3.2 Rebuild Planning, 3 questions
 

My 86 Carrera has 160,000 or 170,000 miles on it. The odometer was broken a while back. She is starting to burn some oil. My wrench says the valve guides are worn so a top end is in my future. Given the mileage on the car, I’m going to do the bottom end as well.

The car is a street car used regularly for autocross and the occasional DE. I considered going to bigger P&Cs to make her a 3.4 but my wrench thought I would be better off leaving the displacement alone and going to carbs. The thought being that the better throttle response trumps the increased displacement. The carbs also allow valve relief in the pistons so I have a wider cam selection.

So my current thinking is to use 46mm PMO carbs. I’m sticking with the motronic ignition and distributer, just having a custom chip burned to cut out the fuel injection. The rebuild will include all the usual upgrades,valve springs, rod bolts, etc. but I’d like opinions on 3 things:

1. P&C compression ration. We have good 93 octane gas in GA, but who know how long that will last. Right now I’m thinking 95mm diameter 9.8 CR. Thought being I could run 91 octane if I had to.

2. Cam grind. I want a fat power band, doesn’t everyone? Wrench suggested an S type cam but my impression is that this is a high rpm power type cam. I’m more interested in low end and mid range power.

3. Exhaust. I need heat too. Seems like my readily available choices are SSi or B&B. Since I’m more interested in low end my wrench thought SSis wouldn’t be a problem, but the 1 ½” pipes seem small. The B&Bs at 1 5/8” seem a good compromise. I’m afraid I would lose low end with 1 ¾” pipes.

Any thoughts would be appreciated.

Hi John,

My thoughts, FWIW.

1) Given the displacement, your summer weather, and the desire to use decent ignition timing, I'd stick to 9.5:1 if it were me.

2) For low and mid-range power, I'd be using a 964 grind; perhaps even a custom one. Remember, such valve timing increases low RPM cylinder pressure, hence the need for #1, above,...:)

3) 1 3/4" is much too big, especially with your power goals in mind. 1 5/8's is ideal, however the choices aren't good at all. B&B stuff is poorly made and not durable. Further, their proprietary header design precludes proper muffler tuning and you must run one of their crack-prone ones. With 964 cams, SSI's won't be a big liability at all and they are very well made. Plus, you can use a good sport muffler (Danske) for effect.

Hope this helps,

Steve,

Thanks for the input. I was planning on custom cams and then custom pistons to handle the valve lift. If I stay with a 964 type cam and 9.5:1 pistons would you recommend stock pistons or something else?

Thanks again,

John,

P&Cs are a budget question, if your current parts need to be replaced then going to 98mm P&Cs with carbs sure help with your bottom end power goals. With the carbs it makes even more sense. I assume you will be replacing the valves based on miles alone, you can get a bit of help with going oversize on the valves, can use same seats and does pick up low lift air flow a bit. Do you have big port heads now or small? I do not know by years like Steve does, the small ports could use some help on a piston increase, the big ports are maybe too big no reason to make them bigger, re shaped maybe bigger not.

It should be a fun ride when you are done.

If you aren't going to go for a crazy cam with lots of overlap, why not just keep the stock fuel injection? Maybe I'm crazy, but I think the throttle response is pretty darn good with the stock setup. You can still do the 964 cam, & a add chip if you must have more power. The cost of a set of PMO's will almost buy some nice oversized P&C's. Unless you have a euro-spec car, you likely will have alusils, which you can re-ring - or not, depending on whom you ask.

Thanks for the input guys. I've got to make some decisions...

I'm going into parts acquisition mode shortly. Probably do the rebuild after the new year.

I'd say stick to 9.5cr , gas won't be getting any better, and there's nothing like the pingetty-ping-pings after getting some sub-standard gas. I carry a dist wrench in the glovebox. get bad gas, turn back the dizzy. re-set at home when I get better gas. 46's may be a bit big for a fat power band, and keep the intake ports moderate. for cams, check out what John did for my 3.0 liter. it's got torque out the wazooo.

ps. if you're interested, I might have available a set of new 95x23 JE's coated and ready to go.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/449608-dougherty-racing-cams-my-experience.html

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