Read on Mummerts site :
"The other enticement for bringing the pistons near flush with the deck surface is reduced risk of detonation. Many complaints of detonation have been noted with the pistons .030"-.040" in the hole and the engine assembled with composition head gaskets."

Can someone elaborate on this for me? How far is the piston in the bore for a 292 stock?

 

Sam, read through this thread: https://www.ford-trucks.com/forums/s...1&page=1&pp=12


See where that gets you. Composition head gaskets are thicker than the original steel shim gaskets, so they will tend to aggravate an existing issue. That's why we determine the head gasket & its compressed thickness when we "set" the deck height.

As far as the 292 dimension goes, they tend (from what I've seen) to be anywhere from .010 to .030 down at TDC. Virtually all Ford engines, at least prior to the late '80s, are in the hole. Most other brands are, to an extent, as well.

 

if you deck your block leaving the pistons from flush to no more than .005 in the hole you can use composition head gaskets.

 

Missed that thread. Thanks for linking to it. Very helpful.

Thinking out loud: Seems then, assuming the original steel gaskets were .025 and factory 292 was .010 to .030 down, then I'm looking at .035 to .055 down. With comp. gaskets at about .045 and 0 deck to +.005 I'm looking at .045 to .050. Guess I'm gonna see what has to be taken off first.

Thanks fellas.

 

Why does say a quench of .040 vs. .65 have the effect of reducing detonation problems? I accept that the lower number has the stated effect, but it seems like a higher compression would produce more detonation tendency. Or does it have more to do with the physical design of the Y head, and the flat area above the piston?

 

To give a short answer: the smaller quench tends to get the mixture moving faster & make it more blended, reducing lean spots in the mixture, and squirts it over toward the plug. Some people say it "blows out" any unwanted flame fronts, but I don't feel that's quite accurate.

When you look at spikes in cylinder pressure, and the place where you get the greatest rate of change of pressure, it is usually in the last 15-20 degrees of crank rotation before TDC.- where quench begins to come into play. The intake closing point affects this somewhat, as well as volumetric efficiency/intake port flow, intake manifold efficiency, RPM, and the use of things like nitrous.

The difference between .040 & .065 may not seem like much, but percentage-wise, it's what- 62.5% bigger? I don't think it can directly translate into a figure like "62.5% more likely to detonate", but there is a correlation.

The long-**** answer: a lot has to do with the chamber shape & finish, though there are many factors. The compression ratio vs. fuel has a lot to do with it, true; for example, you can't produce a true, measured, 13.5:1 ratio, & expect it to run on 87 octane pump swill, no matter what the quench & chamber shape. Cam timing events play a big role, as well as mixture control. Also, don't forget about bore size. However, one thought-provoking example for you:

351C 4V closed chamber heads are able to support some ridiculous ratios without problems. I know of a documented, verified case where the owner has a true 10.7:1 static compression, .035 quench, & runs it regularly on 90 octane without any problem, whatever weather/humidity. This is with a 218/226 degree duration @ .050, & 114 lobe centerline. In other words, low overlap. He thinks it would support 87 octane, but just doesn't have the nerve. I've heard of a lot of similar cases. By the way, he flogs the crap out of it. :=D

Note: overlap "bleeds off" (simplifying a bit) cylinder pressure at lower rpm, thus decreasing the chance to detonate. The higher the overlap, the more pressure bled off.

The 351C OPEN chamber heads, however, have an awful reputation for detonation, in spite of the fact that the compression is often in the low 8 range- PARTICULARLY SO ON un-rebuilt engines. The 351M & 400, which use very similar heads, are the same. Much of this is due to the pistons being .030 or more in the hole, with the rest being IMHO due to the chamber shape (and less quench surface as well). In the cases where they have been used successfully, it always seems that, on purpose or inadvertently, the quench height has been set at a minimum.

In the case of the 351C closed chambers, I think it is a happy marriage of the bore & stroke dimensions, quench, excellent chamber design, & the fact that nowadays, those particular heads are used by performance-minded, detail-minded people (as opposed to "slap 'er on an' get 'er done").

Another example for you is the 385-series Ford: The '68-'71 heads used a closed chamber something like the Cleveland. In 1972 they went to a COMPLETELY open chamber to cut compression. No quench. None. It was a disaster; in spite of the compression being possibly under 8:1, they rattled horribly; warranty complaints were high, and for 1973 Ford returned to the closed chamber, but deepened it .100 into the head. That chamber stayed around until the F.I. heads in 1987. 460 guys that know better won't take a '72 head for anything, not even a plain-Jane rebuild.

To sum it up- quench isn't the only thing I consider (& if you managed to make it through the other thread without falling asleep, you prolly guessed that ), but it is a quite effective tool for street engines. I know a guy who is a successful drag boat racer, using a 698" BBF, and he couldn't care less about quench. The slugs are .070 or more down. He just wants to keep them out of the heads.

Clear as mud??

 

Basically, the greater the quench distance, the less effect it has on "moving" the mixture, and the lazier the mixture is (lazy movement being a relative term inside the extreme tempertures/pressures of the cylinder).

If you look at the past few Engine Masters contests, they were essentially an advanced exercise in detonation control. Jon Kaase, who won two of them with Cleveland-based & 385-based engines, used small bores & long strokes, with (relatively) short rods, in an effort to get the piston up to, then away from, the head, as quickly as possible (astounding rates of piston acceleration/speed). He was fanatical about setting the quench dimensions, including considering moving the plug location in the heads, in an effort to reduce detonation.

 

Thanks Homespun for your excellent effort. It has helped fill in a couple of my many blanks. I understand much better. If a motor with a 292 stroke is bored to 3 7/8 inch for example, there will be less turbulence and mixing because of the larger bore and a relatively slow piston movement.
Now if I can just understand cams, overlaps, centerlines, lift rates, etc. Do you have any good sites in mind for that tutorial?

 

Well, depending on exactly how Ford blue your blood is, you may find this useful:

http://www.motorbooks.com/Store/Prod...ProductID=6654

It appears by the cover to be about the SBC, but most of the info is applicable for all engines. IMHO it's as good or better than anything else currently in print. I still pull it out for reference quite a bit.

The cam vendors have some stuff on their web sites, but it's generally kinda vague. I'll bet there's a good site somewhere out there, though.