PY Online Forums - Bringing the Pontiac Hobby Together - 400 vs 428

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- - 400 vs 428 - Help Me Decide (https://forums.maxperformanceinc.com/forums/showthread.php?t=853346)

Quote:

Originally Posted by hgerhardt (Post 6281994)

Yes, but the 670's were to make the car look more correct.

The 428 is my favorite for street driving: more torque than a 400, but not so much that street tires have no chance off the line. And it still likes to rev.

I'd make a template of the 670 combustion chamber outline and digitize that to mill a dish exactly that shape into the pistons which will maximize quench, which in turn reduces octane dependency.

Not disagreeing with you but always willing to learn, explain how you would "maximize quench". I understand the Mirror Image of the combustion chamber part. Seems like you would need a Gas Ring sealing plan vs a off the shelf cometic type gasket.

Tom V.

And of course proclaiming you have a 428 vs a 400 just sounds cooler at the local cruise or car show!

Sent from my SM-G965U using Tapatalk

Quote:

Originally Posted by Tom Vaught (Post 6282023)

By "maximize quench" I mean take advantage of the maximum available quench area by starting with a flat-top piston and only dish an area which is the mirror image of the combustion chamber.

From what I've experienced, minimizing and equalizing quench height is also important, which is why "zero decking" and squaring the block is important.

As to the sealing tech, I'd just use a standard Cometic gasket. I didn't mean to imply anything fancy here.

Years ago, long before I even knew the definition of "quench", I had a '68 Firebird 400 with the stock #16 heads. I got it from the original owner with 70k miles on it. With its ~10.25:1 CR, I had to retard the ignition timing a bunch to run it on CA's 92 octane at the time. I later rebuilt the engine with generic dished pistons which had a ~3/8" lip around the perimeter and the rest of the area was dished, which meant very little quench area. I believe those pistons reduced CR to 8.5:1. I used an 068 cam in place of the orig 067, so not much difference. I was quite disappointed when that engine had almost the same octane appetite as it had with the stock pistons, and now made less power as well.

It just depends on how the chamber is designed.

I've done the bowl type dish on a 428 build decades ago when I used #13 heads on the engine. The chamber is so large on the 13 that the 3/8 lip around the parameter of the piston took up nearly all of the quench the cylinder head had to offer. That one came out to 9.7:1 compression and it ran great on 91 octane for years with a very small 218 @ .050 camshaft.

The 670 head with it's much smaller chamber, I agree I'd dish a piston differently, just as described which would more likely end up with more of a "D" shaped dish.

Picked up my 428 yesterday at Fastenal (great shipping service BTW).

YH code casting 9786135 turns out to be date-correct for my car L016 Dec 1, 1966. Intake is J216 Oct 21, 1966. Was pulled from a '67 Grand Prix with about 70K miles on it some 30 years ago and sat in a shop waiting for rebirth. Even has the AC brackets, which will come in handy.

Can't wait to start the build, but it looks like we'll be moving soon so it will be a while....

If I had that 428 I'd keep the 670 heads and run a 4.25" stroker kit to complement the 4-bolt mains and then sell the 4" stroke crank for a high premium to one of those guys that believe there's something magical about it. I'd run big dish forged pistons with enough volume to drop the compression ratio down to no more than 9.3:1.