Again: Unleaded or leaded fuel for mid 70's /6?

SlantedMark4

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I have a 1974 225 slant six under the hood. I've found out that hardened valve seats were inducted in 1972. Does that mean all engines from 1972 and up can be operated with unleaded fuel? And how is it with 95 octane fuel instead of 87 octane? 95 fuel is lowest and standard here in central europe. Do I need any additives or am I fine running 95 octane without any additives?
I know this topic is in discussion all the time but I have never really found a good answer.
 

A56

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Not sure about MoPars but my '74 Corvette still used leaded fuel, but in '75 they went to unleaded if I remember correctly
 

RustyRatRod

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In something light like an A body, it's really a non issue. If you had a truck and were doing a lot of towing, the hardened seats would be a plus. Run a lead substitute additive if it bothers you, but that's really not even necessary.

Besides, you cannot get leaded fuel anymore unless it's high octane race gas at 10 plus bucks a gallon.
 

SlantedMark4

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In something light like an A body, it's really a non issue. If you had a truck and were doing a lot of towing, the hardened seats would be a plus. Run a lead substitute additive if it bothers you, but that's really not even necessary.

Besides, you cannot get leaded fuel anymore unless it's high octane race gas at 10 plus bucks a gallon.
Thank you man. So you say running on 95 fuel without (lead) additives is no problem?
 

RustyRatRod

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Thank you man. So you say running on 95 fuel without (lead) additives is no problem?
Or 87. The octane has nothing to do with it. It's all about exhaust temperature. Light cars generally don't have high exhaust temperatures. I've never run additives. Also another tip is, run it just a tad rich and that helps keep exhaust temps down a little more. Not rich enough to blow black smoke, just a little. You'll never have an issue.
 

SlantedMark4

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Or 87. The octane has nothing to do with it. It's all about exhaust temperature. Light cars generally don't have high exhaust temperatures. I've never run additives. Also another tip is, run it just a tad rich and that helps keep exhaust temps down a little more. Not rich enough to blow black smoke, just a little. You'll never have an issue.
Okay thank you mate
 

MoparMike1974

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I dont think the slants ever got hardened seats. The seats were cut into the iron head and induction hardened. Not quite as good as a real hard seat but better than nothing.
75 was the year mopar went to unleaded fuel and catalytic converters. The instrument cluster would have said "unleaded fuel only". In the early years they just slapped a sticker on it.
Octane has nothing to do with the need for hardened seats. The need came when they removed the lead from fuel since lead would do bad things to a catalytic converter. If your concerned try to find a lead substitute to add to your fuel.
 

mvh

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The actual problem that might be caused by unleaded fuel used with non-hardened seats is valve seat recession, which takes a while to develop. I have had problems with this in high performance engines, but it doesn't sound like it is a particular problem with the six. Charrlie is right about the octane number — your numbers and US numbers are not directly comparable.
 

MoparMike1974

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I have seen 318's with the valve seats pounded out and the valves sunk over an 1/8". Several engines over the years. The lead lubricated the stem and helped keep the seat cool.
 

RustyRatRod

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There are endless articles about this all over the internet. The number ONE cause for valve seat recession has nothing to do with leaded gas being removed from the market. It's people not changing their air filters when they are due. Dust and dirt get into the intake tract and through the engine and combined with the intense heat of the exhaust valves the dirt particles basically grind the valve faces and seats into oblivion. Hot rodders seldom see valve seat recession because they are normally on top of filter changes and do things like keeping timing advanced and mixtures a little richer which leads to a cooler running engine. Heat and dirt are the two enemies here.
 

DartLite

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For a moment there I thought this was going into the direction of "Is a breather cap really necessary?" thread from the old slantsix.com days
 

Brooks James

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In something light like an A body, it's really a non issue. If you had a truck and were doing a lot of towing, the hardened seats would be a plus. Run a lead substitute additive if it bothers you, but that's really not even necessary.

Besides, you cannot get leaded fuel anymore unless it's high octane race gas at 10 plus bucks a gallon.
Aviation 100 Octane Low Lead at your local airport
 

Killer6

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Aviation 100 Octane Low Lead at your local airport
Hope Your mixin' that with normal fuel, avgas runs like **** straight, not meant for cold starts/operation at ground level. I had a customer dump that in the tank of His MonteSS, World Product headed 350, Street Avenger Holley. Would barely start & run on a summer morning. Not to mention 100 octane is 14 points too high for a stock Slanty, & their rousing high 7's squeeze. OP, Your seats are induction hardened, & You can run the equivalent of 87(R+M÷2) here in the States.
 

Charrlie_S

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Aviation 100 Octane Low Lead at your local airport
Not a viable choice. There are more factors involved in rating/blending gasoline, then octane rating (of which there are several different methods of measuring). There is octane, reid vapor pressure rating, specific gravity, dielectric constant, oxygen content, and probably some more that I can't think of right now.
 

Brooks James

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Hope Your mixin' that with normal fuel, avgas runs like **** straight, not meant for cold starts/operation at ground level. I had a customer dump that in the tank of His MonteSS, World Product headed 350, Street Avenger Holley. Would barely start & run on a summer morning. Not to mention 100 octane is 14 points too high for a stock Slanty, & their rousing high 7's squeeze. OP, Your seats are induction hardened, & You can run the equivalent of 87(R+M÷2) here in the States.
That's odd, I have thousands of flight hours and the aircraft engines, some as low as 6.5 to 1 cr, always ran great on 100LL
 

Killer6

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That's odd, I have thousands of flight hours and the aircraft engines, some as low as 6.5 to 1 cr, always ran great on 100LL
I'll be the first to admit, I'm not an aircraft engine expert, but don't they have a different fuel/fluid to cold temp start? The fuel evaps/boils more quickly at altitude. That Chebby was running perfectly, I had just got done setting it up with the FAST A/F scanner, best timing was 25° with 93 octane & the fast-burn heads, ran perfectly for about 3 weeks. Then He pulled in complaining it was hard to start, smoking, hard to keep running etc. When I suggested He got a bad load of fuel, He laughed and said "not likely", as He revealed that He had went & filled up with said av-gas. After He ran enough out to add a 1/4 tank pump premium, most of the problems disappeared. Next fillup they were gone.
It's a fact, running more octane than You need results in less mpg & less tq/hp, fact. A gentleman who ran a stock-class 'teener found out after running the stock squeeze combo thru' the ringer, that His best mph/et was on 87. Could He run it on 102 Turbo Blue? Sure, and it would be a pig no matter what timing curve He put in it.
Feel free to educate Me on aircraft operation tho', I'm always glad to learn something.
 

Scody21

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If I’m not mistaken, the higher the octane, the slower the fuel burns and less prone to early detonation from higher compression.

As for the OP’s question, if the engine has not been rebuild and had hardened seats added during the top end rebuild, use a lead additive and pump gas. Leaded pump gas is a thing of the past. If hardened seats have been added, don’t worry about the lead additive. If you can find “super clear” aka non ethanol gas, run that.
 

Rat Bastid

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If I’m not mistaken, the higher the octane, the slower the fuel burns and less prone to early detonation from higher compression.

As for the OP’s question, if the engine has not been rebuild and had hardened seats added during the top end rebuild, use a lead additive and pump gas. Leaded pump gas is a thing of the past. If hardened seats have been added, don’t worry about the lead additive. If you can find “super clear” aka non ethanol gas, run that.

Octane and burn rate are not connected.
 

Brooks James

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I'll be the first to admit, I'm not an aircraft engine expert, but don't they have a different fuel/fluid to cold temp start? The fuel evaps/boils more quickly at altitude. That Chebby was running perfectly, I had just got done setting it up with the FAST A/F scanner, best timing was 25° with 93 octane & the fast-burn heads, ran perfectly for about 3 weeks. Then He pulled in complaining it was hard to start, smoking, hard to keep running etc. When I suggested He got a bad load of fuel, He laughed and said "not likely", as He revealed that He had went & filled up with said av-gas. After He ran enough out to add a 1/4 tank pump premium, most of the problems disappeared. Next fillup they were gone.
It's a fact, running more octane than You need results in less mpg & less tq/hp, fact. A gentleman who ran a stock-class 'teener found out after running the stock squeeze combo thru' the ringer, that His best mph/et was on 87. Could He run it on 102 Turbo Blue? Sure, and it would be a pig no matter what timing curve He put in it.
Feel free to educate Me on aircraft operation tho', I'm always glad to learn something.
I was considering using it and was hoping someone had used it or knew about it's use. You definitely saved me some research.
 

RustyRatRod

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AV gas is ok on the outside. The octane is great. It has other additives though, pertaining to dealing with high altitude that automotive engines do not benefit from. People brag about running it and making power, which I'm sure it can, but it's not the best way to go about it.
 

Rat Bastid

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AV gas is ok on the outside. The octane is great. It has other additives though, pertaining to dealing with high altitude that automotive engines do not benefit from. People brag about running it and making power, which I'm sure it can, but it's not the best way to go about it.

Im going to test some either tonight or tomorrow. It largely depends on how motivated (or not) I am.
 

Killer6

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AV gas is ok on the outside. The octane is great. It has other additives though, pertaining to dealing with high altitude that automotive engines do not benefit from. People brag about running it and making power, which I'm sure it can, but it's not the best way to go about it.
As Charlie pointed out, the RVP is one of the big differences. When it is low, it prevents or minimizes vapor lock, which is why "low RVP" Summer fuel was mandated for high temp regions. One of the few govt. mandates that worked in carbureted gearhead's favor. But when You take that fuel up 5-10,000 feet, it's going to boil like crazy at the slightest provocation. RVP is very low, it has to be. Winter fuel is already in the pumps, close to 2 months now I imagine, it would be the highest RVP blend to ensure fast cold starts.
 
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