Dropping C/R on 993 engine

[ducatithunder]

Hi, new to the board. This is a great idea Steve!

I just got done talking with Charles @ LN about pistons and reconditioning my mahle barrels. Core engine had spun a rod bearing with damage only to the journal and the rod. The rest of the engine looks great. Some wear on the pistons but there is no history on mileage for the engine. Its an unkown so new pistons seems to be smart money right now. OEM Mahle cylinders look good.

So my question is .... Is it worth dropping the C/R down from the 993 stock 11ish to say 10.5-1 or just get the 11.5-1 JE pistons he has in stock? Seems that >11 CR isnt going to work with carbs running 93 pump gas. I am still going to run PMO carbs. RS cams. Heads are out for overhaul and will be getting Ti retainers and aasco HP springs and ground for RS valves. Leaning towards Pauter rods too. Crank is off to Marine Inc for cross drill/groove mods and polish. Will be running JB Racing twin plug dizzy from 2 msd boxes. Im not sure how much timing I can retard to make it work but dont want to loose too much power.

Engine will be going in a longhood RSR build.

Inquiring minds want to know, thanks!

[Steve]

[ducatithunder]

Hey Steve. Thanks for the info. I have looked over your site a few times now. Im in MD and have a very good supply of 93 Octane. That is the lowest I would use. I never get a unknown quality of fuel .. usually just Shell. Im a snob when it comes to that, lol. Charles had mentioned I stay at 10.5 CR. Seems he has true 10.5 CR. I will verify when I start to put the top end together. The pistons will be a stock 10.5 JE with valve relief. If 10.5 is a good CR I will order these pistons and have them fitted when I get my cylinders reconditioned. Seems there are a bunch of guys running 10.5 on older builds no problem. I was curious how the CR of 10.5 would effect the power/torque output of the motor. I guess if the 10.8 of the OE 993 shouldnt be much different. I also figured that the JE pistons would help more with running with carbs then the OE Mahles and now is the time to upgrade. I am planning on running PMO 50s and RS valves and RS cam. I dont want to change the lifters at this point so a SS cam is out of the question. Any cam other then the RS that would work better and still allow hydraulic lifters? Car will be driven hard on the street with events here and there.

[Steve]

JMHO,........

10.5 on 93 is a stretch in a carbureted engine, especially during summertime temperatures. One can do this, however the ignition timing must be curtailed to leave a margin against detonation. Personally, I prefer to use less CR & more timing for an easier-revving engine.

I've seen the results of being too aggressive with CR & timing and its not only not pretty, its expensive, too.

The OEM Mahle piston dome design is VERY good as it offers improved squish (anti-detonation) properties over the traditional double-valve relief ones.

You may use stock, RS or the so-called SS cams; the differences are very modest. Just check your piston-to-valve clearances.

[ducatithunder]

Thanks Steve. I just got done talking with John Dougherty about cam options. He had suggested I back date the rockers to earlier mech rockers he modifies to work with the 993 carriers. Also Said that DC43 - 108 cam would work great with the PMOs and my RPM range would be strong between 3000-7000K. I thought the RS cam would work well but not as well with carbs? 10.5 CR with these cams will be ~325 Crank hp. So if I dont want to run the 10.5 CR JE I could order the 10.1 instead and run more timing. Makes sense. Is 10.1 CR a safer range. How would that affect my numbers? I do want a rev happy engine. Im hesitant to use the OE mahle pistons I have as they dont have relief for valve if I run cam better for the carbs. Still trying to get everything to work with each other .. never ending. Thanks!

[Steve]

Everyone has opinions on the rocker backdate and thats what makes life interesting,...

I would make sure your engine is reliable to 7K RPM as that requires some different hardware. Further, if this will be used for track days, I'd strongly recommend cross-drilling the crank and the matching case mods for proper oiling. Not needed at all for street use.

Good luck on the project!!!

[ducatithunder]

Im hoping that with the Aasco HP springs, Ti retainers that will make the valvetrain safe for 7K. The crank is on its way to Marine Crank for polish, cross drill/grooving mods. Case will be shufflepinned and oiler in case opened up for main bearing when it gets cleaned. Rods will be pauter. Im all this hoping will help protect the engine even thought it wont see alot of track days ... figure now is the time to do it.

Honestly that was the first time I had heard that you could backdate a rocker in that fashion. Seems to work from what Ive been told. At this point I need to get all my parts back and start assembling them. Looks like 10.1 seems to be the best route from further online searching the forums. Not to much compression but enough that I can run a good timing with pump gas. Thanks again Steve!