Originally Posted by Chris Cervelli

As a very general rule engines that are thought of as 'torque' engines have a long stroke and small bore. These are referred to as 'undersquare' meaning the stroke is larger than the bore. 'Horsepower' engines tend to have a large bore and short stroke. They are called 'oversquare' All Porsche engines are oversquare.

A great exception to this rule is the new BMW M3 engine which is a little undersquare and yet revs to 8000 and makes over 100 hp/liter.

Anyway, in the normal non-BMW case, the small bore limits valve size and therefore top end breathing. The long stroke imposes some mechanical limitations on high revs and thus further makes the engine more suitable for low rev 'torque' type operation.

Now keep in mind that the 2.8 engine is still very 'oversquare' (bore bigger than stroke) so I am talking only in relative terms here.

Here is what I expect to see from a stroker 2.8 compared to a normal 2.5 (all other things being equal):

A very large increase in low end torque and a very small increase in top end power. This borne out on the dyno as my 2.8 makes 320rwhp and 375 ft/lbs at only .95 bar. You would also expect to see the peak power and torque occur at a lower rpm.

For the all bore (106mm??) 2.8 I would expect to see a roughly 10% increase up to say 5000 rpm or so, where the percent of increase would dwindle unless the cylinder head's breathing ability was improved. I have not done a 2.8 this way, and don't intend to, because I feel if you can't use factory type pistons, you can't guarantee 100,000 mile durability. (I could be wrong here, I have heard a few sleeving success stories lately)

If we are talking about street cars only, it is obvious that the area in which the 951 needs the most help is low end torque. The stroke 2.8 is going to acheive this much more effectively than the bore 2.8. Plus, you can use all factory type parts are be assured of the durability.

For an all out race engine, the bore 2.8 has more potential. The larger bore allows more room for big valves and should improve breathing automatically. But if you are going this far, why not make a 106mm bore engine with the long stroke? That would be around 3.15 liters I guess.

I have built a ton of stroke 2.8 and found them to be really great for street cars and mild race cars. They have two problems that seem to recur over and over:

They are a lot easier to blow up than a 2.5. The extra displacement puts you that much closer to the fueling limitations with all the stock engine controls. Go lean with a 2.8 and they melt quick. To get a 2.5 to melt you have to work at it.

They are not necessarily better race engines. If you keep all your other stuff and just switch from a 2.5 to 2.8, it is unlikely that you'll go faster. The extra low end torque can be a liability in a race car, since traction should be in short supply. Also your peak power occurs earlier, which means you shift earlier etc.

Now if you put the right turbo on a 2.8 you have a nice race engine. It has some grunt even off boost, so if you get caught down there you are not dead in the water. With the right turbo you can easily make 400 hp at 6500 rpm, which should run very, very well. The torque band will always be wider, so you can be a little lazier with the shifting.

Of course there is the right way and the wrong way to do a stroked 2.8. The right way:

Turbo block bored to 100.5mm
New Mahle pistons for 8:1 CR and 100.5mm bore. These pistons have the pin moved to accomodate the extra stroke. Otherwise the piston would stick out of the bore by about 4.5mm at TDC.
Carrillo rods. The stock rods won't clear the block with the longer stroke. Plus the Carrillos are the best rods you are likely to see.
944S2 or 968 Crankshaft.

The wrong way:

Stock pistons
4.5mm shorter rods to fix the piston problem.
944S2 or 968 crank.

This is really screwed up. The CR is way too high and the rod angle situation is much worse. This engine will never make any power and always want to detonate.

It is too bad Porsche didn't make all the 951 2.8 liters. It would not have cost them any more and it would have made an almost-supercar into a supercar.

Chris Cervelli
Technodyne Inc.

Originally Posted by MAGK944

Thanks V2, I bow to Chris's expertise and will go the piston route instead. However I don't see why you cannot use stock forged rods if you change the pistons, notching the block is common to allow clearance.