964 upgrade to "3.9"

chancecasey · 09-30-2005, 03:06 PM

Hi everyone!

Just discovered this forum, I've been on rennlist but I think my question is a little too technical for them, so here I am!

I've got my 90 C2 cab on jackstands, some initial prep work done and am about to pull the motor for a top-end rebuild. I'm hoping my p/c's are within spec, but at 104K miles, they may or may not be.

Just for kicks, I've been searching around and found new 104mm p/c sets for about $3800. Note that this is not incredibly higher than what a new set of 100mm would cost. I would need a new set if I wanted to insert the cyl/head gaskets that are in later models - but honestly I don't have a huge leak problem there, yet.

Anyway, cut to the chase, can anyone give their opinion on running the 104mm p/c's on stock (machined only for the new cylinders) 964 heads? Likely I would shotpeen/balance the rods and use ARP bolts, and balance the crank as well, but that's it for now. I realize the stock intake manifold, porting, cams, and valves are going to limit the advantages of the bigger bore, but my expectation would be a little more hp, maybe just more low-end torque. Am I necessarily going to "fry" my stock heads, valves, or exhaust system? Anyone think this will negatively affect emissions? (I need to stay street legal in CA)

-Chance

KobaltBlau · 09-30-2005, 03:18 PM

I don't know the answer to your question, but you might call steve weiner at rennsport systems in oregon, I'm sure he could get the proper parts, and he could definitely answer your question.

http://www.rennsportsystems.com/

Kevin Stewart · 09-30-2005, 03:19 PM

try doing a search bit i think your looking at 3.8, Kevin

KobaltBlau · 09-30-2005, 03:25 PM

no, 104x76.4 is really 3.9. Now that I think about it, a really good guy to talk to would be charles at ln engineering:

http://www.lnengineering.com/911.html

he makes 104mm (and 102mm) cylinders, along with many other sizes, and I think his stuff is top notch.

note: the LN cylinders are sold through the host of this BBS, pelican parts, but charles can give you the technical details of what you will need.

chancecasey · 09-30-2005, 04:10 PM

Yeah, Inengineering.com is where I saw these, and I spoke to Charles at length. He was extremely helpful and I'd recommend him to anyone doing a thing like this (no affiliation). Of course, I've got Jerry Woods right down the street from me, but those guys are SO busy, it's hard to just get Rich to let me pay him for an hour of his time - but I'm not giving up yet!!!

The one thing Charles couldn't comment on was emissions. I guess nobody in their right mind would attempt this while needing to pass CA emissions?

Anyone do a 3.8 (or 3.9) upgrade on a 964 and still pass emissions?

Elombard · 09-30-2005, 06:50 PM

If the heads and cams will work ( I think they will but i am not an expert) you should have no problem with emissions. The computer will compensate the mixture so that it is right and your emissions control will work properly. Your exhaust may be too small to extract max power but will probably be OK for street use. Technically I dont hink it does pass emissions but as long as it visually looks the same, the emissions are in tact (cats) and it passes the sniff test - no problem. I looked at a 3.8 964 once and I think it had the stock exhaust, cats and all. There was no way you could tell the motor was not stock that I could see. If the exhaust was a huge issue you could run a GHL or BB exhaust and swap out the cat stock exhaust on emissions day. What is it a 2 hour job max?

Sounds like a fun project!!

chancecasey · 10-01-2005, 09:30 AM

Yeah, I was planning on at least a cat-bypass and g-pipe until emissions testing time - regardless of whether I can use the existing p/c's or upgrade to the 104s.

I guess the concern with emissions and 104s is that the exhaust will flow a bit "faster" (giving the cat less time do it's job) but then again possibly "hotter" (making the catalyst catalyze better). So hopefully the difference will be minor.

I don't have the $$$ to do any major system upgrades, but I figured spending and additional $1600 or so while I'm in there would be a good start, even if the power/torque gain is minor. Later, as time/budget permits, I can add on better intake, heads, and valve train to really get some punch out of her.

I'll post pictures as I go along either way, if anyone is interested.

KobaltBlau · 10-01-2005, 11:09 AM

there is no replacement for displacement, new P/Cs aren't cheap but in porsche naturally aspirated $/hp terms they are usually a good bet.