Dumb question re 70 LT-1

[Ed H.]

To the best of my recollection - 35 years ago - this beast averaged about 15 mpg. It's now in the vicinity of 11. The carb was rebuilt by Holley (It was only after the fact that I discovered what I thought was the original carb is not... it's a 3310). I have an inquiry in to Holley as to the jets and power valves fitted - no response, yet.

So, my question, and I realize the 'grin factor' carries some weight, LOL, but with 3.36 diff, what sort of mileage is reasonable? What carb are those of you who are driving your's, using?

I will be taking it for A/F ratio testing etc, but we're away soon, so it won't be till after July, probably.

Thank you!

[Allen N.]

Re: Dumb question re 70 LT-1

The GRINS are worth however much gas you are using!

Sorry, I have no answer for your actual question.

[Patrick B.]

[Ed H.]

Re: Dumb question re 70 LT-1

Not original. Was in when I bought it.

[Bill S.]

Re: Dumb question re 70 LT-1

A gallon of new gas blended with ethanol does not do as much work as a gallon of old gas did. The BTU content of old gas vs new gas would at least partially account for your drop in mpg if not the whole 4 mpg.

[Terry M.]

Re: Dumb question re 70 LT-1

To the best of my recollection - 35 years ago - this beast averaged about 15 mpg. It's now in the vicinity of 11. The carb was rebuilt by Holley (It was only after the fact that I discovered what I thought was the original carb is not... it's a 3310). I have an inquiry in to Holley as to the jets and power valves fitted - no response, yet.

So, my question, and I realize the 'grin factor' carries some weight, LOL, but with 3.36 diff, what sort of mileage is reasonable? What carb are those of you who are driving your's, using?

I will be taking it for A/F ratio testing etc, but we're away soon, so it won't be till after July, probably.

Thank you!

My 1970 LT1 has the standard 3.70:1 and when new (note Bill Strobel's comment about today's gas v yesterday's gas) I got 18 mpg from Chicago to Fort Collins CO. Almost all interstate in the Monfort lane. Speed limits were not widely enforced for those truckers, and I followed them closely.

[John S.]

Re: Dumb question re 70 LT-1

Ed,
I have the same story with my 67 300 hp car. When I first bought it 10 years ago I was averaging 17 mpg on the freeway going 70 mph. Now I am lucky if I get 13! Only difference is the gas. It is all an ethanol mix here in Cali and I hate it!

[Richard G.]

Re: Dumb question re 70 LT-1

John;
Your mileage dropped by 26%.
I don't believe the ethanol in the gas could account for this high of a percentage drop.
guessing its a combination of things.

[Joe L.]

Re: Dumb question re 70 LT-1

John;
Your mileage dropped by 26%.
I don't believe the ethanol in the gas could account for this high of a percentage drop.
guessing its a combination of things.

Richard------

It can't. The mileage drop between "pure" gasoline and ethanol blended gasoline (10%, the highest allowed) should not be more than about 1.9%

[Mark E.]

Re: Dumb question re 70 LT-1

Drove my '70 454/M20/3.36 from Dallas to our Austin Chapter meet a few weeks ago. 75 mph speed limit much of the way on backroads. Averaged 14 mpg round trip while driving it fairly hard. Too much fun getting into the secondaries.

[Duke W.]

Re: Dumb question re 70 LT-1

E10 has three percent less energy than straight gasoline, so the lmileage drop would be tough to measure.

The 3310 is a generic carburetor and is likely very rich.

If the compression ratio was lowered at some point that could increase fuel consumption by 5-10 percent.

The ported vacuum advance and lazy spark advance map probably costs at least 5 percent relative to the optimum map, which would be the same as OE on the 365/375 HP 327s.

A change in axle ratio without installing the proper speedo gears or a non-OE revs per mile tire could cause either an indicated increase or decrease in fuel consumption.

The driving environment is a big factor. As the ratio of highway/freeway driving to around town driving goes down, fuel consumption will increase as it will when average trips length is reduced.

So there are a lot of variables, but the first things I recommend you do is work on the carb and spark advance map.

Duke

[Kenneth B.]

Re: Dumb question re 70 LT-1

My 1970 LT1 has the standard 3.70:1 and when new (note Bill Strobel's comment about today's gas v yesterday's gas) I got 18 mpg from Chicago to Fort Collins CO. Almost all interstate in the Monfort lane. Speed limits were not widely enforced for those truckers, and I followed them closely.

I haven't heard the term Monfort lane in a long time. That's what we also called it in Indiana. Believe those trucks hauled meat from CO. to the east on the toll road/ I 80. Wonder if people from other states used that term. Those trucks were probably where they got the idea for Smoky & the Bandit.

[Ed H.]

Re: Dumb question re 70 LT-1

So here is what I think I know. Still no response from Holley...

http://www.mortec.com/carbtip1.htm says:

Primary jet - 72
Secondary - 76
Primary power valve - 10.5
Secondary pv - 8.5

GM says

Primary jet - 70
Secondary jet - 76

But nothing in manual re power valves.

As usual, thank you all for your assistance and amusing comments. ;-)

Of course my vacuum gauge, unused for a millennium, LOL, is on hiatus.

[Timothy B.]

Re: Dumb question re 70 LT-1

Ed,

My guess is that with a 3.36 rear axle gear you are primarily running on the Holley's idle circuit while cruising. The idle circuit is very active up until approx 2500rpm, jets and power valves all contribute to the main circuit.

The good news is that the Holley is not difficult to tune once you understand it's internal workings. Even the factory carburetor may be on the rich side because it's always safe to err on the safe side.

[Kenneth B.]

Re: Dumb question re 70 LT-1

I really wonder why anyone with a HP Corvette gives a rats ass about the gas mileage. Just asking?