How much power with boost.....reliably

[quattrorunner]

I am going to be opening the engine and most likley splitting the case in order to freshen things up on a high milage engine(200000) it's an sc with 8.5 to1. I don't want to add the cost of I'll be boosting it and was wondering if 400 hp is possable with the 3 liter and still be relativly reliable like 5 years between rebuilds and no serious track time. I know this is high for the 3.0 but I'll be using efi and intercooling of some sort.
Will it be best to machine the top of the pistons to reduce compression or is it fine? In the end, I will do whats best for the engine which means that if it's not healthy to try for this figure, I just won't do it. But the reason for the turbo is HP. So I just want to know what I can reasonably expect to achieve.
The nature of the rebuild is not to go crazy with internals but to make it robust enough to turbocharge. What is involved with the machining of the crank in order to cool the underside of the pistons with oil? Any other thoughts that are worth mentioning?Thanks, BA
Here is a photo just cause I love my car.

[Steve@Rennsport]

BA:

Nice car,...

You've asked a somewhat complex question and the first part of an intelligent reply to you involves asking what your engine life expectancy is?

In short, how long do you need the engine to last before rebuild time?

The answer to that frames all the rest,...

[quattrorunner]

Well llike I said, maybe 5 years and 12000 per year. So 60000 miles. That sounds like not much so I feel it is reasonable to get higher power and less longevity.

[Steve@Rennsport]

If you need 60K out of that, prepare to scale back your HP objectives.

As long as these engines are operated at 1 bar or less, see 11.2-11.5:1 AFR's, run 92 or better fuel, and have full control of oil temps and charge-air temps, you can come close to your objectives depending on how you use the car.

This assumes that you keep the CR around 7.5-8:1 and replace the stock cast SC pistons with Mahle forged ones. Deck heights must be maintained between .039-.040. I'd strongly recommend twin-ignition too, given the state of your pump gasolines.

With all of the above, you can usually run between 325 & 350 HP (flywheel) without trouble.

[quattrorunner]

Thanks Steve.
Real life figures are what I wanted.
I will choose maybe a lower expectation, like 300hp. Is this more realistic for my stock pistons with no twin ignition? I could have the machine work done while things are apart. Maybe even machine the head for twin plug but I don't have high $ plans for motor work planned. Just boosting which is easier than building a highly strung NA engine.
Thanks for your valuable experience.
I guess my machine shop knows how to machine for oiling the underside of the pistons? I know nothing of machine technique.

[Steve@Rennsport]

If you must retain your stock cast pistons and run on pump gas, then keep your boost levels around .4-.5 bar. The real trick on these cars is proper fuel enrichment & ignition retard while on boost.

If the heads are off, thats the right time to have them machined for the lower plugs. You already have piston oil squirters so thats nothing to worry about,...

[quattrorunner]

" You already have piston oil squirters so thats nothing to worry about,..."
O.......cool.
Well, not to keep beating the bush but can the stock cast pistons be trimmed or machined in some way as to help with the compression or is this not the problem with these pistons? Maybe they are suseptable to weakness? It's all good to know before hand so I don't have to tear it apart only months later.
I don't plan to be on cis for long so low boost levels should be OK till I go EFI. Like 5 or 6 psi. I had an audi 90 quattro run on 6 psi with the stock CIS system till EFI. AFR stayed in safe ranges for the most part.
Thanks for helpfull responses Steve.

[Steve@Rennsport]

You cannot safely remove enough material from the piston crowns to reduce the CR to what you need. Remember, these are cast, not forged, and not really suitable for forced induction applications.

Air-cooled engine operating conditions/temperatures should never be compared to a water-cooled engine's,...............

[quattrorunner]

OK thanks, I'll keep this in mind. BA

[sammyg2]

I've been running my stock 3.0 SC engine with .5 bar for a few years now, the reason I don't run higher boost is because I think I'm getting close to the reliability limit on pump gas. I don't want to tear it up and I'm afraid that might happen if I crank the boost up much farther.

I'd say that around 325 hp is the safe limit for stock (turbocharged) SC engine on 91 octane pump gas.
Obviously if you start modifying or changing the internals the sky is the limit.