This engine was recently rebuilt w/ new apex seals, springs, etc. and had < 4 race hours on it after break-in when it lost compression in the rear. The previous week it was on a dyno and was showing a wavy line at peak rpm. During the qualifying session I could hear it starting to grunt at that point. 5 laps into the race it blew.

Now, looking through the exhaust ports, all apex seals are intact. So, what could cause loss of compression aside from broken seals? I'd like to be able to suggest to the builder that there was a problem w/ assembly. Do I have a case?

 

Side seals?

I beleive if you're missing an apex seal, you'll get 1/3 pulses (since one apex seal affects two chambers), but if you have a broken side seal, it'll have 2/3 pulses (since the side seals are only for one chamber).

Or something like that.

 

We're all shooting in the dark trying guess what's wrong.

Tear the engine down, and you'll get your answer.

It sounds like bad assembly to me.
If it wasn't for the rear rotor with no compression, I would've put money the front thrust bearing slipped.


-Ted

 

No compression with the apex seals intact would be strange, it is more likely that one of them cracked and a small chunk fell out causing damage to the rotor housing. You can't see the complete apex seal through the exhaust. Can you see any gouging on the rotor tips or feel any gouges around the inside of the exhaust port?

 

I'm betting wrecked side housing due to debris or fragged side seal.

A hosed apex seal usually takes out the entire rotor from FOD wrecking the rotor housing and/or the chunks peening the following apex seal into its slot. But, this would be easily visible through a spark plug hole or exhaust port, whereas side housing damage would not.

 

There is virtually 0 compression in 2 chambers so I believe that suggests an apex seal not a side seal or side housing. I don't think the engine is grenaded - it just has no compression. I am told that an apex seal can stick in its groove or can be so loose as to skip across the surface. I was thinking that the wavy dyno line and the sound I was hearing at peak rpm was the apex seal losing contact w/ the housing surface. I know I will find out what is wrong when we open it up but I don't want to have this builder even get started w/ it if he recognizes no scenario in which he is responsible.

Is there anyone here who has built an engine? Is there not some mistake that can be made that might result in loss of compression w/ good apex seals? Improper clearance?

 

Intermediate housing?

 

Well, I went back to make a better inspection of the seals w/ a good magnifying mirror and it does look like that one of the seals picked up some metal and is jammed. It actually looks like it is soldered. Wonder where the metal came from.

 

the other apex seal that blew.

they chip at the corners a lot. you can't see it from the exhaust port.