Missed shift, compression/leakdown results OK for track use?

[logician]

A recent missed shift (5->2) in my 2.7L race car got me worried about the possibility of a bent valve, and prompted me to measure compression, and then leakdown. The engine burns 91 octane, has 40 Webers, 9.5:1 compression, and mod-S cams -- modest mods. Here are my results:

1: 160psi 4%
2: 160psi 3%
3: 160psi 3%
4: 160psi 2%
5: 160psi 3%
6: 145psi 8%

Obviously #6 is off. Actually, I had not previously measured compression or leakdown, so I can't be sure whether this is a result of my missed shift, or whether it's been that way since I bought the car. My question is how bad is this? Can I continue to run it as is? Or do I risk damaging the engine, i.e. burning the valve or head? My usage is track, so mostly WOT. What is generally considered good/acceptable leakdown compression and leakdown? Thanks in advance for your help.

-Juan

[Steve@Rennsport]

Juan,

Can I assume that the observed 8% leakage in #6 came from the valves (I or E)?

Without more info, I'd not hazard a guess but offhand I think you are OK.

[logician]

Steve,

Thanks for the reply. Not absolutely sure about whether the leak is through intake, exhaust, or rings. If I had to say, I would say it was intake, as I could hear the sound get louder as I opened up the throttle while listening at the venturi. But not sure if that just wasn't the air sound reverberating through the whole head.

I take it a small intake valve leak would not cause further damage, just lose power?

-Juan

[Steve@Rennsport]

Originally Posted by logician

I take it a small intake valve leak would not cause further damage, just lose power?

-Juan

Yessir, thats true. Amazingly, sometimes the power loss is very small.

[Eagledriver]

A leaky intake valve should not cause a problem. A leaking exhaust valve will cause the valve to burn. This will happen slowly over time and will be evident with compression/leakdown numbers slowly deteriorating. Just keep an eye on it.

-Andy

[Walt Fricke]

Juan

Had you not had the missed shift, I suspect you and most others would not be overly concerned with these numbers. But you did, and I'd worry also. Sometimes a bent valve will sort of true itself with use. I had exhausts which did that. The stem was noticeably S shaped after a further year of street driving post missed shift. That damaged the guides, which sloted to accommodate the bend and whatnot. In addition to the possibility of burning a seat ("just" burning a valve is not, by itself, so bad long term, as bent valves have to be replaced, but that's simpler than replacing a seat), the worst disaster is having the head of the valve break off. That blows the motor up. Since yours is a race motor, I think more worry is in order, even if a slight tweak leading to a small leak may not have compromised the valve that much.

So I'd spend some more time listening to the air leaking. You can skip the gauge parts, since you have your measurements, and now just need air under pressure. Maybe alternate between #4 and #6, the better to differentiate between one which looks like the best, and the worrisome one. You need to calibrate your ear, so to speak. Do you have a long small hose you can put in your ear and up the tailpipe as well as down the intake past the butterfly, and into the case?

My experience with low leakages has been that they are past the rings. Even with a nominal 0 percent leakage (Total Seal rings) I always hear some air sound in the case. Which doesn't mean that is your situation, of course.

The exhausts seem the most likely to strike a piston (and get bent) when missing a shift. Do your heads have screw plugged air injector holes in the exhaust ports? That might make a good listening location if the plugs aren't hard to screw out.

If taking another listen still leaves you uncertain, with some extra effort you could pull your right side exhaust header, pressurize the cylinder, and spray soapy water up into the exhaust port and look for bubbles. Again, probably a good idea to do it to more than just your problem child so you have a comparison. When I have bent an exhaust I've known it without having to go that far, but it should work.

Walt

[logician]

Andy, Thanks for the comments. You guys are getting really fast these days! I think I'd have to pedal really really hard to keep up. For now I've decided to get my car running again, but keep the 2.7L and tranny. Not really optimal, or technically legal, for a spec car. I might make a cameo appearance this season if I get the car sorted out enough.

Walt, Thanks for the ideas. I like the idea of looking right in the ports. Actually, I may be taking the carbs off for other reasons, which would give the opportunity to examine that intake valve, maybe using the soapy water suggestion.

-Juan

[cstreit]

I would occasionally us ethe rubber glove test when doing this sort of exploration on an engine. Place a rubber glove over the intake, oil-breather, and exhaust while doing the leakdown. Usually you can pinpoint the location as that glove will inflate bigger/faster.

[MBEngineering]

HI Juan
I would not worry to much about the compression No', the leak down is more important and checking the rocker gear for bent adjusters and there is always the stretched rod bolts ???? and the nuts backing off, have you ARP rod bolts fitted?, if not I would be pulling it apart and replacing the big end bolts and bearings, before this happens;





regards mike

[MrPerkles]

ill bring my super glue and fix that for you saturday mike

[logician]

Good news! I rechecked the leakdown today and it seems the previous poor result was due to contamination in the valves. Cylinder 4 read 2%, same as last time, and 6 read 3%, vs 8% last time. So the engine is just fine. Whew!

I also have another interesting data point to contribute. These numbers were measured with an aircraft style leakdown tester:

Eastern Technology Corporation

I like these aircraft testers because unlike the automotive ones, their calibration is specified -- the measurements are relative to a .040" orifice at 80psi.

So today I measured these two cylinders with a Snap-on tester after measuring with my aircraft gauge. The Snap-on read 3x the leakage percentage of the aircraft gauge, so it indicated 6% and 8%. Nothing wrong with either reading, just different calibration.

To my knowledge there is no standard calibration for automotive leakdown gauges, so different gauges will read different percentages for the same amount of leakage. However, the 3x factor that I measured today gives one data point for calibration of the Snap-on gauge.

-Juan

[Walt Fricke]

Good indeed.

Rechecking, especially after doing something like running the engine a little, is one of the standard procedures if one hole is off. The suggestion is that a bit of carbon might have gotten on the seat, preventing full closure.

I've heard, on this very forum, that there is no standard calibration for these testers. Were I less lazy, I might take my low buck tester apart and measure the orfice, just to see.