KILRWAIL

I finally got to Road America after years of anticipation, but I came home with a blown engine. It was the annual TRAC event hosted by Chicago Region, where I met a lot of nice people who were very helpful and welcoming to this visiting Canadian. I had six good stints on track over the first two days until the trouble began.

My engine is a built 3.2L with PMO carb’s, based on a 1979 case, with 120/104 cams, MSD ignition and a 9.5:1 CR. Oil appeared on the fiberglas engine cover on the left side and I called my mechanic to discuss the possible reasons. The engine seemed to be running fine otherwise. Oil pressure was strong; temperature never exceeded 120 C; the Innovate AFR gauge which monitors both headers never exceeded 13.5. We concluded that it might be the low oil pressure switch that was leaking up the centre. So I made a diaper out of paper towels and duct tape and did another stint. There was much less oil this time, so I felt that we had correctly identified the cause. In my last session, I lost power dramatically going into Turn 5 and pulled off the track. The only smoke was coming from a small amount of oil dripping onto the left header.

After the 1500 km drive home with truck and trailer, I took the car to the mechanic for diagnosis. We got the engine to fire, but it ran really roughly. Here’s what we found:

The #1 spark plug had a broken ceramic core and the electrode was half burned off. The #2 plug looked normal. The #3 plug had a broken ceramic core. The plugs on the right side were fine.

Compression in #1 was 25 psi, in #2 we had 105 psi and in #3 it was zero.

The engine will be dismantled shortly to find the reasons.

This engine was rebuilt twice last year after melting pistons and breaking rings, which we attributed to a worn/dirty/defective CIS system that caused an excessive lean condition. Hence the switch to carburetors and the installation of the AFR gauge. When it failed this weekend, it had 35 hours of track use since going back into the car in April. I have never missed a shift or exceeded 6500 rpm.

Any ideas guys?

beentherebaby

Sounds like lean out causing detonation from what you report but it could actually be an ignition issue. It could be many problems including a fuel starvation issue under cornering/braking loads, fuel pump/line issues, etc.? The AFR gauge may or may not be very accurate but under WOT you definitely want to be in the 12.5:1 range.

Proper engine dyno testing may help determine what the real issue is and if it's a vehicle dynamics issue vs. pure engine issue?

ivangene

I spent about an hour watching an engine machinist (does the machine work on engines) diagnos an engine failure. To say the least it was amazing what "he" swa -vs- what I saw and he showed me everything and explained what and why...

in our case we had -30hp and worked for weeks to recover it, got close but out at the track she came apart thru connecting rod #4 and some colateral damage.

once apart he started explaining the tolerance stack ups and the clearances in full detail. With the eye of a forensic (spelling) detective, he pointed out there was no carbon ring on the top of 3 pistons indication that there wasnt enough space and they may be in contact or soo close. The connecting rod bearing also showed wear at the bottom indicating they were "giving" due to lack of clearance. Down to the crank the same points had simillar wear so it points to a case of not enough clearance.

then onto the pistons where he looked at the rings for a long time. He explained how the cross hatching (hone) could cause excessing wear. If too smooth the oil will not get trapped and the second ring runs too dry cauing heat, the piston grows 0.02" at top completely round, but only 0.005" on the skirt and not uniformly (known and designed that way) Excessive heat (over 500F) making the piston grow too much and have interferance was also happening and this he clamed he to do with the way it was honed, the mixture or pinging (ignition without spark) due to heat...

Lastly the oil, we talked at length about having too much oil as a problem as much as too little. This related to the crank "slapping" oil and making it have to work too hard....

so, very sorry about your motor.... I would get a real detective on it. Check all the parts for cause. Look at impact at the tops of pistons, wear on the rings, any rubbing, marring from expansion due to heat. A good set of eye will figure it out. Hope this was somewhat informational too you.
Best regards

whalebird

I would certainly tune the carbs. Lean sounds like the cause to me. I saw a similar situation in a 2.8 RSR motor with webers and twin ignition. The car was a real beast, but the owner was tired of street driving on aviation/race gas. he wanted it detuned for drivability. So he started playing with jets and timing. It was fine on the street - then he decided to do some DEs. it went for a year or so, but let go on the track. This motor was a supurbly built engine by Larry Sheldrik in Asheville NC, and had 10 years of continued usage in "proper" tune. I am convinced the detuning leaned it out and the sustained high-rpms on the track just killed it (this thing would pull to 8K). It was in an original 74 coupe chassis (a real sleeper) on stock 15" fuchs with 205/60s and charlie bars. I should have bought this car when I had the chance. Its sitting in a garage now with a rod hanging out of the case(leg out of bed).

whalebird

Great post Ed. I found it informational

johnsjmc

Plug damage suggests either detonation or inappropriate plug heat range. What plugs are you running? I also suspect a lean condition under sustained load . When one bank /carb is affected it suggests the fault will ultimately be found in the fuel system not the ign. Too much ign advance would cause detonation but spread the damage to both banks.

whalebird

A close eye on cam-timing upon reassembly can't hurt either.

whalebird

Lets not forget about throttle linkage.

ivangene

Thanks WB - I am telling you watching this guy was the highlight of my year... (welll that might be a stretch, but....)

For me the more I learn the more I realize I never knew anything to start with.... just awesome to be around people who really know what they are doing

race911

Consider the track. You're wide open at Road America for, what, 95% of your lap there? Not to mention it's pretty much all 4th and 5th gear? If there was a margin for error, that is a place that will suck it all out. I lost a piston up in Seattle, which then had a nearly one mile acceleration zone. That was a Euro compression CIS 3.0 using 92 octane on a warmish summer evening. Severely used race/track cars require the experience of guys who have seen it all under severe race/track conditions. I won't name names, but I think we know how small the pool of talent is.

KILRWAIL

Thanks for the input folks. A neighbour who is an experienced SCCA racer, aircraft mechanic and engine builder suggested measuring the case carefully to see whether it's ever been twisted, since this is the third failure in two years. He also mentioned what he called "over torquing", such as I experienced climbing the hill on the front straight at RA, whereby the rev's couldn't climb very quickly in third gear - resulting in excessive oil pressure in the case from the sustained high torque. It will be interesting to see how much damage has been done to the pistons and rings. More to follow...

whalebird

race911. Ineresting point. Have been both a streetcar and race car builder, I've always thought that streetcar work does not apply to trackcar development, BUT, trackcar work lends itself well to streetcar work - there is a ton of great streetcar guys out there, but...

Steve Weiner-Rennsport Systems

Peter,

JMHO,......

13.5 AFR and close, is FAR too lean for an air-cooled engine to survive. You gave little information about jetting, ignition timing and what gas you were using so its difficult to offer anything constructive beyond this.

Obviously, one needs to do the forensics to confirm this by VERY careful inspection of the combustion chambers, piston crowns, spark plugs (what left), piston pin bores, and rod bearings to confirm.

Don't waste your time on "case twisting" issues: even on the old 750 HP 935 engines, the case didn't twist, it mere fretted on its parting lines but I suspect your power level isn't close to that,...

whalebird

I love it when the smart guys chime in. As for the case, It may not apply, but a lot of drag/circle car guys won't build an engine on a new block, they much prefer a "seasoned" block.

KILRWAIL

Steve - Thanks for joining in. I use 91 octane pump gas; the jets are 145's, chosen to keep the AFR between 80% and 90% of lambda at 6000 rpm; the timing I believe was about 10 BTDC, going to 35 at WOT; the MSD has a limiter at 7000 rpm which I did not achieve. The jetting was reduced from 160's as received from PMO, in increments of 5, to reach the target AFR which was recommended by both the builder and an experienced Porsche racer. Until this weekend the AFR would not exceed 13.2 and I'm surprised that you say an air-cooled engine can't tolerate that. What would you suggest? I'm just the dumb guy writing the cheques, so I have to rely on others for this kind of advice.