12 mpg 383.....help!

[Sparks280zt]

OK, so here is the deal. I knew my car was not getting any decent mileage numbers, so when I left my college apartment today to head back to my parents for the long weekend, I topped off the tank.

 

I drove exactly 80 miles (all highway) running 65-70mph the entire time. I babied it the whole way, never over 3k rpm. My cruising rpm was between 2300-2600 rpm. I pulled into a station and filled up and the car took 6.7 gallons. So as the math indicateds, 11.94 mpg! Ouch

 

Here are the engine specs:

 

Engine:

 

Carb: Holley HP 750

 

60 over 350, squared, line honed and torque plate honed Keith Black hypereutectic pistons. Speed pro rings. 6" Elgin 5140 rods with ARP bolts. Internaly balanced, clevite bearings, ARP bolts used throughout engine, Dart Pro 1 200cc heads, cast crank, crane cam 228@.05 480 lift. 140 amp alternator, weiand water pump, AZC radiator and taurus two speed fan.

 

 

Now I know carb'd v8's aren't known for amazing mileage numbers, but I feel like I should be atleast getting 16 or so. I PM'd grumpy the engine specs and he felt like 18-20 mpg was acheivable.

 

Trans is a t56.

 

The fuel pump is a Holley Red and I am not sure if it is the 67 or 91 gph, but I glanced under the car and I didn't see a fuel return line. Could that be the problem?

[ModernS30]

OK, so here is the deal. I knew my car was not getting any decent mileage numbers, so when I left my college apartment today to head back to my parents for the long weekend, I topped off the tank.

 

I drove exactly 80 miles (all highway) running 65-70mph the entire time. I babied it the whole way, never over 3k rpm. My cruising rpm was between 2300-2600 rpm. I pulled into a station and filled up and the car took 6.7 gallons. So as the math indicateds, 11.94 mpg! Ouch

 

Here are the engine specs:

 

Engine:

 

Carb: Holley HP 750

 

60 over 350, squared, line honed and torque plate honed Keith Black hypereutectic pistons. Speed pro rings. 6" Elgin 5140 rods with ARP bolts. Internaly balanced, clevite bearings, ARP bolts used throughout engine, Dart Pro 1 200cc heads, cast crank, crane cam 228@.05 480 lift. 140 amp alternator, weiand water pump, AZC radiator and taurus two speed fan.

 

 

Now I know carb'd v8's aren't known for amazing mileage numbers, but I feel like I should be atleast getting 16 or so. I PM'd grumpy the engine specs and he felt like 18-20 mpg was acheivable.

 

Trans is a t56.

 

The fuel pump is a Holley Red and I am not sure if it is the 67 or 91 gph, but I glanced under the car and I didn't see a fuel return line. Could that be the problem?

 

Don't know much about V8 cars gas mileage, but I have a friend who gets 12 mpg with a 78 Firdbird. 350w/edelbrock and an automatic. That's city mileage though. I could see the fuel return being an issue if it is supposed to be there, plus if there is a return line hole on the tank with no line, gas could spill out, maybe not much, but it would make a difference.

[grumpyvette]

without a fuel pressure gauge and an adjustable return style fuel pressure regulator with a return line installed your basically restricted to guessing, and chances are very good youll have major problems, you need the engine timing set correctly and you need to learn how to tune carbs and correctly read spark plugs

step one is getting the fuel pressure rock steady at 5 psi, then get the carbs floats and jets set correctly, so you get plugs that look like this

 

good

 

too lean

too rich

 

 

http://forum.grumpysperformance.com/viewtopic.php?f=55&t=211

 

http://forum.grumpysperformance.com/viewtopic.php?f=55&t=635

 

http://forum.grumpysperformance.com/viewtopic.php?f=70&t=202

 

http://forum.grumpysperformance.com/viewtopic.php?f=70&t=1015

Edited January 14, 2010 by grumpyvette

[Sparks280zt]

OK the fuel pressure gauge under the hood is showing 6.5 psi at idle.

[grumpyvette]

OK the fuel pressure gauge under the hood is showing 6.5 psi at idle.

 

it NEEDS to read 4.5-5 psi MAX and you NEED to set the float levels per the carb manufacturers instructions, having the pressure at 6.5 psi will in many cases cause the needle & seat valve to constantly leak excess fuel, into the carb while the engine runs

Edited January 15, 2010 by grumpyvette

[Sparks280zt]

OK it's parked right now.

 

Here are some pics of a spark plug I just pulled:

 

 

[grumpyvette]

if it was a clearer picture I could tell a GREAT DEAL MORE about conditions in the combustion chamber but its obvious the lower plug pictures running real rich

[Sparks280zt]

Yea I can't figure out how to get the camera settings to work to get it clearer. But you get the idea.

 

I will try and re-set the float levels tommorow, but without adjusting the fuel pressure down? I suspect it won't help much?

[grumpyvette]

Yea I can't figure out how to get the camera settings to work to get it clearer. But you get the idea.

 

I will try and re-set the float levels tommorow, but without adjusting the fuel pressure down? I suspect it won't help much?

 

 

 

IT MAY HELP! but the cure is getting all the systems in the carb operating exactly as designed

getting the fuel pressure and float levels correct and then the jets and power valves correct for the application and adjusting the acellerator pump cam and throttle plates for the application, and maybe changing the boosters and squirters, and vacuum springs in rare cases

[310z]

It sounds like grumpyvette is getting you dialed in on the carb end of things. Which sounds like were you need to be. However, when you are done with that you may look at running taller gears in the rear end. With the kind of low end torque a 383 makes you could drop 500 to 900 RPM at cruising speeds and that would help also.

 

I replaced a 350 (250 hp) in my Classic Century Resorter boat with a 388 that sounds similar to your 383. Prior to the rebuild I could pull skiers all weekend long on a 3 day weekend and still have a 1/4 of a tank left. With the 388 (430 hp) I can get one day and the next morning skiing in before heading for a refill. Horse power cost money, my craving is usually beyond my wallet.

 

I do think grumpy's estimates are obtainable and that most of your gains will come through his suggestions.

[jt1]

When my Z was a street car at about 325 whp, It would get 20-25 mph if I kept my foot out of it. With the T56, you don't have any gearing issues, so it just needs tuning. Cruise mixture, power valves, timing curve. It takes some patience, but you can get there.

 

jt

[HarrisonTX]

You're turning 2300-2600 RPM and going 65-70 mph? Jesus, what rear gears do you have? With a t56 I'd think you'd be about 1300RPM.

[Sparks280zt]

Harrison, that was in 5th gear. In 6th, it is about 1600 rpm at those speeds. Car seems to run bad under 2000 rpm so I left it in 5th. I think the rich issue is the reason it runs sloppy under 2000 rpm.

[Sparks280zt]

Oh and can anyone suggest a good adjustable FPR for my set up?

[sticky280zx]

holley makes a good shiny chrome one....dont know about on your motor but most fpr's are the same and the one i had on my carbed 280 motor had a return line on it as well

[Alxxpress]

Tune that carb bro, you think of useing a wide band air fuel meter? I use them when I tune EFI cars but I know you can get a good idea of whats going on at cruis speeds with your carb car. I think this tool can help.

[HarrisonTX]

Tune that carb bro, you think of useing a wide band air fuel meter? I use them when I tune EFI cars but I know you can get a good idea of whats going on at cruis speeds with your carb car. I think this tool can help.

 

I have an innovate LM1 for my carb tuning. It really takes the guess work out of stuff. Run you somewhere around $270. Helps you tune at idle, part throttle, and full throttle. Its a Wide band A/F ratio gauge.

Avoid narrow band gauges, they are worthless.

The LM1 requires a bung to be welded into your exhaust (they all do)

and a little wiring.

 

Thanks for clearning up the RPM issue. I though that was crazy high for a 6 speed.

 

If it only turns 1600 at those speeds here is a simple solution - Drive faster.

[310z]

If it only turns 1600 at those speeds here is a simple solution - Drive faster.

 

I like the solution.

[Pyro]

ignition timing and air/fuel mixture are what you need to look at when going for mpg.

 

Holleys come jetted rich. I bet your engine would like a couple jets leaner. read a plug, and make the adjustment. do a web search "how to tune a holley the scientific way". Nice write up and free.

 

ignition timing is the next thing to setup. 48 degrees of timing going down the highway is what the engine needs for good mpg. That should be the total timing (mechanical plus vacuum advance). do another web search "ignition timing 101".

 

you should be getting at least 20 mpg on the highway with your combo.

 

here you go. http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

 

http://www.bob2000.com/carb.htm

[DavyZ]

Matt,

 

Sorry I didn't show to Austin's to help dial in the carb (see my PM to you). I could have brought you an Edelbrock or even another Holley, a 650, to let you run it and see what the difference is. If you are going back to his house on another weekend, please let me know and I can bring a carb or two.

 

 

Davy