Hi guys,

This is my first post, but I've been reading from Uruguay this great forum for the last 3 years, so first of all, CONGRATS!!!

I have a 1991 Corvette withe a Supercharged ZZ$ 350 engine on it. It has various bells and whistles such as custom exhaust, MSD ignition, bigger throttle body, air filter, etc, etc.

The car was sold to me 1 year ago with a claimed 390 RWHP, but it doesn't come even close. I had my mechanic meassure the compression of all 8 cylinder and it appears one of them is really low. Basically 7 cyclinders have an average of 100 lb/ft and one has 40 lbft!!!

Do I definetely need to rebuild the engine? If I do so with new forged pistons, rings, etc, do you think I will notice a dramatic change in performance?

Many thanks for all your comments!!

 

Sorry to hear of your problem, it may be one of the following.
Bent valve, damaged valve seat, broken piston rings cracked piston or head gasket. I would start by removing the rocker cover check you can turn the push rods, you should be able to rotate them.. turn the engine and watch the rocker is up. It could be a simple overtightened rocker nut, however a supercharged engine can self destruct if it is not set up properly. Best to start with removing the head and if the head gasket is ok and valves seal and are not bent, the piston or rings may be damaged.
As in performance i know there was a dramatic increase in power everywhere when i put the blower on my vette.
You will need to find the cause of your problem or your rebuilt engine may suffer the same fate....

 

Thks for the comments gerard!! I will follow you steps. The problem is not on the valves as i've checked them already..

Just to correct the compression of the cylinders, I have 7 of them with an average of 135 psi and 1 with 90 psi. Is this sooo bad?? How much HP can it be detracting?

Thanks!!

 

Most likely will be rings or the ring landings on your piston,
that will ruin your bores if not fixed asap.
My vette hates standard 91 octane and will ping (knock) on that.
I run 98 octane fuel in my vette and have no problems, has been supercharged for many years with the standard engine.
(We have 91, 95 and 98 octane fuel here in Australia)
Now the fuel prices are around $1.60 per litre so costs over a hundred dollars to fill the vette. The premium fuel being 14 cents more per litre. so 60x 1.74 = $104.00

I recommend you get the best fuel you can if you are using an intercooler, you will be able to run around 6 to 8 psi boost on a standard engine. Any more boost and forged pistons will be needed,
If it is not intercooled you should already have the ignition retarded, and use a water injection system to control combustion temperatures.
Any pinging knocking is the pistons slapping the cylinder bores as they tilt side to side, the rings will be struggling to maintain contact with the cylinder walls and that is how they break rings and landings or the piston skirts etc. With a blown engine you really need to pay attention to any noises, i have had bad fuel at times and the engine will protest violently.

 

Remember when comparing octane #'s the US uses the R+M/2 method. I believe Australia uses the R or Research method (which we used back in the 60's). Sunoco 104 back in the day was the R method and the 93 or 94 we have today (R+M/2) on the East Coast is almost as good.

That being said you have a cracked piston(s). A simple leak down test will show leakage to the crankcase: I bet !!! Tear it down and fix.

BR greg

 

I agree with Greg.
Detonation from a bad tune or bad gas plus cast pistons = broken ring lands.
The engine might still run like this, but not well.
Leak down test would tell for sure.
And unfortunately, the broken piece of piston usually damages the head also

 

I disagree with the post claiming you can only run 6 to 8 psi before you need forged pistons.

If you need to rebuild it, by all means, go forged. However if its a perfectly fine bottom end, you can get away w/ 10 psi With modern meth/water inj systems and failsafe cutout.

I'm talking about a centrifugal blower.

 

Hi, thanks you for all the comments. What is the leak down test? Is it performed by injecting compressed air into the cylinder and see from where does it leak? or is it done with fluids?

Sorry for not understanding some english terminology, but I am from South America!!

thanksss

 

Compressed air in the cylinders. Valves must be closed. I air comes out the dipstick tube or valve cover vent, you have compression getting past the rings.

 

thanks for the clarification jack!!

 

Actually a pretty good explanation of a leak down test on Wiki.

Better than I could explain it.


http://en.wikipedia.org/wiki/Leak-down_tester

 

Hi guys,

My mechanic adjusted the valve and the compression of the "broken cylinder went back up to 125, very close to the others at 135.. Is this ok? I read elsewhere that the compression should be closer to 180-200 on a zz4 350. Is this so?

Also i dyno the car and was dissapointed. 262 rwhp... Pls take a look at the results in the below youtube video. Does anyone know why does the torque fall and then goes up again?

The procharger boost is only 4 psi. Isnt that too low? What could be happening?

http://www.youtube.com/watch?v=Qpsdjd1-6kU

Thanks