[VEGA 3503508.5]

Hello again guys,

Easy question-

I'm about to buy a compression tester for my sbc 350 with 305 heads. I only see metric 14mm and 18mm fitting testers on ebay. I need the screw in type because of clearance issues. What is the correct fitting/adapter size that will screw into my 305 heads?

please advise.

 

[nitro_baller4692]

14mm on a smallblock chevy

 

[VEGA 3503508.5]

So it is metric .wow.

Im guessing the 350 heads use the 18mm then -since those spark plugs are bigger.

thanks...

 

[johnsongrass1]

It depends on the head, not the size of the engine.

 

[nitro_baller4692]

if your buying a tester i would buy one that has both sizes in the kit.or make your own

 

[VEGA 3503508.5]

FYI the 14mm bolted right up...

It was very easy and cheap to do the test.

125 to 135 on all exept #8 which had 115.

Kinda low for ~~10:1 CR 305 heads on a 350. no?

 

[kuhn-77]

I would say thats really low, b/c on my 388 SBC with 350 heads i run 10:1 compression and all my cylinders are in a range of 190-200 psi ..........

 

[JohnyR17]

Not necessarily... are you running a very large cam with lots of valve overlap?

 

[VEGA 3503508.5]

The guy who built it (friend of a friend) said it has a mild RV style cam. He didn't have the specs.

For some reason...He didnt have ANY tools either and he wanted to swap this engine out of his Blazer for a 383. I told him that I'll lend him my engine hoist, jacks, etc if he gives me the 350. So I can't complain.

Well at least I can stop using 91 octane. lol... :thumbup:

 

[killerformula]

14mm and 9/16ths interchange. Little tighter than 13mm-1/2 inch, but it works.

K

 

[johnsongrass1]

Was No 8 the last cylinder tested, did you show a gradual decrease in the readings for each cylinder on your comp gauge while doing the test.

 

[onovakind67]

FYI the 14mm bolted right up...

It was very easy and cheap to do the test.

125 to 135 on all exept #8 which had 115.

Kinda low for ~~10:1 CR 305 heads on a 350. no?

Did you have the throttle wide open?

 

[VEGA 3503508.5]

I floored the gas pedal. I let the thing crank until the highest reading. About 5 power strokes was enough.

2-4-6-8 then 1-3-5-7. The readings were random but always about 130...except #8.

I already knew about #8. When I changed the head gasket I ran my finger through the cylinder walls just to feel for imperfections and #8 had some slightly WAVY spots. I guess the block needed to get machined but the guy just did a re-ring job. I knew a rebuild was needed but necessity is a mother so I slapped everything back together.

I was expecting 180-200 readings and I got 130 on the good pistons. I have 450 casting 305 heads. This goes against everything I've read on this site. Maybe I'm just missing something.

The car still hauls serious *** on the 7 good pistons.

 

[454C10]

Gage could be wrong. I have seen lots of low readings from compression gages. I have a few gages and one reads 20 or 30 psi lower than the other one. Could also be caused by a very retarded camshaft install. How does the engine run?

FYI, cam overlap doesn't effect cranking pressure. How does the point in the 4 stroke engine where both valves are open effect cranking pressure? The engine starts making cylinder pressure during the cycle where the exhaust is close and the intake valve just closes. This is way past the overlap period in the camshaft profile. So, it is the intake valve closing point that effects the cylinder pressure. That is why advancing a cam will rise cranking pressure due to an earlier closing intake valve. Or a long duration cam will lower cranking pressure due to a late closing point. It is called dynamic compression ratio.