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Author Topic: Summit Street/strip 750 cfm elec. choke or edelbrock performer 750 elec. choke?  (Read 10574 times)

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clayton

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i currently am running an edelbrock performer 750 electric choke carb on my 79 trans am 403 6.6 liter.... the carb runs pretty good it just seems like it is inefficient on this engine, which has a holley aluminum intake.  i have adjusted the air/fuel mix and ive set the accelerator pump pin about where it should be and the car still has a slight stutter off of idle.... the real power doesnt come out till around 30 to 40 mph at about 3/4 throttle or more.... im guessing the carb is either struggling for air. or its just not going to deliver the right amount of fuel to get a decent amount of response off of idle.....Im wanting to know if anyone has used the summit brand street/strip carbs advertised in the summit catalogs, and if this would give my engine a better performance gain than the current edelbrock.  the summit carb sells for much cheaper than what i paid for the edelbrock brand new 6 months ago so im interested in knowing if it would be beneficial to sell the edelbrock and switch to the summit 750 electric choke!  also would cutting the fake vents out of my spoiler help or could it have a non desireable affect? ive heard of people doing this on their shaker scoops to increase airflow and response.... and another thing i need help with is if i should install an electric fuel pump for carburetors? the car has a painless wire harness so i have an entire circuit on the fuse box and the wiring to have one installed... or will the manual fuel pump be good enough for a street/strip carb? all advice will be greatly appreciated...

Todd Stahr

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I am more of a Qjet fan and I havent had any good experience with Edelbrock carbs on modified engines. Edelbrock carbs work for stock engines and they are quite simple though and have few adjustments. On the other hand learning to build a Qjet takes some time and tuning them while not being complicated is made a bit tougher because Qjets have a wide range of tuning possibilities. Holley carbs are between those two extremes and there is a wealth of information available for them.

Im not sure which Summit carb you are referring to, but I assume its the one similar to a Holley or Autolite 4100 found on Fords in the 60s and 70s. If its the Qjet it might need some rework to operate correctly, quite a few reman Qjets are just slapped together and I havent bought any from Summit, so I dont have first hand experience with them.

Regardless of what carburetor you choose, you will need to spend some time getting it working correctly. Sometimes it will be close enough right out of the box that it will run just fine for you. Its a good idea to get a book about the carb you choose and learn to make adjustments on it the correct way, that way you can find out what does what on the carb.

By cutting the fake vents on the spoiler I believe you mean the shaker scoop. Yes you can remove those and the engine will receive cooler air as a result. Since its a rear facing scoop you only need to worry about water entering the open scoop when its parked, not when driving. A performance difference is not easy to see unless you are on a dragstrip, most often cars will pick up about .2 seconds, that is two tenths of a second, in the quarter mile. What you will notice is the engine sounds louder when you open the throttle under hard acceleration. Part of the reason the scoops were closed in the early 70s was because of noise limits placed on new cars at that time. The rest of the spoilers and vents on the 2nd gen Trans Am actually work quite well, so dont cut them off.

Carburetors require a low fuel pressure, often under 5 PSI. Electric fuel pumps can require a regulator to limit the fuel pressure to the carb. If pressure is too high, it will simply push its way past the needle seat and either flood the engine or cause it to run very rich due to an elevated float level. Some pumps simply dont have enough pressure to push past the needle seat, but they do require something else. A circuit that will kill the pump in the event of oil pressure loss or an accident should be considered mandatory. You do not want to have the fuel pump continue to run if the car is in an accident and a fuel line is severed. Also if left on with no return line, pressure can build up and push past the needle seat spilling fuel onto a hot engine. Its better to have the fuel pump shut off if you arent capable of doing it at the time.

I run electric pumps on some of the cars I own, one style doesnt need a regulator but the rest do. They work quite well and most last provided you dont try to use a race only pump continuously on the highway, as they tend to overheat if run too long. I plan to use the stock pump on the red 79 TA I am working on this month, and it will get a rebuilt Qjet in place of the Holley 600 a previous owner installed.

One thing about the 79-81 Trans Am is they had highway gearing and a 2.41 will not feel like it is accelerating until 30-40 mph. The range of gears installed in 79s was 2.41, 2.56, 2.73, 3.08, and 3.42. If you have any of the 2 series gears it will feel rather lethargic from a stoplight but it will have a low cruise rpm on the highway. You have two choices if you want to be a bit quicker off the line, both have drawbacks and considerations. You can either make more torque with the engine, or you can swap gears to a higher numerical ratio. A gear swap will increase the cruise rpm on the highway, and going too far will limit the engine life and cause overheating. For a stock 403 nothing higher than 4.11, gears and really a practical limit of 3.55 is better suited to the powerband and internals of the 403. 4.11s are if you plan to do mostly 1/8th mile racing, you will have excessive rpm on the highway without an overdrive trans, and those gears will let the engine rev past its powerband too quickly. Making more torque requires engine changes, either something simple like nitrous, or a complete rebuild will be needed to get more torque from the 403.

Highway gears are not all bad though, I know of a 3700lb 81 Formula that has 2.56 gears and a mild 455 that runs 13.0 to 12.8 in the 1/4, if you make the torque the car will move. Most gear swaps will only pick up at most half a second, but they will change the way the car drives and operates.

I will detail some of the work on a Qjet for a 403 in the coming weeks, and I might do a Holley because I need one for a Ford. Keep an eye on the red TA in the project section.
Performance and general repair tech. Traditional Pontiac V8 is my specialty, as well as automatic transmission repair/diagonossis. 2nd gen speciality, and with the addition of the 98 Formula 4th gen LS1 I am getting to know them real well. Holley and Qjet advice as well.

Chalk mark restoration and date codes arent my thing. W.O.T. is my thing..

Is it strange to have 9 firebirds?

clayton

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wow. thanks for all the helpfull info....i should have clarified though that i was asking about the shaker scoop vents....and ive been hearing that removing the fake vents can have a bad effect, that it could cause more turbulence or make the carburetor struggle for air as the air pressure above and behind the vents increases with speed.....the claim is that basically instead of making it easier for air to be drawn in it, removing the vents has a reverse result as the air is moving too fast in front of the windshield.....is any of this true? if so will it have dramatic downfalls? \

and for the other question regarding the carburetor, the summit carb is here on this link: http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM-M08750VS&N=700+115&autoview=sku

its a 4 barrel carb just like the edelbrock but from the specs list it seems to have more performance potential than the edelbrock....im wondering if it would be beneficial to switch to this one, considering it could possibly not cost me much...

and as far as being fast off the line....im not going to be actually racing this thing at all with this particular engine but im wanting to get rid of my low end bog. which as far as i can tell a problem from my air/fuel delivery (carb)... so im not going to go as far as changing gear ratios or transmissions. and i know my tachometer is way off so i cant say what rpm the car cruses at. but i know it was well around 2500 and maybe a needle width lower around 60mph.... (i sent the gauges off to be repaired recently) so im happy with the gear ratio cuz the car will peel the tires if i wanted to from a stop but from a slow roll when i stomp on it its pretty reluctant to deliver much power as it will after about 30 mph.....

and as far as the fuel pump question....i was only planning on installing a pump that is made for carburetors that doesnt need a regulator if i could.... and the electric fuel pump will only be powered by the ignition just like modern vehicles and as far as an accident i guess i would have to find some way to install a kill switch..... but i wont bother going that far if i can be assured that the manual fuel pump is sufficient enough for any carburetor.... and more fuel pressure isnt needed for any more performance gain...

overall im not looking to build this car into a full out dragster im basically starting off with an almost stock 403ci 6.6 liter 79 trans am that has been rebuilt (engine, transmission, rear end, all running gear)  and im just wanting to know all the possible cheapest performance upgrades as i restore this car the way i want it....im not going for a factory restoration obviously so things like this is what i have tons of questions about.... how to make this almost stock engine run better as well as last longer and be a good daily driver if i wanted to use it to drive to work (9 miles round trip)


Todd Stahr

  • Firebird & Trans Am Club Tech Staff
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It just so happens I am working on a 79 TA right now, and its one of the primary projects for this summer. Some performance and other enhancements are planned after the paint is back on it and the rust is repaired.

As for the scoop, if there was nothing sticking up behind it and it had either a smooth shelf or a drop off after it, it wouldnt get much air into it. As it is, the windshield forcing the air upwards creates a high pressure zone where the air sort of piles up as it tries to go over the car. Regardless, there is a greater increase from cooler air that hasnt been heated by the radiator and the engine itself under the hood going into the carb than there is from any sort of ram effect. Just getting the cooler air into the intake will achieve the same effect. Oldsmobile had a ram air system that used two large hoses with ducts mounted under the front bumper, they were connected to the air filter assembly. Similar systems are still used today to limit preignition since cooler air temps while running gasoline will  limit a tendency to spark knock, as well as a small performance boost even from the worst designs. So even if you dont open the scoops, you can still run a duct from the radiator support and draw cool air as long as it isnt getting air from behind the radiator. The downside to opening the scoop is it is a permanent thing that is not easily reversed. It could affect an emissions inspection and possibly resale value, but really there isnt anything stopping you from having two scoops, since the top portion is replaceable.

That carb is similar to the Autolite carb I mentioned earlier. It isnt a modular design and the top must be removed to perform any tuning, just like a Qjet and some changes on the Edelbrock carb. As for performance potential, that one and the Edelbrock are about the same, they will work on a stock engine with minimal changes. Its how the timing is adjusted, the quality of the fuel, condition of the engine, and the ratio of fuel to air that will decide how much more power you get from the 403 until you go into it and change things. The list of things that affect how it drives and how well it responds to driver input is a rather large list, that includes gear ratio, transmissions, stall speeds, weight, and a host of smaller seemingly minor things. I havent worked with the Summit carb at all, but I have a couple Autolite 4100s that were on my Fords. They are good for a daily driver and are geared more towards economy than power. The 4100 is a good carb for smaller displacement engines,but they arent rated at 750 cfm either. I dont know what differences there are between the Summit carb and the Autolite, but they appear to be minimal.
Performance and general repair tech. Traditional Pontiac V8 is my specialty, as well as automatic transmission repair/diagonossis. 2nd gen speciality, and with the addition of the 98 Formula 4th gen LS1 I am getting to know them real well. Holley and Qjet advice as well.

Chalk mark restoration and date codes arent my thing. W.O.T. is my thing..

Is it strange to have 9 firebirds?

clayton

  • Message Board Member but not a NFTAC Club Member
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  • Posts: 80
ok well i already cut the vents out just because i really wanted to...and if i wanted to reverse it i could just cut some plates out of plastic or stainless steel and bondo them in and paint it again its not a big deal to me really....

and for the carb. im just looking for my performance options on this carb or if it could be easier to find another one to mess with....i do still have a weber 1000 cfm competition series that was on this thing when i bought it but it needs to be completely rebuilt and im sure that carb is not a good choice but im feeling like this edelbrock is holding my car back straight from the box.... ill look in to mods for this carb and maybe even a higher volume mechanical pump....thanks for all the help

Todd Stahr

  • Firebird & Trans Am Club Tech Staff
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  • Posts: 88
A 10th Anniversary TA owner installed this kit on his scoop, it isnt too expensive and appears to work much like the early systems from 70-72.

http://pontiactransamshakers.homestead.com/
Performance and general repair tech. Traditional Pontiac V8 is my specialty, as well as automatic transmission repair/diagonossis. 2nd gen speciality, and with the addition of the 98 Formula 4th gen LS1 I am getting to know them real well. Holley and Qjet advice as well.

Chalk mark restoration and date codes arent my thing. W.O.T. is my thing..

Is it strange to have 9 firebirds?

clayton

  • Message Board Member but not a NFTAC Club Member
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  • Posts: 80
those kits looked pretty trick. but i have a few other things i should take care of on the care before buying one.....

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