Thread: Did a compression test today...

I've been noticing some boost issues lately with my setup. Seems like I can swap anywhere from a 2.7" to a 3.0" and still only get 10-11 psi of boost. I can't get higher than that for whatever reason. I did the brake cleaner trick, can't find vac leaks, bypassed the solonoid still can't find anything wrong. Idles at 19-20 in vac which is normal. My only thought is maybe my 170,000 mile L67 is tired. I did a compression test today, no leak down though. Wasn't quite sure how to do that. Here are my results starting from the back head to the front...


#2 = 155 psi

#4 = 155 psi

#6 = 157-158 psi

#1 = 146-147 psi

#3 = 149-148 psi

#5 = 158-160 psi


I believe the spec is within 10 psi right? If so, #1 is on the edge but I don't think thats my problem either. 1 and 3 are the lowest and 5 is the highest all on the same head too. Figured i'd show this and maybe get some input. I'm no expert but I don't think I see a huge issue here, unless i'm missing something. Thoughts?

Shawn...talked to Brian...he said it might have something to do with the BBV if you can't get anymore boost.

Looks good to me on the compression test. I'd keep beating on it like a hillbilly on his wife.

Compression looks fine.

I looked at yer mods list, and noticed Stock MAF & Screen ?? I'm not speaking from experience, and mostly out of my arse, but wouldn't that limit air flow to a certain cu ft/rate ???

Pretty sure his TB is hogged out...and a MAF would cause rough starting/idling

Originally Posted by bluegtp91

Shawn...talked to Brian...he said it might have something to do with the BBV if you can't get anymore boost.

What actully fails on the BBV? Just curous. A few months ago I did a vac test on the BBV out of the car and it held fine. Anything else I need to check?

BTW, I have a spare BBV some where, gotta find it I guess.

Originally Posted by Street Wolf

Glad you didn't find your problem Shawn.


Seriously though, that being done already ruled out the worst case scenario, and obviously no belt slip, sooo.... BBV can't hurt. I'm sure we have one laying around if needed.

Appreciate the offer, I have one somewhere myself. Though I think thats not the issue either.

For the leak down just get the cylinder to tdc then fill it with air. bubbles in the rad = head gasket, coming out the intake = intake valve same for the exhaust, coming out of the oil filler= leaky rings.

Originally Posted by Gregorius

Compression looks fine.

I looked at yer mods list, and noticed Stock MAF & Screen ?? I'm not speaking from experience, and mostly out of my arse, but wouldn't that limit air flow to a certain cu ft/rate ???

Nick is right, I have an old ZZP HVTB Stage 2 TB which is 72mm throughout with a matching 72mm plate. It was fixed up by GBP too sometime ago.

As for the stock MAF, i've had no reason to replace it, I was never maxing it out last I checked. As for the screen, that helps the MAF read better and should be left in all MAFs IMO.

and for the compression test the spec is no less than 100 psi which IMO is way to low. All the cylinder should be within 20% of each other IIRC.

Originally Posted by 01GTP

For the leak down just get the cylinder to tdc then fill it with air. bubbles in the rad = head gasket, coming out the intake = intake valve same for the exhaust, coming out of the oil filler= leaky rings.

Thanks for the info, i'll have to rent the proper compression tool to do that, mine has no air chuck. And I have to get my lazy ass to do it too, lol

I honestly have no idea what fails man...hence why I called Brian LMAO!! I would think it just kinda rots out over time just like the "T" fittings...but I really don't know.

And for the compression...it is a rule of thumb that each cylinder needs to be within 10psi-20psi...20psi being on the low end of the spectrum IMO.

Originally Posted by Iron Indian

Thanks for the info, i'll have to rent the proper compression tool to do that, mine has no air chuck. And I have to get my lazy ass to do it too, lol

Got an old spark plug? Smash out the porcelain and put an air fitting where the porcelain used to be, if you want to be cheap. OR if you get a leakage test it can tell u how much how fast. IN your case with the good compression numbers I do not think you will find much, but it is worth a shot.

Originally Posted by Iron Indian

Nick is right, I have an old ZZP HVTB Stage 2 TB which is 72mm throughout with a matching 72mm plate. It was fixed up by GBP too sometime ago.

As for the stock MAF, i've had no reason to replace it, I was never maxing it out last I checked. As for the screen, that helps the MAF read better and should be left in all MAFs IMO.

Ok okokok. My first thinking was just more spins with out more pressure makes me think of a restriction up-stream. Or a bad bypass. I think simple first. And I'll do it again.... Collapsing Intake hose or filter??

It's a diaphragm that uses the pressure difference between the top half and bottom half to regulate itself. It's spring loaded so that under WOT (atmospheric) it closes the valve and you get boost. You should be able to see if it's fully closing or not by revving it at the TB. The engine's vacuum is what pulls it open, applying tension to the diaphragm. If the diaphragm is wearing out it might not go back to the fully open position when no vacuum is applied to the top side.

Shawn i think what you are seeing may be normal.

When i had all my mods most i ever saw unless it was freezing out was 12psi

Originally Posted by Toofastgs

Shawn i think what you are seeing may be normal.

When i had all my mods most i ever saw unless it was freezing out was 12psi

I was thinking this at first but in reality it can't be it. You old setup was headed, cammed, Iced... the whole 9 yards. I'm nowhere near as modded as you were.

I've got a stock long block, stock heads, etc. I'm only running 1.9 Rockers. I do have TOG Headers and 3" exh. I'm Intercooled and i'm playing with Alky too. I still have alot of restriction left in the motor, such as the stock heads, cam, etc. I should be able to boost stack it easy enough I think.

Thanks for the suggest, i'm just not at your mod level yet. If I was i wouldn't think anything of it. You had a GOOD FLOWING motor....

Thanks for the info so far guys, anyone else?

Is all the coating on the rotors still?