I want to either rebuild or buy a 302 to replace my 360 in my '68 F100. I would like to be able to run 87 octane most of the time with super for those fun occasions. Is there a direct link between compression and the octane you can use? If so what would be a good comprimise to get good H.P. but use the CHEAP stuff, as gas is ridiculous now? Any help would be great.
Thanks
Martin

 

You could run about 9.5 to 1. You would have to turn the distributor back for cheap gas, and turn it back up for the higher octane gas. I have heard you will have better luck running higher compression with cheap gas, using aluminum heads.

 

Why you dumping the 360. Are you shooting for less torque? Ok done picking now. Actually a 302 with a good roller cam can do everything a 360 can. I would try and stay around 9:1.

 

I agree with ratsmoker, stay about 9:1 or so. Is it going to be carbureted or fuel injected. The reason I ask is I think you can get away with slightly more timing with a carb because the air injested into the motor has more time to cool from the fuel's latent heat of evaporation. When the injector injects the fuel, it has about 4" to cool the air and the velocity is much higher by the valve rather than below the carb. The reason I beleive this is true is each of my friends 5.0's have to run on premium or it pings, and the timing is only at 10 deg. That's at 9.0:1 compression. 87 octane= 8.5-9.0:1 comp.

 

instead of a 302, why dont you just get a 390 crank and rebuild your engine?it will have way more power and torque than a 302 and will even button up to your factory transmission and motor mounts to boot. 9.0:1 or maybe 9.5:1 compression is about as high as you can go on the cheap stuff

 

I agree with the above post...turn your 360 into a 390. Unless your going for decent gas mileage.

 

There are many things that effect your fuel octane requirement. Personally, I've gotten away with 9.7:1 static, but the real things that will govern what you can do with what fuel are 1) dynamic compression ratio and 2) you ignition timing advance. Although the dynamic compression is loosely related to the static compression, it depends more on the cam profile. Cams with a tight lobe seperation angle build dynamic compression, but unless volumetric efficiency exceeds 100%, (which may happen in super high-dollar race engines, most street motors run around 88-90% VE), the dynamic compression ratio will always be lower than the static. If your peak dynamic compression exceeds around 190 psi (also known as cranking pressure), then you will have to run high octane fuel, for a standard timing curve. You will be able to tolerate lower octane fuel if you back off the timing. Most pump gas will make peak power with around 36-38 degrees of total timing. The efficiency of the combustion chamber will also have an impact on the octane requirement. Most factory cylinder heads will have hot spots in the chamber from inefficient design that occur with high compression, and it may preignite low octane fuel. Most aftermarket heads have very intensely researched chamber shapes that minimize these hot spots, and reduce the octane requirements. I would say that with a factory cam and heads, the upper limit for cheap gas is around 9.5:1 with 36 degrees total timing. This also depends on where you live, and what elevation. Some areas have 86 octane cheap gas, and fuel will be much closer to its vapor pressure at great elevations, and will therefore be much more prone to detonation. This is the long and mostly complete answer to your question. There are plenty more things that have an effect on this, like spark plug heat range, winter fuel (extra volitile blend) availability, piston shape (quench principle), and so on. I'm gonna stop now because I think my keyboard is running out of ink. TK

 

OK TorqueKing and any other that has insite on this ...
What Octane Fuel should I use in my 14.5 to 1 Compression Motor.
I'm Currently using Phillups B33 Racing fuel but I think it's for 16 to 1 Compression..Could I get away with less then 116 Octane.

ShaZam

 

it depends on your cranking pressure. Get a compression tester and hit the starter, and tell us what your peak compression is, and what kind of ignition timing are you trying to run?

 

On my rebuild I am running 200 psi cylinder pressure, timing is set at 14-15 BTDC and using 89 octane. I've yet to get this thing to ping. (Well, maybe a TEENY hint in 4th gear, pedal mashed at 20 MPH). This is generally thought of as 93 octane territory. During rebuild I employed everything I could for ping resistance (tight quench, polished combustion chambers, retained EGR, CompCams 31-255-5 114 deg LSA, MSD 6A), and it seems to be working well. I'm more than tickled as I thought this was going to be a major hurdle on building a high compression street motor. I'm also thinking the Windsor Jr's have a well designed chamber that might be critical too.

Randy

 

OK took a compression test and here's the figures. Highest 270 psi lowest 250 psi ... I run 38 Dreg. Fixed (Welded dist..)
Now with this info ........what Octane should I be running ..

 

So you've got 40psi more than my 2.5L was running, and I was
running super no lead...

20 years ago, I had a 2.0L running 12.5:1 on a mix of super unleaeded
and 104 octane av-gas, depending on your engines ability to resist
knock, you may be able to get away with a 1/2 and 1/2 mix...

 

run the 116 octane, and back your timing down to 34-36 degrees. I've seen some test that seem to show that the higher octane fuel needs less spark advance than lower octane fuels, and tend to make peak power between 32-36 total timing.

 

OK


Thank You ............

 

Using cast iron heads i'd keep the comp ratio no greater than 9.5:1

using aluminum heads can bump up the comp ratio 1 point....10.5:1

Getting the alum heads heat dispersant coated can also help with high advance predetonation....

Another approach is gettig the headers, down pipes, valve and piston crowns thermal barrier coated...

Also consider a cold air inlet filter....the cooler tha air going into the engine, the less chance of the mixture spontaneously igniting....

I would also consder the Edelbrock rpm air gap intake that separates the intake carb plenum from the oil lifter valley....this drops temps of air inside the intake by quite a bit....

then there's timing tweaking......to match driving habits and octane being used...