I'm in the planning stages of building a pump gas engine for my 69. Planning on doing a 400/512 combo, and using my old INDY SR Heads. The downside of the SR heads, is they have been cut to 70cc's. The 4.250 stroke, 4.375 bore with zero deck, and a piston with 24cc dish, the compression ration still come out to 11:1. Is this safe for regular 91 octane pump gas? I do not want e85, and not willing to mix gas. I want to be able to pull up to the pump, fill the tank, and enjoy driving the car.

Sure it'd be ok, assuming you have quench and over 270@050 cam. Cleaning-up the sharp edges on the piston and notching the bores combined will help reduce some static comp. If there's signs of trouble 28* timing and a good cooling system help...

2800 feet elevation here and there are a few guys running around with 11:1 +/- and aluminum heads on the street with pump gas. Most stations stop at 91 octane, but there is one brand (Stinker Station?) that has, or used to have, 93 octane. Based on how some of these engines sound at idle they have pretty good sized camshafts in them.

11-1 is a no brainer with an aluminum head. We have pushed them to 12-1 with no real issues. but need to take that into account when working over chambers and cam choices.

I think you'll be OK, another thing to consider is reverse cooling if you think you may have issues.

Jeff my old 360 that I did several drags weeks with was 11:1 with good quench and never had any issues.

Down here in mid South Texas heat no way , running a huge cam helps but I think that is counterintuitive , can you open up the combustion chamber some ??? just a thought.

I would re- think that combo. Once it's buttooned up and doesn't work you'll either mix gas or start over. I prefer to error to the conservative side on drivers.

We run 11:1 in the 505 RB.

No issues on 91 but typically run 93.

My 540 will be 11:1 with a 275 duration cam, not worried about running it on 93.

In your case you will have to pay a little more attention to the timing curve though as you'll be able to heavily load the motor well below peak torque. With a normal 4500-ish rpm street/strip converter, the engine easily blows through the area where cylinder pressure is highest and it's most sensitive to detonation, so the timing curve isn't as sensitive.

Jeff, talk to Dwayne about opening up the chambers. When he ported my SRs years ago he opened them up some, but I can't recall how much exactly. Mine were somewhere around 64 or 65 cc when I sent them to him.

This has been beat to death, but consider these things. First the static compression is not the only factor involved. I have successfully built pulling truck engines in pump gas classes with 12 to 1 static compression. The dynamic compression is much more a factor in octane requirements.
second most engines do not detonate at WOT they do it under lower rpm and high loads, the ignition curve will solve those issues very successfully. Third how does an engine builder juggle these factors? I use performance trends engine software, it will calculate the dynamic compression ratio, cylinder cranking compression, idle vacuum, and will generate a timing curve to keep the engine out of detonation. Cost is reasonable, I use it on every build I do. And I have a dyno in my shop, predicted results are within 10% on every build I do. Just remember garbage in garbage out, you got to take the time to input good numbers into the program.

The cam duration is less important than the intake valve closing point ABDC (nominal, not @ .050").
Probably OK,, but other variables to give a Y/N:
rod ratio (low # = better)
spark curve (slow = better)
thermo temp (low = better)
SLR (high = better)
converter (loose = better)
weight (low = better)

Jeff, the 408 smallblock i built had 10.5 compression, iron EQ heads with good quench, cam was a hyd roller 235/249/107 in at 105, timing curve was lazy, 16 initial 30 total but not in till 4,000 rpm, the flash of the converter. A little different animal than you are building i know, but thought the info useful. A friends 451 lowdeck running quench pad pistons and iron 452 heads machined for .040 quench distance has 11-1 compression, runs 93, hydraulic flat tappet cam with 268 at .050 duration, has run 11.26, full weight 68 roadrunner at Rockfalls. That combo has been together for several years. Pump 93 fuel .

There is no correct answer as some engines ping on low compression while the new BMW M 1000 RR has a 14.5:1 SCR and will run on 91-93 octane just fine with barely any manipulation of the timing and fuel curves. The piston to head clearance is less than the thickness of the head gasket due to the small beautifully contoured dome required to reach this SCR. The BMW engine also has squish pads in four places, and undoubtedly has lots of compute hours getting the mixture motion most homogeneous in order to propagate the combustion process very rapidly from start to finish. BMW went to a much more oval port design in the new M engine and this was almost certainly done in order to fill the cylinder with a closer-to-ideal mixture in the combustion chamber that allows almost magical results.

A person really needs to derive a burn characteristics model to state a decent estimation of maximum SCR in a given engine and chassis combination as vehicles with less mass and loose torque converters or a stick can get away with more SCR as the piston spends less time at critical points of the combustion process.

The OEMs and heavily financially backed race teams spend a minimum of a few thousand compute hours before a test engine is built. These are typically single cylinder engines because it's far cheaper and yields valuable data. Shmoes like us have to use best practices in port design, valve jobs, combustion chamber shape, piston crown, squish, cam timing, cooling, vehicle mass, final gearing, and fuel and timing curves to arrive at a reasonable estimate.

There is an untapped market for cylinder heads that accurately state that head x has a higher threshold for knocking under various scenarios. Flow numbers mean nothing without providing a homogeneous mixture ready for a nice, fast, and complete burn. That's what the OEMs do and have been very successful in that area.

I would tend to fall with jwb123 and say it would be worthwhile to calculate your dynamic compression. Wallace calculators has one, but the trick is getting your actual intake valve closing, as you really want the degree it is actually on the seat, as normally you are only given at .050 numbers and then you have to account for lash (if you have a solid FT). While the numbers are not an absolute at least you may have a feel whether you are on the edge (a 7.9 vs, 8.9) so maybe having good quench or reduced timing is necessary.

Cam is going to be a solid roller. I have an OLD .750 lift Super Stock grind cam, that will be reground to something around .650 lift. I'm trying to use as many things I have laying around over the years, as I don't have a ton of spending cash. Planning on just getting a short block done for now, and then will focus on the heads when that time comes.

Torque converter? Never owned one, and I'm not going to. Car will have a TKX 5 speed, with 4.10 gears, and a 315 tire.

The events on an old cam will likely cost you some.

Back in the day, those old rollercams at .750 lift were 280+ at .050, should be enough room to make plenty of cam for a hot street motor.