1969 what type of fuel?

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Drfn8r
Last seen
Joined
Dec 10, 2022
Thunderbird Year
1969
hey everyone i have the 429 thunderjet in my car and was recently told I should be using high octane fuel along with octane booster due to this engine being used pre unleaded gas just wondering if thats what the rest of you do or if the regular stuff will work meaning not kill my engine or ping
 
I think they have pretty high compression so you probably should be using high octane, but I don't think you need any octane boosters. Unleaded gas has nothing to do with the octane needs.
 
Depending on how much you plan on driving I would be more concerned about the lack of hardened valve seats. It will probably run just fine on mid grade, maybe even regular. You can try a tank and see how it goes. One tank won't really hurt anything.
 
i think it was advertised as 11:1 compression ratio and I really only plan on driving maybe once a week about 20 miles maybe per week thanks for the feedback i got pretty nervous when they told me id been running wrong fuel through it so far ive only been starting it long enough for tuning purposes and to warm it up for other checks nothing long
 
i think it was advertised as 11:1 compression ratio and I really only plan on driving maybe once a week about 20 miles maybe per week thanks for the feedback i got pretty nervous when they told me id been running wrong fuel through it so far ive only been starting it long enough for tuning purposes and to warm it up for other checks nothing long
11:1 needs premium but 20 miles a week of easy driving on regular probably won't hurt it much if any but I'd switch to premium. For the little driving you do the extra cost of premium is pennies. You don't need to worry about the valve seats. If the car started life using leaded fuel for its first 10,000 miles, it will be fine unless you plan to go racing with it.
 
In stock configuration that engine was 9.5 to 1. Leaded fuel provided anti knock and valve seat lubrication.
The need for hardened seats came about from the lack of lubrication from the unleaded fuel. Short trips and light load will extend the life of the valve seats.
However you seem to be thinking that this 55 year old car has not been worked on. It's highly possible that it's already been updated, after all this car was a daily driver in the mid '70s.
 
very true ill see if i can find the previous owners son and find out if the seats or top of engine was ever worked on or changed. that 9.5 to 1 is way different than the info i got would you still recommend the premium
 
I would try a tank of each different fuel, the car will tell you what it needs. I'm betting mid grade would be fine, but regular could also be acceptable.
 
very true ill see if i can find the previous owners son and find out if the seats or top of engine was ever worked on or changed. that 9.5 to 1 is way different than the info i got would you still recommend the premium
My 57 is 9.5 (or 9.7) to 1. For a while I ran it on regular fuel but after adjusting the timing a little more advanced, I need to run mid-grade to prevent problems when it gets hot. But the extra advance makes it run better overall so it's worth the extra cost. I'd say for your case it may be similar, it will depend on how your car is timed/tuned whether you can get away with regular, but I doubt you would need to go higher than mid-grade... unless it really is 11:1 !!
 
My 57 is 9.5 (or 9.7) to 1. For a while I ran it on regular fuel but after adjusting the timing a little more advanced, I need to run mid-grade to prevent problems when it gets hot. But the extra advance makes it run better overall so it's worth the extra cost. I'd say for your case it may be similar, it will depend on how your car is timed/tuned whether you can get away with regular, but I doubt you would need to go higher than mid-grade... unless it really is 11:1 !!
lol thanks for the info im thinking your correct. im gonna do like harley said and try a tank of each but for the rest of the time im tuning and things ill start using mid grade.
 
i think it was advertised as 11:1 compression ratio and I really only plan on driving maybe once a week about 20 miles maybe per week thanks for the feedback i got pretty nervous when they told me id been running wrong fuel through it so far ive only been starting it long enough for tuning purposes and to warm it up for other checks nothing long
Spark knock is your enemy. A little bit is ok under hard acceleration. But spark knock is telling you to run a higher octane of fuel. You can retard the timing to lessen spark knock but that affects your performance too. I would definitely run the higher octane fuel (91 and up). Also ethanol gas is hard on our carburetors. Ethanol free is better. Just saying. A warmed up engine will be best to check for spark knock.
 
Spark knock is your enemy. A little bit is ok under hard acceleration. But spark knock is telling you to run a higher octane of fuel. You can retard the timing to lessen spark knock but that affects your performance too. I would definitely run the higher octane fuel (91 and up). Also ethanol gas is hard on our carburetors. Ethanol free is better. Just saying. A warmed up engine will be best to check for spark knock.

We might be differing in out definitions, but knock is never good. Never. Ping is OK if it's only occasional like when you "give it the gas" and hear a ping under the extra load to pass someone. A sustained ping may not break things, but it will greatly increase combustion temperatures and can lead to burned valves. It would be bad continuously to climb a hill of if it's pinging all the way. A knock isn't just noise, it's a violent out of control and mistimed firing of the fuel air mixture and can quickly destroy pistons, crack rings. It will feel a lot like a bad spark plug missing.
 
Altitude and temp will affect pre ignition. Im above 5000 ft and it is cool here.. So Im going to try Regular.... Altitude lowers the compression ratio...
 
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