As tight as you can get it without smacking something!! Most racers will keep trimming things during rebuilds until things just touch and stay there.

Of course this is in cases where there are rules and you have to look for every nano-HP. Average street stuff is different.

From my experience, tighter seems to help detonation resistance. I live in Houston and it's hot and I try every trick I can to make 'em run on 93 octane without backing off timing.

I sincerely doubt you would ever measure HP increase going from .045 to .040. It's one of those "can't hurt" areas that all add up to 5 or 10 more HP. But you will be able to feel how it responds to the more aggresive timing you can put in etc.

Just for reference, my 540 is at .028. Pistons are .009 out of the hole and gaskets are .037. I have .0045 wall clearance on a relatively short piston in a 4.500 bore.

When I tear it down, the "quench" area of the piston is perfectly clean and shiny. The other side shows slight "kissing" of piston and head. You can't feel it, but you can see the chamber shape on the piston and a different texture in that area. I think I'm about on the edge.

It's been running like this for 5-6 years at well over 7000 rpm as well as many street miles. Works fine and I run 39* timing on 93 octane on the street, track and dyno.

Would it run with .060? Sure...but not as well.

BTW- I deck everything at literally around at least .002 OUT of the hole. Most builders end up with .005 down even when they tell you they "0" decked it. I then pick gaskets to make the clearance.

JIM

 

The tight squish, besides reducing the tendency for detonation, causes the air/fuel mixture to burn more rapidly. Fast burn. This reduces the need for excessive timing lead. Have you ever set your timing curve on a dyno? With your extreme squish, quench, piston to head clearance, I would expect your engine to make more power with less spark advance. One undesirable consequence of a lot of advance, is that the "fire" starts sooner, and pressure builds in the cylinder as the piston is still raising on the compression stroke. When the optimum lead is less, there is less pressure put on the piston, before it is desired.

One good measure of an efficient combustion chamber is the lower the amount of advance required to make the maximum power. We've all been to the track and heard someone comment, all excited and pleased with themselves, "I'm running X° of advance". Where X is some huge number. I just walk away and think to myself, "Too bad".

I suspect that 427Hotrod might go faster with less timing.

RACE ON!!!

 

The tight squish, besides reducing the tendency for detonation, causes the air/fuel mixture to burn more rapidly. Fast burn. This reduces the need for excessive timing lead. Have you ever set your timing curve for maximum power on the dyno? With your extreme squish, quench, piston to head clearance, I would expect your engine to make more power with less spark advance. One undesirable consequence of a lot of advance, is that the "fire" starts sooner, and combustion pressure builds in the cylinder as the piston is still raising on the compression stroke. When the optimum lead is less, there is less pressure put on the piston, before it is desired.

One good measure of an efficient combustion chamber is the lower the amount of advance required to make the maximum power. We've all been to the track and heard someone comment, all excited and pleased with themselves, "I'm running X° of advance". Where X is some huge number. I just walk away and think to myself, "Too bad".

I suspect that 427Hotrod might go faster with less timing.

RACE ON!!!

 

Real race cars actually pull timing out as rpm goes up. Any motor making one HP per cube or more could realize power gains from these types of ignition systems.

 

Yes, I own the book. I unfortunately loaned it to a friend, and haven't seen it since. Plus I move every three months, and books aren't high on my list of things I like to pack and carry.

 

Trust me...been through lots of timing loops. When heads were unported during the 540's first incarnation, it picked up 17 hp on engine dyno moving from 36 to 39*. It didn't like any more. Stayed the same at 40* and fell off after that.

After we ported heads and opened chambers a good bit for breathing, it is actually much less sensitive to timing. It makes the same power at 37* as it does at 39*. Again, it doesn't like any more. The reason it's at 39* is because I stuck in a new MSD dist. and even with the biggest bushing, it still doesn't allow the initial timing I like for street driving. One of these days I'll take care of that, but for now I just use the higher number.

I've also tested it on track and on chassis dyno (first version) and got same results. It does alright down in the 34* range, but it's faster at 39*.

I used timing retards on some previous motors and might play with it again someday on this one. The other ones really did come alive in high gear with some timing pulled out.

You also have to remember, I'm using Brodix open chamber heads which are based on the older big 'ole sloppy style chambers...nothing like new style trick stuff. That's why I have to do what I can.

It's running 141.27 mph trap speeds on 93 octane, all motor with 3.07 gears...so it's doing "fair"!! I've got lots of tricks up my sleeve for winter though!!


JIM

 

I think THAT is the explanation.

RACE ON!!!