What are the harmful effects by running unleaded in an engine requiring leaded? I've never really thought about this since leaded fuel was before my era but I'm sure somebody on here has some knowledge on this topic since there are some gearheads who like old cars. The reason I ask is I've taken on a project of keep up with the 1950 model Mack fire engine that requires leaded fuel. The only answer I've been given is since the engine is only used in parades, etc. and not daily driven that the effects won't be harmful. I know a lead substitute can be bought. Is it worth it or just a waste of money?

12/13/2005 9:42:22 AM

lead was primarily used as an octane booster. so from that standpoint you are ok... but one of the other benefits was protecting valve seats. might need to look into an additive for this or modify the car. since you aren't driving it often, i'd say you probably are ok too. but if it is in good shape and you want to make sure it stays that way, get the additive to be safe.

12/13/2005 9:57:01 AM

when i was regularly driving a car with an older engine (1965 cutlass convertible, orginal 330 4brl w/ 75k miles) I had a bottle of concentrated lead substitute, took one capful per tank and lasted about 100 tanks. It was $25 or so, but much cheaper than using one $4 bottle per tank.

I'm sure you know about the hardened valve seat issue, so over long periods of time you could cause some damage

12/13/2005 10:11:07 AM

how often will it be run though. i mean a parade only type classic sees what, a tank of gas a year or something? i think it may be a moot point.

12/13/2005 10:41:41 AM

I thought lead was added to increase the self-ignition temperature of the fuel, thus decreasing the probability of knock. This allowed for higher compression ratios. I suppose if you used a high enough octane fuel, then you wouldn't have to add lead.

[Edited on December 13, 2005 at 2:41 PM. Reason : reread]

12/13/2005 2:36:25 PM

friend of mine had an old plymouth fury
he had to run premium in it to keep it from knocking, and he said the lead substitute didnt do jack

12/13/2005 2:46:52 PM

^^^ Yeah, but the other way you could look at it would be to say that since it barely sees any use it won't cost squat to use the lead substitute.

[Edited on December 13, 2005 at 2:47 PM. Reason : s]

12/13/2005 2:47:13 PM

My dad has '29 model a that has been running on unlead fuel for the past 30 years, and probably seen 50,000 miles. I did a valve job on it two years ago on it, and the seats looked good, and one of the valves was slightly burned. My advice, its a parade truck, it will run forever the way it is.

12/13/2005 3:42:51 PM

lead was taken out cause it fuxors cat. convertors.

12/13/2005 5:50:49 PM

thanks for all the advice. after talking to some others, i did find out that what y'all said about lead being an increase in octane is correct. ill do some more research to find out what octane should be comparable.

12/13/2005 7:42:50 PM

yeah you might need to change the valve seats or get them hardened or just run octane boost or a lead additive.

also some older engines ran higher compression (11:1+ range) and with timing not being computer controled thus less acturate leaded fuel would keep the motor from detonating.

12/13/2005 7:48:57 PM

well, the truck runs surprisingly well. i drove it from wakefield to new bern ave. last friday and from new bern ave/corporation pkwy. to the memorial auditorium and back today. i don't hear any spark knock from the engine. it does have a miss on idle but then again the fuel is over a year old. i'm gonna pump that shit out tomorrow and put in some 93 to see if that helps.

this is the engine.

circa 1950



same truck today.


[Edited on December 13, 2005 at 7:59 PM. Reason : pics]

12/13/2005 7:52:38 PM

i've yet to see any valve seats in terrible condition in the motors i've torn down from the leaded gas era that have been running unleaded since the 80's. coronakid swears up and down it only takes a few thousand miles to trash them, but i've never seen it or really heard of it other than rumors.

12/13/2005 7:57:06 PM

the 312 in the thunderbird pre-rebuild had pretty messed up valve seats.

12/13/2005 8:01:43 PM

^^the truck has been maintained, albeit poorly, but it has been ran once or twice a year. other than being washed every now and then, nothing else has been done to it. it was finally pulled out of service in the mid 80's as a reserve so there is no telling how many miles was put on it since then. the odometer only shows 42,xxx miles. i'm sure if damage to the valves was going to be done that quick, it would've already happened.

[Edited on December 13, 2005 at 8:02 PM. Reason : ^]

12/13/2005 8:02:31 PM

^^yeah, but how many miles did it have on it? i'm talking about low mileage motors that have run on unleaded for more than a few thousand miles of that.

12/13/2005 8:05:59 PM

Cool truck! It's pretty amazing that it made it to the 80's and was still in service, even if it was a reserve.

What's the usual service life for a fire engine, if there is such a thing? I guess with as specialized as they are, and as few miles as they rack up, their useful lives must be pretty long.

12/13/2005 9:33:05 PM

i really don't know honestly i mean totale around 85 k but i think most of that was on leaded ...
it was in a barn for YEARS

12/13/2005 9:38:25 PM

^^that all depends on the municipality. in raleigh it's 100K or 20 years.

12/13/2005 9:54:16 PM

that lead substitute left some nasty orange buildup on my old MG Midget valves

12/13/2005 10:11:19 PM