Poor gas mileage...?? - Team Camaro Tech
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post #1 of 6 (permalink) Old Nov 25th, 14, 08:40 AM Thread Starter
Mark
 
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Location: Essex England
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Poor gas mileage...??

I've owned my '81 Z28 for a few weeks and the gas mileage seems a little high to me. Since I'm new to the car I've driven it easily so far.

Here's the spec:

ZZ4 crate engine, trick flow ally heads held down with an arp bolt set on top of ultra thin metal head gaskets raising compression to 10.5:1, trick flow cam, trick flow hardened push rods, Weiand stealth warrior dual plane inlet with a 750DP Holley carb fed by a 110gph Holley fuel pump, upgraded Davies unified ignition HEI distributor, new rotor arm, new leads and new plugs, full length headers into 3 inch collectors with exhaust dumps, power now estimated @ 460+ bhp all kept cool by a 3 core ally be-cool rad and steel flexi fan with billet reduction pulley, also new battery and alternator just fitted

Drivetrain
Rebuilt TH350 with B&M 3200 stall converter, shift kit, external cooler, and quick silver shifter
Rear end is a rebuilt 10 bolt posi with 3.90 gears

I'm getting 4-6 mpg which I figure is too low, but then again considering the spec and gearing is it?? I will say a fair amount of town driving has been involved, and I have noticed at 70 mph the engine is around 3500 RPM.

Any ideas? Thanks
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post #2 of 6 (permalink) Old Nov 25th, 14, 09:54 AM
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Dave
 
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Re: Poor gas mileage...??

Two things jump out to me if you are concerned about mileage.

First thing is a 350 cube motor will run quite happily with a 650 cfm vacuum carb.

But the biggest is your 3200 RPM torque converter. You didn't mention the cam's specs but one must assume that you have a thumping cam installed. If you are driving on the street, you have no need of that cam or the 3.90 gear for that matter. The configuration should be much more aligned with how the Chevy engineers put the car together originally.

But even then a 350 wouldn't be your first choice in mileage. I would have gone for the 153 cubic inch displacement "Iron Duke" four cylinder in line engine Chevy used in the Chevy II instead. That is of course if you are really concerned about mileage.

Now as to your mileage figures: I agree with you, some thing is indeed seriously wrong. I owned a 4.56 rear gear, four speed, L-88 powered first gen (actually several of them before I went to the LS-7 454 when the parts first became available) that still got better mileage than you are now. I am sticking with my first assessment however in that the converter without a lock-up is the problem.

Big Dave
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post #3 of 6 (permalink) Old Nov 25th, 14, 10:44 AM Thread Starter
Mark
 
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Re: Poor gas mileage...??

Quote:
Originally Posted by Larger Dave View Post
Two things jump out to me if you are concerned about mileage.

First thing is a 350 cube motor will run quite happily with a 650 cfm vacuum carb.

But the biggest is your 3200 RPM torque converter. You didn't mention the cam's specs but one must assume that you have a thumping cam installed. If you are driving on the street, you have no need of that cam or the 3.90 gear for that matter. The configuration should be much more aligned with how the Chevy engineers put the car together originally.

But even then a 350 wouldn't be your first choice in mileage. I would have gone for the 153 cubic inch displacement "Iron Duke" four cylinder in line engine Chevy used in the Chevy II instead. That is of course if you are really concerned about mileage.

Now as to your mileage figures: I agree with you, some thing is indeed seriously wrong. I owned a 4.56 rear gear, four speed, L-88 powered first gen (actually several of them before I went to the LS-7 454 when the parts first became available) that still got better mileage than you are now. I am sticking with my first assessment however in that the converter without a lock-up is the problem.

Big Dave
Thanks for the info there Dave

I don't really wanna spend too much on changing various
Bits and pieces. I can handle 10-12 mpg quite comfortably, just not 4-6 mpg!!!
The guys that owned her before used her for the occasional Sunday afternoon drag racing event.

I do like the car's performance and I'm keen to keep that, maybe I'll indulge in some of those events sometime. You mentioned something about a lock up, where is this located and what does it do? I'm very very new to these sort of engines, my knowledge and experience is very limited indeed.
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post #4 of 6 (permalink) Old Nov 25th, 14, 11:33 AM
DjD
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Re: Poor gas mileage...??

Since it's a mechanical secondary carb, try unhooking the secondary linkage and see if it gets better mileage on just the primaries.

...Dennis

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post #5 of 6 (permalink) Old Nov 25th, 14, 11:44 AM
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Scott
 
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Re: Poor gas mileage...??

Quote:
Originally Posted by DjD View Post
Since it's a mechanical secondary carb, try unhooking the secondary linkage and see if it gets better mileage on just the primaries.
I'll second the direction Dennis has suggested, that double pumper is not designed with fuel economy in mind. And what Dave suggested is very true as well, you'd be much better off with a 650 CFM vacuum secondary carburetor for street driving and it might even perform better than what you have on there now. One thing is for sure, you would pickup some mileage. If you really want to stay with that existing carburetor, install a wide band data logger and get the carburetor tuned so you know it running really rich which it very well may be right now.

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post #6 of 6 (permalink) Old Nov 25th, 14, 07:25 PM
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Steps
 
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Re: Poor gas mileage...??

Several things effect mileage
As dave mentions above.. the carb
1/carbs have a narrow efficient rpm range.. big carb no bottom end efficiency... or torque
2/the number of rpms/ engine volume/ total amount of mixture/ end of the day fuel required per mile... the high stall will be slipping, engine turning over at the slightest load slope accerlation way faster than it needs to
3/ Correct timing AND mixtures at any given rpm and engine load.
4/ cams also have a narrow rpm range, and outside that they become inefficient.. and cruise cams have bottom end torque rather than hp
5/ headers tend to be designed for high rpms and loads , not cruise.
6/ valley cover/ inlet manifold for more economy have longer and narrower runners
7/Fuel type matched to the dynmamic compression of the engine.. cam design.

Quote:
I don't really wanna spend too much on changing various
Bits and pieces. I can handle 10-12 mpg quite comfortably, just not 4-6 mpg!!!
1/Dump the hi stall.. but keep for a quick swap if heading to the track for a play
2/ do the same with the big carb.. cruiser a 600 vac sec will do the job the carb should give u around a low 15s AFR at cruise and idle and 11.5 to 12.5 WoT.
3/ get the timing in ball park total all in around 32 deg around 3200 rpms,
4/ VA that has around 10 deg in it and is all in at the engine vaccum at highway 60 mph cruise... and still gives around 16/ 18 deg on manifold vaccuum idle degs.

That alone should get u up in the 16/ 18 mpg

My Spelling is not incorrect...it is creative

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