Zero decking with dished pistons

[irishman]

Anyone tried a zero-decked 400 block with something like the new KB dished pistons? Just wondering is the dishing would wipe out any gains achieved by decking. This is strictly daily driver/red light racer stuff I'm talking about, not all out performance.

Thanks,
Jerry

[irishman]

Anyone tried a zero-decked 400 block with something like the new KB dished pistons? Just wondering is the dishing would wipe out any gains achieved by decking. This is strictly daily driver/red light racer stuff I'm talking about, not all out performance.

Thanks,
Jerry

[Will]

Irishman,

I have dished pistons and a zero-decked 400 block in my '72 Formula. When calculating the dish size I needed, I first figured what the maximum depth could be so that I could keep the diameter of the dish as small as possible. Since quench is only occuring around the outside of the cylinder where the top of the piston comes close to the flat part of the head that protrudes into the cylinder (outside the actual combustion chamber in the head) I wanted to maintain a "rim" on the outside of the piston to achieve as much quench as possible. The most efficient dish that wouldn't take away from quench at all would mirror the shape of the combustion chamber in the head, but it's awful complicated to figure out the shape and volume of such a dish. I really don't know how much this helped since I don't have any back-to-back comparisons, but the theory is sound. As long as the "un-dished" part of the piston top is coming up to the flat part of the head above it, you will see some benefits of the quench area. At least, that's the theory... ;-)

[irishman]

Will: Yup, helps a heck of a lot. At least now I get a mental picture of what the "ideal" dish would look like. I'm thinking if I was to use clay, bolt on the head with a headgasket to define the diameter accurately, I'd end up with a 3 dimentional mold I could use to locate and shape the dish to create that mirror image. I'd been hoping KB would come through with their dished pistons, but I heard today that's on hold, due to perceived lack of demand. Are you able to run 93 octane with yours without detonation?

[Frank]

I ran high compression (10.25-10.75)GTOs in they early 80s when 91 was the highest octane available. Never had a problem with detonation probably due to Pontiac's excellent combustion chamber design.

[Gach]

Irishman, a 400 will tolerate 10.1 compression allot better the a 455. You don't need to run a dish piston, as long as you keep the head cc no lower then 72 cc. For guys who are more performance oriented, and are trying to squeeze ever little bit then can, I even run the piston out of the hole
.006-.008 depending on the head gasket. Cleaning up the chamber to get rid of all the hot spots will also lower the compression, some. Many ways to do it. Our 64 Goat ran R-Air ll heads, .030 over with pistons .006 out, and never had a denotation problem. Another thing is picking a cam with
the right opening an closing will help bleed off some of the compression, while at the same time help pull good clean air into the cylinder, for better combustion, and better filling.

[Lee]

Gach has alot of experience, but I'll go against him on this. I've helped with two REAL 400 street engines that used "13" castings. By "real", I mean that both of these cars are daily drivers, main mode of transportation, driven often on freeway, and don't have the heads off every 1,000 miles to freshen the valve job and clean the carbon off. One block was zero decked, the other was not. The zero decked block worked out to about 10.0, the non-decked around 9.8. Both of these motors require 93 octane, and occasionally experience detonation. Playing ALOT with the timing and jetting has minimized detonation, but long drives on hot days still brings on the ping. I built another 400 recently with dished TRW's and 72cc heads. It worked out to right at 9.25:1. It does fine on 93, and the owner has admitted that he puts some 89 in on occasion without any detonation that he can hear.

In summary, it depends on the end user. If you have the ability and desire to tune, you can probably get 10.0 to work on the street for you. If curving the distributor and rejetting the carb multiple times sounds too hard or time consuming, then don't go over 9.5.

[Gach]

Lee, good post. But denotation is cause by allot of things, poor machine work, poor guides, the list goes on. I've built over 40 motors with 10.1 compression, one of them has over 35,000". heads have never been off. This kid has driven 150 miles, race and driven back many times. No problems
what so ever. If it's do right you won't have any problems, what I've found is the guide work is critical, right valve seals also, but I agree with you, you don't know what the end results will be with the ueser of the motor, I've been lucky I pick guys who know a little some thing, like yourself. I was problemly wrong to sugguest that anyone could do it. I agree with you, if you don't have good
tunning abilty stay with a good safe compression ratio. I some times forget, that I have the abilty to over see every thing, and check all the machine work.

[Lee]

Gach, totally agree with you on that. Compression ratio is a prime factor in detonation, but not the sole factor. Heck, my wife's Volvo has over 11:1, and it runs fine on 87 octane!! I didn't believe her, but on a long trip I played with different grades of gas, and the mpg didn't even differ with the different grades. Goes to show what modern chamber and port designs, along with computer controls can really offer us.

Sharp edges, wrong plugs, incorrect timing, incorrect timing curve, lean fuel mixture, oil contamination (loose valve guides, bad valve seals, trashed rings,...), chamber design, port design, on and on, all have an effect.

[irishman]

Gach and Lee - whether you know it or not, with regard to the tuning skills, you guys were talking about me. At larger things like swapping our a rear or pulling heads, no problem. It's things that require finesse, like fine tuning a Q'Jet or working the curve on a distributor where I simply don't have the skills for it. Plus, since this is going to be my driver car, my attitude is I have to do it as right as possible the first time out because I can't be out in the apartment parking lot in the dark with a timing light or vacuum gauge after work.

Where I'm heading, is using a 400 block I have that'll clean up beautifully at .020 over, zero deck it, .39 head gaskets, mildly dished pistons, meticulous attention to ring gaps and cylinder wall finish, #13 heads with new guides, stock valves, factory-style oil seals, all of the sharp edges around the shroud eased, and the chambers themselves highly polished, a Melling 068 cam, Cloyes double roller chain, and 60lb oil pump. For an intake, I've got both a '70 cast, and a Performer. I'll probably use the Performer only because of the weight savings. I'm leaning toward a Carb Shop Q'Jet built specifically for the engine, to eliminate the guesswork on jetting. This'll feed into Ram Air exhaust manifolds into a 2 1/4" mandrel bent system, with Flowmasters 2 chambers. The Flowmasters are for fun, and so it'll at least SOUND mean. The radiator is already sporting a new 4-row core, and I'm replacing the factory shroud with an A/C version to run a 19" 6/7 blade fan, without clutch. I picked up a high torque mini starter because it's just plain embarrasing when a Poncho won't start from heat soak. I'll run either an HEI or Pertronics, and use a rebuilt NAPA distributor. I'll again have that set up on a machine for the curve to match the engine. Behind this is a fresh Turbo 400 with a converter about 4-600 over stock, and a 3:55 posi rear and 28" tires.

I know I won't run with the big dogs, but I figure it should start every day, take me 2 or 200 miles on 93 octane, idle in traffic and maybe give me 13-15 mpg on the highway, and if I dial it in right, will use a pair of rear tires every 1000 miles or so. And, I might just give a few Rustangs a surprise at a redlight. Thanks, guys - I've learned a lot from your posts, and it's really appreciated.

[This message has been edited by irishman (edited 12-29-1999).]

[Bob Dillon]

Lee, is the Volvo EFI? Aluminum head?
That makes quite a difference, in my experience.

[Will]

Irishman, For the 250 miles my engine ran, it ran fine on 92/93 octane. Once I get the valvetrain problems worked out (and the weather improves) I'm going to try 89 octane to see how that does. For reliability and ease of tuning, I went with the lower CR for now, though I have another set of heads in the works that will put the CR up around 9.6-9.7:1 compared to the current 9.27:1.

[Lee]

Bob, yes, EFI, aluminum head & block, extremelyt efficient cooling system, multi-valve... But still, I'm talking 11:1 on 87-octane! Even the "#1" Pontian engine builder from the west coast hasn't been able to do that :-)