To Re-Ring Or Not To Re-Ring.... That is the question!!
 

Hi all,

I am going to be rebuilding my '88 944 N/A. It's had 90k on the motor plus 6 years of racing. I was just taking some initial measurements to see what I am up against before pulling the motor and here is what I got:

Leakdown Test:

Cyl #1 - 4%
Cyl #2 - 2%
Cyl #3 - 5%
Cyl #4 - 5%

Compression Test:

Cyl #1 - 170-175 psi
Cyl #2 - 170-175 psi
Cyl #3 - 165-170 psi
Cyl #4 - 175-180 psi


So with this information would you re-ring but not hone the cylinder walls? Or just leave the rings in and do the rest (head, rods, mains, etc)?

TIA

IMO no need for a hone but I would lap the bores with an-30 and fit new rings. If you don't lap the bores the new rings will have a hard time bedding-in. Also check the condition of your piston skirts, if the iron coating is worn anywhere it's a good opportunity to Swain coat them. Finally, dont forget to gap your rings. You'll know a lot more when you have everything apart and measured.

If that's all you got out of the leakdown, you need to re-do it.

When you have each cylinder hooked up, you need to listen and see where the air is escaping; this will tell you what components are to blame for the lack of sealing. Rings will result in the hiss of escaping air audible in the crankcase - listen at the oil fill. Intake or exhaust valves will, naturally, result in air hissing audibly in the intake or exhaust (ports, if the engine's already out of the car, or at the air filter and tailpipe if still installed).

It is very possible that your leakage is only in the valves, meaning you need only a head job and re-lap the valves. Or, alternatively, maybe just refreshing the head/valves would be enough to, say, get you down to around 2% leakage... though you may be at the point where you are OK with putting fresh rings in anyway.

You must be pleased with the leakdown test,sounds like you have a strong motor thats been well looked after.
I wouldn't change the rings ,just keep it as a top overhaul and do your rod bearings,front /rear and cooler seals.

+1 on leave it alone.

With the compression and leakdown numbers you have, that's great. It's a strong, healthy engine that's not in need of anything at all. 90K miles is nothing at all. In 944 terms, that's nearly new.

I can't image how rings or head work would bring any appreciable improvement over what you have now. You are sure the leakdown numbers are correct?

Unless you've seen/heard of some problem from the bottom end, I wouldn't bother doing the rod bearings.

Save your money and spend it on suspension or maybe rubber.

On a racecar - yes, I've rebuilt that early (around 5% leakdown). Racing is different.

But re-check the leakdown first, you may only need the head refresh.

It is definitely time for a rebuild as I am getting metal material on the drain plug. It is a motor with 90k plus an additional 6 years of racing on it.

As for the leakdown procedure

1. I warned up engine
2. Pulled plugs
3. Removed coil wire
4. Removed fuel pump fuse
5. Screwed in air hose from cylinder to Longacre leak down tester.
6. Turned motor to TDC
7. Hooked the air line to the tester.
8. Hooked up air compressor line to tester
9. Used the regulator on the tester to increase pressure to 100 psi
10. Read the gauge

I repeated this on each cylinder. Any hissing was from the intake and exhaust manifolds.


Thanks for your thoughts gentlemen! Keep em coming!!

OK - so you'll need a rebuild due to metal, valve job due to leakdown results - seems like rings is a no-brainer then, at this point. Whole fresh motor.