yamato 302 compression test

[piranah racing]

hi, could somebody tell me what i should be getting on a compression test with a yamato 302. I tested mine and only got 90psi on each cylinder.

What is the best it'll get to with new rings, i thought about 150 maybe?

My other engine showed 120...

Thanks for your help

[ZUL8TR]

Y 302 compression

Owners of the 302 chime in.

90 is low.

If stock per rules with 25.5 cc minimum combustion chamber volume with 14mm head expect about 125 - 130psi. A bit less with the 18mm head and 26cc minimum.

A lot depends on accuracy of gage you are using and if cold or hot. I test cold to keep that variable somewhat controlled.

Try test again and pull at least 4 times and note results. Then squirt some outboard oil in cylinders and rope over several times to distribute the oil around the rings then repeat the test. These compression numbers will be higher than no oil and will indicate how well the reings are leaking compared to first test.

Open throttle full on all tests.
Both cylinders should be very close compression if all well.

[Aussie Steve]

Right around 120 with my CCed 14 mm head

[HydroKyle93R]

New head gasket?

Also, with a 14mm head, @ 25.5cc's, I would agree that 120-130 is the ideal compression range.

[crfabbro2F]

Mine has been at 132 for 2 years since new. I wouldn't go below 125.

[form-e one]

dont forget all comp testers different,,,,,, my harbour freight test my 3 rude at 100 psi,,, the snap on tester we use at work 180 psi,,, you just dont want more than about 5 psi diff in a race more

[Danny Pigott]

You have to get the ring seal right. All the comp test tells you is that they are all as close to the same as possible. A leak down test will tell the real story

[FJOHNSON]

Leak down

Compression tests should be used as a quick test to see if you have a problem. but if you really want to know, do a leak down. Test it at tdc and rotate the engine to just before the opening port. You will know for sure if you need to repair cylinder.

[form-e one]

yamato"s,, 95% the time just head gasket...

[ZUL8TR]

Leak down

You have to get the ring seal right. All the comp test tells you is that they are all as close to the same as possible. A leak down test will tell the real story

Describe your leak down test set up and procedure and what pressure conditions fail the test.

Thanks