What Octane?

Topic: What Octane? (Read 16973 times) previous topic - next topic

What Octane?

December 26, 2011, 12:55:43 PM

Hey guys. I think there are a lot of us here that are running very similar engine combos. The engine combo I'm talking about is 5.0 with GT40 heads & intake with TFS stage 1 cam. If you're running this combo, or GT40 iron heads & intake with an alphabet cam, what octane gas do you fill up with? Just curious.

What Octane?

Reply #1 – December 26, 2011, 08:36:29 PM

What Octane?

Reply #2 – December 26, 2011, 08:43:30 PM

Quote from: TOM Renzo;376289

That depends on many factors??? Need a build sheet. Normally an octane rating is a direct result of Pre ignition or Knock.

Thanks Tom, but I know that. Things like rod-ratio, the shape of the combustion chamber, whether you're running aluminum heads, or not, all factor in to what octane is required to prevent detonation. Hell, I can even go into detail on stoichiometric mixtures (fuel/air ratios) if you'd like. I'm just curious to see what people here who are running that pretty common combo are putting in the their tanks. That's all. :)

What Octane?

Reply #3 – December 26, 2011, 09:21:52 PM

What Octane?

Reply #4 – December 26, 2011, 10:22:18 PM

I think you either misread, or misunderstood my question Tom. I didn't ask what the octane requirement of my engine would be. I simply asked what octane fuel guys here who are running that combo are using. Dynamic stacking can obviously vary from engine to engine. Even if the engines are using identical parts. Dynamic stacking however will NOT cause those two engines to have appreciably different octane requirements if the engines are timed the same. That just doesn't happen.
I appreciate you trying to help, but you're answering questions I've known the answers to for a long time. I'mjust asking to see what guys here are running. That's all. I've wrenched on everything from jet engines (I'm a licensed Aircraft Mechanic), to heavy equipment, quads and cars. It's safe to assume I have all the basic mechanical theories like "use enough octane to keep your engine from pinging" down.

What Octane?

Reply #5 – December 27, 2011, 02:12:46 AM

With stock engine and h.o. intake/throttle body, I was running 14-16* with no pinging on 85 woth "up to 10%" ethonal. My alttitude was. Between 4200-5000ft above sea level or so. I never advanced it until it pinged, but left my dist loose and ran up and down a hill until it felt good and until just before my mph going up the hill started to decrease.

The hill I ran was about 6% incline with a traffic light at its base and a 60mph speed limit. I would guess just under 1/4 mile. Just playing with the timing and some stock style $1.99 copper plugs gapped at .40-.45I went from 64mph on the hill up to 76mph. However, I did have to pull my dist out when I had a leaky timing chain cover. I replaced the timing chain while I hade it all apart, and didn't have a timing light to verify timing until later.

Most of what you run has more to do with cranking cylinder pressure and alttitude then anything else. I left my car lower then it had to be because I did over 100miles a day commute, and in a day could go from 3200ft to just over 10000ft in a single day. I did try to run higher octane gas and did not notice any real improvement, or change in power or pinging from different grades.

I would check your compression pressures and make an educated guess from there. You can gain some power by having the heads milled down and usually not have to worry too much about ptv.

What Octane?

Reply #6 – December 27, 2011, 06:48:44 AM

What Octane?

Reply #7 – December 27, 2011, 07:10:18 AM

What Octane?

Reply #8 – December 27, 2011, 08:52:05 AM

My silver POS has GT40P's, an E303, 1.7 RR's. Mustang H.O. block with reliefs in the pistons, stock pistons, just to be thorough. Explorer intake. Have the timing at 16 degrees, runs like an animal. One day about a month after I bought the car, accidentally put 87 in it instead of 93. Made no difference. LOL!!!! Sooooo a cheap as I am, been running it like that since, for over a year now, beating the living daylights out of the thing...no observed ill effects so far. My friend's 90 Vette L98 TPI motor pings with regular gas on a hot summer day if he's got the timing bumped a couple degrees, forget what those run. Naturally he hates me. As said, clearly a case by case thing, but there you go there's my case.

Actually, another case. The red car, S.O. bottom end, flat top pistons, E7 heads explorer intake bone stock valvetrain. Timing again 16 deg, always careful with that engine, always have 93 in it, but once or twice I tried 87 and same result, couldn't get it to ping.

What Octane?

Reply #9 – December 27, 2011, 11:56:18 AM

Since most 5.0 engines run well on 87 octane, I'm not surprised to hear about your cars both running well on it. Since you get more power out of a lower octane fuel than you do a higher octane fuel, it makes no sense at all to run a higher than necessary octane fuel in your car. There are a lot of guys out there who advance their timing tot he point where they have to run a higher octane fuel to keep their engine from pinging. That's counterproductive and is a practice that should be avoided. The higher octane fuel doesn't burn as fast, or completely as a lower octane fuel would. That means less power and more carbon deposits in your engine. The higher octane fuel is more expensive. It's best to run as much timing as you can without causing your engine to ping while running the lowest octane fuel possible. Doing so delivers the most power/cleanest running engine.

You've got a win/win scenario Zonda.

What Octane?

Reply #10 – December 27, 2011, 12:17:09 PM

Yeah exactly. I'll probably mess around with it some more in the red car...like I said up until May of 2010 when I started to tear it down, so about 4 years and maybe 20k miles of running that timing and always 93 octane for fear of even trying, because it's very important to me that the original block and bottom end stay with this car and stay in unmolested condition for the rest of my life. It has great compression, all is fine, 134k miles and all is well. No reason why it can't stay that way. And once it's back together, it'll be driven far less than it was before. Car will probably never see 150k. It did great and the spark plugs were clean and red-ish, which indicates some fuel additives so I've heard. But everything nice upon inspection. It's a good idea though, and if it does good with 87 it'll go over to 87.

I love these engines. And I love Ford EEC-IV and their fuel injection LOL. It's like old guys that won't drive anything without a carburetor. I'll probably never update past this stuff!!!

What Octane?

Reply #11 – December 27, 2011, 03:06:06 PM

I'm running 93 with the timing at 16* BTDC. I haven't tried 87 yet. It may or may not work with 87. All combinations respond differently. Try it and see what happens

What Octane?

Reply #12 – December 27, 2011, 07:47:55 PM

What Octane?

Reply #13 – December 27, 2011, 07:59:55 PM

I ran 93 in mine when I first built it cause I didn't know any better... A years or so later, I found it had better throttle response and didn't ping on lower octane, so I generally ran 89... Of course mine has alu heads so I could likely run it on 87...

What Octane?

Reply #14 – December 27, 2011, 08:32:42 PM

Quote from: TurboCoupe50;376353

I ran 93 in mine when I first built it cause I didn't know any better... A years or so later, I found it had better throttle response and didn't ping on lower octane, so I generally ran 89... Of course mine has alu heads so I could likely run it on 87...

Hey Tom. Where have you been? This place hasn't been the same without you.

Yea. There are a lot of canards floating around about cars like the mistaken idea you'll get more power by using a higher octane fuel. As your experience with switching to a lower ocatne gasoline proved, you get more power by running less octane.

There are obviously situations where running higher octane is worth it. I'm not saying that can't be the case. Being able to run higher compression is a perfect example of a favorable trade off for having to run higher octane. For nothing more than bumping the timing, it's not a worth-while trade off.

As far as higher octane fuels not burning as cleanly as lower octane fuels. That's easy to prove too. Say you had some extremely bored Aircraft Mechanics that decided to do a little test one day. Let's say they took two blue and yellow 55 gallon drums next to the hangar and put super high octane AvGas in on drum. They then put an equal amount of 87 octane gas in the other and then lit the fuels off. When they looked into the barrels, it was obvious the AvGas didn't burn as cleanly as the 87 octane gas did.

Everything I claimed can be and HAS been proven. It's not speculation.