Mileage went from 20 to 10 with new motor...why?

[PotvinGuy]

I used to get 20 MPG with my bucket, 350 SBC, 4bbl on small blower, 2100 lbs, 3.0 rear. Then I replaced the motor with a new GM crate 350, the 260 HP version. Now I only get 10 MPG. I can't find any fuel leaks, the motor runs fine and the plugs look great. Any ideas what is going on?!


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[GT63]

I used to get 20 MPG with my bucket, 350 SBC, 4bbl on small blower, 2100 lbs, 3.0 rear. Then I replaced the motor with a new GM crate 350, the 260 HP version. Now I only get 10 MPG. I can't find any fuel leaks, the motor runs fine and the plugs look great. Any ideas what is going on?!


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Has your foot got heavier? LOL

Did you change the fuel cap? Could the gas be evaporating out of the tank?

Maybe it just the difference in intake and heads.

 

[Mr. Fixit]

I used to get 20 MPG with my bucket, 350 SBC, 4bbl on small blower, 2100 lbs, 3.0 rear. Then I replaced the motor with a new GM crate 350, the 260 HP version. Now I only get 10 MPG. I can't find any fuel leaks, the motor runs fine and the plugs look great. Any ideas what is going on?!


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Although not all of the issue, the motor will need to break in and the mating parts will need to become like close family. Get 500 miles on it, change the oil and check the milage again.

 

[one finger john]

If I hadn't looked at the accompanying picture I would have thought that the replacemwnt was a NA motor with a quad. It appears that you are paying the price for going from a blown, inje ... edit. Just saw that the potvin motor was equiped w/a 4 barrel. I thought it was injected. Sorry. Could we have a side by side comparison of motors? Is that a 144 or the larger 177 B&M blower? Doe the price of 10 mpg equal better performance? 10 mpg seems 'bout the norm for blown sml. blocks.

John

 

[RPM]

Double check the timing, is it the distributor? Advance curve will affect fuel mileage big time.

My blown SBC gets about 10 mpg

 

[Old Rotor Flap]

While this won't provide and answer to your mileage question, maybe can tell us something about the blower drive you had.

As I recall. the Potvin front drives were direct drive off the front of the crank at a 1:1 ratio. I didn't think you could change that ratio.

If you were driving at 1:1, what compression ratio pistons did you have in that motor?

Was it a 4-71 or 6-71 blower?

Thanks.

 

[PotvinGuy]

While this won't provide and answer to your mileage question, maybe can tell us something about the blower drive you had.

As I recall. the Potvin front drives were direct drive off the front of the crank at a 1:1 ratio. I didn't think you could change that ratio.

If you were driving at 1:1, what compression ratio pistons did you have in that motor?

Was it a 4-71 or 6-71 blower?

Thanks.

It's not relevant to the problem I'm having, since I replaced the Potvin setup with the current config and was getting the 20 until I put in the new long block. But I'm always glad to talk about the Potvin. It is crank driven at 1:1 and uses a 671 blower. That motor was 7.5 CR, and had port EFI. The thing on the blower is just a throttle body to regulate the air. That setup was scary powerful, and I only used 2 of the 4 barrels on the throttle body! Never had the nerve to link the other two. I'm going back to the Potvin soon, maybe in time for GoodGuys in TX. It got better mileage, too.

Haven't made any significant changes to the car since the new motor. Same tires, tranny, gears.

And is 10 MPG really par for a bucket with a 350? That doesn't seem right. My Buick Park Avenue gets 20+ and it must weigh twice as much.

 

[72442conv]

It's not relevant to the problem I'm having, since I replaced the Potvin setup with the current config and was getting the 20 until I put in the new long block. But I'm always glad to talk about the Potvin. It is crank driven at 1:1 and uses a 671 blower. That motor was 7.5 CR, and had port EFI. The thing on the blower is just a throttle body to regulate the air. That setup was scary powerful, and I only used 2 of the 4 barrels on the throttle body! Never had the nerve to link the other two. I'm going back to the Potvin soon, maybe in time for GoodGuys in TX. It got better mileage, too.

Haven't made any significant changes to the car since the new motor. Same tires, tranny, gears.

And is 10 MPG really par for a bucket with a 350? That doesn't seem right. My Buick Park Avenue gets 20+ and it must weigh twice as much.

10 MPG is about right for a blown small block. I have an 8-71 blown 351 Cleveland, 4 speed toploader. 10 MPG is normal for me. 12.5 MPG is the best I have gotten out of it.

 

[GT63]

It's not relevant to the problem I'm having, since I replaced the Potvin setup with the current config and was getting the 20 until I put in the new long block. But I'm always glad to talk about the Potvin. It is crank driven at 1:1 and uses a 671 blower. That motor was 7.5 CR, and had port EFI. The thing on the blower is just a throttle body to regulate the air. That setup was scary powerful, and I only used 2 of the 4 barrels on the throttle body! Never had the nerve to link the other two. I'm going back to the Potvin soon, maybe in time for GoodGuys in TX. It got better mileage, too.

Haven't made any significant changes to the car since the new motor. Same tires, tranny, gears.

And is 10 MPG really par for a bucket with a 350? That doesn't seem right. My Buick Park Avenue gets 20+ and it must weigh twice as much.

I get about 10 MPG too. And you've seen my set-up. 6-71 on top of a 355ci SBC. If I'm real easy on the loud pedal I can get 12, but that's no fun. :flex:

 

[one finger john]

So cut the roof off the Park Avenue. Instant roadster. It would be nice to get complete information on both engines and any changes to the running gear so that INFORMED opinions can be made on your MPG loss. When I was a service adviser for Honda of America customers would call and ask "My car won't start. What do you think is wrong?" Would tell them "Don't know till you bring it in and we take a look at it". Same thing here. No definitive info, no definitive answer. Not a mind reader. Can't look into a crystal ball. Need more real time information.

John

 

[gfigms]

I used to get 20 MPG with my bucket, 350 SBC, 4bbl on small blower, 2100 lbs, 3.0 rear. Then I replaced the motor with a new GM crate 350, the 260 HP version. Now I only get 10 MPG. I can't find any fuel leaks, the motor runs fine and the plugs look great. Any ideas what is going on?!


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I think Oklahoma has 3.2 gas ... just like their beer! :flex:

 

[Lee_in_KC]

My 6-71 SBC355, TH350, 3.42 rear, 31" tires hotrod gets 6 mpg toolin' around town, 9 mpg highway. I set it up to run rich when I built it to stay away from detonation. I've been intending to put it on a chassis dyno and put a good tune on it ever since, but just haven't gotten around to it. I've had the calibration kit for three years! :flex:

 

[PotvinGuy]

So cut the roof off the Park Avenue. Instant roadster. It would be nice to get complete information on both engines and any changes to the running gear so that INFORMED opinions can be made on your MPG loss. When I was a service adviser for Honda of America customers would call and ask "My car won't start. What do you think is wrong?" Would tell them "Don't know till you bring it in and we take a look at it". Same thing here. No definitive info, no definitive answer. Not a mind reader. Can't look into a crystal ball. Need more real time information.

John

John, if you read my responses, you'll see that there were NO changes to the running gear. The engines are both Gen I 350 SBC. I took the induction and ignition and headers off the old motor and put them on the new motor. We might assume minor differences in cams and CR, but that couldn't account for a 50% reduction in MPG.

I see several responses (Hi Grant!) saying 10 MPG is nominal. Maybe I dreamed the whole 20 thing like whats-her-name on Dallas. I'll keep looking.

 

[one finger john]

Sorry if frustration appeared to be anger. The Potvin/fuel injected edition was likely the 20 mpg motor. If it wasn't, then it is either ignition, carburation, or converter. If you are using twice as much gas (one gallon to go ten miles, two gallons to go twenty vs one gallon to go twenty miles as with the earlier engine), are you getting 20% more horse power (at least that for the amount of fuel burned)? Have you looked at the plugs to see how they are burning? I would imagine that they would be black as night and on the verge of fouling.

Just trying to be helpful, John

 

[deckofficer]

Dropping from 20 mpg to 10 mpg is a lot more than I would expect from a new tight motor. Back in the 60's I used to rebuild my motors and was amazed at the increase in effort to hand spin (without plugs) the same motor with new bearings and rings. My mileage would drop also from 16 mpg to 12 mpg pushing a 4000 lb car. After a couple thousand miles it would work back up to 16 mpg. I would guess that a tight engine pushing a car that only weighs 2000 lb would show a higher percentage drop. Your local gas stations might have gone to the "summer blend" which could account for 1 or 2 mpg. Put it all together, it might mean that large of a percentage drop.