What would you do next with this wedge dome setup?

[BGCarrera32]

So here's the deal...Mahle 98mm wedge dome, recommended negative deck height is 1 mm, which I am currently at. Notice that I lapped the new cylinders into the heads, which clearly shows the amount of "overhang" in the combustion chamber per-say (red arrows). I'm concerned that even with the 1 mm deck, the side of the piston dome could smack the head at the corner of the head dome, and I'm unsure as to what I need there for clearance.

This was a 3.2 now becoming a 3.4, so cylinders went from 95mm to 98mm bore. Altering deck height from 1mm to 1.25 drops me from 9.86:1 to about 9.6:1.

Thoughts?



At 1mm quench:

[MBruns]

I would machine a small chamfer to meet bore size, we do it all the time
Mike Bruns JBRacing.com

[304065]

"Deck height" is the distance that the "deck" of the cylinder protrudes above the top of the piston.

"Negative Deck Height" is when the "deck" is below the top of the piston. The 964 has this, the piston edge actually protrudes into the head, I believe.

You have positive deck height, but just not enough chamfer. Didn't the factory chamfer the 2,7 heads for this precise reason? I bet there is a photo here of one of them vs. a 2,2-2,4.

[BGCarrera32]

So what would be the pitfall of increasing the base gasket thickness to increase the deck height from 1mm to 1.2 or 1.25mm? Whether I add a chamfer, or add gasket thickness instead, I'm increasing chamber volume which is going to reduce compression.

I am not sure how much hp loss there is between 2 or 3 points of compression on a big bore 98mm, but I would bet it will be notable.

Not that I have a choice either way, just curious.

[Steve@Rennsport]

JMHO.....

You really don't want to increase deck height beyond 1mm as that increases the propensity for detonation even though the CR is decreased.

[Eagledriver]

Originally Posted by BGCarrera32

So what would be the pitfall of increasing the base gasket thickness to increase the deck height from 1mm to 1.2 or 1.25mm? Whether I add a chamfer, or add gasket thickness instead, I'm increasing chamber volume which is going to reduce compression.

There is a big difference. If you chamfer the heads you loose a small amount of volume (ie: the amount of metal you shave off). If you drop the deck height you'll lose the amount that you would by shaving the entire top of the piston. The chamfer is the proper fix. I'm also not sure .2mm would do the trick. You'd still be very close to contact.

-Andy

[BGCarrera32]

Thanks guys. I took a little blob of modeling clay and stuck it to both the high and low dome side of the piston tonight, rotated that cylinder up and over TDC. When I pulled the head off, the clay was sheared away on the high dome side, looked like I had the full 1 mm on the low dome side. Looks like I will have to chamfer as this seems uncomfortably close.

Next question is that I have approx. .060" near the exhaust valve seat. How close can a guy get to the steel seat?

[Eagledriver]

I don't think there is any minimum required there. The steel seat is very thick. The only limit would be to not have the cutter hit the steel seat.

-Andy

[rdimarco]

IF the piston had a wide flat around the top edge (unlike these) could chamfering the head out to cylinder diameter partially re-create undesirable excess space at the edges similar to increased deck height? Some JE 98/100mm pistons are flat at the edge and they do not need chamfering for clearance, maybe no chamfer with those helps with squish? Thanks.

[MBruns]

Its a 20 deg. chamfer, sometimes they touch the seats most time not.
If you have a neg. 1mm deck (which is the edge of the piston before it begins to make the dome) below the cyl. sealing surface and that 20 deg. chamfer to bore dia. you will have plenty of room, on your bore/stroke if you were to add a.5 mm shim under the cyl. it changes the CR by .25 of a point and the chamfer would be such a small cc change and almost no change in CR.
Mike Bruns JBRacing.com