Difference between 21 to 20mm head dome?

[edz250r]

Hey Guys,

I have an ESR billet head with 21mm dome. Its running the trx thin gasket that came with the head and I currently have 165 psi of compression. I want to get as close to 180psi without going over because I still want to run pump gas. I called ESR and asked what they thought about running the cr 250 gasket, but they said that it would give me too much (about 195psi) compression and suggested moving down to the 20mm dome (they said it should put me at about 180psi).

What compression are you guys getting that are running a 20mm dome with the trx thin gasket? I would love to compare your number vs what I have been told before I start switching domes...

Thanks!

[Rich250RRacer]

I run the 20cc dome and one thin piece of an ATC head gasket which is about the same as the CR gasket and have about 195-200 psi compression. I know alot of people may scratch their heads but I have never run race gas. Always BP 93 octane with Bel-ray MC-1. No ping, no detonation, no problem.

[wilkin250r]

Automotive compression testers don't always work well on small-bore 2-stroke engines, they will often give a reading lower than actual.

The problem is the size and length of hose from the spark plug holder to the guage, the volume of that hose is added to the volume of the combustion chamber. On a typical Chevy 350V8, the combustion chamber is around 85cc, but it becomes 88cc when you put the compression tester on. This isn't a big deal, it will only change your reading by a little over 3%. Basically, a 135 psi reading will read about 130psi, not a huge issue.

However, with a 20cc dome, increasing to 23cc when you add the guage, a 185psi will only read around 160psi.

The smaller and shorter the hose, the more accurate your compression tester will be.

[edz250r]

Wilkins - Very true. I actually put my store bought compression gauge on my engine and only got 145psi, but my engine builder had a different one that they use at the Honda dealership and he got 165psi. He recommended that I go with the thinner gasket and thought I should be right around 180. He also said that what bore its on also makes a difference in how much compression you will get which is one thing that ESR didnt ask me. I'll try the cheaper method of simply swapping out the trx thin gasket with the cr gasket and hopefully that will give me the extra 15psi I want.

[Rich250RRacer]

What is the brand of the compression tester they use? I would like to get my hands on one of these.

[beerock]

Originally posted by Rich250RRacer
What is the brand of the compression tester they use? I would like to get my hands on one of these.

snap on makes one with a bunch of extension hoses, just use the one that fits like the plug....

[edz250r]

Hey guys...just an update. My engine builder swapped the trx thin gasket that came with the ESR head and put on a cr 250 gasket and the compression increased from 165 to 185psi while still using the 21mm dome. I'm going to still run pump gas and hope I dont have any problems.