Eng. Rebuild-10.3 to 1 comp ratio OK?
 

I will be doing an engine rebuild in the fall on my 3.2 Carrera and wondered if the 10.3 to 1 pistons will give me any trouble or measurable gain over the stock ratio. I have 94 octane available here in Michigan with no smog issues.

Make sure you dont have to use race fuel, 4 bucks a gallon, Kevin

You're right on the borderline. The trouble with modifying the 3.2 engine is that you can't do anything if you mess up. On earlier engines, you could retard the timing to reduce pinging. The 3.2 has no knock sensor, so if you increase your compression too much, then you could risk knocking, for which there is no solution, except to run race gas at $5 a gallon.

It is for this reason, that I don't recommend running higher compression pistons and cylinders on the Motronic engines. Anything with an adjustable timing - fine, but with the Motronic, you may find yourself having to replace your fuel injection.

I personally don't recommend running 10.3:1 on the street without twin plug ignition. Anything over 10:1 is asking for trouble (especially here in CA with our crappy gas).

-Wayne

Well, I'm confused now. How does the stock-to-Euro upgrade factor into this then? Still the same Motronic control... (I'm looking at Wayne's top engine picks section in his rebuild book).

-BG

The factories stated compression ratios are higher than the measured compression ratio. I don't have the figures for the Euro motor but here are some examples:
Factory (Real) Motor
9.5:1 (9.2) 3.2L US motor
7.0:1 (6.5) 3.3 Turbo
11.3:1 (10.4) 3.6L
-Chris

I ran 10.3:1 in my 3.2 for years without so much as a single ping. The Euro vs. US pistons are worth about 30hp.

Dave, you're running 93 or better octane, right? Toolman should be OK up in MI, too. But those poor souls on the left coast... :rolleyes:

What altitude are you at? What about winter/summer differences? Are you gong to twin-plug? I think this is pretty risky depending on the above answers.

Altitude is about sea level, I only drive the car in the summer, and no on the twin plug. What if I have Steve Wong make a chip with a little less advance? I am thinking like BG Carrera, Why is it possible with a euro spec car and not a motronic 3.2? How about fly cutting the pistons a little to reduce the compression ratio to about 10 to 1 ?

in days of old, with the intro of lousy gas.. some of the 60's high comp engines had to use a cam that gave less cylinder pressure.. not sure on my lingo.. and not sure if it's an option..............Ron

Toolman, Euro cars require 98 RON octane which is equal to our 93 octane, so if you use 94, you should be good to go. If you want to maximize your power, put in a europremuffler or backdate your exhaust, and a chip should be programmed with it utilizing the European ignition curves, with a modified fuel curve to maximize acceleration and throttle response. This will make your car leap whenever you jab your throttle. If you don't need to replace your pistons and cylinders though, I don't think the change to higher compression pistons is worth the expense. You can get to within a couple of hp of the European 231 spec with the changes I mentioned.

So why not go to twin plugs? It's not a big deal. Or not--at current usage has it--"that big of a deal."

Stephan

I ran my 3.2 with Euro "10.3:1" pistons for years also. I used 93 octane gas. (With US chip and DME.) The PO had made the change. I didn' t know it had Euro pistons until I had the motor apart.
Twin plugging is a lot of work and money for a little gain. It's a fun project though.
-Chris

less adv. would be a fun exp. -- but think about the downside if the exp. "fails" I would be safe and go about 9.5 -- but hey, it's your car, and your rebuild.

Basicly, you are trying to figure out where the edge of the envelope is, and it moves around a lot -- i.e. there is substantial individual variation among motors, and among different gasolines.

Running twin plug costs $$$$. <---Notice the 4 dollar signs. Not only do you have to find/modify the heads, but you've got to find a twin plug distibutor. You could crank-fire which works great, but once again it becomes a dollar and sense issue. Why don't you just forget high compression and go to a bigger bore? I mean, you're already buying the pistons ... ;)

Having a custom chip designed with a reduced advance is one backup solution, in case the engine pings...

-Wayne

On a Motronic 3.2 it really isn't "that big of a deal" when done with a rebuild. If your heads are at the machine shop anyway, it is only ~$400 more for the extra work. A 3.6 distributor is essentially plug and play, with a splitter from Andial.
http://www.andial.com/content/911/twin_plug.htm

Tom

What's the rough cost of the andial twin plug kit?

I am going to buy pistons and cylinders anyway, they only cost a few more $. I already have the Euro premuffler and a Danske sport muffler, the PO installed a weltmeister chip. I am using about quart of oil every 200 miles and the car only has 75000 miles on it. I am going to try to get as much performance as possible for the buck. If I go with larger displacement, then I will have to go to a higher flow exhaust system. I think higher compression is the next step, but I really value the opinions on this site.

I did this a few years ago but here's a basic breakdown of parts costs to twin plug a 3.2:
- Andial splitter box $600
- twin plug headmachining and cut holes in valve covers $400
- 2nd set of plug wires, about $200
- custom Cyntex chip, about $450

I have a bit more info on my web site: 3.4 twin plug
-Chris