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Old 07-01-2008, 11:48 AM
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save fuel daily driver holley carb

Hi ALL,

I use as a daily driver an 85 trans-am with a carbed 355ci . The engine is about 370 hp ( mostly world products ) and running a 4160 Holley 750 and MSD HEI street fire with 12 degres initial. I've chaged main jets from 70 to 67 and yellow secondary spring instead of silver. Vacuum advance is hooked to ported as manifold was raising my idle to 1500rpm and idle speed screw all the way out could not operate anymore...I'm thinking of putting a little wire in the metering block IFR and or changer discharge nozzle from 031 to 025 and maybe the orange cam to the white one all that to try to save fuel and maybe get more crsip on the throttle.

What do you guys think or have experienced??

Thanks lot.

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Old 07-01-2008, 11:56 AM
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You need to drop down to 600 cfm with 62 jet and long yellow spring along with 45 power valve and use .028 nozzle and with white (218) pump cam.
I had a 73 Chevelle with 355 9:1 CR stock cam advanced 4 with Eldebrock CB3X intake and 375 heads with 600 Holley jetted as posted above got 22 mpg.
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Old 07-01-2008, 12:01 PM
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If timing changes make that much difference, you definitey need a new timing work-up on this engine.
Check out Performance Distributors tech web site for explanation of timing curves and ported vs. manifold vacuum.
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Old 07-01-2008, 12:17 PM
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this engine was built under a combo given by world products " Typical Application Street: 274 Cam: 230/236 .487''/.490'', Dart II Sportsman #955-061010 dual plane intake; 750 cfm carb, 1-3/4'' header. 430HP on 9.8:1 355; 480HP on 383 10.5:1 (varies with displacement and compression ratio)."

I've read that some performance engines liked only ported vacuum and could not handle manifold. Seems like what mine's doing..Fire up perfect cold or hot, idle at 900 rpm and 600 in gear with 9' vacuum.
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Old 07-01-2008, 12:33 PM
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The cam is way too big for good fuel mileage. Other than that the timing curve needs work.
Big cams like a lot of initial spark timing. 4.5 power valve.
Because you have jetted down the primaries, you can crank up the secondary jetting a bit to restore the overall WOT jetting AFR.
If you have a secondary metering plate with no rear jets, (holley 3310) remove it and redrill the "Jet" orifices .078" to .081"

Experimenting the idle feed restrictions (little V wires) may get you some fuel savings.
Get a heated 3 or 4 wire O2 sensor and weld it into the the headers collector so you can read the voltage output with a DC multimeter while idling and cruising at low rpm.
That will give you some direction on what effect the little V wires are doing for you.

Last edited by F-BIRD'88; 07-01-2008 at 12:45 PM.
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Old 07-01-2008, 02:50 PM
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So smaller discharge nozzle and smaller carb cam would not help fuel efficiency? Right now my plugs are perfect light brown. On cruising highway around 2300rpm and hitting a little the throttle the car takes off nice but I can smell gas so would that mean somehow to rich on the pump or nozzle or both ?
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Old 07-01-2008, 03:29 PM
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Quote:
Originally Posted by jean-karim
So smaller discharge nozzle and smaller carb cam would not help fuel efficiency? Right now my plugs are perfect light brown. On cruising highway around 2300rpm and hitting a little the throttle the car takes off nice but I can smell gas so would that mean somehow to rich on the pump or nozzle or both ?
No it won;t help fuel mileage. You can try playing with the accel pump etc but.....

More timing at idle will help, 12 deg inital is not enough for that cam at idle. You're dumping unburned fuelinot the exhaust at idle with only 12deg timing and a big cam. try 24deg at idle and reset the carb. If you smell gas while driving and or idling, you may have a hole in your fire wall under the the dash. Have a look see.
The fact that you likely have eliminated/disabled the fuel vapour purge canister system doesn't help.

The cam is too big for fuel efficient crusing at 2300 rpm. Sorry, part of the joys off having such a big cam.
If you Primarily cruise and or commute in this car and fuel mileage matters, then the cam is too big. Something around 204 to 216@.050" will be amuch better cruise/power daily driver compromise.
A change to something like this cam

or this crane cam

or this lil Isky cam

Will improve the overall mileage and still make lots of power. Low end grunt will be much better. Stock converter is fine.

You're on the the right track with the idle fuel feed restriction tuning to fine tune the idle low rpm AFR with the little wires but by leaning out the idle circuit overall you'll probabily want to increase the accel pump shot to maintain throttle response. Right now the carb is set up with a "universal" "bolt it on" idle calibration for as "performance" tune. That means that carb will "bolt on" a run on any typical street motor, but is not exactly optimised for any particular car. You'll need a AFR ratio guage O2 sensor to dial it in. You can lean the idle circuit (idle feed restriction and idle air bleed and primary float level height) down quite a bit, but if you go too far the thing will miss and spitter at cruise. The more you lean down the idle circuit the more it will want a tamer camshaft and the longer it will take to "warm up"
Optimising the intake manifold under plenum exhaust track heat temp is critical.
Hotter manifold plenum allows leaner idle and cruise jetting (better fuel vapourization). Too hot and you get fuel perculation, hot soak issues.
Too cold (especially in winter) requires more choke, richer winter jetting and llonger "warm up".

Last edited by F-BIRD'88; 07-01-2008 at 03:50 PM.
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