Max pump gas psi - iron vs alum

So i finally decided to move on from my 7:1 low compression 413 RV motor - picked up a 440 block locally and plan on piecing it together over the coming year. Purdy stoked.

So the question is - what would be a max safe cylinder pressure that i could run 93 octane? In all likelihood, i'll be running either my 906's or the 915's i picked up recently. Not looking to be on the hairy edge of detonation, the car is 99% street with a few visits to the track a year. Not a daily driver, just a weekend toy.

I do plan on using an wide band A/F gauge to tune it.

Gracias

cast iron (9.5to1) pump gas

mopar12372 said:

cast iron (9.5to1) pump gas

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I've read that as well. I'm wondering though, at 9.5:1, by playing around with cam selection and intake valve closing, you can get anywhere from about 150psi to 190psi in cranking pressure. Hughes says 165psi iron heads and 195 psi aluminum. Gregory C has had issues tuning at about 190 psi with AL and ive read some guys suggest a max of 175psi, but not sure if that suggestion was for AL or iron.

I am currently running iron 915s with 13 to 1 C/R in a street strip motor. But I am at 5600 feet which makes a difference. Lowest octane that it will run on with out pinging is about 96. I get that with a blend of 91 octane premium, and 100 octane race fuel. You can also raise the octane a little by putting in E 85, at about a 20% or 1 to 5 ratio. These motors really come alive at C/R over 12 to 1, but you have to have lots of quench (.035) and above 95 octane fuel. I am swapping these heads for some Edelbrock Performer RPM heads. Will have the same quench at .035, but C/R will be 12.4 to 1. So will see what octane requirements it will have with the alum heads.

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I once had a 440 with .020" quench, and 11 to 1 C/R. Ran great on the street, but at the drag strip a friend over revved it and threw a rod. But there was no evidence of the pistons contacting the cyl head. A rod bolt had broken. Found the nut in the valley pan.

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Running tight quench will reduce the octane requirements. It improves the combustion process, and helps to keep the engine from pinging. I have run as tight as .030" quench recently with no problems.

ya , i have a buddy that runs stock class in the nhra with piston 75 in the hole / dish running a wierd cam like your talkin about lots of quench.
10.85 et . BUT with that said those guys are as bad as smokey yunik so who knows what is really goin on . haha

Makes you wonder what kind of C/R they running. You sure he has dished pistons? Thought stock class allows flat tops?

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Oh and cranking compression readings on my 451 are 210 to 220 PSI.

IMO,
More involved than just cranking psi from Camshaft durations effect on static C.R., because of Cylinder Head efficiencies above 100% @rpm,(or lack thereof below).
example;
What "should" work GREAT at 175psi on Aluminum, can rattle like a pop can under loads from poor quality A/F charge(port problems) as delivered and ignited in the cylinder...
and vice-versa....

"efficient" ports... as measured/tested/plotted from swirl meter data combined with good A/F monitoring are the best bet when pushing anywhere close to detonation pressures.... NOT real practical for DIY guys ?
IMO again,
If you are at home on your own, get the Most Head Flow you can for the application, stay very safe on the Dynamic C.R. with Cam events.... and let the HEAD make the power.
just my opinion

mopar12372 said:

cast iron (9.5to1) pump gas

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Ummm, where did you hear that? Depends on cam timing, size, chamber and piston design, etc. I have consistently run 10.5:1 on 93 with iron heads. Static compression ratio and cranking p.s.I. are not really indicative of what it can burn.

i see you asked a simple question with a very complex answer.as stated there are several factors comming into play here.if i were you i would shoot for about 10:1 static with a good quality piston or about 9.5:1 if you are going low budget.you should be safe at those levels and not need a doctoral thesis on air flow management.that is by no means making fun of any statement made on this thread.i am just trying to keep a simple answer to a simple question.hopefully you plan out the entire build before you purchase even one part.

from my experience 180psi can be edgy with an iron head 440, and thats with a good quench. 160psi is pretty safe. this is from my experience with the two 440's in both my cars. the short blocks are identical with exception of camshafts. both use kb184 quench pistons at 9.3:1. the 160psi engine is be replaced with an alum head engine with good quench but the static compression ratio will be less than 10:1. something people don't think about is carbon build up in the chambers. just .010" of build will raise static compression ratio .5 point, and .010 of carbon isn't much. another thing to think about is that last year the EPA changed gas formulation. there will be only one type of gas available, not 50 different blends. i suspect the "one" type will be california 91 octane. i don't adhere to the alum head 1.5 point compression increase stuff. pressure is pressure. i don't care if the head is iron, alum, or plywood; too much pressure at the wrong point in the power stroke and you have detonation.

Yeah, fellas, I figured I was oversimplifying. I figured I'd run my stock 906's for a while, maybe fool around and give em a bowl blend and gasket match. Run the kb184's for something in the low / mid 9:1's then if I wanted to grow, do some 75cc RPM's which will bump it to 10:1 or so, then if a cam swap has to happen at that time, so be it.

Using a quench dome piston, like the KB hypers, VS a non quench, open chamber combination is going to be a big factor in regards to intake valve closing time/cranking compression.
I like to use the pistons with the oversize dome and machine it to the chamber for the correct quench. This way, you're not having to mill the hell out of the head to try to get it tight to the piston.
I have run 10.5:1 compression, quench piston, iron head 440's with 226°/234° cams on a 110° LSA with no detonation problems. On the other hand, my personal engine is fuel picky as all hell with 11:1 with no quench. I knew it would be, and I run a race fuel blend.

180-190 psi cranking compression is not unreasonable in an engine that had good fuel atomization.

The 440 in my Belvedere is a 10.1:1 / 190 PSI motor and it will ping like crazy on a hot day. Driving around light footed is fine. With a splash of 110 all problems are solved and the car flat gets with the program. The engine combo is KB 237's .015" in the hole with a 915 head and steel shim gasket. Intake is factory iron with a 750 AFB jetted 4% richer in the secondaries. At the time I didn't have access to an O2 sensor but things seemed to work well enough that the 4000 lb car ran high 12's with slicks.

Now this same engine with a six pack. The short answer is less power and lean. But no pinging on pump gas. I realize I have quite a project on my hands to make the six pack work but those results were quite an eye opener. My initial guess is fuel distribution difference between the iron 4 BBL and the six pack. The six pack has now moved to the orange RR so tuning will resume when that gets running.

That sure seems odd that the six pak is making less power than the stock iron intake and a 750 Carter. Got to be something wrong with the jetting in the carbs, or maybe an intake leak somewhere? Have you tried going a few steps richer on the jets? Good luck.

I fought with my six pack for 10 hours on the dyno to get it right. Drilling metering plates, trying different vacuum curves with light load, etc.

I finally got it right on. A nice fuel curve and consistent egt's. The hi-vac cruise timing is probably over 50° or thereabouts. A 160° thermostat makes it much less prone to detonate.

I can't imagine ever getting it close in the car trying to "road tune" it.

of all the 6-paks i've ran thru the years power problems were always fuel delivery. i've learned how to modify mech pumps to feed the carbs without too much fuel pressure/regulators or nusiance electrical pumps. i love'em; don't leave home without one!

I had a detonation problem with my 493. Aluminum heads and almost 11.0 to one compression. My cranking compression of 188 average knocked at WOT on California 91 octane gas. I changed to a solid lifter cam in an effort to reduce cranking compression but I had the specs mixed up. The solid cam raised my average psi to 191 and it knocked worse.
This year I added thicker head gaskets and with the compression reduced to 10.07 and 160 cranking psi, I'm not knocking anymore.

So the aluminum heads didn't help at the 11 to 1 C/R? I really couldn't see how alum would allow more C/R over iron with out knocking. That is, given the same C/R. That seems to be a myth that you can run more C/R with alum vs iron with out raising octane.

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I keep my Hi Compression 451 as cool as possible. Also using slightly richer jets on the sec side of the carb, that's when it will knock, when the secondaries tip in. There are alot of tuning tricks you can do to eliminate or reduce pinging. Cooler temps, richer mixture, higher octane fuel.

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Once 2 summers ago, on a 105 degree afternoon, going up a gradual incline, it started knocking like crazy. Had to pull over and cool it down. No probs since then. Heat is definitely a big factor. And was only at 12.6 to 1 C/R then. But had .117" quench with the 906 heads.

Having to rely on octane boosters or mixing race gas with pump gas S U C K S.
It is much better to build an engine to run on common pump gasoline. These cars are supposed to be fun to drive. They shouldn't be a pain in the ***.

rev.ronnie said:

I fought with my six pack for 10 hours on the dyno to get it right. Drilling metering plates, trying different vacuum curves with light load, etc.

I finally got it right on. A nice fuel curve and consistent egt's. The hi-vac cruise timing is probably over 50° or thereabouts. A 160° thermostat makes it much less prone to detonate.

I can't imagine ever getting it close in the car trying to "road tune" it.

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Oh man, it's a disaster. I'm not even close but putting stiffer springs in the vacuum pods helped a lot. There is also a bleed off hole for the vacuum signal port that goes into the main venturi - just like any vac secondary carb. I have three sets of carbs and found two different size holes. I want to say a .043" and .067". The ones on my car were the smaller and I drilled them out to the larger. That slowed down the outboard carbs quite a bit more, which was a good move for me. My other set of carbs has two different size holes and this is a set I had sitting around since the 80's.

On a chassis dyno I was able to determine that the outboards would never open fully under load with the black spring, but that produced the most consistent mixture. A two step lighter spring opened fully but went super lean right at opening, with slow recovery as max RPM approached (and still never recovered to a normal mixture). Another thing I noticed was how non linear the outboards opened. I'd say I got 75% open almost immediately and the remaining 25% went pretty slow (lighter spring). The lean spike seemed to be from the rapid opening causing a venturi signal delay. On the black spring test I hooked up a piece of wire and when they stopped moving at maybe 50% I pulled them open manually. Now the RPM was maybe 4500-5K and it really liked the manual opening from the higher RPM! It was like a N2O shot! These tests were done at high gear acceleration from 3000 RPM to 6000 RPM.

The most recent battle has been dealing with the primary mixture. I can get the idle in neutral and in drive OK but as soon as I tip in it goes to 12:1 and pretty much stays there on cruise. Rolling on the throttle the mixture starts to correct toward stoich. Readings are done with a tail pipe O2 sensor, which is not the best way to go, but better than nothing. I do plan to put a bung in each collector and add TC's to each primary tube. I may end up with a Race Pack to fully dial this in.

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lewtot184 said:

of all the 6-paks i've ran thru the years power problems were always fuel delivery. i've learned how to modify mech pumps to feed the carbs without too much fuel pressure/regulators or nusiance electrical pumps. i love'em; don't leave home without one!

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I have a 7 PSI mech pump but fuel delivery could very well be at the top of the list of stuff to check. One thing carbs like is a consistent fuel level.