I blew the edge off #4 exhaust valve 200 miles from home. I limped home and ordered a new casting w/o jet valves from Clearwater. Resumed the trip and put on 1,000 miles or so then checked the compression pressure yesterday. Old head ran 145-160psi with the new head 2-4 cylinders are at 186 and #1 is at 198psi. The FSM says it should be 170. I ran each test 4X with a good/accurate gauge. Anything to be concerned about?

It seems to be running OK except for a rattle/knock under light load that's in the lower end, probably #3 cylinder, but that was there before I replaced the head.

You may need to run a little higher octane fuel to avoid spark knock.

+1

Does it say the 'service limit is 170' or does it say it should be at 170psi? What's the stock compression ratio on those?

anyhow, definitely run higher octane for now at least.

That's what I'm thinking. I'm running at 7 degrees BTDC and was thinking (before the test) about kicking it up to 9 degrees...not now. After this tank I'll switch to premium gas, at least while the prices are low, thanks.

What about the specs on the compression?

FSM says Standard Value: 170 psi, Limit: 136 psi. Compression Ratio is 7:1. It says to check the compression on a hot engine (176-194 degrees) I checked it cold.

If your cam timing is off signifigantly your compression will be high, same goes for a worn cam or really loose valve adjusters.

Do you have mechanical lash adjusters that might be set loose?

Even having the wrong rocker arms can cause the same problem.

Also if yours is a truck motor the CR should be in the 8 to 1 range, not 7 to 1 unless you put in a set of turbo pistons.

I have gotten 180 PSI cranking pressure on a 7.8 to 1 2.6 engine and that had a turbo on it.

Are you sure the noise you are hearing is detonation? Did it get better with higher octane gas? How does the power on this motor feel? Does it pull clean to 5500 RPM?

Kevin C

Thanks for your input Kevin. When one of the other guys asked what the compression ratio was I was sure that it was at least 8:1 but thought I'd better check so I pulled out the FSM and I saw that it was 7:1. It wouldn't be the first time that I found an error in the manual. I've never seen a non-turbo engine with such a low C/R. I'm assuming that turbo pistons are dished to reduce the C/R, mine are not, they're flat and level with the top of the block.

The heads from Clearwater come complete, ready to install with the same non-adjustable hydraulic lifters (less the adjusting screws for the jet valves). They told me that they use used cams but mic them and put them on a cam polishing machine before assembly.

As far as the cam timing goes I used a tie-wrap to prevent the timing chains from moving, so the timing is the same as it was before the install. It makes me feel better that you got 180psi with a turbo because I doubt if there was any boost at such low cranking speed, but then I don't know much about turbos.

The rattle sounds like the timing chains, because I had the problem with the tensioner plunger stuck all the way in which I replaced when I did the silent shaft elimination kit. The chains made the same sounding rattle then. It doesn't rattle under load, only when the load starts to lessen does it appear. I believe that detonation should be greater under load.

The engine pulls well throughout the range, after finally getting my Weber 32/36 jetted and adjusted right.

I'm going to adjust the mechanical chain guide adjuster for the chains today and see if that helps. And, I'm going to double check the C/R Thanks for your suggestions/questions.

Did anyone mention to check your pressure guage for accuracy?

Good idea. I just checked it against my air compressor gauge. The air compressor output read about 98psi, the gauge read 105psi, so that would put me in the 170's. I feel better now, thanks for the suggestion.