing the piston down while it is still coming up on the compression stroke.

Does the air/fuel mixture also burn faster with lower octane?

Why do I get the impression that the timing is advanced more when lower octane and/or higher compression ratio is used?

Thanks for helping

[Flieger]

Steve,

Are you going to be at the Ventura Show this year? I thought it was very interesting meeting you and seeing the motor pieces you brought.

Competition Engineering's tech seminar was better, though

Just kidding.

It would be great to have you there.

[Steve@Rennsport]

Originally Posted by Flieger

Steve,

Are you going to be at the Ventura Show this year? I thought it was very interesting meeting you and seeing the motor pieces you brought.

Competition Engineering's tech seminar was better, though

Just kidding.

It would be great to have you there.

It was really It was fun bringing down some cars, some engines, and gaggle of custom parts to display. The best part is meeting people like you and getting a chance to put a face to a voice and/or e-mail.

I would love to be at this year's Autofest but since the show was a last minute resurrection, I won't be able to make it this year.

LOL,...I'd like to see more tech seminars (we have done a lot of them up here) as that would build a LOT of value into the event and I'd be happy to participate next year on that. (Hard to follow Walt, tho)

[Eagledriver]

Jon,

Welcome to the Spec class. When I was in your position I chose the later US spec pistons (9.3) compression ratio. The class leading engines are all running Euro 9.8 pistons. They can only run about 30 degrees advance with those pistons. I'm running 32 degrees with mine. Not sure 35 degrees would be safe.

-Andy

[914obsession]

I'm looking at doing something similiar. I have a 3.0 (1978) that I am rebuilding. I plan on using 40 Webers, many have told me that I can run the stock pistons and CIS cams, if I do, what sort of horsepower/torque am I looking at getting? Can I have the base of the cylinders cut down a bit to raise the compression ration? If so, how much would this improve the engines performance? How much would I need to take off to get to 9.5/1? Has anyone used the GT3 coated rod bearings? Is it worth the time to moore and moon the case and cylinders? What about knife edging the crank?

Thanks,

Dan

[JP911]

Originally Posted by Eagledriver

Jon,

Welcome to the Spec class. When I was in your position I chose the later US spec pistons (9.3) compression ratio. The class leading engines are all running Euro 9.8 pistons. They can only run about 30 degrees advance with those pistons. I'm running 32 degrees with mine. Not sure 35 degrees would be safe.

-Andy

Good info on the spec engines. Steve pointed out that he would go for less compression with more advance, which is counter to what the class leaders are doing here (are the Euro pistons legal?). Does it come down to a judgment call/personal preference? What are the different performance characteristics between greater advance + lower comp vs. less advance + higher comp?

Thanks to Andy and Steve for the info!

FYI to Flieger: when you run lower octane fuel you must pull back the advance.

[Steve@Rennsport]

Originally Posted by JP911

Does it come down to a judgment call/personal preference? What are the different performance characteristics between greater advance + lower comp vs. less advance + higher comp?

FYI to Flieger: when you run lower octane fuel you must pull back the advance.

Jon,

The balance between optimal advance values and practical compression ratios is a delicate one as the threshold of detonation (thus massive power dropoffs), changes with atmospheric conditions, cylinder head temps, and each batch of pump gasoline. As you can imagine, its a dynamic that one must build a margin for to prevent piston & ring damage.

All things being equal: higher CR makes more power but only if one can maintain sufficient ignition advance to support the fuel one will use. Less CR with more optimal advance makes for better throttle response and acceleration (especially important for racing when you cannot change gear ratios).

Each engine's octane requirements will vary according to AFR's, CR, advance curve, and atmospheric conditions so there is not a "set" value to shoot for. One determines most of this information from extensive engine dyno tests where all of the variables can be monitored.

JMHO, of course,.....

[304065]

Just a couple cents to add to Dr. Ing. Weiner's thoughts above.

Racing, and winning, is about optimizing the variables that are within your control. In classes with limited displacement and few other rules, the way you get there is to rev the living daylights out of the engine.

It seems to me that since the rules limit your ability to use twin plug and require you to use CIS pistons, you want to maximize every other possible variable.

Is pump gas required? Why couldn't you use the Euro 9,8:1 pistons, offset bore the small end bushings to reduce the deck to 0.25mm and order up a few jugs of Rockett Brand racing fuel?

[JP911]

Originally Posted by john_cramer

Just a couple cents to add to Dr. Ing. Weiner's thoughts above.

Racing, and winning, is about optimizing the variables that are within your control. In classes with limited displacement and few other rules, the way you get there is to rev the living daylights out of the engine.

It seems to me that since the rules limit your ability to use twin plug and require you to use CIS pistons, you want to maximize every other possible variable.

Is pump gas required? Why couldn't you use the Euro 9,8:1 pistons, offset bore the small end bushings to reduce the deck to 0.25mm and order up a few jugs of Rockett Brand racing fuel?

Wish I could run race gas, but 91 octane is a requirement.

[304065]

Can I change my answer? If you can't run race gas then I would think that as much control over the mixture as possible would be advantageous in preventing detonation. How about EFI? As Steve has said here, the big bucks are in the tuning of that system, but it seems to me the only way out is as high a compression as you can tolerate on single plug and VERY precise control of mixture. PMO does that well but not AS well as a feeback system.

[JP911]

Originally Posted by john_cramer

Can I change my answer? If you can't run race gas then I would think that as much control over the mixture as possible would be advantageous in preventing detonation. How about EFI? As Steve has said here, the big bucks are in the tuning of that system, but it seems to me the only way out is as high a compression as you can tolerate on single plug and VERY precise control of mixture. PMO does that well but not AS well as a feeback system.

Interesting that you should bring this up. As Andy mentioned earlier, the frontrunners are using Euro pistons. They are also using an earlier 3.6 intake on top of custom-made individual throttle bodies (they're short to allow for the use of the 3.6 intake) with an aftermarket ECU. Based on your comment, I would guess that the more precise mixture control is what gives these guys the edge. They get precise mixture control, allowing for greater advance.

If money were no object, I would go for the Euro pistons + the 3.6 intake setup, but PMOs are currently in the cards. Perhaps I should save up some duckets for the high-end induction.

[304065]

Aha! I just KNEW there had to be an arbitrage somewhere. Cash for speed!

In truth, Dr. Weiner knows a great deal about the "3.8 RSR" style intake, which evidently combines the advantage of ITBs with the resonance tuning effect of the single plenum. This is probably the reason it is being used.

While precise mixture control alone will not prevent detonation, it sure must help.

[camgrinder]

What about retarding the camshaft timing? I assume camshafts are not legal.

[JP911]

Originally Posted by camgrinder

What about retarding the camshaft timing? I assume camshafts are not legal.

We have to stick with the stock cams. Tell me more about retarding the camshaft timing and how this would help.

[camgrinder]

Retarding the camshaft closes the intake valve later on the compression stroke. This can sometimes reduce cylinder pressure while the engine is running. It can also increase VE in the middle to the top of the torque curve and increase cylinder pressure. More top end HP.
I would probably use a 9.5-1 piston, run the cams around .8-.9mm and put the heaviest springs I could in the distributor. Delay full advance as long as I could and work my way back testing on the dyno.
You could also fatten the cams up a little by running the lash tighter on the intake side. Even if you cut the lash in half, you would still be running on the clearance ramp once the engine was warmed up.

It really depends on how you race the car, what rpm the engine lives at. You can increase cylinder pressure a bunch, and keep the engine above peak torque and get away with less octane. Like the old 18-1 compression restrictor plate Nascar engines. They ran from 6000 to 7200 on the track. With the plate, VE was way down below 85- 90%. Once in a while the driver would go full throttle out of the pits below 4000 and the high VE at low speed would smoke the piston. If he lightly accelerated past 5500 and then nailed it, everything worked fine. Now they are limited to only 12-1 and 100 octane fuel.