Hello, longtime stalker, first time joiner. I have a 87 GT with a 393w and a 76mm(.96 exh) on3 turbo kit. I made a trip to the dyno yesterday and couldnt reach over 5psi. I can pull 10psi on the street. Anyone ever experience this? My google results were not that great so I figured I would ask. All opinions welcome! #ad


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Exhaust leaks, wastage issues. That 393 should light that 76 instantly off idle almost.

 

It does spool fairly quick even on the dyno, the boost just isn't there like on the street. Waste gate isn't opening on the dyno so I'm thinking the dyno needs to be loaded to pull street boost levels. That make sense?

 

Not really, I think you have a large exhaust or intake leak. Only reason you see boost on the street is the load overcomes the leak.

 

I feel a leak that large would be noticeable tho. The cold side should make a vacuum leak and screw up my afrs, and the exhaust side should make a bit of noise I would think. Not saying ur theory isn't plausible, I just think I would have found that before now. Where could a leak that large not be seen or heard?

 

BOV, header gaskets, a split seam on a pipe. It probably only happens under high pressure, so you wouldn't really hear it at WOT.

If your wastage never opened on the dyno, it only enforces my thoughts.

Pull your plugs, make sure you don't have any dead cylinders either.

 

Look at your couplers too. Had a friend last year that had a split that wasn't visible without taking it apart, he was losing boost as well.

 

Will do. I don't suspect it is a leak issue, but I would like to rule it out.

 

Here's a better question, what size is your wastage, what boost controller do you have. What's the Engine setup.

That 76 is very small for a 393, it should be able to make boost in its sleep.

 

It's the 44mm gate from on3. 50mm bov. Mechanical pressure regulator style boost controller. Engine is a 8.5:1 393 full roller (retro lifters, rockers, cam). afr225s, edelbrock victor efi upper and lower intakes. Cam is comp cca357758.

 

I'd really check that gate, the ON3s are known to have shaft seizing issues. You just pop them apart and use high temp anti-seize on the shaft.

 

If the dyno doesnt load you may have issues making boost

Happens to my car. My gate spring only is down 2-3 lbs on average. I have to turn my co2 controller settings alot higher to maintain boost on dyno vs street

But dont rule out boost leaks or exhaust leaks

 

and pull the screen off the turbo inlet...

 

Very interesting. I have heard of the boost with dyno's with no load or very little load....

 

Im going to take the wategate off this week and put a different spring in to check for any change, and check for a sticking shaft. Although, if it were sticking open causing an exhaust/boost leak at any point, I feel like I would have heard the open exhaust coming from the front of the car. The exhaust system is fairly quite and I can hear the engine well, especially in the dyno room (which is where I cant reach max boost). The turbo guard in front of the turbo isn't affecting anything so its gonna stay.