How far in the hole are stock pistons?
 

How far down from the deck are pistons from the factory? Everyone zero decks them now, but what were they from the factory? How thick are factory head gaskets?

There’s nothing carved in stone with the deck clearance on factory blocks. But from what I have seen there anywhere from .019” to .024” assuming all the rod center to centers are the same in a given motor along with the piston compression heights and the crank stroke .

Factory head gaskets could be anywhere from .048” to .028” in uncompressed dimension.

If your only building a street strip motor or one that’s race but not competing in a class then to me a deck clearance of between.019” to .024” is not worth messing with for the misecul gain in compression it will buy you.

Isn’t it a good quench that you’re looking for?

I thought I read somewhere once that Pontiacs were around mid teens for in the hole from factory.

The 428 & 455 Stroke & generous Quench Pad (head & piston) make/ compensate for great quench even with piston down the hole.

Good quench or adequate quench depends on what factory head you’re talking about.

The heads up to 66 and the B body heads up to mid 67 had the most quench area.

These where not the greatest in terms of unburned fuel emissions, but they had better tolerance for octane level for a given compression ratio.

The 1967 670 and RA997 casting gave up some of that quench area.

The 061 casting of late 67 I call the mule head because it was used by the factory to see how far they could go with an open chamber and still not have a head that was prone to detonating.

The good thing about these 061 castings and there shallow chambers is they have about .040” more short turn height to them so when porting them to flow intake flow levels above 260 cfm it’s easier to keep them from breaking into a tumble condition and going into flow regression.

The 68 and up chambers with there increased quench are over the 061 casting shows that they found out which direction they needed to go to have control of emissions, a good burn rate and a unshrouded chamber.

Where talking non RAII ( 1968 1/2 ) D port heads only.

They vary a lot. The blocks typically are not anywhere near square at all so you get different deck height front to rear and side to side. Meaning a lot of the time the block is twisted. Even brand new blocks aren't square. I've got one right now that is about .014" different front to back on one side and back to front on the other side, meaning it's twisted. That doesn't sound like much but it does affect compression cylinder to cylinder going from 10.2 to 10.7 and everywhere in between.


That's a pretty big change and why I've always said it's important to deck and square up these blocks

Isn’t an ideal quench like around .040 ish? If I’m .035 in the hole and use a .045 head gasket isn’t that like .080 quench?

The reason I'm brought it up was everyone says you need a tight quench for good octane tolerance and power. But many engines from the factory came with somewhat large quench distance. So is it really as important as people think? If there are 2 engines both 11:1, one has .040 quench and the other has .075, will there really be a difference in octane tolerance and power?

I can tell you when we tried a 0.100 down deck to use my 69 cc RAIV heads on a 455 in the 80s it HAD to have at least 5 gal race gas in it or would rattle if lugged.

I’ve taken a few all original 455 engines apart and all were between zero and..005” in the hole. Not saying they all were but that’s what I’ve seen.

My 421 HO was built with .045” Butler head gaskets and the pistons .007” to .008” above the deck.

Currently waiting for my 2276cc air cooled VW bus engine to be delivered, pistons are between .038” and .041” below the top of the cylinder, they don’t have a head gasket.

You should be shooting for .040” no matter what, it makes a difference in power and efficiency.

No offense, but besides everyone saying that , is there any real world testing that shows that. On paper a lot of things look good, reality not so much sometimes.

More details on the combo? Compression, cam specs, octane of the pg used?

No offense taken.

Yes there is.

There are so-so engine builds that have been improved by tightening the deck.

Believe whatever you want, but it does matter. There’s absolutely no reason not to optimize the quench distance when you’re spending the money and taking the time to do an expensive engine build.

I have search quite a bet I can't find a test that compares just quenched distances. Almost all the ones I find have other variables changed, like compression.

That's a play right out of Butler's playbook ..... And everyone thought it was great because "Call" Butler recommended it.

Its not good to have that .035 or whatever in the hole area acting as part of the combustion chamber? Easier to cause detonation? Mine ended up .005 in the hole with a .036 headgasket for a .041 . quench. Im asking because Im far from any type of expert.,,,just regurgitating stuff Ive got from here over the years

Basically if your doing a A B test of heads in which one has a faster burn rate chamber then that head will tolerate the making of a bigger quench / deck clearance if formed by dropping the piston.

Installing and setting up a long reach plug which provides a faster burn rate while needing less timing goes a long way towards dealing with the bad effects of having a tall deck height.

There are other chamber shape factors that come into play here also .

It’s been found that sharp traditions from a chamber floor to the chamber wall will collect fuel that burns late and can make for a second flame front lighting off which is part of the definition of detonation.

Agitation that a tight quench clearance provides does not help with a late burn issue from fuel pockets like I described above.

Skip,
My was your cam timing?

If you look at the Pontiac factor cams they The had very long seat duration for their 0.050" numbers. Yes I know Pontiac rated their cams at less than 0.006". When I look at the Crane version of the RAIV cam it has around 66 hydraulic intensity.

_______I__N__T__A__K__E
Rocker_Arm_Ratio_=_1.650_________Valve_Lash_=_0.00 000________Valve_Angle_=_14.0

CAM_______Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Ar ea____ICL
_________0.00600____27.83_|__90.08_|_297.90_|__26. 00_|_121.13
_________0.01000____22.59_|__77.14_|_279.73_|__25. 93_|_117.28
_________0.02000____14.84_|__65.34_|_260.18_|__25. 78_|_115.25
_________0.04000_____5.27_|__54.22_|_239.49_|__25. 46_|_114.47
_________0.05000_____1.51_|__50.12_|_231.64_|__25. 27_|_114.31
_________0.10000___-14.12_|__33.66_|_199.54_|__24.05_|_113.89
_________0.15000___-28.35_|__19.09_|_170.75_|__22.12_|_113.72
_________0.20000___-43.43_|___3.95_|_140.51_|__19.43_|_113.69
_________0.25000___-61.22_|_-13.98_|_104.80_|__15.29_|_113.62
_________0.30000___-89.25_|_-41.90_|__48.86_|___7.44_|_113.68

_________Max_Lobe_Lift_0.31374

Stan

The 455 piston comes closer to the deck than the 400.... From what I've seen. Going from memory the 455's are usually.005 to .012 down (closer is on the Super Duty's). The 400's are around .020. From what I remember the 455 usually used the thicker head gasket. The 400's used thinner bead gaskets.

Steve25 have you compared the 061 intake port to a 670? Any difference?