Mach 5

I am installing Solex cams in my otherwise stock '72 2.4T longblock. The engine builder that sold me the cams assured me that Solex cams would work on this motor (claiming they have previously done this) primarily due to the low CR of 7.5:1.

When properly timed (4.2-4.6mm @ TDC), they do not cause the valves to hit the pistons, but there is only about .6mm valve/piston clearance on the intake and 1.5mm on the exhaust. If I retard the timing to 3.6mm @ TDC, I get about 1mm clearance all around.

Now, I realize the conventional wisdom is at least 1.5mm all around. However, I also understand that the only factory reference is for at least .8mm all around. Given that I do not plan to rev the motor much past 6K (I'm actually installing a 6500 rev limiting rotor), I would be inclined to "gamble" on the 1mm, knowing that I at least exceed the factory minimum.

My primary concern is on the retarded timing. I have read that retarding may boost top end at the expense of bottom. I don't mind this. However, will retarding the timing this much cause running problems or damage?

I appreciate any experience anyone may have with this.

Thanks.
Roger

geo.aigel

Stupid question: Is machining small valve reliefs into the pistons an option? I would be very uncomfortable with such tight tolerances. Even with a rev limiter, mis-shifts can happen....

George

Mach 5

I'm not pulling the heads. If I pulled the heads, I could probably use some thicker gaskets. On the other hand, if I pulled the heads, I would probably just replace the pistons with 9.5:1 JEs!

Maybe I should send the cams back and exchange them for a pair of Es. Which is what I originally wanted before the shop convinced me Solexes would work. Hell, it only took the shop 2 months to get me the first pair. I should have these back by Halloween and the motor reassembled and back in the car by Christmas! (Too bad I live in Chicago.)

This process has taught me one lesson for sure. Trying to do any significant 911 engine work yourself is a VERY SLOW (and expensive) process, at least in the Midwest, where you have to mail order for every part or service you need.

OK, I'm done "venting".

geo.aigel

I keep asking stupid questions if I may. I am really not very familiar with the flat six (yet): How do you measure your piston to valve clearance if you don't pull at least one head? Are those theoretic numbers? If they are, isn't that scary?

Cheers, George

Steve Weiner-Rennsport Systems

Mach 5:

The ONLY way you'll know what your piston-to-valve clearance is by measuring it. Guesswork or extrapolation is potentially expensive.

If you are not going to operate your engine beyond 6500 RPM, the Solex cams are not really optimal. I'd use E cams in your engine for more usable torque and better acceleration.

With your low CR, retaining cylinder pressures is paramount,..... I would have installed the "S" 8.5:1 P/C set, though.

Mach 5

Yes, I measured it with a dial guage. Yes, I'm going to pull the Solexes and exchange them for Es.

The motor has good compression and the valve guides seem good too. Therefore, I really don't want to tear it down. I just want more "life" past 5000 rpm. The Es should be fine for this. When it does need complete rebuilding, I'll go with a higher CR, port the heads, and put in S cams.

Someday I'll drive this car again...someday.