feasibility of compression increase for 3.2?
 

So I've got my 3.2L torn down to the case and I'm pretty sure I'm looking at new pistons, though detailed measurements & inspection of the pistons have yet to be made. There are some performance options on the table and I need some advice, particularly about the feasibility of using higher compression pistons. The best gas I can get reliably is 91 octane. I'm worried about detonation, because that's what got me into this situation in the first place -- lots of breathing mods with no chip to effect a compensating increase in fuel delivery.

If I were to go with higher compression pistons, I would definitely dual-plug - that's a given. So, if I had this setup...

* 3.4 or 3.2 pistons @ 10.5 (Euro) compression
* dual plug
* custom chip
* breathing mods I already have: C2 cams, ported heads, K&N cone intake, stock headers, cat bypass, dual out B&B muffler

...would I be in danger of creating detonation? As I understand it, my motronic has no knock sensor. (If that's the case, what's the little sensor in the #6 head?) And this is a car I would need to be able to take on long road trips through the desert - assume 110 degrees outside temp.

Other things to consider are the fact that my heads have been milled down just a little in the past to clean up the sealing surface. And I have to do that again because I scratched one head's sealing surface again pulling it off. This may raise my compression slighly(?). I have no idea by how much.

My mech suggested also that this time I get a MAF unit to accurately meter the air coming in. Would that help any to stave off detonation / make the higher compression more feasible?

Thanks in advance!

Hugh,
Making a twin plug a 3.4 is not cheap (splitter box $600, used dual distributor $250-400, twin plugging heads ~$400, extras plug wires $200?, sheet metal mods, custom chip $400-450, 2nd coil, bracket for two coils, 3.4 P&C's ~$2500. BTDT
By the time you are done a 3.6 conversion may be more cost effective (but not necessarily cheaper).
The sensor on #6 is the head temp sensor.
-Chris

And you plan to be able to pass smog inspection?

Ya think it will? I personally I have no idea, that's why I'm asking.

If this car is yr daily driver that's something to think about. if not, then you just might want to dump that "stock header" which really aren't headers at all (i.e. equal length tube for each cyl.)and at least go to an SSi set up and a real hi-flow muffler.

Come to think of it, I believe that the SSi's are at the top of their ability when used with a 3.2L per Bruce Anderson. So if you are going to increase yr disp. above that you will want a header system with an increased diameter over stock if you want to realize as much power gain as possible. Anything less and you'll just be choking off power. Kind of like trying to exhale with a stuffy nose !! :)

Good Luck!

You are unlikely to get by with 91 octane without a chip that will retard the timing somewhat significantly. This will, to some degree, counteract your modifications. Of course, you might consider using toulene to boost the octane of your local pump gas.

I too find it extremely difficult to believe that you'd pass the CA visual inspection with a dual plug set up. My recollection is that 3.6 conversions are easily detected (e.g., due to the distributor placement, etc.), so I'd have to imagine that an extra set of plug wires would set off alarm bells for even the thickest inspector. OTOH, I don't personally know anyone who's failed over this, so I'd certainly defer to a local on this point.

10.2:1 is the max my engine builder would go on pump gas here in the Pacific Northwest. I think it is 92, still only one step up from water.

This is with twin plug and MSD ignition.

Randy Jones
1971 911

Here's my biased opinion :D

Decrease compression using a 0.5mm head gasket and 0.5mm cylinder base shim and turbocharge :D

The money you spend on bigger P&C's + twin plugging can get you a Protomotive Stage 2 turbosystem and you'll be running between 450-485HP :eek:

(these are designed for CA's 91 Octane fuel, so detonation is not an issue).

I dunno - if you have twin plug and fully mapped ignition, you should be able to go a little higher.

How about looking here:

http://www.rennsportsystems.com/~porsche/engines%202.7-3.2.html

http://www.rennsportsystems.com/~porsche/2a.html

Steve Weiner at Rennsport is a very nice guy and probably more than willing to sell you some of the stuff required ;) (although if Pelican has it, buy it here :D).