gas and octane question

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74Ddart

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I've been talking at work and thought I would get answers from FABO. What octane gas would you use in a 273 four barrel with mild cam. I'm thinking 93.
Also do I really need an additive to replace the lead that was used in the day?

Thanks
 
what is your compression ratio that will tell what gas you need to use for no backfiring. If it is 10.5to 1 or higher you need unoxygenated or 93 with boost
 
is it a factory commando motor? or just a 273 with a 4 barrel intake and carb?
commando motors were 10.5:1 compression regular 273 motors were 8.5:1 either way if it was run with unleaded gas the exhaust valve seats are probably shot and should be replaced with unleaded seats
 
A lead additive should help and be used since the lead is used to cushion the exhaust seats on valve closeings. Intakes do not need this, they run cooler and there is also some cushion from the gas flowing through unlike the exhaust side which only see crazy hot exhaust.
 
Thanks for all the information. My engine was professionally rebuilt when I bought it in January and has less than 1000 miles on it. It has an Edelbrock 600 CFM and a Weiand intake manifold. I would guess that the exhaust seats would have been replaced to accommidate unleaded gas. I got the old Moroso valve pan covers and the commando air filter with the car, so I'm guessing it was original. Now the engine has the Mopar performance air cleaner.

Joe
 
you need to check the receipts or talk to the re-builder and find out exactly what was done and what parts they used, that way there is no guess work
 
I've been talking at work and thought I would get answers from FABO. What octane gas would you use in a 273 four barrel with mild cam. I'm thinking 93.
Also do I really need an additive to replace the lead that was used in the day?

Thanks

I'd use at least 91 w/ octane boost. Sometimes its a crap shoot getting quality gas, since gas quality & additives can vary from state to state. Here in Reno, NV, they start adding E85 & oxygenating the fuel around November & it plays havoc on my hemi. If I run 100 unleaded from VP @ $5+ per gal or their leaded 110 ($7.99 gal), I have no problems w/ the fuel boiling at a lower temp & causing vapor lock. I'm about to install the factory return line w/ vapor separator to help & will see if the cr-p Chevron 91 will be ok.
If you can get 93, you'll probably be ok.
 
Do a cranking compression test. Also I wouldn't worry about the lead additive. If you had a truck and towed or plan on driving it 100,000 then I'd use it.
My 273 has 150 psi cranking and a static of 9 to one. I run 87 and 34 degrees total timing. Never once had it ping. If it doesn't ping with the fuel you are using, I wouldn't worry about it.

Although non-oxygenated is better fuel all around. Especially if you don't drive it often and the fuel sits in the carb for a while.
 
i cant imagine he has a hi compression motor with a 600 cfm carb.
 
If you`re not running a ton of advance and super high compression then run the lowest octane you can without detonation. Start with a mid grade gasoline, say 89 octane. Drive the car in a high gear at a slow speed and mash the gas without downshifting. Listen for what sounds like all of your valves rattling. If you don`t hear it then your 89 octane should be okay, if you hear a rattle then move up to 91 or 93 octane. Your car will tell you what it needs, you just have to listen.
 
a 600 cfm is correct for a mild 273 cu with a max rpm of 6500
cu times max RPM divided by 3456
273 times 6500=1774500 divided by3456=512.45486
 
hey this was really helpful. I just jumped back in to a 65 dodge dart GT 273 with a two barrel. Based on that article I think I will just try 89 with no additive and see how she runs. Any suggestions? Thanks!
 
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