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Old 11-15-2012, 05:08 PM
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SBC 392 or 396 fueling for economy advice please

Hello hotrodding gurus,

I'm looking for some advice and guidance on what to do about improving my SBC setup please. I acquired the car already modified, and the seller didn't have all the details about what had been done, as it was all done back in the early 1990s. What I have (some of this might be inaccurate, but it's the best info I've got):

1985 Trans Am with replacement engine & original T5 manual box & axle.
SBC 392 or 396 ci 4 bolt mains
Dart II aluminium heads 2.2" inlet, 1.6" exhaust
Unknown camshaft
Wiseco forged flat top racing pistons
Manley steel rods
Stroked steel crank
Trap-door sump.
Edlebrock inlet manifold, possibly Performer or Torker.

Was originally the sister build to a race engine with a 1050 carb and appropriate cam, but this was too wild for the street, so the cam was changed to something less harsh, and the carb was replaced with a 750 modified to something like 925, by Demon we think. Now running a standard Holley 3310 750.

It seems to have plenty of power and goes well, but I'm getting pretty terrible miles per gallon from it, and it's not very smooth to drive below 2500 RPM. I'd like to get better fuel economy for general street use - fuel costs over 8 US dollars per US gallon over here I am happy to swap to a bigger carb for occasion visits to the strip.

Assuming the engine needs 750+ CFM to achieve its full performance, will it do any harm to run a much smaller carb for economy on the street, and would that even work?

Am I right in assuming a spread-bore carb will give me better economy with light use of the gas pedal, but still have plenty of fueling capability if I put my foot down? If so, how much better economy would I get with a spread-bore carb compared to the same CFM square-bore?

I suppose it is probably a good idea to measure the compression ratio and cam profiles.

Can anyone recommend a suitable cam, carb & inlet manifold please?

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Old 11-15-2012, 05:54 PM
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I think the head spec should be 2.02" inlet.
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Old 11-15-2012, 05:56 PM
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If you have a race oriented engine,sell/trade for something. To detune an engine will cost as much as building a new one and you will have parts you dont want but have to use them.
Maybe figure exactly what you want as a finished product?
Big cams and big right foot are hard on fuel
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Old 11-15-2012, 05:57 PM
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Quote:
Originally Posted by Pum View Post
Hello hotrodding gurus,

I'm looking for some advice and guidance on what to do about improving my SBC setup please. I acquired the car already modified, and the seller didn't have all the details about what had been done, as it was all done back in the early 1990s. What I have (some of this might be inaccurate, but it's the best info I've got):

1985 Trans Am with replacement engine & original T5 manual box & axle.
SBC 392 or 396 ci 4 bolt mains
Dart II aluminium heads 2.2" inlet, 1.6" exhaust
Unknown camshaft
Wiseco forged flat top racing pistons
Manley steel rods
Stroked steel crank
Trap-door sump.
Edlebrock inlet manifold, possibly Performer or Torker.

Was originally the sister build to a race engine with a 1050 carb and appropriate cam, but this was too wild for the street, so the cam was changed to something less harsh, and the carb was replaced with a 750 modified to something like 925, by Demon we think. Now running a standard Holley 3310 750.

It seems to have plenty of power and goes well, but I'm getting pretty terrible miles per gallon from it, and it's not very smooth to drive below 2500 RPM. I'd like to get better fuel economy for general street use - fuel costs over 8 US dollars per US gallon over here I am happy to swap to a bigger carb for occasion visits to the strip.

Assuming the engine needs 750+ CFM to achieve its full performance, will it do any harm to run a much smaller carb for economy on the street, and would that even work?

Am I right in assuming a spread-bore carb will give me better economy with light use of the gas pedal, but still have plenty of fueling capability if I put my foot down? If so, how much better economy would I get with a spread-bore carb compared to the same CFM square-bore?

I suppose it is probably a good idea to measure the compression ratio and cam profiles.

Can anyone recommend a suitable cam, carb & inlet manifold please?
Before you buy any new parts you need to find what you have. You have a lot more homework to do. After finding what you have fixing it may be a very easy and simple one part swap or it could be cheaper to sell your current engine and buy a good street engine.


First thing you need to do is find out what you have.
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Old 11-15-2012, 06:21 PM
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Originally Posted by ap72 View Post
Before you buy any new parts you need to find what you have. You have a lot more homework to do. After finding what you have fixing it may be a very easy and simple one part swap or it could be cheaper to sell your current engine and buy a good street engine.
First thing you need to do is find out what you have.
^times 2
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Old 11-15-2012, 06:32 PM
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So it sounds like you got a good build there.

What type of distributor is on the car. Do you have vacum advance hooked up. Often removed in race engines. If not you can save some mpg right there with a carefully tuned setup.

Multi spark also saves fuel. Smaller carbs can use more gas depending on the driver. For me my foot is in it way to much. Larger carbs make good power so usally your in it less. But not really always the case.

How is the gear. This can be the biggest fuel savings of them all. Even the next size higher could save a few mpg.

Good spark plugs/gapped and good wires. Can all help with mpg. Royal purple light syn oils can help also. And most importantly make sure its tuned right. If its blowing black smoke when you floor it you might be to rich. Did you tune the carb. A dyno tune with wide band may solve a lot of issues. It would suck to change all that stuff and find its only a little better. Or buy a four banger to drive to work and rod for the weekend. Its never going to get great milage its still a v8 sports car.

If you really need to up the milage get an LT or LS engine from the junk yard with trans and computer/wiring harness setup for a swap. You will be suprised how cheap it can be to swap and how much better mpg and power you cna get out of the lt or ls style engines. Ls and 6 speed could put you over 20 mpg and still run low 13 or high 12's could pay for itself in gas milage alone. Also your race motor is probalby worth a few buck in your area.
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Old 11-15-2012, 06:36 PM
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Manifold will say torker or performer on it.

Bet its a torker they fit under the stock hood and are single plain for larger power plants. But they arent good for mpg tq or hp. Middle of the road is not always the best place to be. Performer would be a good choice for fuel saver but the performer rpm may not fit under your hood.
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Old 11-17-2012, 12:27 PM
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Thanx for all the replies and advice.
Intake manifold is Edelbrock Torker (1990 vintage, as was engine build.) I'm liking the idea of a Performer + spread-bore quadrajet of some sort, but will wait till I know all the facts about the engine.
Having done further research I think the engine is probably 396 (4.030" bore x 3.875" stroke) as this seems to be the most usual config at that sort of capacity.
The 1990 vintage Dart II heads appear to have been available in 64cc or 72cc chamber capacity - which is more likely? I'm guessing 64cc.
With 3.875" stroke, 64cc heads and flat top pistons, compression ratio should be around 11.5:1 yes?
I'll check distributor and vacuum advance.
Has thick, good condition spark leads.
I agree that I need to find out for sure what it all is, so I'll get the cam profile, bore, stroke and compression ratio measured when I can get it in the shop (which will be spring when I have my other car back on the road, and hopefully also have a job.)
I quite like the idea of swapping in a used LS engine if I can sell my engine for decent money - I'll keep that idea in mind too.
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Old 11-17-2012, 01:08 PM
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got to say the LS swap is good for fuel milage and power. Its no dog and even the lame ones are good power for street machine and are easily upgraded to more power later on down the road.

The real trick is finding the right deal. Avoid the demand for the corvette falvors they are too much money but the truck blocks are just as much fun. Best thing to do is research it and save your money for the day the deal comes along.

Check all the local junk yards. You will have your pick of engines to research. might as well get the 6 speed as well. It will help save on gas aswell. Not sure if the computer controlled auto tranny is as easy to swap but would also be pretty sweet.

Whatever you do dont spend 15K for ls7 427 swap. It would be freakin awesome but really not needed or worth the money since that kind of power is not impossible from the cheaper packages.
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Old 11-17-2012, 01:25 PM
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how is that t5 still in one piece
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Old 11-18-2012, 12:00 PM
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T5 needed a rebuild, and 1st & 2nd synchro replacing. It doesn't get abused enough really
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