Thoughts on twin pluging Euro spec 3.2

tholyoak · 02-25-2021, 06:10 AM

I am rebuilding my 89 3.2 using new Euro Mahle pistons and cylinders that are advertised at 10.3:1.

I understand that the actual compression may be lower than the advertised compression but was wondering the logic on whether I should twin plug the engine due to the increased compression over the North American spec engine.

I will be changing the cam profile as well to 993SS or similar during the rebuild as well but retaining the motronic injection.

We have 91 and 93 octane fuel available but I wouldn't want to limit myself to having to run 93 only, as it isn't as widely available.

Thanks,
Todd

Mark Henry · 02-25-2021, 06:24 AM

I can do the head and cover mods, no broken fins.
PM if interested.

That said I think you can get away with (9.3-9.5CR) and single plug.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by tholyoak

I am rebuilding my 89 3.2 using new Euro Mahle pistons and cylinders that are advertised at 10.3:1.

I understand that the actual compression may be lower than the advertised compression but was wondering the logic on whether I should twin plug the engine due to the increased compression over the North American spec engine.

I will be changing the cam profile as well to 993SS or similar during the rebuild as well but retaining the motronic injection.

We have 91 and 93 octane fuel available but I wouldn't want to limit myself to having to run 93 only, as it isn't as widely available.

Thanks,
Todd

There is a small adjustment screw on the ECU that allows you to adjust for higher compression pistons. 3 degrees of timing advance/retard I believe. That said, compression performance is overrated. Do a volume test to get an accurate compression number and shoot for 9.5-9.8:1. Base gasket of varying thicknesses will make that happen.

Jeff Alton · 02-27-2021, 06:26 PM

Yes, measure. The Euro pistons we have measured were never over 10:1 despite what the book says. Base gasket will get you down to where you can still run single plug easily. That all said, we have quite a few Euro spec cars we service, all original single plug. No detonation with good fuel.

Cheers

BURN-BROS · 02-28-2021, 09:51 AM

I speculate that Mahle advertises the compression ratio @ 1mm deck and does not account for the CC between the crown and the top Ring. So it's gross Dome volume. Once you account for the .6cc loss and a standard 1.25mm deck the Compression is about 9.92 provided everything is in spec. YRMV

tholyoak · 02-28-2021, 04:13 PM

Thanks all for the comments. I will certainly measure the compression as suggested. The additional cost to twin plug it really isn't that great so it might make sense to just have the machining done while it is apart.

-Todd

Mark Henry · 03-01-2021, 05:45 AM

The machining is only part of the cost, then you need the kit to fire 12 plugs. This can be done DYI for under $1k but most systems are $3k to over $10k. One avenue I'd explore is if MS can be run ignition only, then you have the future option of going full EFI.

One thing you could do is the maths for both single and twin plug, set it up for single plug and once you decide to take the TP plunge you can loosen the jugs, clip a shim and you're good to go for twin plug.

tholyoak · 03-01-2021, 05:58 AM

Thanks. I'm well aware of what is required to run twin plug and on the motronic it really only needs a 964 distributor, a bosch ignitor and a remapped chip. No need to get complicated and run an aftermarket engine management system or even the Andial splitter setup according to Steve Wong (https://www.911chips.com/engine.htm).

Personally, if I was going to go to the expense of an aftermarket ECU I would go with a VEMS system, do coil on plug, and ditch the air meter.

Todd