Octane requirements

[747mopar]

I'm sure one of you know of an octane versus cranking pressure chart to tell what octane is required for a certain cr. I did a cranking PSI test on my motor and all cylinders are in the 155-160 psi range, I'm running iron heads and have been using 93 octane but I think I can get away with less octane (less money).

 

[69Bee]

Try it, and if it runs fine, then you are good.

 

[Cranky]

I would think you probably could depending on timing and temps. Is this a stock engine with little to no quench? That also plays into the equation. Anyways, wouldn't hurt to try and see what it can use.....if it starts pinging, you can back off of the timing a bit and that might help it quit or go back to 93. Also, if it runs above 180 or so, that can help it to ping too.

 

[747mopar]

It runs right at 165 (165 thermostat), has flat top pistons, 452 heads, timing is 16 btdc and 36 fully advanced. It seams perfectly happy but cheaper gas means more driving.

 

[Meep-Meep]

This is a very complex subject and there are a lot of variables. As an example I had a 66 F250 with a 352 and a mild cam. Cranking pressure was 160 PSI and I ran regular. The actual CR I didn't know but can safely guess it wasn't over 9:1. The truck ran great on 87. My 66 Bel with a 440 is 10.1:1 and 190 PSI. It does not like pump 91 but I can get away with it if I drive it like an old lady. My 66 hemi Charger is 220 PSI and I have no idea the CR. Again, drive like an old lady and it's fine, but ping city when you get on it. It also seems to generate a lot of heat under the hood and the carbs just heat soak. With your cranking pressure of 155 I'd certainly try 87. An engine will survive pinging pretty well. What it won't survive is preignition. Preignition is a case where a hot spot ignites the fuel charge long before the piston gets near the position where the spark hits. This causes insane pressures and immediate destruction.

 

[747mopar]

I think next time I fill up I'll try 89, if it goes well Ill do 87 and see what happens.

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This is a very complex subject and there are a lot of variables. As an example I had a 66 F250 with a 352 and a mild cam. Cranking pressure was 160 PSI and I ran regular. The actual CR I didn't know but can safely guess it wasn't over 9:1. The truck ran great on 87. My 66 Bel with a 440 is 10.1:1 and 190 PSI. It does not like pump 91 but I can get away with it if I drive it like an old lady. My 66 hemi Charger is 220 PSI and I have no idea the CR. Again, drive like an old lady and it's fine, but ping city when you get on it. It also seems to generate a lot of heat under the hood and the carbs just heat soak. With your cranking pressure of 155 I'd certainly try 87. An engine will survive pinging pretty well. What it won't survive is preignition. Preignition is a case where a hot spot ignites the fuel charge long before the piston gets near the position where the spark hits. This causes insane pressures and immediate destruction.

Like the new avatar!

 

[Garys1969RR]

I am getting cranking compression pressures of 210 to 220 PSI. A 451 cubic inch at 13 to 1 C/R. Last fuel mix was 3 gallons of 91 octane Premium, (all the premium is 91 octane in Denver, as opposed to 93 at lower elevations). 2 gal of E 85 which is 105 octane, and 1 gallon of 100 Sunoco race fuel. This mix should be about 97 octane. The only time it would ping, was at wide open throttle going up a steep hill at 70 MPH. The engine was fairly warm. Got it to the track, cooled it down to about 130 degrees coolant temp, and no ping! Engine temp plays a big part in this. Cooler=less tendency to ping.

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Now going 4 jet sizes larger than stock on an 850 CFM Holley double pumper. And with this carb jetted richer than stock, I will use a 50/50 mix of E 85 and 91 Premium with the 850 on the engine. That should work well, and give slightly higher octane numbers. This mix averages out to 98 octane. Bottom line is experiment until you find what works for your particular combination of cam, C/R, Altitude, and fuel choice.

 

[747mopar]

When building this motor I put extra emphasis on a cool intake charge (shaker scoop) and a cool running motor (big aluminum rad, good fan and shroud, high flow water pump and a 165 thermostat). Like I said before it stays right around that 165 so heat shouldn't be to much of an issue.

 

[Cranky]

Oil is designed to run best at 180. If it were mine, I'd install an oil temp gauge and see what it's running. It might be running cooler and cool running oil won't evap the moisture out of the crankcase very fast.....

 

[747mopar]

Good point Cranky.

 

[383man]

Many say that you dont want to go over 190 cyl pressure and run pump. Yours should be fine on 93 like it is and as was said you might get away with less with your cyl press but you dont have quench so you will just have to try it and see if it pings at all. It all depends so much on cyl press which is dependent on the comp ratio and the camshaft and cyl temps along with quench so there is a general rule but nothing is written in stone as its to many things affecting it. Ron

 

[sleepar]

The cheaper the gas the more it cost in repairs later.... I run $12-$14 a gallon 100-108 octaine vintage leadded gas or the racing gas with no ethanol,mpbe, water etc I. It.... if you can't afford $14 per gallon explain how you can afford to fix all the problems associated with the cheap fuel.... cheap and old mopar don't work well together in every instance except maybe the air in the tires versus nitrogen....

 

[383man]

The cheaper the gas the more it cost in repairs later.... I run $12-$14 a gallon 100-108 octaine vintage leadded gas or the racing gas with no ethanol,mpbe, water etc I. It.... if you can't afford $14 per gallon explain how you can afford to fix all the problems associated with the cheap fuel.... cheap and old mopar don't work well together in every instance except maybe the air in the tires versus nitrogen....

Well I built my 440/493 to run on 92 pump which it does fine and has done with no problems for the three years its been in the car and I have not had to fix any problems because I run pump gas. I do agree the pump is not getting any better but I wanted to drive my car anywhere and get gas in any station. I used a dished piston for 10.6 comp with my aluminum heads with quench and it runs fine on 92 pump as I have never had race gas in it. I drive it 45 miles to the track and run 10.70's thru the pipes on pump just like I drive it. Sure I would rather build it to run on and use race gas but I would not be able to drive it much when you compare right now 92 pump is about $3.95 a gallon and the race gas is $10.00 a gallon and up. If you built the eng right to run on pump you should be able to as many people out there are still running strong engines on pump. Ron

 

[747mopar]

The cheaper the gas the more it cost in repairs later.... I run $12-$14 a gallon 100-108 octaine vintage leadded gas or the racing gas with no ethanol,mpbe, water etc I. It.... if you can't afford $14 per gallon explain how you can afford to fix all the problems associated with the cheap fuel.... cheap and old mopar don't work well together in every instance except maybe the air in the tires versus nitrogen....

What are you talking about "if you can't afford $14 a gallon gas"? Let's see here, if you are building a motor "for street use" and build it to only run on $14 a gallon fuel then I'd say your an idiot "seriously". When I built this motor I made sure it had hardened seats, plenty of cooling and a low enough CR to run pump gas so I can AFFORD to drive the wheels off of it I'm only now considering trying a lower octane. To say that you have to run $14 a gallon gas to not have problems is ridiculous, maybe if your running an original pre 70s high perf motor but not if you built your own with pump gas in mind.

 

[HT413]

The cheaper the gas the more it cost in repairs later.... I run $12-$14 a gallon 100-108 octaine vintage leadded gas or the racing gas with no ethanol,mpbe, water etc I. It.... if you can't afford $14 per gallon explain how you can afford to fix all the problems associated with the cheap fuel.... cheap and old mopar don't work well together in every instance except maybe the air in the tires versus nitrogen....

If I were to build my motor for $14 fuel, I'd just go 13.5:1 and get the extra power. You can spend your money however you want, but you're out of your mind.

 

[ma mopar nutz]

If I were to build my motor for $14 fuel, I'd just go 13.5:1 and get the extra power. You can spend your money however you want, but you're out of your mind.

maybe he only drives it a 1/4 mile at a time O and on and off the trailer !:icon_confused:

 

[sleepar]

So every car made before UN leaded fuel was either a racer or a trailer queen. OK you can get the good gas everywhere if you look for it and have it delivered to you're house if you have a tank.... you get better mileage and the the car runs so good on it you will not recognize it as being the same car.... try a tank or 2 of good stuff and you'll quit messing around with mixing and additives that cost you more in the log run.... true it's not a daily driver..... but if your car sits for months at a time the fuel is fresh no problems with carbs full of crap... I mixed fuel for years and its a waste of time MO. for those who drink beer it's like drinking the slop tray after a hundred beers were poured and it sat overnight compared to a full pint of whatever you prefer.... beyond the hardened seat issue sinking valves....

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I got 15 mpg with the pump gas and 26 on the 108 leaded.... same driving same setup except I had to turn the idle down about 200 rpm. And the car can sit for years and crank up with a good battery.... if you don't drive it every day at least put it up with it...
This was with a '69 Buick skylark special (gs) with a 350 factory stock motor.

 

[Meep-Meep]

So every car made before UN leaded fuel was either a racer or a trailer queen. OK you can get the good gas everywhere if you look for it and have it delivered to you're house if you have a tank.... you get better mileage and the the car runs so good on it you will not recognize it as being the same car.... try a tank or 2 of good stuff and you'll quit messing around with mixing and additives that cost you more in the log run.... true it's not a daily driver..... but if your car sits for months at a time the fuel is fresh no problems with carbs full of crap... I mixed fuel for years and its a waste of time MO. for those who drink beer it's like drinking the slop tray after a hundred beers were poured and it sat overnight compared to a full pint of whatever you prefer.... beyond the hardened seat issue sinking valves....

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I got 15 mpg with the pump gas and 26 on the 108 leaded.... same driving same setup except I had to turn the idle down about 200 rpm. And the car can sit for years and crank up with a good battery.... if you don't drive it every day at least put it up with it...
This was with a '69 Buick skylark special (gs) with a 350 factory stock motor.

I'd like to see a 69 Buick with a 350 get 26 MPG. They didn't do that when they were new.

 

[747mopar]

So every car made before UN leaded fuel was either a racer or a trailer queen. OK you can get the good gas everywhere if you look for it and have it delivered to you're house if you have a tank.... you get better mileage and the the car runs so good on it you will not recognize it as being the same car.... try a tank or 2 of good stuff and you'll quit messing around with mixing and additives that cost you more in the log run.... true it's not a daily driver..... but if your car sits for months at a time the fuel is fresh no problems with carbs full of crap... I mixed fuel for years and its a waste of time MO. for those who drink beer it's like drinking the slop tray after a hundred beers were poured and it sat overnight compared to a full pint of whatever you prefer.... beyond the hardened seat issue sinking valves....

- - - Updated - - -

I got 15 mpg with the pump gas and 26 on the 108 leaded.... same driving same setup except I had to turn the idle down about 200 rpm. And the car can sit for years and crank up with a good battery.... if you don't drive it every day at least put it up with it...
This was with a '69 Buick skylark special (gs) with a 350 factory stock motor.

Ok maybe you were referring more to the guys talking about mixing high octane fuel because I'm just filling her up and going on my merry way. I simply don't want to keep filling it up on 93 if it will run fine on 89 or 87, kinda silly to buy something you don't need. I don't like this crap gas anymore then you and am already planning on finding somewhere that sells the good stuff ( no ethanol) for winter storage.

 

[Meep-Meep]

Current pump gas is what we are stuck with and if we want to put some serious seat time in our cars, not building the engine for it is just insane. Sure, good gas is a good thing for our stuff but leaded gas is not legal to run anyway. What happens if you get busted? Probably the same as getting caught running red diesel in my new Dodge. What do you do on a road trip? I say build for and run pump gas and if you want to store the car fill with good stuff and get it mixed through the system before parking it.