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
Wiseco forged flat top racing pistons
Manley steel rods
Stroked steel crank
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?
I think the head spec should be 2.02" inlet.
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
First thing you need to do is find out what you have.
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.
how is that t5 still in one piece
T5 needed a rebuild, and 1st & 2nd synchro replacing. It doesn't get abused enough really ;)
Hello hotrod gurus again. I just stumbled across this old thread I started, and thought you might like to know what has happened, after all your helpful advice. It's been a while, and progress has been slow, due to much of my spare time being taken up by other activities, including fixing my track Lotus that some idiot crashed into.
Edit: distributor is HEI and vacuum was disconnected; reconnected it and this has helped mid-range manners.
I've measured the camshaft; it's 232 degrees intake, 240 exhaust @ 0.050" lift. Lobe separation is 114 degrees. Hydraulic flat lifters and Kenne Bell 1.5 roller rockers. The cam profile seems a little bit excessive, even for the spec of engine, and I'm thinking of changing to something a little milder, with hydraulic roller lifters - I understand that hydraulic give better road manners than solid, and rollers will snap the valves open and closed more quickly/efficiently.
Intake manifold changed to Edelbrock Performer RPM - noticably smoother at low end of rev range.
Carb changed to a Sean Murphy Induction Quadrajet. A beautiful, superb carb. Sean built to the specs of my engine, and idle vacuum reading (12.5" Hg.) Much smoother and more civilised, with no apparent loss of performance.
Several spark leads were found to be badly heat deteriorated, due to being extremely close to the exhaust headers. Replaced with ACCEL Extreme 9000 Ceramic Spark Plug Wires.
Along the way, lots of non-performance related jobs done, such as replacing all door and tailgate rubber seals, new passenger door (to provide something to attach the new seals too ;)), new gas rams front and back, re-trim head lining, and lots of little tweaks and repairs.
Preliminary indications are much improved economy (need to check over a longer period), possibly up into the teens MPG, with no noticeable loss of power or performance. Much easier and nicer to drive.
Thanx for the advice previously given, which helped guide me. Regards, Pum.
with out knowing compression ratio its difficult to choose the correct camshaft profile.
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