And...

Losing some weight off the car will make the HP feel like more than it really is. Nice light wheels too help. Reducing that rotating mass.

While there are numerous benefits to having lighter wheels the effect on acceleration is minute

5# extra on each of the rear wheels yields a net loss of ~1.4lb-ft, this is virtually imperceptible

on the other hand .5" of drive tire height is worth ~6lb-ft, definitely noticeable by most drivers

100# from the chassis is worth ~8lb-ft, if taken from the ends it's especially noticeable

as 1# from the front edge reduces the polar moment in pitch and yaw by ~9lb-ft
similarly 1# from the rear edge reduces polar moment in pitch and yaw by ~5lb-ft

I agree, I've done a couple things to reduce weight and a few more things to do in the future.

Thanks for the replies, I'm thinking I'll go ahead and get the SSIs and probably the CDI+ as I would use those no matter what direction I went.

Are these components that I could buy along the way or should I wait to do everything at once?

Hi Bill,

Are those displacement data correct for the 1984 SC/RS (Typ 954)? I have read in various places (e.g., https://www.stuttcars.com/porsche-911-sc-rs-an-oft-overlooked-gem/) that Porsche/Prodrive opted to use the 3.0 liter motors due to the class (Group B) displacement to weight ratios.

------

As an aside, the Typ 954 on paper: https://rallygroupbshrine.org/the-group-b-cars/rally-cars/porsche-911-scrs/

sounds like this on the road: https://youtu.be/PKKKUleaSAE?t=174

OP, there was a thread a while back that showed dyno plots of a big port motor (with 964 cams) vs a small port motor (with 2021 cams). Both cars had SSI. The results were surprising. I’ll dig it up but in short, the small port motor had more area under the curve all the way up to high 5000s before it ran out of breath. Better volumetric efficiency of the small ports plus the higher compression.
The big port motor had slightly more legs but peak power was only bumped a few hundred RPMs.

For a stock motor with bolt ons, I don’t think there’s that much of a difference. If you were to ever open your motor up, you’re onto a winner bc you’ve got big ports. Add compression and the other things you’re talking about, and you’d easily be north of 204hp that the Euro 9.8:1 compression cars generated.

I have SSIs, 2021 cams (set to full advance to bring torque curve lower) and CDi+ on my small port motor. Makes a torquey motor torqueyer still, but doesn’t get you the big crescendo that these cars maybe miss a little. Car hits 4000RPM, feels like the party is about to get hopping, and then the cops turn the lights on at 5800RPM type vibes.

no, SC/RS was 2994 just like the SCs and Carrera 3.0

amended
http://forums.pelicanparts.com/uploa...1655850461.gif

Great analogy, reminds me of my youth! :D

Great advice. I have a 75 915 with 7:31 in my chipped 84 M491 with headers and it is actually a very quick car. MUCH quicker than with the original transmission. Dog slow as stock,. AMC Pacers can run away from it.

Gearing has to be the cheapest, most fun hp going. And you get a better shifting 915 during the rebuild.

Ya it was baffling I expected much more. Both the dyno guys told me it is about 200 - 220 at the crank. Apparently they are thinking 15-20% loss when measuring at the wheels. I found another dyno sheet on here with roughly the same specs and the same results. Everything was dialed in on the engine we played with ignition timing we gave it so much until power actually started to drop. Cam timing was set to the middle rand of what web recommended.

Like I said the car was a blast to drive just very disappointing #s on the dyno

Going from heavy 17" wheels to the stock 16" - although they had the same circumference (gearing), felt like a massive power increase. It made the response a lot more snappy.

I have the stock 16x6/7 fuchs now and have thought about going with a set of 15s with a chunky tire, maybe find a smaller rolling diameter tire.

Cheapest way for you to go is to move the 7s to the front and purchase 8ET10.6 or 9ET15 x16 Fuchs for the back, then rationalize the tires

205/50 & 225/45 on 7 & 8 is very close to a 7 & 8x 15 205/50 & 225.50 setup, see chart below

http://forums.pelicanparts.com/uploa...1655928776.jpg


or you could go 205/50 & 245/45 on 7 & 9

seen here
http://forums.pelicanparts.com/uploa...1655929029.jpg

when doing engine work do it all at once or there will always be a choke point left

buying a set of 98mm p& C isn't going to be that much more expensive than just a set of 9.8 95s

don't forget that all the other little stuff adds up quickly, head studs, timing chains, gasket kits etc are all on the list + more

I'm redoing one of my 993 motors & transmission/clutch now and so far ~$12000 in parts

Thanks, Bill, I appreciate the tire charts and I think you're right, I'll do the engine work all at once and probably go with the 98mm. I was originally thinking of higher comp pistons and use the same cylinders and save some cash.

I just did a top end on my ‘83SC with stock CIS and SSIs. Worn valve guides, high oil consumption, and not being able to keep up at Ruchlos Rallye last year finally made me pull the trigger.

Went with 3.2 P&Cs from Mahle, 10:1 compression, 964 cams, lightened flywheel, heads redone, new valves all around, cam towers honed and decked, rods re-bushed, everything except splitting the case as mentioned. My wrench pulled a rod, declared the crank good, and sealed what we needed to on the case.

Night and day results, super happy with the power all thru the range. Really felt the results about 700 miles into the break-in, and still getting better about 2,000 miles in. Almost no oil consumption. And plenty of smiles to go around. Plus, no problem keeping up at this year’s Ruchlos.

Good luck with your rebuild.

The "at the wheels" number could be disappointing if you aren't prepared to do a little math, And if your dyno guy hasn't set up the correction factor for the atmospheric conditions.

Depending on the dyno and the operator, it could be valid to apply a correction factor to the base reading. SAE standard J1349 specifies the standard conditions for testing to be 77 degrees f, 0% humidity, and 29.23inhg barometric pressure.

I recently had Project Heavy Metal dyno'd and it was 85 f, 38% humidity and 29.1 in hg pressure. This gives a correction factor of 1.033

The 240Hp reading at the wheels on that day corrects to 247.96 standard.

The J1349 standard also allows for 11% drivetrain loss which would give me 275Hp. Generally people use a 15% number instead so that would be 285Hp.

Enough about my car - in your case, depending on the weather the correction could be 5% or more. So it's possible the 170 at the wheels could be as much as 180 and the flywheel Hp number is over 200Hp.

I'll assume that you've decided on 225 hp as a goal out of cost constraints. Obviously if you have the budget one can get well north of that.

I have an '82 small port. I was where you are - my engine had a top end less than 25K miles ago - so I decided to do only bolt-on mods. My research suggested that changing cams, pistons, & cylinders adds about as much horsepower at 3x the cost (if you have someone else do it, maybe 2x if you do it yourself). (And keep in mind the small port has higher compression already). If your engine is due for a top end it might make sense, but it sounds like yours is not. (Of course if you do both bolt-ons and internal mods and you can get well above 250 if done right.) As for me, I ditched the CIS, went with an ITB setup with a megasquirt controller, and CDI+ ignition, plus a gutted cat and SSIs. I'm still tuning the megasquirt. I have an appointment at a dyno shop on the 13th, I'll post the results here :

http://forums.pelicanparts.com/porsche-911-technical-forum/1118237-rhd-itb-megasquirt-install.html

It was a lot of work and I had my share of headaches, as you can see in the thread. Right now, with my pretty crappy base tune, it's much peppier up to 3K. Higher rpms under acceleration still needs some work, but I'm sure the tuner will be able to get it all worked out. I won't be at all surprised if it's over 225 at the crank, up from a listed 185 stock. Fyi the PO had gutted the cat for me. The first thing I did was the CDI+ leaving the advance stock. I felt the difference, but it wasn't dramatic. Next the SSIs also gave an improvement, but nothing dramatic. Adding the ITBs and changing to full control of the CDI+ made a big difference, even with the crappy tune I have now.

The one thing I believe - all these components are part of a system. Changing the headers might not do anything if everything else stays the same, but the same headers can add big HP if the right set of mods on top of it are added. Unfortunately, it's all a bit of guesswork, and there are as many opinions as there are 911s, but the input available on this board gets you 80% there!

Yes, budget is a concern plus I've always been of the opinion that when adding horsepower, reliability or life expectancy tends to go down. I want a fun, reliable engine that will last me a long time before needing a rebuild.

The PO had resealed the top end and that was about 4k miles ago, I've only put a few thousand miles on since I bought it and have yet to add oil, doesn't even smoke on a cold start and there are no oil leaks, or enough to drop on my floor.

Where did you buy the components and did your shop do all the work or send items out for machining?

I see P/Cs Patrick motorsports and LN sells P/C with Nikasil liners.

Any recommendations?

BTW, I've rebuilt a few engines but not an aircooled Porsche engine so there's a bit of a learning curve.