Shaven 885 Head on ETA Bottom = 9:1 C/R Has anyone actually CC'd their own head?

[4x4_e30]

So, about putting an 885 head on an ETA bottom end. Apparently it's "common knowledge" that this combo leads to 8.3:1 - 8.5:1 C/R. I had mine shaved to 4.909" (minus .020" from stock). Upon further research, pretty much everyone has just regurgitated what they've read so many times nobody bothers to do any investigation themselves.

I only found one other report of someone actually checking the volume:
http://www.e30tech.com/forum/showthr...54#post1191454

So I busted out the plexiglass, grease, and medicine syringe. Cylinder #6 came out to 37cc, #2 was at 37.75cc. Although my technique was not 100% exact, repeated measurements showed a tolerance of +/- 1/8th of a mL. The stock ETA head comes in at 38cc.

I won't lie, using the 885 head with ETA bottom end greatly reduces the squish band area compared to the 200 head. But the compression ratio is actually a little higher than stock ETA.

So, let the flaming begin!

[Northern]

Saw this post on FB, had no idea why you cared.

With the eta bottom end, it makes sense now.

[whodwho]

Combustion chamber volume is not the only factor in the equation that makes for the CR, next measure the difference in the dome/dish of eta piston vs the b25i piston.

[4x4_e30]

The ETA bottom is still sweeping the same exact volume as it would with the 200 head. So if I push the same amount of air into a smaller place, why would I need to measure the "i" piston when it's not even a part of the system?

[redlightpete]

The ETA bottom is still sweeping the same exact volume as it would with the 200 head. So if I push the same amount of air into a smaller place, why would I need to measure the "i" piston when it's not even a part of the system?

Unless you have a completely flat piston that is flush with the top of the head gasket, you need to factor in the combustion chamber volume below the head.

[LJ851]

Guys, it's not rocket science. If you remove the head from a given bottom end and replace it with another head having basically the same CC combustion chambers, the compression ratio basically doesn't change.

[digger]

Guys, it's not rocket science. If you remove the head from a given bottom end and replace it with another head having basically the same CC combustion chambers, the compression ratio basically doesn't change.

this.

if nothing else changes its just the delta of the head cc between the 200 and machined 885, which in this case is basically zero.

taking 0.5mm off the head is ~3cc a basic calculation will show this.

alternatively you could use a thinner gasket but that will cost more and its good to use an OEM gasket where possible

[digger]

It should be noted that a half a point of CR is probably good for about 3 or 4bhp. But if you ran a 272 cam or similar it would work a bit nicer with the higher CR. however the piston to valve clearance gets worse with both skimming the head and again with a 272 so be careful