Pump Gas Dome

[250Rhonda]

So my 250R came in yesterday and it still had some gas in the tank. I smelled it to see if was still good and got a big whiff of C12 race gas. I called the guy up I bought it from and he said that he only ran it on C12 and he had the high compression dome in the Pro Design Cool Head. My questions is what CC dome should I put in it to be able to run it on pump gas 91 octane? The timing is stock so that is not anything to worry about. Any help is appreciated. I believe 21 CC is the correct dome size.

[wild250rman]

what head gasket is on it? or is it o-ring type?

[quad janern]

I think the 21CC is what you need

[250Rhonda]

Originally posted by quad janern
I think the 21CC is what you need

That is what I have been reading online.

[TLR-Online]

Originally posted by 250Rhonda
That is what I have been reading online.

I would def speak to a professional engine builder. There are LOTS of factors in determining the correct dome size.

C-Leigh Racing should be able to help you out some.

[86Rrider]

Pro Design offers 18cc, 19cc, 20cc, 21cc and 22cc domes for the TRX cool head.

I have one on my '86 with the 22cc dome, using a CR250R head gasket and I run premium pump gas with no issues at all. So, you switching out to a 21cc or the 22cc should allow you to run pump gas.

However, if one of the pro-engine builders on this site see this or you contact them, they could probably tell you exactly what affect each dome size change will do for you.

[TLR-Online]

After talking to an engine builder VERY briefly, this is just some quick/basic info that you will need to correctly determine the dome size.

Altitude
Gas you want to run (87, 89, 91, 93, 100, 110, ect)

Port Timing on the cylinder if its has been ported
If the cylinder has been decked, how much?
Head Gasket Thickness (need to setup your squish first)
Bore Size

You maybe able to just pick a dome and it be ok... but everyones setup is so different, i wouldnt want to take the chance.

[mx91a]

I believe for pump gas you want to be under 13:1 UnCorrected Compression Ratio (UCCR).

Here is the formula to calculate your needed head volume:

Head Volume = [Displacement/(UCCR-1)] - Gasket Volume + Deck Height Volume + Piston Crown Volume


Volume is in CC. Deck height volume is negative if the piston is below deck and postive if it comes above.

This should get you a close estimate, then round up to the next available dome size to be safe.

[250Rhonda]

Well I spoke with the previous owner and he said I can run pump gas in it as long as I jet it correctly. I think I am gonna stop by the airport and pick up some 100LL:D . I know people are all controversial about that stuff but honestly its 100 octane rating is based on the "motor" or "pump" rating so it is what we would consider a true 100 octane fuel. tTe only issue that comes from it is it has a de icing and de-watering agent in it that can give the fuel a sluggish response in comparison to true Race Gas, also it is a little "dryer" of a fuel so a little more oil in your gas mix and a increase in jetting is necessary. I have ran the stuff in my 11:1 454 big block and you can literally feel the difference versus 93 octane (aluminum heads) if you think Im talking out my ***:p