J. Mummert stated in his Y-Block Porting articles that he does not like to relieve all the way to the gasket surface when unshrouding, as the "G" chamber overhangs the block least, thus not canceling the benefit over more open chamber models. It's a bit unclear if he was referring strictly to loss of compression by relieving all the way (which would obviously remove more material), or is there some other factor in play re: "block overhang" that I'm just not getting (assuming no material is removed outside the gasket line)?

6 VOLTS/POS. GRD. NW INDIANA

Dan, you might be worrying needlessly about dynamic compression. According to the E-4 Isky I checked on the Cam Doctor the intake closing is considerably later than Isky claims. Its closer to 90 ABDC. To get to their claimed duration of 260 degrees and intake closing around 60 ABDC the lash would need to be .028".

At .016" lash the seat to seat duration is nearly 290 degrees.

If you're still concerned you could put your new valves, springs ect in a set of -113 heads. 73cc will put your static C/R at 9.25:1.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

That's really good news.  I have an engine similar to Daniel's and I have been following these posts pretty closely and trying to get all this straight in my head for a couple of days now.  Thanks John for the "stress reliever".  I can sleep tonight.

Butch Lawson

Manchester,  TN

Ted,



Wow. 90 ABDC really drops the dynamic CR way down (using web formula program link). If final volume measurement proves all chambers within 2 CCs, I might not bother with any more grinding (though it is tempting since they are still laid out on the bench). Only inquired about relieving to gasket surface since none of the sectioned head photos demonstrated thickness of that particular part of chamber. While breaking through not likely, I DID read about cracking danger if thinned too much. A lot of meat quite accessible near the top if more material HAD to be removed to lower CR. Also, if more precise chamber uniformity might prove exceptionally beneficial (though not sure it would, if unnecessary lowering of CR resulted), damage to valves (final state of assembly) a lot less likely if grinder bit kept high.

6 VOLTS/POS. GRD. NW INDIANA

Oops. Not Ted, but JOHN! (senility setting in).

6 VOLTS/POS. GRD. NW INDIANA

John,

A bit puzzling. Don't know enough about the manufacturing process to guess, but wonder if deviation from advertised specs. is consistent? If they are cranked out by hand/one-at-a-time, then set-up errors could be quite random. If made in larger lots, then date of manufacture might be a determining factor? What is your experience with other brands re: consistency? Is it common practice to double-check each cam and validate specs?

6 VOLTS/POS. GRD. NW INDIANA

John,



P.S. Unless that was MY E4 you checked, it occurs that simplest way to insure peace-of-mind before heads are buttoned-up might be to just measure my intake closing point. Once a damper is bolted on and TDC verified, installing a pushrod and chalk-marking the point where dial indicator stops moving should allow (using tape measure & calculator) an accurate enough estimate to determine if dynamic CR is really under safe limit? Also optimistic (assuming intake closes significantly later than Isky specs.) smaller aluminum head CCs would not then present any future problem? Going over all the relevant alum. postings, I didn't notice a specific comparison guideline for DCR/pump gas limits for the new heads?

6 VOLTS/POS. GRD. NW INDIANA

Dan, I checked a 505 Isky and it needs .036" lash to match the 290 degrees they call advertized duration. So they seem to be consistant. They may be figuring in deflection of the cam, pushrod, rocker shaft ect and coming up with a calculated running duration. Or, they might just be telling us what we want to hear. This is something that cam grinders often do since they know most of us have no way to check their work.

The reason we got the Cam Doctor in the first place was I bought "284" degree cams from 3 different manufacturers and had them tested. They were radically different and would have behaved very different in an engine. I realized that we could not rely on the manufacturers for accurate information.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

[quote][b]John Mummert (5/28/2010) "they might just be telling us what we want to hear".



John,



Confused. If "what we want to hear" is that the cam has MORE duration than it actually does, that would be the opposite of my problem, as more duration than advertised would then keep my DCR under the safe limit?



"most of us have no way to check their work".



Measuring the intake closing point now would seem to be worthwhile insurance. Murphy's Law dictates my E4 will likely be one of the few that IS close to advertised specs.! Just need to figure out how to remove complications of lash, valve train slack, etc. so that when the lifter stops moving, I can interpret/translate that crank degree data to the Isky spec. sheet #s, since those measurements likely relate to a fully assembled motor? Would hate to lower static CR now just to be safe, only to discover later on I have ended up with unnecessarily low DCR, and thrown away power/torque.






6 VOLTS/POS. GRD. NW INDIANA

I think your idea of using the crank can work if you degreed your cam so that you know where it is in relation to the crank, and you have positively located tdc on the damper.

Keep in mind, the cam turns half the speed of the crank. If you forget that, you will come up with some really weird numbers.



Lawrenceville, GA