100LL Avgas vs pump 93

[Rich G.]

I have access to Avgas and once in a while I put some in the 66 327 350 or the 68 427 435. I've always had the feeling both cars ran worse on the Avgas, but I chalked it up to imagination.

With the gas lines here for the past couple of weeks I've been driving both cars on short trips because I could get Avgas at the airport with no hassle. The first thing I noticed is the idle dropped on both cars by about 400 rpm. I bumped the idle up so they would stay running. Driving was no difference. Also, they became harder to start. They both used to start instantly, especially when cold. Now it was like a flooded start every time.

Today I finally ran low on the BB and put pump 93 in. After driving 5 miles I had to drop the idle about 400 rpm or so. What had been 900 on the Avgas turned into 1300 on the car gas. I have not put car gas back in the 66 yet.

I assume the timing needs to be adjusted if one wants to burn a higher octane, slower and longer burning fuel and that both these cars are set up to run best on car gas, but that's only an assumption because I don't know a heck of a lot! I know the timing on the 66 is factory spec with the correct cam and correct vacuum advance. Not sure about the 68 because it runs too good to be fooling with. BTW, I am happy to run car gas forever. This is a curiosity question.

Any thoughts?

Rich

[Duke W.]

Re: 100LL Avgas vs pump 93

100 LL avgas doesn't burn any faster or slower than pump gas. That's a myth.

Many report hot percolation problems with E10 pump gas, but avgas solves the problem because is has a higher temperature distillation curve that is much more resistant to percolation, but this can cause harder cold starting.

The symptoms you report really don't make any sense. Hot idle behaviour should not change, but cold idle might be rougher due to the lower rate of vaporization.

Avgas should perform about the same as pump gas in normal driving, but start easier when hot, maybe a little slower when cold, and cure any detonation. Avgas doesn't need different timing, but since it's about 104 PON the spark advance map can be much more aggressive.

You can reduce fuel consumption in your 435 HP by switching from ported to full time vacuum advance, but you'll also need to change the VAC to a 12" B20 or B26, and lighter springs in the distributor will both reduce fuel consumption and increase low end torque. Maximum advance at high load/low revs is usually limited by mogas detonation.

The 327 could benefit from the same changes. The OE 8" VAC is more agressive than necessary. and the centrifugal curve is way more lazy than it needs to be.

The 435 HP distributor was set up for emission control, not performance or fuel economy. The 350 HP distributor isn't set up for emission control unless the car was originally sold in California. In either case the spark advance maps are far from optimum.

Duke

[Rich G.]

Re: 100LL Avgas vs pump 93

Yes Duke

I know it doesn't make any sense, which is why I never asked about it before because I thought it was silly, but it is 100 percent repeatable.

The idle number I spoke of are hot after a long drive. I'll see what happens when I get car gas back in the 66.

Rich

[Rich G.]

Re: 100LL Avgas vs pump 93

Yes Duke

I know it doesn't make any sense, which is why I never asked about it before because I thought it was silly, but it is 100 percent repeatable.

The idle number I spoke of are hot after a long drive. I'll see what happens when I get car gas back in the 66.

Rich

[Duke W.]

Re: 100LL Avgas vs pump 93

Let's see if anyone else reports a similar issue, but I don't think anyone else has ever reported a significant loss of hot idle speed on 100 LLavgas. There might be some issues if the heat riser is blocked - not just the valve wired open, but completely blocked with shims, which could reduce hot idle vaporization that could cause lost idle speed due to an excessively lean mixture.

Are your heat riser passages blocked with shims?

Duke

[Rich G.]

Re: 100LL Avgas vs pump 93

Duke,

On the 66 SB, definitely not. On the BB I don't think so, but not 100% sure.

Rich

[Michael W.]

Re: 100LL Avgas vs pump 93

100 LL avgas doesn't burn any faster or slower than pump gas. That's a myth.

Absolutely- the octane number of ANY gas is unrelated to it's burn speed. This becomes obvious if the phenomena of detonation is understood.

[Duke W.]

Re: 100LL Avgas vs pump 93

I should have said that there is no significant difference in the rate of flame propagation for verious gasoline blends in normal (detonation free) combustion. If combustion is abnormal (detonation is present) it's a whole different story.

I covered both normal and abnormal combustion in some detail in my San Diego presentation.

Duke

[Jerry B.]

Re: 100LL Avgas vs pump 93

I drive a 62 fuely with a 355 on the weekends. I use the 100ll and it runs perfect.