Stupid question of the day

[rtstrack]

So I purchased my first cobra with a 428 big block in it, only to realize that I don't even know what kind of gas goes in it. I was told that the block was from the late 60's, but i am assuming it has been rebuilt since then. I think original engines back then ran on LEADED gasoline, but i have no clue, that was 20 years before my time. I was also told from the prior owner (a used car dealership) that they were using "racing fuel" in it. Can't i use 87 or 93 pump gas in it? ANY help would be appreciated.

[undy]

If the heads are original and rebuilt then they should have hardened exhaust valve seats installed during the rebuild. That's the only reason you can't run unleaded as the lead coated the exhaust valve seats to protect them from the heat. The only way to tell on the seats is to pull a head. Other than that I'd recommend running only 93 octane as with OEM iron heads you'll probably run into a detonation problem running 87/89.
Hopefully you can find some history on the rebuild. Other than that keep your fingers crossed if you buy it.

Dave

[mr0077]

I was racking my brain trying to figure out what I'd do in your situation, and concluded the first thing would be to try to contact the guy that built the car and ask what he did to the motor (and then try to figure out if the motor build in your car is the same one he did). If that is not an option, then waaayyy more info is needed to help you. What idle RPM have you got and is the idle rough (lumpy or racey)? If a 700-800 rpm relatively smooth idle, I'd think it was a mild build and could use 91-93 octane, but if it won't idle below 900 RPM and sounds and acts pretty radical, it may require higher octane fuel. You could check the compression, but interpreting those readings is tricky and depends a lot on cam overlap and compression ratios.
You might try running the tank down, putting in a few gallons of 93 octane and drive it gently, listening for pinging. If you get pinging or rattle on light throttle opening (in gear) you probably need higher octane (but it could be an ignition timing issue), and you could either add racing fuel or Xylene to get the octane up. If no pinging or rattle, continue to use 93 carefully.
If you don't feel comfortable interpreting and testing, take it to a good mechanic in your area and ask them to give you their opinion. Lots of variables involved in this without taking the engine apart to be sure of what you have.
Contacting the previous owner or engine builder would be the best option.

Any other ideas out there? Not an enviable situation but an interesting question...

[rtstrack]

So if i run 93 octane I should be fine regardless of the rebuild? I'm mechanically challenged if you couldn't already tell.

[Ron61]

Undy gave you good advise. My 69 NASCAR Cobra with the 428 SCJ has high compression and the book says to run nothing less than 105 octane leaded gas in it. The highest rating we can get here, and it is unleaded is 90. So I keep a barrel of 110 leaded racing gas and use that when I drive it which isn't a lot any more. Aluminum heads would help a lot over the iron ones as they get rid of heat faster.

Ron

[rtstrack]

Originally Posted by Ron61

So I keep a barrel of 110 leaded racing gas and use that when I drive it
Ron

Are you serious? My wife it going to kill me

[Excaliber]

93 octane should be fine, it was mentioned all ready the KEY is to determine if it is pinging under deep throttle (detonation, knocking it's known by many names). A compression ratio higher than about 10 to 1 generally requires you to do one of two things:
1. Run race gas or aviation gas which has a higher octane count than premium fuel. Doing that is a real drag, hard to get gas, expensive and generaly a pain in the butt.
2. Retard the ignition timing and NEVER 'lug' the engine, keep the rpms about 2500 at all times.

As to unleaded or leaded, no big deal. If you have the old leaded seats I wouldn't worry about it UNLESS your taking an extended trip at freeway speeds, several hundred miles in a day. Most of us never do that with our Cobras anyway. The extended run time at cruise rpm eventually gets the valve seats hot enough to cause damage, otherwise a bit of stop and go allows them to cool off.

Determing your compression ratio is going to be all but impossible without the engine build specs. Listening for the ping and adjusting timing accordingly is the only logical
solution here.

It's pretty hard to hear the ping over the roar of the sidepipes, the ping should be most notable when starting off on a hill at low rpm. If it sounds really loud with only moderate throttle when starting up the hill, yup, you got high compression. Depending on the hill, throttle and clutch slippage SOME light ping may be expected anyway.

I was running 12.5 to 1 compression, thats REALLY HIGH and was able to use 92 octane gas OK by retarding the timing, avoiding taking off on a hill (steep incline) and never letting the rpm's drop to low. HOWEVER, when I would run race gas and advance the timing the engine made noticably more horse power, ran better, much quicker and more comfortable to drive over all.

NOTE: That engine had forged pistons which are much stronger when it comes to 'ping'. Cheaper pistons will crack like an egg if they sustain much pinging. Also it had iron heads, alloy IS better for this problem.

[rtstrack]

WOW...you guys have been amazing with all of the responses. I really do appreciate all of your help. Now if someone could just translate them all into english for me I'd be all set. I think I have the basics and will listen for the ping. If I do hear anything I'll have to bring it to a mechanic who can help me adjust the timing. Thanks for every thing I really do appreciate it all.

Regards
Ryan

[427 S/O]

This may help, I contacted the department of weights and measures in Georgia, which is under the department of agriculture. Their the ones who place that little tag on the gas pumps after inspection. They measure dispensing accuracy and octane levels, I ask if the octane levels were accurate and he said they all match minimums with the exception of BP, which test out at 94-96 on a regular basis. That's all I run....

[Danr55]

RTS, I don't know where you're located, but I suggest that you find a bottle of Lucas Octane Booster and keep it handy. If you use the highest octane fuel you can find (91 or 93) and add a bottle of Lucas with each tank of fuel, your engine won't run it's best, but it will be much better than it will without the Lucas additive. I run a 1971 Cleveland motor that also runs on race fuel, but that's sometimes hard to find when you're 150 miles from the big city. The Lucas additive helps. Be sure to get the stuff in the red bottle. Each bottle is good for about 23 gallons and will bring the octane level to an acceptable level.

[Excaliber]

OH, forgot, heres a BIG clue as it concerns octane and compression ratio. Engine 'run on', sometimes called 'deiseling'. There are several causes but the primary one is fuel octane to low for the engine.

You turn off the key and the engine continues to run, all though it runs poorly, it still runs, it won't shut off. This is very very hard on the motor, put it in gear and let out the clutch to STOP the engine from running.

Higher octane fuel (high enough for your compression ratio) prohibits this run on. If it still runs on with 93 octane perhaps your idle speed is to high or the octane really isn't high enough!

Mixed feeling on octane booster from me. Do bear this in mind. When it says it raises the octane by one point that is not from 92 to 93 octane. That my friend is from 92 to 92.1 octane. Not a big jump...

[SSSammy]

Before you get too wrapped around the axle .. open the hood and tell us if you see aluminum intake and aluminum heads. If so, it is probably a modern engine rebuild on a vintage block, and you can run unleaded 93 and not worry further. If you see iron intake and heads, then all the above advice applies.

Sam

P.S. Go to the Lone Star Forum here and also their factory site and intro yourself and ask if anyone knows your car. Give the chassis number. There aren't that many of them, somebody will recognize it and be able to give you some history on it. Failing that, call the factory and ask Brian the same question.

[mr0077]

Ditto what Excaliber said on octane boosters...use Xylene (116 octane) from your local Sherwin Williams paint shop with 91 or 93 octane if you need to boost it, but race gas is cheaper.

[vettestr]

All good advise already. Failing to learn about the engine from the sources mentioned you could do a little investigation yourself. On a new car to me I would like to know what the compression reading of each cylinder anyway. Look at your cylinder pressures to guess at compression range. Check valve train to tell if you are running a solid or hyd. cam design. Check your initial and total timing for a hint of what it shows or may be lazy to cover high compression. Then give it a listen and get a little aggressive with timing and drive it with a load (low RPM and in a higher gear) to induce pinging to see where you are at. If the purchase price does not cover the risk after all the facts you can find, pass it up till you find one. If you buy it then.........

Put a good tune-up in it and drive it like you stole it............... Enjoy!

[Ibr8k4vetts]

If you let us know where you are located maybe someone local can help

[davids]

91-93 Octane.. Dave

[rtstrack]

Thanks for all of the input so far. I'm located near saratoga springs, ny. I literally just got the car last night, but at first glimpse the headers look like they are steel and not aluminum, but i will double check today. I'll also check with the lone star crowd to see if anyone knows anything about the car.

Regards
Ryan

[427 S/O]

Toluene is used frequently as a boost, mix 2 gal toluene and 8 gal 94

[SSSammy]

RTS, the headers are certainly steel, I'm referring to the cylinder heads .. the big castings the spark plugs are screwed into. If you have aluminum heads (not painted cast iron), they were not used in the old days, so it will likely be a modern rebuild with good valve seats.

The intake is the big casting that the carburetor is bolted to. If that is aluminum, not painted cast iron, same theory.

Sam

[Excaliber]

I thought the plugs screwed into the big casting the carb bolts to?




...just kidding