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MADMAT

MADMAT

Location
Ohio
I've got 2 SJ's I have to deal with here.

Mine has a Greater Yamaha head with domes and my friends has a Twist head with domes.

We are both at 150 and want to go to 180.

John@greater said to try UMI for mine because he no longer sells the domes I need to get to 180. Who is the best one to call for UMI domes?


Will any other domes fit the Twist? Also, the motor with the Twist is about to get bored and ported by Paul. He will need the head and domes to machine them to the new bore? Can he work with this or is my friend going to need a new head.

Just trying to prepare before I bug some peeps on the phone.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
35cc seems about right for your average 701 head to make 180 psi.

My ADA's 35cc make 180-185 on a stock bore.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
Roughly, it should be the same volume from manufacturer to manufacturer.
There aren't too many interpretations of volume, and the dome geometry is pretty similar between different makes.
 
sicki

sicki

Ride hard, put away wet!
How the heck are you getting such low compression on 33cc domes. Thats race gas with my Pro-Design head. Pro-design i believe makes most of the heads/domes for other companies, I know Pro-Tec for sure. Im running 37cc and getting 180psi. Mike Serlin ran 35cc domes in his Pro-design and some idiot told him that would be pump gas. I was right, he had to by 37cc domes from Pro-tec. You guys should upgrade from your Harbor Freight Tools compression gages.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
Harbor freight gauges...LOL, how true.
Check what the manufacturer specs as compression for a given dome size, and confirm with a gauge.
 
MADMAT

MADMAT

Location
Ohio
Waste Land said:
You already have a head on your ski and its only at 150? should it be much higher even with the lowest domes?
Click to expand...

I thought rec. domes were around 150.

Is it a general rule that 1cc=10PSi? At least in this situation.
 
djkorn1

djkorn1

kidkornfilms
Site Supporter
Location
Cleveland Ohio
You might need rings or a top end. You rode the hell out of that motor for 3-4 years?? It might be time for a rebuild. That suckers got some hours on it. Better to do a quick rebuild now than in the middle of summer and blow your crank too.
 
MADMAT

MADMAT

Location
Ohio
We ran it for 2 summers (04 and 05). We checked em'. I think they are fine but I'm considering it anyway.

The motor is race ported so if you use the highest cc domes you will have low compression....right??

I think the previous owner set it up for reliability and recreational use before he sold it. He probably kept his badazz domes.
 
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MADMAT

MADMAT

Location
Ohio
OK. A little more info. I am currently at 150psi with 38cc domes and race porting.

That is with the little adapter attached to my compression checker. So I may actually be at 160psi.

I think I am going to try 36cc domes and be done with it. This should put me at 170 or 180 which is still pump gas, right?
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
If I am not mistaken, race porting means higher than normal exhaust ports, which will lower your cranking compression - that's exactly what you're seeing.

Your dynamic - running - compression is most likely quite a bit higher, so I wouldn't base my octane needs solely on cranking compression, especially on a ported motor.
 
ski4

ski4

gonzo
Location
cleveland
amen korn,
matt, dj says it was running like a top

don't mess with it now, spring is right around the corner ( remember last summer) maybe this will be the year we all make it the whole season with no layups.
 
MADMAT

MADMAT

Location
Ohio
Neva!! Need more power!! Must have more powa!!

Its just domes and orings. And a lightened MaGoo flywheel.

I'm just gonna drop like 1 or 2 CC's.

I almost made it thru all of last summer with no drama.
 
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