Compression & Leak Down Test Results

[Andy804]

I have the following numbers for my 1998 2.5L Boxster that is for all intents and purposes a spec Boxster race car (no engine modifications). Would appreciate any insights as to the meaning of these numbers. I have a suspicion that the engine is not exactly healthy. But would it affect how well the car accelerates? Car was dyno'd with 2 runs at 178.74 and a run at 180.58 but with stock exhaust so horsepower numbers might actually be within range of other spec Boxsters if it had a full race exhaust set up.

Compression Test in psi
1. 135
2. 130
3. 145
4. 145
5. 140
6. 130

Leak Down Test
1. 8% past rings
2. 9% past rings and intake
3. 5% past rings
4. 5% past rings
5. 5% past rings
6. 9% past rings and intake

[JFP in PA]

Originally Posted by Andy804

I have the following numbers for my 1998 2.5L Boxster that is for all intents and purposes a spec Boxster race car (no engine modifications). Would appreciate any insights as to the meaning of these numbers. I have a suspicion that the engine is not exactly healthy. But would it affect how well the car accelerates? Car was dyno'd with 2 runs at 178.74 and a run at 180.58 but with stock exhaust so horsepower numbers might actually be within range of other spec Boxsters if it had a full race exhaust set up.

Compression Test in psi
1. 135
2. 130
3. 145
4. 145
5. 140
6. 130

Leak Down Test
1. 8% past rings
2. 9% past rings and intake
3. 5% past rings
4. 5% past rings
5. 5% past rings
6. 9% past rings and intake

Several of the leak down numbers are approaching the upper limit, suggesting that you may need to plan for a rebuild.

[thstone]

I'd call this a somewhat "tired engine". Nothing is hugely out of whack, but the compression should be somewhat higher (more like 160) and the leak down numbers somewhat lower (more like 5%). And yes. this could affect acceleration.

Most Spec Boxster's dyno in the low-to-mid 190's at the rear wheels. The stock exhaust might account for 5-7 hp. Do you have the under drive pulley installed? If not, then that might be another 3-4hp especially if you still have a/c. Do you still have the cat's? There is probably another 2-3 hp there.

If we use the upper range of those numbers: 7+4+3 = 14. 180+14 = 194. Which is about right. If we use the lower range, the you might be down a couple of hp compared to a stout SPB engine..

I also sent you this info via FB messenger.

Here are the compression and leak down numbers for my last engine just before it failed. It was clear that cyl's 4 and 5 were not happy. The engine still made decent power but the engine felt "off", like it wasn't its old self.

[Andy804]

Thank you for the explanation, Tom. A/C is out and under-drive pulley was supposedly installed per the build sheet when I bought the car as a BSX. In your opinion, would having the lighter flywheel provide a noticeable improvement in acceleration as well?

My suspicions about my motor came back recently because I was looking at a partial lap video of a beginner driver in a spec Boxster at Willow Springs this past weekend. The driver was completely lifting going into turn 8 and feathering the throttle through it, braked earlier into turn 9, then turned in a lap time not much different than mine. If I had done the same thing, I certainly would have been at least two to three seconds slower if not more if you extrapolate that over the entire lap. Then I thought about a few instances when I would get a run on another Boxster ahead of me coming out of turn 9 but it didn't matter once we got going down the straight.