quick qustion about 2.8 liter motor and C.R.
 

I remember reading in B.A. book that using the 2.8 RSR p/c set on a 2.7 case and heads yields some really high c.r. like 12.5:1.

How is it that many of you are running these on the streets. I think even with twin-plug this is too high, No?

...or is it possible to get the c.r. to somewhere lower than 11:1 to make a suitable twin plug street car?

I am thinking of having a 2.8 I own freshened up and put into my car and adding either MFI or EFI instead of carbs.

JZ

I think that "RSR" has been applied to any pistons intended to be run in a twin plug aplication with non-RSR heads. So, an original RSR piston would produce a 12.5 CR (with a standard head), while the pistons folks have refered to would produce a 10.5:1 CR (or thereabouts) using standard 2.2/2.4/2.7 heads.

What about these old motors like 911RST for instance. He is running a 2.8 and I don't think anything in the 92mm size was available other than the 2.8 RSR pistons?

JZ,

Carefully read Bruce’s book. Page 139, 2nd Ed. has a graph of the CR vs. Deck Height (piston-to-head clearance.) With normal street clearance of say 1.0 mm (0.038”) Bruce’s graph indicates 11.5:1 with an unmodified 2.7 head. The RSR piston won’t clear the 2.7 or earlier heads without modification that removes metal from the head. Removing metal lowers the CR. Any of these engines today should have twin plugs. That additionally lowers the CR slightly. With a 2.7 head (or 2.2 & 2.4 heads) there is the availability to machine around the perimeter of the head to additionally lower the CR. It is reasonable to get the CR below 10:1.

The RSR piston is configured to fit RSR valves and RSR/906 cams so there is a lot of extra clearance when used with “S” cams and 2.7 heads & valves. I have seen some pistons where the perimeter of the valve pockets were cut down to lower the CR. The piston was designed for 49 & 41.5 mm valves where the stock 2.7 heads have 46 & 40 mm valves.

Fitting 92 mm RSR P&Cs to a 2.4S is not a bolt-on operation. The clearances must be carefully measured and set to the desired numbers. The compression ratio must be measured and set to the desired number. Of course all the other 911 engine building issues need to be addressed. While this is not for the “feint of heart”, careful work will yield an outstanding performing engine with very light weight.


What would I do today?

I would put up with the necessity to use high octane race gasoline and have the CR as high as reasonable.
I would have the deck clearance in the 0.80 mm range.
Of course twin plugs and a 7R case with all the mods.
MFI and “S” cams.
Maximum fan cooling (1.82:1), front oil cooler(s), and the Rubbermaid Solution for track events in hot weather.
Everything as light weight as possible.

No one should be afraid of regular “maintenance rebuilds” on their trick 911s. The cost is minimal compared to a failure. For the cost of gaskets, bearings, rings, and a precision valve job, your 911 can stay on top of the curve. Besides, its fun.

Best,
Grady

BTW, the 2.8S engine in rs911t we built in about ’74 and has never had the case split.

Thanks alot Grady.

I have raised the issue of using a Callas built 2.8 twin plug motor that I own in prior posts. It has not ran in a long time but was pulled from a good running car. I would plan on having it rebuilt prior to using it if I go that route. One thing is certain if I do choose to use this motor it WILL NOT HAVE CARBS! I have had a carbereuted 911 for years and think they are just a thing of the past for a variety of reasons.

I am torn between re-building the 2.8 changing either to EFI or MFI ...deciding what to do with ignition. (currently has a marelli twin plug)...weber 46's too.

I am currently running a pretty strong 3.0. Elgin Mod S cams 9.3:1c.r., carbs (40s) ...212 RWHP. ...I am tired of fiddling with the carbs and am going to be changing to f.i. of some sort.

If I am going to change to F.I. my choices are using the 2.8 as a platform or selling the lot and going with a 3.6 conversion. I figure between the two engines I've got about 14-15K worth of goodies I could sell to help fund the 3.6 conversion.

...tough choices. I wish I could drive a 2.8S car and an early 3.6 car back to back.


JZ

one other thing, How would you deal with the impracticality of running race fuel on a street car?

Whats a Rubbermaid Solution ?


Kurt Williams

JZ,

The big issue with using a 2.8 mag case compared to any later engine is weight. Light weight has all sorts of advantages:
Better acceleration, duh.
Better breaking.
Better cornering.
Better weight distribution.
Possibly lower CG.
and more.

Of course the disadvantage is that 2.8 is about the limit of displacement. We all agree there is no substitute for cubic inches.

Talk to Jerry Woods and Rich Dalton (408-369-9607) about converting your carb 2.8 to EFI. I think you will be pleasantly surprised.

Here in Denver, race fuel is readily available, tax paid. If you live where that is not the case, you can always have it shipped in. With a large, or auxiliary, fuel tank it isn’t a problem. With logistical organization, even cross country trips are possible.

Kurt, try the Pelican search function.

Best,
Grady

I believe that as of two years ago Mahle was making 92mm pistons in the CRs I described in my previous post. It sounds like your pistons might be quite old, making it unlikely that they are other than original RSR.

Exactly my point Scott. I WAS referring to original 2.8 RSR pistons.