Ford Mustang Forum banner

compression and octane?

2439 Views 20 Replies 11 Participants Last post by  1968poney
i am sure i set my 302 to either 10.5:1 or 11:1 what octane should i use can i get by with 93 from a pump or do i have to add lead? i did some research and saw that i should be useing 104 octane but i have no idea what they mean with all this RON and PON stuff can anyone tell me if i should go with 93 or should i mix lead or raceing fuel? i hope its 93.

Thanks Tom
1 - 20 of 21 Posts
You can't buy gas from a normal station for either of those compression ratios today. You barely could - ever.

Although each engine may be a little different an 11:1 ratio nominally requires 104 research octane number (RON = R) fuel. When measured by the motor method (MON = M) the octane of that fuel is lower at around 94. The pump at the corner station is marked with (R+M)/2 meaning you would need to find a pump selling 99 octane gas - good luck. The cans of 'lead' you add aren't all that effective and I doubt you can add enough of them to make up the difference. When they talk about increasing octane by so many 'points' they often mean 0.1 octane, NOT 1.0 octane points. Racing gas and/or avgas come to mind. You need a good job to pay for either but they will still likely be cheaper than the cans of additive.

I re-built my engine at 9.5:1 for just that reason. In the 60s Ford called that a 'regular fueled engine' as opposed to the premium fueled 10:1's or the super-premium 10.5:1's. My 9.5:1 will need 93 octane at sea level but at least that is still available but its now called 'premium'. I mention sea level since at higher elevations the octane requirement is lower. Living at more than a mile above sea level my 9.5:1 can burn regular which up here is only 86 pump octane. No, its not any less expensive, its just lower octane.

11:1 has a typical thermal efficiency of 34% while a 9.5:1 is only 32.5%. To me that less than 5% loss in HP with the lower compression would seem worthwhile today since it allows cars to be driven on the road without needing to own a major bank.
See less See more
Auctally quite a few gas stations sell 104 but you have to buy a minimun of 5 gallons and it has to be pumped into a container and not your car. Sunoco is one of them, but not all of them sale it. Plus most good hot rod shop carry it.
...and it has to be pumped into a container and not your car.
Does that mean its leaded?

Availability obviously varies depending upon where you live. We don't have Sunoco stations here.
so what your saying is 11:1 requires 104 octane and in order to not have ticking i have to use 104 and will not be able to slide by with pump 93. someone said you can get pump 104 from sunco is that available in NJ or not a chance and when you mean pump into a container i have to bring a container to fill my car. . . thanks and a off topic question i might make a new thread but the compression was incressed on a 1980 stock 302 by useing 1966 289 heads because the compression is higher will all the main components handle that compression stock that is or do i need to change some parts to a stronger material?

Thanks Tom
Very few people remember Texaco and their fuel names

Firechief and Skychief...

also known as Reg 88 and Super 98

Those were octane ratings back then although they were slightly off
this bites i built this engine and brought the compression too high and now i have to pay for it so theres no chance on being able to use pump gas im not racing it its just my every day driver.

this bites i built this engine and brought the compression too high and now i have to pay for it so theres no chance on being able to use pump gas im not racing it its just my every day driver.

Unfortunately, that may be the case. 11:1 in this day and age is veeery high for anyone wanting to run on the normal gas you wil find at most stations. My Mercedes with ONLY a 9.4:1 calls for 'premium'. My Mustang, as built, is around 9.6:1 which I thought was about as far as I could go on 'pump' gas. The normal econo-whatever is more like 8.5:1 these days.
You could could try this product it is also leaded and alot cheaper than race gas. I had to play with some distributor and and a few gallons of VP red which is leaded 105 octance that cost 11$ a gallon if my stills pings after a full tank of premium I am buting this myself. My 351w is at 10.3 to 1 with alumiun heads if you have cast iron heads you should try this product. Kemco Oil & Chemical Inc
The Kemco product does seem to be accurately described. To boost pump 93 premium to 99 octane would require nominally about 1.25 gr/liter of TEL which agrees pretty much with their 2.4 oz/gal; i.e. 1.2 quarts of additive per 16 gallons of premium. That boosts your cost about 75 cents a gallon above pump premium.

However, keep in mind that since 1996, it is a violation of federal EPA regulations to use leaded gasoline with more than 0.05 gr/liter TEL as a highway motor fuel. 1.25 gr/litre exceeds the rule's limit by a factor of 25x. The words they didn't prominently display are 'For off road use only'.
so adding sditives are illegal is what ur saying but how would you even get caught is there gas police and so basc. im scrwd. with 11:1 rrrr so even if i add the attitive i would have to bring it with me every time i need gas and its going to use alot of gas and this is an every day driver and i really dont want to take the engine apart after i just finished it what a problem i have to just get a better job it seems:gringreen ehhh i prob will stick with the compression but now i found out the stock starter wont even get it going i need a stronger one any recomendations i saw mini starter will they work some go up to 17:1 compression

Thanks Tom
If you're running iron heads, you NEED either VP or similar racing fuel or an octane booster. If you are running aluminum heads, you MIGHT get by on 93 if you are only at 10.5 compression. Aluminum heads run cooler and you can get by with more compression on pump gas.

If you want to reduce your compression, pull the heads and see fi you can open up the chambers some without completely screwing up the flow. Or, if running domed pistons, yank em and put in flat tops. Yo may have to get different heads altogether if wanting to use this car as a daily driver. Also, you didn't mention your cam specifics.

A third alternative is to spring for the big money carb that allows you to run on alcohol/E85. This will also allow you to run higher compression because it burns cooler. Efficiency drops though.

Good Luck!
Do you live near a small airport ?

I have a 1970 429 CJ in my truck and every year on its birthday I go to the local airport and buy about 40 Litres of Avgas (110* octane) and mix it with the 92 I get at the gas bar in town.
The engine just asks for more throttle and I am looking for a runway to drive on.

I was sure you Americans had access to 95 Octane at Chevron stations ?
gtjoe got it right. if you have aluminum heads you would probably be ok with 93 octane and not a lot of timing. but with iron heads it would be a little tough..
i have iron heads stock 1966 289 heads cam is also stock to a 1980 block i need 104 octane and it will be a daily driver i want to stick with the compression and ill just take the money up the . . . but as far as the air port thing they use 110 octane for what and would airports even sell it unless you owened a plane there as far as small idk i only know 2 airports close to where i live jfk and newark im 45 mins from and jfk about 2 hours if i do find a way to get 110 how much do i mix 93 with 110 to get 104 any clue

Thanks Tom
I'm curious, what did you do to get you to the 10.5 or 11.0 If you just have small chambered heads,l get yourself a different set with something around 64cc chambers and your problem is solved. Running 11:1 compression with a stock cam gets you nowhere performance wise. You have to match your intake, exhaust, compression, cam, etc.

As for 104 octane, try going to your local parts house and buying a can of "104 octane booster" It's going to be cheaper than running to an airport or buying VP.
I got by fine with 10:1 iron quench chamber heads and ran good advanced timing on 93 octane premium. But at the cost of gas today I would opt for EFI and electronic igniton and burn thte cheaper lower octane stuff. Carbs and distributors are asking for problems with todays cheap crap gasl. Anything over 10:1 is cartainly asking for problems on todays crap gas. I ran 12:1 back in the sixities with ethal gas which was leaded. That old leaded ethal gas was 103 octane and would take all the ignition advance you wanted. Lead makes a huge differance and today's unleaded gas is just crap. Even today's higher octane gas does not compare to the old leaded ethal gas. Your running race car only compression numbers and good luck with that high of a number on the street. You can compensate some by adjusting your ignition timing but there again your losing power. So why have such high compression in the first place? With Obama mania gas quality will get worse in the future. I would back off the compression to 10:1 max and upgrade to a SEFI ECM system. Technology can compensate for the poor gas.
See less See more
Do you have easy availability of E 85?. If you do swap out your carb easy fix. I run 11.9 in my 351 c 4v. E 85 will work up to about 13.5 compression so you will have no problems at all. Of corse I live in the corn belt and have three ethonal plants within 10 miles. Use this link to locate a station near
Tom ,decompression is your answer man, my 351C is a closed chambered 4v engine and fuel is a huge problem. I am at 10.5 or so with iron heads and stuck at 4-6BTC everythign else PINGS and pisses me off. I use 104 boost with chevron 93 and on a hot day I still hear the faint ping sometimes. My $.02
I am also running a 351 C with ross flat top pistons australian heads ported and polished crane hyd roller cam 284-292 adv duration lift is 586-609. weialand dual intake with 770 street avenger holley carb. we think it is app. 11.5 and I was having pining problems bad. My engine turner had to recurb the dist. adjust time and tune the carb. We now have it so it will run on 93 octane with 104 boost will ping in hot weather. He thinks it is about 75% of its actual power but at least I can drive it. I asked him about the airplane gas and he said it is a dry gas and if you use it you will need to add trany fluid to it.
1 - 20 of 21 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.