Hello,

What would you say would be the max compression ratio that could be run on pump gas on a 416 small block with

open chamber heads
NO quench
hyd roller cam 254/260 @.050
.640/.640 valve lift
104/112 CL

would 9:1 be the ticket?

or better go 9,5:1?

Id rather go with the CR ratio that will let me set up the proper advance without pinging for max power on pump gas rather than being able to run on pump gas with no pinging by backing up on timing and not being able to run the engine to its full potential.

I hear that 1 full compression point equates to about 4% increase in HP aprox. So I guess going from 9 to 9,5:1 on the above combo wouldnt translate to about 10 extra HP----thats if it doesnt ping on pump gas at 9,5 of course.

I want it to run setup to make the most power on pump gas all the time....sooo.....

what compression would u run?

What is the M+R/2 octane rating of your intended fuel?

If your "pump gas" is comparable to the 93 E10 I have where I live, I'd say 9.5:1 CR is about as high as I'd run. I'd be pretty careful to find out how much ignition timing it'll take on a hot day, too.

Our rating is 98 but its mon only i think or something like that.

Id say its similar to the 92 octane that u get in the USA.

From my experience with PING monsters in hot Southeast Texas, I would go 9to1
I built my alum head 512 with quench at 9.7, just to be on the safe side.

There is much more going on in the chamber that changes with quench, including valve shrouding, squish turbulence, etc. Even if your quench distance is beyond conventional limits (best before .060", gone at .125"), having the surfaces approach each other to a closer distance at some points (a hemi does not) always helps.
1/2 point isn't worth 2%.

What do you mean when u say half a point isnt worth 2%?

Is it worth more is it worth less?

altitude??

LA motor with stock iron heads with the spark plug in the stock location I wouldn't run much more than 9.5 to 1 compression ratio with a stock type heat range extended gap plug like the OEM Champion RN9YC or the smaller plug socket size like the RC9YC with no more than 34 degrees total timing all in by 2200 RPM
If your near sea level elevation than start with 30 degrees total advance and go from there
All the 1968 to 1971 stock factory assembled 340 motors I took apart barely had 9.25 to 1 to no more than 9 .75 to 1 compression ratio when we did the math on those OEM factory built motors

My 406" LA iron-head Duster with 214/224 @ .050 and .470/.504 was 9.5:1 real CR and it was at the edge of detonation on Calif 91. I pulled some timing out to be safe. Still ran well (street car), but never hit the track to compare ET/MPH. Stock converter, 3.21 gear (9.25 axle), 28" tire, guess about 3400 lbs.
Elevation, duration & overlap will lessen that a bit as each increases, but it's way better to play it safe than to rattle the thing.

With my 906's it was right around 10:1. I usually ran a mix of 5 gallons 100LL to 10 gallons 93 Shell. That was the only way to keep from having a slight ping under heavy load in the hot summer. Spring and Fall I could get away with just 93. That was with a MP .528 solid cam.

Overlap doesn't reduce CCP (later intake closing does), but reduces knock slightly because of residual exhaust gas - less power. This will vary with local exhaust port pressure, and system backpressure.

So, Poly, opening the exhaust early would be better for reducing pressure? (All else being equal for comparison's sake)
Late closing of intake would seem prone to creating reversion, right?
I understand it from the visual of 2 triangles, intake lobe & exhaust lobe, and where they are in relation to each other (overlapping I closing/E opening). So, the shape/angle of the sides of those triangles would be the clever bit.

Aluminum or steel heads?
Cam is pretty decent size.
I would try about 10:1 to start. You can add thicker head gaskets to drop compression from there if needed.

Overlap will reduce low speed ping because of dilution but will also hurt low speed TQ so it is not the solution anymore than retarding timing would be.

Retarding the whole cam could help if you really were pushing the envelope that much, you get a later intake closing point so less low speed cylinder pressure as well as a later ex opening giving you more time to push on the piston.

I think with a stock open chamber head on that big of an engine you are not going to efficently fill the chambers at higher speed anyhow so I would shoot for 9.5 - 10.0

Opening the exhaust early subtracts from the duty cycle (less energy turned to crank rotation, more lost in the pipe) in favor of reduced pumping loss.
Closing it later (during OL) leaves more exhaust gas in the chamber, which both dilutes and heats the new charge. Typically cylinder pressure is higher than we want (I want zero!), so the exhaust forces the new charge backward until RPM is higher (with high vacuum this is even worse). This is one reason why "split overlap" (equal duration IO BTDC and EX ATDC) is nothing like equal events in terms of pressure balance.
Again, it's a compromise like everything else.

Cast Iron Heads.

Sea Elevation.

I think Im gonna play it safe and stay around 9:1

On an engine like this which should be putting around 500 hp (small block stroker with w2 heads). Each point increase in compression would equate to how much HP gains? Lets say if the engine made 500hp at 9:1 if it was bumped to 10:1 and race gas was used so it doesnt ping....how much HP would be gained from the full point increase in compression?

Its hard to get a tight quench with open chambered mopar heads. If you measure them you may find the chambers are not as equal as you might think both volume and height. Octane needs of the engine have more to do with dynamic compression than static compression. I use an engine program to decide what kind of fuel the engine will need. Performance trends, it calculates cranking compression, advance curve, and lets you try different octane fuels. It also calculates a ton of other stuff. Results on a real dyno are close if you take the time to put good numbers in the program. And it costs about what a good 1/2 ratchet costs.

It's not hard to get quench. Stick the piston out of the bore.

There was a test on a 440 stacking gaskets to controle cr and from 9 to 1 to 10 to 1 gained 20 hp so maybe you would get 15hp.

I think he's referring to the "as cast" 452 etc. chambers in which the exact depth and especially the contour of the quasi-quench surface is irregular. The KB instructions begin with "mill this down until all 8 match".