3 Broken Exhaust Rockers on 1-2-3 Bank

[kucharskimb]

At the AX on Saturday, I suddenly lost power and started hearing a gah-gah-gah sound out of the exhaust. I coasted the rest of the way and shut it down out of the way. I troubleshot plug wires, carbs and the intake valves on the 1-2-3 side, but everything seemed fine.

This evening I was able to drain the oil and look at the exhaust valves. As soon as i pulled the valve cover, on of the rocker tappets fell out. Okay, fine. I had suspected something like that. But, wait!! All three were broken off.





The motor still turns over. And it actually was running on three cylinders before shutting it down.

I had just completed a valve job and had double checked all backlash gaps after tightening everything down. I used the .004" feeler tool. And checked all valves a third time on a second complete revolution of the motor before putting the covers back on. I guess what I'm saying is that I am confident that the valves are/were adjusted correctly.

I can't find anything in the sump/screen or on any of the drain plugs.

1. What could have caused this?
2. Could I have damaged rings or valve seats? --I'm guessing no, since the air would have been stagnant.
3. After re-checking all valves/rockers, would it be safe to simply replace the 3 broken ones, or should I replace all 12?

Thank you for your time.
Matt

[gtihop]

If I were to hazard a guess, I'd check the valve timing on that bank.

[john walker's workshop]

valves hit pistons and the rockers are sacrificial. trouble is, not sacrificial enough. bent valves most likely.

[Steve@Rennsport]

Originally posted by john walker's workshop
valves hit pistons and the rockers are sacrificial. trouble is, not sacrificial enough. bent valves most likely.

Mister Walker is spot-on,.....

You overreved the motor and when the valves floated, the pistons hit the exhaust valves and thats what broke the rockers, just like whats supposed to happen. There are tell-tale marks left on the tips of the valve stems that will confirm that,...

I'd guess that your valve springs might be tired and do not have the same seat pressure as when they were new.

Without a recording tach and a tell-tale (such as one of those nice Autometer ones), you have no idea what your true peak revs were. Those are strongly recommended for cars like yours.

Do a leakdown test to see if your valves are bent.

[kucharskimb]

i'll check the timing and see where things are at.

should i attempt a compression check or leakdown check for each cylinder, as well? would this verify bent valves or just verify good seats?

i remember setting the intake valves at 118degrees after TDC for my E cams. This was splitting the difference between 2.2 and 2.4 E settings. I'll check the cam timing to see if anything has moved.

could this have been cause by valve float? i'd say i was running at 6-6500rpms intermittently during the last AX for durations as long as 3 seconds. i know the E motors were rev limited to 7100, so i never gave it much thought earlier.

[kucharskimb]

ah... steve beat me to the punch.

i'll get more info tomorrow and report back. thanks john and steve.

[cstreit]

As mentioned I'm pretty certain you'll find a series of bent exhaust valves. It's unfortunate but not catostrophic. Be sure to have your valve-spring pressures checked and definitely consider updating to a higher pressure spring to help avoid this in the future!

Best of luck!

[Eagledriver]

Other possibility is a skipped tooth on the chain on that side. Tensioners good? If the timing retarded (skipped tooth) the exhaust valves will not close in time for the piston to arrive at TDC.

A leakdown will confirm if the valves are bent. Bent valves don't seal very well

Sorry about your problem.

-Andy

[2.7RACER]

Hi Matt,
What is this?...............
"I remember setting the intake valves at 118 degrees after TDC for my E cams. This was splitting the difference between 2.2 and 2.4 E settings. I'll check the cam timing to see if anything has moved."

The 2.2 '70/'71 T motor intakes open at 15 degrees BTDC and close at 29 degrees ABDC.
The 2.4 '72/'73 T motor intakes open at 16 degrees BTDC and close at 30 degrees ABDC for US 911's.
Where does the 118 degrees after TDC come from?

[kucharskimb]

doug
i know i have a T car, but i'm running E cams.

117 degrees is the centerline of the intake lobe. I should have written 97 degrees after TDC, plus one degree to split the difference between the 71/72 settings. 118 ATDC was incorrect. thanks for the catch.

For the E cam, the numbers are 20 BTDC and 34 ABDC and 18/36 for each respective year.

TCD to BDC is 180 of crank rotation.

20 + 180 + 34= 234 / 2 = 117 for centerline

so after TDC, the intake lobe reaches centerline 97 degrees later, and my cam is retarded one degree further to 98.

in the meantime, i'm still trying to wrestle up a leak down tester. i know the 1-2-3 side is probably bent. but i guess i need to check the 4-5-6 side to see how far i have to tear down.

yes, i plan to replace all springs.

if the leakdown is good for the second side, should it still be torn down? should i simply plan to do a case up rebuild (top end)?

[2.7RACER]

Good morning Matt,
Very clear explanation of what you are doing.
This does assume the cam lobes are symetrical.
I'm still trying to understand, if the cams were timed properly, why the pistons hit the exhaust valves rather than the intakes.
At crossover the intakes extend further into the chamber than do the exhaust valves.
Unless the clearance for the intakes cut into the pistons is greater, which would make sense.

[Eagledriver]

If the exhaust valves float they will hit the pistons first. The intake valves are on the down stroke at TDC so they are firmly pushed against the cam lobes. The exhausts are trying to close as fast as the springs will push them. If they aren't pushed fast enough they will float and still be open too much when the piston arrives on it's way to TDC.

Having said that the intake valves often hit during an over rev as well. I haven't figured out how that can happen.

-Andy

[2.7RACER]

Hi Andy,
I agree with what you are saying re: exhaust valve float hitting piston and bending ex valves.
In this case the exhaust valve rockers broke, indicating the exhaust valves were in contact with the rockers at the stem end and in contact with the piston at the valve head end, at the same time.
Once this engine is torn down we should get some answers.

[kucharskimb]

i picked up a leak down tester today, but ran out of time to use it. i'll get numbers tomorrow.

as for the valve springs, i'm not well versed on porsche parts. are there different ratings for the same size springs, or do i have to go up to a, let's say, 3.0 spring?

thanks for the assistance.

[camgrinder]

Matt,

Did you check your exhaust centerline? The numbers you quoted look like Solex cam numbers. Im guessing with an intake centerline of 98, your exhaust will measure 106. With a lobe seperation angle of 102 degrees.

The little Porsche book lists duration numbers that look like solex cams for the later E. I have never seen a set of these E cams and I have been grinding Porsche cams for 16+ years. I think this spec in the little book is wrong. Every MFI E cam I receive in I profile, just in case a set shows up.

[Eagledriver]

I believe all factory valve springs are the same. The difference in settings is the installed spring height. The idea here is that the springs may lose some of their rate after so many years, miles, and heat. For your application I'd go with and upgraded spring from Camgrinder.

-Andy

[kucharskimb]

okay, guys. here are the numbers.

all numbers are on a COLD engine at 100psi pressure. oil is drained from the engine.

1. 92psi
2. 92psi
3. 92psi
4. 92psi
5. 94psi
6. 92psi

all numbers were about 2 psi higher the first time i took them.

all blow by was past the rings and was heard through the top breather plate with the vent hose removed. i used a length of rubber hose inserted into the case and held to my ear. i could hear nothing through the exhaust or in the carbs.

the engine is still in the car. as it is, i can't get a dial indicator positioned correctly to verify cam timing. i'll be using the .040" opening to try to verify the book numbers that i have, ie

IO 20BTDC
IC 34 ABDC
EO 40 BBDC
EC 6ATDC

comments so far?...

matt

[Steve@Rennsport]

Matt:

Assuming that you used 100 psi to perform the leakdown tests, I'd say that you are a very lucky fellow.

Quite clearly, the engine worked precisely as Porsche intended to and thats with broken rockers instead of bent valves. Frankly, thats the exception, not the rule as most episodes of valve float results in a set bent exhaust (and sometimes intake) valves.

Count your blessings, say a prayer, make an incantation, and install new rockers. You might consider a set of better valve springs (we use Aase ones) and Ti retainers for insurance. A Wevo shifter also makes a big difference in helping prevent the "money shift".

[kucharskimb]

Well, the only reason I came out okay is because I canceled my Parade Registration after I thought the engine was going to have to be rebuilt...

Yes, I have some spring shopping to do.

Now, with new rockers on an old cam, what is the mating/break-in process?

Would it be everyone's opinion to replace:

All rockers
All springs

Can the springs be replaced without removing the heads?

I hope to have the engine out within the next 5 days. I'll post the cam timing, and then go from there.

[2.7RACER]

Hi Matt,
Sounds like better than expected news.
I've heard you can pressurize the cylinder with air to hold the valves closed while changing springs and keepers.
I haven't tried this myself.
I'd be sure to attach a flywheel lock so the engine doesn't move with the pressure.
Good luck,