Zero10

When I started planning this build I was pretty sure I would be safe using N/A pistons at stock boost levels with stock DME/KLR maps, but now I am starting to worry about knock with this setup based on the opinions here. Can anybody with direct experience chime in on this, is it safe to run stock DME/KLR chips on N/A pistons so long as 91 octane or higher fuel is used?

If not I don't think it is worth the risk. I will only find out that there is an issue after putting dozens of hours into this engine and car (not to mention nearly $2k in parts) and I don't want to put that all at risk unless I know 12psi is safe at 9.5:1CR.

gt37vgt

well i would say 95 octane for 12 psi 9.5:1....

Zero10

Is that 95 RON or MON? I see you are from australia and I thought a different octane measuring system was used over there?

blown 944

I can't really comment on 91 octane.

IMO if you are worried about it, I wouldn't do it.


I have abused the crap out of the 83 piston w/o problems and the turbo pistons obviously hold up. The only problems I have had, are with the later NA units (broken ring lands at high boost)

dand86951

Based on experience with forged 968 Kobelnshmidt pistons with approx 9.3 CR and running 8-12psi I would not recommend it. With the small K26/6 your boost rise at partial throttle will be very quick and the stock chips and other aftermarket chips seem to have fairly advanced timing at partial throttle and around 3000 rpm. With this set up you would get to 10 psi on partial throttle at 3000 and with advanced timing knock could be a problem. You would be right at torque peak.

As blown944 said the ring lands and location and piston strength in general may not be enough to handle the extra torque and if you add a bit of knock it may not work as long as you want.

Also, I believe there is a shop in Calgary that can do the Alusil bores. I can't remember the name, but check with Alpine Autowerks.

Zero10

There are a couple shops in calgary that can do the alusil bores. I had my last block done at Precision. I don't plan on having any block work done to my new one though

I'll get hunting for some used group 1 turbo pistons then, sounds like the N/A ones are just too risky without a custom tune and I don't want to sink an extra $500+ in taking care of that detail here. I really wanted them to work but it is just too much risk.

Thanks for all the advice.

dand86951

I have a set of used turbo pistons, don't remember which group though but will check this weekend and I'll PM you.

Ski

Budget permits....I'd get a set of pistons, Wossners from Chris or the Mahle slipper skirts from LR. Trying to match bores with pistons is taking a chance. Block work being $450-$650, depending on what you have done, with pistons, that's probably around $2k extra but, it will be worth in the end.

Zero10

Unfortunately the budget does not permit an extra $2k on this project. I could squeeze a bit of cash for a used set of pistons but I cannot do $2k extra. If I can find a set of used turbo pistons with no physical damage and the skirt coating intact I should be set.

When the current motor was built I spared no expense, new pistons, bored and honed cylinders, head work, everything. After I got it stitched back up I found myself back inside the engine twice more, once from the rings being upside-down, and once to find out why my compression was falling badly and there were aluminum bits in the oil. On the second check I found the skirt coating on my new (1000 mile) pistons had failed and 3 bores were severely damaged along with the pistons. Despite spending in excess of $3000 just building that engine I was still left with nothing. This time I am staying factory sizes, factory pistons, factory as-much-as-possible

Tom M'Guinn

If budget is a primary concern, I still think your best option is to find a nice leak-free factory-assembled 951 motor from a running car. If you put used pistons into a used block, your chances of getting proper clearances are not great, and the rings may or may not seat well. Why take the chance? I don't think you can build any decent motor -- even with unmachined used parts -- for less than the cost of a used motor. Just my two cents.

Ski

Depending on your timeline...I'm pulling the new motor out of the track car and we're going 2.8L. The pistons in there are the new Mahle slipper skirts and basically have just run in time on them, one oil change. The car was due to get painted but both Jim and myself ran into time issues. They're 101mm but I'd let them go for $900 if you want them. We're keeping the block, the crank that was worked, rods, etc.

FRporscheman

+1

I'm in a similar position. I'm itching to get my 3.0 built after 2 years of waiting but I just can't afford the pistons and machine work right now. Instead of waiting another year, or cutting corners, I'm going to just install a used 951 motor to get me going.

SprintStar

[QUOTE=Olli Snellman;6662126]S block is the direct fit to 8V head. With 3.0L S2 or 968 blocks you have to use 2,7L head or modified 2,5L head.
QUOTE]

Hmm... So can I make my '86 951 into a 16V turbo by using a S head?

I think the intake manifold needs to be modded but it should breathe a lot better?

Sprint.