GT40 Compression

[bojans]

Our Nautique is now at over 1,700 hours, I figure it is time to do a compression test. Nowhere that I have seen suggests the appropriate compression PSI. Does anyone have some insight?

[SocalWaterSki]

I look for consistency rather than the actual numbers. It's been pretty well accepted that gauges are not real accurate. My Snap On reads 14-15lbs higher than a Matco of similar design.

[bojans]

That makes sense and I have heard that before. So how do you know when it is time for a rebuild? I would want to catch any issue before it resulted in catastrophic failure.

[E_T]

@bojans if one of they cylinders is drastically lower than the others then you know you are getting blow by on the rings or other issues. If all are with in 1-3psi of each other you should be ok,

[Bruce_Butterfield]

I checked my GT-40 with about 1700 hrs and compression was between 150-160psi. Socal is right, the variation is more important than the actual reading. Any variation is usually an early indication of something going wrong in one of the cylinders.

 

The first common warning sign is oil consumption. If you are keeping your oil change intervals regular with good quality oil, and aren't seeing any measurable loss, you should be good to go for another few thousand hours. The only real thing to watch is to use an oil with a high Zinc-phosphate content due to the flat tappet valves. I forget all the details, but remember Valvoline VR-1 racing oil was one of the better ones.

[SocalWaterSki]

I should probably mention... my engine is literally just getting put back in the boat tomorrow. It's been out for the last week getting rebuilt. When I pulled it, the numbers were 152-155-158-149-152-155-148-156... So, the numbers looked good in my eyes.... especially with almost 1800 hours. I pulled it because I'm having new interior put in, I'm changing the exterior, and I had the means to have it done at the same time... I'll have photos up of the interior and exterior if anyone wants to see them

[BraceMaker]

There's also value in the leakdown test, but I've never bothered on anything that wasn't losing oil, gaining water, or had an odd compression test on one hole.

[Siouxcitysmitty]

When I checked mine at 1300 hours, all were in the 155 to 162 range. The range of 1-3 psi recommended above seems a little tight to me. Maybe 5-10 from lowest to highest. You'll also get lower readings if you're at higher elevations.

[RichardDoane]

@bojans - if it still runs strong, isn't smoking or consuming oil, and doesn't have any leakage issues, put that baby back in and run it another 1700 hours, you'll know when it needs an overhaul by loss of power/lack of compression/etc.

[A_B]

Everything I see written is not over 10% variance between highest and lowest reading, which looks like you are well within. I would worry more about ECM failure or fuel relays with the GT. Those were a bugaboo for me.

 

Good video here with information:

 

 

 

[uflbret]

AB is right on. 10% is the magic number for variation. If you get into numbers over that you probably need to move on to leakdown test. Compression testing kits are cheap and its easy to do. Sure helps to have a helper work the ignition while you are running the tool. Make sure to pull all the plugs before starting test.

[DW]

To get a good snapshot of ring and valve seat condition, I would do a leakdown test along with a compression test. You can get much better insight on component condition via the leak down test and determine exactly where you are loosing compression. When doing engine health diagnosis, the tool of choice for me is the leak down tester.

 

The feedback on compression #'s above are spot on. Compression ratio's in the 9:1 range result in ~ 170# of compression pressure. It is very easy to get a lot of variability in a leak down test so the raw number is not as critical as the consistency. Engine temperature, oil temperature, oil viscosity, number of pulses, gauge / gauge type variability all provide variation in readings.