I have always assumed any engine I rebuild would run on regular gas... The other day I filled up at a different station across town and they had 93 octane at the pump!!

That got me wondering what would an engine built for 91-93 octane would be like day to day compared to one set up for 87 octane?

Also I have this idea that if the engine was built for 91 octane, I may well be able to get away with 87 Octane in the winter, and spring, and 93 octane in the heat of the summer here in Vermont.

What are your experiences and thoughts on this subject?

This question is a "math problem" you can solve, with actual measurements, from your engine...

Effective compression in the 7.8 to 8.0 range will run well on 87 octane fuel
Effective compression in the 8.2 to 8.4 (?) range for 93 octane fuel. (actual)

Do the math...
https://uempistons.com/p-28-effective-c ... lator.html

Yes... the engine will produce more power (or economy) if you can "dial-it-in" to run on a higher grade fuel.
That is exactly what today's "new" engines do... with computer controls.
You would have to occasionally re-adjust the timing on your "old school" SL6, to compensate for the "seasonal" fuel quality octane level variations.
DD

Thank You DD!

I will limit DCR to 8.0, as it is probably easier to go up in compression later on, than the other way around.

Jase

I build almost all my motors for pump premium. The cost and time in building the engine and car far outweigh anything you might save with a few bucks each fillup. I'd rather have more fun...

Lou

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Home of Slant6-powered fun machines

Yes, that sounds terrific .. and, I would like all the compression I can get away with.. and stay out of detonation when loaded heavy, pulling a hill in the middle of summer... Not that I PLAN on working the truck on the hottest day I can find, but if I build it othewise, then that decision may come back to haunt me..

The good aspects of the vehicle that will support some compression:

-5.83:1 Ring& Pinion gears,
-Manual transmission,
-Willingness to adjust timing and even change thermostat between summer/winter, to optimize the seasonal tune up.
-Good amount of radiator space for HD cooling
-55 mph top speed (at 3200RPM)

The challenge aspect:

-5600 LB empty curb weight

What is this??

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Same question as Rick, I had a old military power wagon with those same gears, but might have weighted 4500 #

I'm building my truck to run on pump premium as well. It's not my daily driver, and it will need all the power it can muster when I'm pulling a camper. If my estimications are correct, I'll be in the 9:1 to 9:3 range.

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If it was easy, everybody would be doing it.

Don't forget the highest you can get in some states is 91...…...

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Yeah, ^^^^^ that. And if You're far away from home base, You may buy an unforseenly awful tank-full of garbage fuel that would bring an stock squeeze mill to it's knees. I know, I've had it happen, in a '76 350 GMC 4x4. Stock low comp. popping and pinging that forced Me to stop for more fuel early, and I stepped up the octane to abate it.

My understanding is that the same engine doesn't require as high an octane rating at higher altitudes. Lower ambient air pressure leads to lower cylinder pressure for a given DCR, reducing the tendency to ping. This is the reason for lower octane availability in places like Colorado, although I suppose there could be exceptions not related to altitude where 93 isn't available.

I can definitely say that the same (bad) combination of DCR and ignition timing that burned up a head gasket in an hour of endurance racing in Houston lasted about 8 hours in Eastern Colorado, though I don't recall what grade of fuel we were running at that Colorado race. I think we used to run 87 at home back in those days, but we may have stepped up to mid-grade or premium just to be cautious if all the numbers on the pump were lower than what we were used to.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Dodge M37. It is more like a high speed tractor, than a truck.



I would love for it to weight 4500!! What truck did you have?



I like the idea, but I am afraid the engine will not like being fully loaded in the middle of summer, Presently I am thinking I will build for the high side of 87 octane. Something like 8.0 DCR, and put enough cam in it to drop the cylinder pressure a bit... Then run it in the middle of august and see how it behaves...

Jase
My Power Wagon was a 1947 WDX without a winch. I know it was advertised at 8700 # gvw with a payload of 3000# but mind was a lot lighter than that?

With 5.83 gears, you are very unlikely to ever be more than 4 miles from home!

I could well be dead wrong about the weight of the M37.... The data attached to the the glovebox door, suggested 5600 as the base weight, and up to 2000 payload. Just looking around the world wide web, I saw an article about a WDX having a 5200 base weight, with winch. I can't imagine how an M37 would weigh more, as it has a shorter wheelbase.



Yes, it is or was happiest at 35-40 MPH.. with the flathead, hopeful that the Slant 6's willingness to run for an extended time at 3200-3500RPM will give it a bit longer legs, and a 15-25 mile range.