leaded vs unleaded in 69

[Kenneth G.]

Hello all,

As I have posted previously, I just got my 69 L46 350/350 hp back from restoration. It is basically a stock motor (11:1, stock cam, orig carb all emissions etc). I decided to use a higher octane gas than regular pump premium, and was considering the Sunoco Gt-100 unleaded gas (100 octane). Although I think this would work well for my car, it is not available in my immediate area (45 minute drive to get it). However, there was a speed shop very close to me that sells "VP racing fuel" in 5 gall. containers. This is 105 octane leaded gas, called "VP Red" (website is www.vpracingfuel.com). My motor has hardened valve seats so I could run unleaded or leaded if I wanted to (?). My question is: what are the the downfalls/benefits of running leaded gas in this situation? Will the unleaded run better/cleaner? Since both are relatively close in octane (100 vs 105) and would probably both work well in my application, all things equal, which is the better choice? Thanks.

Kenny #31871

[Donald T.]

Re: leaded vs unleaded in 69

Kenny,

Are you sure you even need more octane? Your motor may run just fine on 93/94 octane pump gas. As long as it does not knock or run on, you are in good shape. You certainly shouldn't require 100 octane. 100 octane was recommended at the time, but that was rated using ROM. Current pump gas is rated at (ROM+MON)/2, so the recommended 100 converts to around 96 octane by the current standard.

You can mix pump gas with race gas (either leaded or unleaded) and raise the octane level. Use just enough race gas to stop knocking/run on. There is no benefit to higher octane than is necessary to prevent knock. Using unleaded would be preferable because it will burn cleaner, and leaded fuel is not legal for street use.

[Kenneth G.]

Re: leaded vs unleaded in 69

Don,

Did I say I would run leaded gas on the street? I would NEVER do that!

Kenny#31871

[Kenneth G.]

correct site in original post

the correct website for VP racing fuel is www.vpracingfuels.com

[Donald T.]

Re: correct site in original post

I'm with ya. I have a 65 327/350 also with 11:1. It has aggressive timing and ignition curve. As a result I sometimes get a little run on with 94 octane pump gas. I use enough lead supreme 130 to raise octane to 96, with good results. Lead supreme contains real lead (TEL). I also use it stictly for off road use only

Good luck with your car, should be a lot of fun!

[Brian Monticello]

Re: leaded vs unleaded in 69

Ask your builder if he calculated thee ACTUAL static compression ratio comes for your setup. This is based almost entirely on the piston shape (dome, dish, flat), piston/deck clearance, bore and stroke, head gasket thickness, and combustion chamber volume. You might find that you are below 11:1. Like the above suggestions I recommend filling up with some 93 or 94 and seeing what happens.

Brian

[Jim T.]

Re: correct site in original post

I also have a 68 L79 327/350 HP with 11:1 and the engine runs great on pump 93. I use 10 degrees BTDC for timing. I don't lug the engine and usually run it in 3rd gear when driving on city streets where the speed limit is 30-40 MPH. The owners manual printed in 68 when there was leaded high octane gas recommends using 2nd gear driving in stop and go traffic less than 30 MPH.

[Dan M.]

Re: leaded vs unleaded in 69

Brian,

I have a stock 1969 L-71 (11:1 compression) and I blend 5 gallons of (VP 105 RED) to a tank of regular unleaded. My experience has been you get a higher finished octane by using regular versus premium based on my own actual lab work. The reason for the regular if there is a lower amount of aromatics a plus when blending leaded fuel for octane. The car runs great with no knock and or run on.

Dan
NCRS #35068

[Duke W.]

Re: leaded vs unleaded in 69

I'd like to hear more about the lab work that leads you to your conclusion.

Duke

[Dan M.]

Re: leaded vs unleaded in 69

Duke,

My profession give me assess to test equipment for quality control of octane ratings as well as actual test engines where CR can be varied in the presence of knock detection equipment. Not sure I understand the chemistry at play here but it was explained to me that the benefits of the lead decline as the aromatic content is increased. Our home brews always had better results blending leaded with unleaded regular; all else being equal.

Dan

[Duke W.]

Re: leaded vs unleaded in 69

Aromatics tend to have higher octane than some of the other HC families, but I'm not familiar with the effect of TEL on aromatics versus the other families.

Aromatics tend to be on the voltatile end of the distillation curve, and some are considered carcinogenic. My understanding is that, in the case of current RFG II gasolines used in CA, aromatic content is strictly regulated, and I'm not sure if premium has higher aromatic content than regular.

Interesting subject if you can come up with any more science.

Duke