How does one build a 2.9 twin-plug?

[356KarmannSUPER]

I would like to build a high-compression, 2.9, MFI, twin-plug motor. What case and p's and c's? Crank? How big should the ports be? I am hoping for something north of 280 hp and fairly torquey, probably E cams. Crankfire? What would the longevity be like? Advice?

Thanks!

[Tim Walsh]

hmm. 7R case with P&C's from mahle, standard 2.4,2.7 crankshaft. S cams or possibly even a touch hotter. Overall a very pricey but fun motor.... I want.

280 HP out of a 2.9 might be a little high. The 2.0S with MFI only made 85hp/L so applying that to a 2.9 you get around 250

[Steve@Rennsport]

Hi:

I would ask one major question. Do you want a street or race engine? Both are very different things. You asked a lot of questions,....

I would also add a note about the 93mm P/C's used for a 2.9.

Due to the fixed stud spacing on these engine cases, these cylinders have very thin walls and cannot be counted on for a reliable ring seal when hot, since these do not stay perfectly round at operating temperatures. Further, the head sealing surfaces are VERY thin and I've seen some CE ring failures due to lack of enough mass in this critical junction.

I do a demo in the shop for some customers who inquire about such things. Using a 90mm Mahle cylinder with a ring-less piston inserted into the top of the bore, I'll ask the customer to hold the barrel between the palms of his hands and squeeze inwards. The piston stays at the top of the bore until the person relaxes pressure and the piston falls out the bottom. These cylinders do flex and this demo is with the thicker walled 90mm ones,..............think about the 93mm's.

IMHO, the largest practical bore size for these engines using this stud spacing is the 92mm RSR setup and these still require careful machine work to ensure good sealing between the heads and cylinders.

Caveat Emptor on the 93mm ones.

[89turbocabmike]

Steve, you are a never-ending source of info that just makes sense

[Wayne 962]

2.9 Engine with MFI - that would require quite a bit of customization of the pump & distributor, and would also require some hard-to-find and expensive 2.4S stacks and throttle bodies (or modified ones). You could spend in excess of $5k on the fuel injection alone. I would recommend Engine Management (TEC-3 or something similar) if class rules don't prohibit it.

As for 2.9s, I'm not a huge fan of them, mainly for reasons that steve mentioned (thin cylinder walls). Depending upon what people's restrictions are, I recommend that they run a 1976-77 Turbo case with the larger cylinder stud spacing. You can create a short-stroke 2.8 or larger by running the small crankshaft from the early cars with some of the larger pistons from the later cars. The engine's power profiles are different, but you have a bit more options, and a much stronger engine.

-Wayne