Which domes for pump gas

[JimTMich]

I have a 2000 sxi pro motor with total loss ignition 19 degrees timing, 81.5mm boar (stock was 80), stock exhasut system, dual 44 carbs. rec porting. What domes should I be runnign with my ada head? I have 30cc, 28cc, 26cc..... I would like to run on BP or Shell 93 octane pump gas. Also, are the domes I have ok with the 81.5mm boar?
Thanks
Jim

[Brooks]

The squish on the domes is cut to 81mm, which would be okay, but the location of the oring groove would be the problem. You should go with the 800 domes, they are cut to 82.5mm. As far as what size you need, it is hard to say since the cylinder has been ported. The height of the exhaust port directly affects the compression. I would guess a 26cc dome, as the larger bore may make up for the porting, if it wasn't ported too radically. You could always just do a compression check with the domes that you have, as the 26cc dome you have would measure the same compression wise.

[JimTMich]

Can I really set up the motor using cranking compression?
The exhaust port height looks as though it was only raised about 1/2 mm if at all.
On the o-ring issue, your saying there is not enough metal sealing surface at the 81.5mm boar between the head and cyl before I get to the o-ring. Makes sence.
So my question is, can I set up using compression as my guide or is there something else. I have read that you should not use compression as a setup means and that you should calculate/measure your way into a compression ratio. I don't know what ratio works with 93 octane though. 180psi.14.7 = 12.2:1. Don't know though.
Any input you have would be appreciated.
Thanks

[Brooks]

Yes, you are correct, you should not setup a motor using cranking pressure. You should set it up by compression ratio. I would say that if you are using 93 octane, I would go for somewhere around 12 or 13:1, uncorrected.

[JimTMich]

Tried some domes last night

I tried the 28 cc domes last night, got 145psi
Tried the 26cc domes and got 160.
This is on a new 81.5mm bore and new pistons. Has never been run. The walls and rings probably have some fogging oil on them.
Remember light porting. (SXI PRO doesn't need to have the exhasut port raised much)

So, should I get some 26's or 28's for 93 pump gas? I'm at roughly 900 feet elevation.

So, between the elevation, light porting (.5mm exhaust port raised), new bore, larger bore that would explane the 180 spec vs 160 measured. Should I go with a 26cc 800 domes?

Thanks
Jim

[Brooks]

I would say the 26's. It does sound as though your exhaust port is raised more the .5mm. Or, it could be that the rings are not seated and sealing. Once you've run it and broken the engine in, then get a good reading. The larger bore would more than compensate raising the exhaust port only .5mm.