Thread: Blown 350, Better Gas Mileage Possible

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Hey guys I am a refuge over from the CobraClub and need some help.

I have a B&M supercharged 350 and am getting horrible gas mileage. On the order of ~5 mpg in town and ~7 mpg on the freeway. Currently I am running 9 pounds of boost on 91 octane stuff, compression is 7.2:1 rear end ration is 2.91 cruising around 2400 rpm at 65

I need to get better gas mileage for the simple fact that am taking longer trips in the car and can't get a fuel cell bigger then 10 gallons to fit in the trunk.

I dropped the boost down to try and get some better numbers (4 psi). In town numbers shot to ~8 mpg and freeway to ~11 mpg.

I still want to drop the boost down some more, but am worried mainly because I can;t find anybosy else doing this. I am afriad most of all if roms that the blower spin at are two low the idle will go all to heck?

Also after talking to Holley, they believe that I can run a 600crf supercharger specific carb down from my 800 double pumper. Anybody know what that will do to the gas mileage.

I know my jetting is right because my wideband is kicking out readings around 13:1 accross the board, but my cruise seams to float around 12:1 which seams a little rich to me.

So any pointers would be great, I need to get at least 15 mpg out of this thing to continue driving it. If I pull the blower and put a more traditional setup on the motor whats a well built 350 putting out about 300hp with a street cam get for mileage. (Cars 2200 pounds)

you put a 600 cfm vacume secondary holley on it and the gas milage cant help but go up.

Oh and the word refugee has 2 'e's on the end of it.

storm the best way to save gas with a blower is not kick the blower in so much. the blower is not doing anything untill around 1/2 throttle anyway, so riding down the road at light throttle there is not enough air to make any boost, so its not doing anything. ive had 454s with 671 blowers that got the same gas mileage with or without the blower.depending on the way i drove it) i leaned the carbs. on down got good gas mileage but could not hold the boost on for any amount of time. if you wont gas milage the blower is going to be for looks only. i think that gear might be hurting you a little also. i ran a little lower gear so it didnt take as much throttle to move the car. go over to the gallery and look at the 70 chev. conv. i was getting around 15 freeway mpg with that. i dont see what the boost has got to do with the idle eather. if your sitting at the stop light idling you could take the rotors out of the blower and it wouldnt make any dif. hope this helps

Originally posted by southerner
you put a 600 cfm vacume secondary holley on it and the gas milage cant help but go up.

Oh and the word refugee has 2 'e's on the end of it.

I know I can't spell for anything, if spellcheck had not been invented then I would be stuck in the 3rd grade for the rest of my life.

Any clue as to what the gas mileage whould do when I switch to the 600m say 10-20%. I would like to spin the blower around 1 psi with a huge pulley and use the blower as a huge air beater for most of the time when I daily driver the car.

I can change the gears in the "winter's" and swap the pulley on the blower in about a hour and go to the track and beat on the car, drive home and pull them off in the same time. Much better then pulling the blower every time I get back from the track.

hey StormBringer you didnt say you was drag racing this car to. you cant run 1/4 mile with 6 lbs. of boost and then drive the car to work (without changing something) and get 15 mpg, cant be done. been there done that.

Originally posted by lt1s10
storm the best way to save gas with a blower is not kick the blower in so much. the blower is not doing anything untill around 1/2 throttle anyway, so riding down the road at light throttle there is not enough air to make any boost, so its not doing anything. ive had 454s with 671 blowers that got the same gas mileage with or without the blower.depending on the way i drove it) i leaned the carbs. on down got good gas mileage but could not hold the boost on for any amount of time. if you wont gas milage the blower is going to be for looks only. i think that gear might be hurting you a little also. i ran a little lower gear so it didnt take as much throttle to move the car. go over to the gallery and look at the 70 chev. conv. i was getting around 15 freeway mpg with that. i dont see what the boost has got to do with the idle eather. if your sitting at the stop light idling you could take the rotors out of the blower and it wouldnt make any dif. hope this helps

Sorry it seems your post snuck in while I was typing the other one. I lowered the boost because I thought that I could move the curve to the left so there would be almost no boost at the lower rpms that I was using to cruise on the freeway. The carb seems to be happier at the lower

Just how lean did you run the carbs, I am a horrible by ear tuner so I tend to use the wideband for most of what I dod.

Also on the gearing any suggestions. I have a 3.13 and a 3.33 and a 3.69 or so in the garage right now. Thing is I don't want to run my freeway cruising rpms to high.

i just saw where your car dont weight 2200 lbs. with that weight that gear should be ok. i had AFBs on it and dont remember the jet and metering rods i used, but i turned as if i didnt have a blower. you was saying something about low in boost. driving down the road at 55 mph part throttle you should not have any boost. what carbs are you running?

I don’t see the need for different gears, seeing as your car only weights 2200 lbs. But as for the carb, why not try a 600 cfm for the street and the 800 for the track. If your meter is reading 12:1 at crusing speeds, you are running rich, but I am sure that I really didn’t have to tell you that. You could try re-calibrating the 800 intermediate circuit.

As for another 350 engine, forget it. If you have the huffer, use it.

Use a clutch to kick in the blower. If you can, get a AC compressor clutch and modify it so that you can kick in the blower when you want it. My old strip car was set up to kick it in when I was at the line. That should keep you from using so much fuel when you are not racing. I know Weiand used to make something like that. Perhaps you could check with your blower manufacturer to see if they have something similar.

Another thing I used for a while was a window washer scaffold brake. Its a centrifugal setup that has weights. If you set the thing up with larger springs it should kick in at higher RPM's

Originally posted by Tommycat
Use a clutch to kick in the blower. If you can, get a AC compressor clutch and modify it so that you can kick in the blower when you want it. My old strip car was set up to kick it in when I was at the line. That should keep you from using so much fuel when you are not racing. I know Weiand used to make something like that. Perhaps you could check with your blower manufacturer to see if they have something similar.

Thought about that but I don't think it will work, if you look inside the blower if it is not spinning the rotors can mesh and stop all air flow or cause major restriction to the flow.

I am going to pick up a supercharger specific 600 this upcomming weekend and give it a shot, if it does not work I am sure that I can fit it to something else in the garage.

Will try leaning out the current 800 for the time being, anybody got a airfuel range I should be shooting for at cruise on a setup like this.

Sorry must be different design from the blowers I had

Originally posted by StormBringer
Thought about that but I don't think it will work, if you look inside the blower if it is not spinning the rotors can mesh and stop all air flow or cause major restriction to the flow.

I am going to pick up a supercharger specific 600 this upcomming weekend and give it a shot, if it does not work I am sure that I can fit it to something else in the garage.

Will try leaning out the current 800 for the time being, anybody got a airfuel range I should be shooting for at cruise on a setup like this.

Actually I lost a blower belt on my set up once, started the moter up and she kept on running the intermeshig rotors just kept turning and went along for the ride in the airflow caused between the maifold and carb. Mind you I was using the GMC 671 with the epicyclodal rotors and they seemed to windmill quite well.

As for using an air conditioning clutch forget it, they are not strong enough to handle the horsepower a blower needs to drive it.
I would run the secondary jets 3 steps richer than present and go from there. I mean 3 steps richer than stock jetting.

Somewhere above you said you had talked to Holley about referencing the carb off the manifold. I think what you mean is referencing the powervalve off the manifold pressure/vacuum. Holley makes conversion carbs that have this built in. They are abit spendy but they work very well. I think you would find a major inprovement in gas mileage as well as performance.

I drove the wheels off my T-bird super coupe which had a blower from the factory. It had as high as 24 inches vacuum with 22 at normal cruise. It got 26 mpg all the time and ran like crazy flat out.

Years ago I built a blown Hemi Dart which we used manifold referenced power valves and it ran very nice and got reasonable mpg considering it was almost a racecar. It made all the difference inthe world as far as setting the cruise condition on the carbs.

you also are absolutely right on the a/c clutch..no good. It would take one powerful clutch to hold that kind of torque. Even a 3in flat timing belt is near the edge which is why we use the metric belts.

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Dodge CTD dualies rule. Kings of smoke

Originally posted by Streets
You should have used a "mechanically engaged" huffer specific clutch then, instead of the weak A/C type that doesn't work all that well at all.. The one I used on my 383" stroker worked very well on the street but, alas, it was no fun when not boosted!!

First time I have heard of one, who puts them out ? (makes em)