FRporscheman

I know this will sound stupid. But I'd just like to hear the opinions of the experienced, and of the pros on here. So just explain to me why this won't work.

The issue with using a 3.0L crank in a 2.5L engine to stroke it is that the pistons will go 4.5mm higher. So one can either buy special pistons with 4.5mm higher wrist pins (Rennlist says ) or buy 4.5mm shorter rods (Rennlist says ). Why can't you just use a 4.5mm thicker headgasket and let the pistons stick out a little bit? As long as the upper ring stays in the cylinder, right? Which it would. A copper (etc) headgasket could just act like an extension of the block, making the block/cylinders taller.

A thick HG would be way cheaper than rods or pistons. I know, not the best approach, but I was just thinking and got curious if it would work.

I appreciate all you guys' insight and help. I'm just lookin' to learn.

Chris White

You would also have to deal with the timing belt - 4.5mm is a lot to add to the deck height and expect to use the stock belt length.

A set of custom pistons does not cost as much as it did several years ago - under $1k for Alusil pistons based on a custom design (relocated wrist pins) - plus you can get them in any compression ratio.

The other issue is that you are exposing a lot of head gasket material to the combustion area. Typically the head gasket is slightly recessed from the cylinder wall and is protected from the full combustion heat by type of quenching action - but it that face area goes from 1mm to 5.5mm then the actual flame front may contact the head gasket and transfer a lot more heat.

User 52121

I'm certainly no expert when it comes to hardcore engine building... but 4.5mm is an awfully thick head gasket. You'd be putting a lot of faith in the strength of a gasket vs. going with custom rods/pistons and putting your faith in the strength of the block itself. And considering most people who build a 3.0L aren't building it for the grandmother to drive... you probably want faith in the parts in there.

rlm328

I would be more than happy to be your head gasket sales man. Pistons are a grand as are decent rods. The 2.8 is not a cheap build if that is what you are looking for.

badcoupe

the thick head gasket puts the quench height as very undesireable number ideal is .038 or so. A higher quench will add to detonation, the reason thicker head gaskets can still add detonation issues even though the compression ratio is lower. Plus like mentioned above that puts the timing belt out, you'd have to get a longer belt or get an adjustable gear and degree the cam in.

67King

Badcoupe is exactly right about squish. As for rod length. Guess I am inclined to disagree with the consensus, with a bit stipulation - I don't know the rod ratios. Longer is better for power (shorter for torque), but that is usually a very secondary factor. The problem is, if you give up compression height in the piston, you start getting more and more into the oil scraper rings. Maybe not a big deal on a track car, but a street car? No, way.

What goes on is that as your rod gets longer, you move the delta V further into the intake event, when your valve is more open, making it easier to fill the cylinder.

I looked at this last summer for a for a turbocharged 968 application. Guy insisted on making the rods as long as possible. So he was left with as little commpression height as possible, and the wristpin went through all of the scraper rings. It was really ugly. And how much good did it do for him? Virtually none, see first attachment. I didn't like it because it was for a street car.

Basically, it all boils down to diminishing returns. If memory serves, the 3.0L engines are already near a 1.8 rod ratio. See second attachment. Notice that as you go from 1.4 to 1.6, you see a big difference. As you go from 1.6 to 1.8, you see about half the benefit (with the same step size). The 2.5 is what Formula 1 engines typically run, so don't bother going there. But trying to get beyond the 1.8 (which is what I think the 3.0L's had) is just not going to do much for you. I'm going off of memory, but when I stroked my 2.3 (turbo Ford) in my XR, I had to go long to avoid going to a 1.4. But had I been anywhere near a 1.8, I would have left it alone.

333pg333

Arash, just pony up for the custom pistons and be done with it.

Chris White

An interesting side effect (although very small) is that the higher the rod length ratio is the lower the vibration caused due to piston location. Really small difference - but worth noting if you are an engine geek!

JohnKoaWood

Chris, your geekness is showing!

To the OP, either do it the proven way, or strike out on your own and post results, I know there is more than one turbo / SC installation to an N/A going on, and several of them have considered this exact same "fix"...

On a side note, anyone notice Fix and Fail are right next to each other in the engine builders bible?

Chris White

Hmmm....but Fail comes before Fix....in life as well as in the building bible!

V2Rocket

after reading the horror stories of thicker gaskets making detonation worse, i think ill just pony up for a few gallons of 100 unleaded each fillup

Own Goal

Ok, as long as we are all playing the “how’s about game”; on the old digital caliper looks like about .175” or so problem. Maybe chuck up the pistons in a lathe and do a face cut of as much across face as needed to prevent slapping the head. Probably need to then go over to the mill and cut some valve recesses. Then again all this depends on if there is enough “meat” in the domes to cut this much. IF SO, guess last thing would be to send out and have tops coated by Swain Tech or someone. As long as playing absurd deal out. Or maybe have someone water jet you a 3.0-4.0mm (or so) titanium or stainless spacer with stock head gaskets on each side. Or, order up the right pistons for less $ unless you have access to the machine tools .

badcoupe

the spacer oce again raises quench height. Milling the pistons wouldn't leave much dome thickness and under a good load the top of the piston would burn through most likely since it couldn't handle the heat and pressure esp under boost

JohnKoaWood

But could adding a spacer aid in my quest to turbo by N/A?

badcoupe

that would make the engine more detonation prone. I run NA pistons in my turbo with no ill effects never put a headgasket down, but I watch the WB very closely and tune accordingly. I run a 60-1 hifi .63 hotside 15-20 lbs of boost. The quench height is distance from the top of the piston to the bottom of the the head. ideal is in the range stated above. Thats why sometimes you deck the block to the point the piston comes out of the block to get this number correct, many things account for what you do here chamber size and shape headgasket thickness etc.