Holley carburetor question

[eddie c]

I recently did some work on a guys car,its a 68 Camaro with a 350ci
engine,700r4 trans,and a 4.10:1 rear gear. The engine has a mild cam,
not exactly sure of lift and duration. Anyway when the guy had the engine built a few years ago it had a 780 cfm Holley. At that time the guy was very happy with the performance of the car,now the car saw about three different people work on it over the first few years. Each time ,things were modified or changed,I dont really know what was done,however one of the guys that previously worked on the car changed out the carburetor, he put a Quadra Jet with an adapter plate on it and after that the car really performed poorly. Now I come into the picture,and suggest that we go back to a Holley because the intake,Elderbrock Scorpion, is designed for that type of base.Using the carb size calculator,I take the engine size and max RPM which is 6500 and determine that a 650 cfm Holley would be the best choice. After changing jets acc.pump nozzles and spark plugs
the car starts running really good,unfortunately no et slips,just seat of
the pants. Well long story short the guy says he wants to go back to a
780 cfm carb. I have explained to him that the carb size recomendation
was within the correct limits. What do you guys think? Will this car
run better if we go to a 780 cfm carb? Thanks for reading such a long post. ed

[Paul Merolla]

In my opinion, yes it will. That formula doesn't take into account the fact that when the carb is flowing its "rated" cfm, it is causing a restriction and vacuum is increasing. Bigger carb = less restriction = more power...to a point. Too big and you'll lose signal. My street car is almost identical to the way you've described your customer's ride. When I started taking it to the Friday night street drags, it had an Edelbrock 600 cfm on a Performer RPM intake. It ran fairly consistent 12.90s every week. For an experiment, one Friday I bolted my buddy's 850 Holley on. The new norm became 12.80, then after lots of jet changes and squirter changes, the car ran a best of 12.63. I thought that was cool, but he wanted his carb back so I bought a 750 DP almost new and spent week after week chasing that elusive 12.63 to no avail. It settled in at .70s and that was it. Sorry for the long story...but my logic tells me the bigger the better (within reason of course), as long as you can keep the A/F where it needs to be.

[eddie c]

Hi Paul; I appreciate you going into detail and dont mind the long story at all. Now what do you think about seat of the pants feel,can most people feel two tenths of a second,or is it really not distinguishable without an et slip? ed

[Paul Merolla]

Yes, you can feel it - at least in my car. Properly set up, a double-pumper is a lot of fun on the street!

[Phillip marvetz]

The customer is always right.

[Reed Granrt]

Originally Posted by Paul Merolla

Yes, you can feel it - at least in my car. Properly set up, a double-pumper is a lot of fun on the street!

Depends on what is in the seat of your pants. By the way , what does .2 look like. I aint never seen one. HeHe

reed

[Alan Roehrich]

Originally Posted by eddie c

I recently did some work on a guys car,its a 68 Camaro with a 350ci
engine,700r4 trans,and a 4.10:1 rear gear. The engine has a mild cam,
not exactly sure of lift and duration. Anyway when the guy had the engine built a few years ago it had a 780 cfm Holley. At that time the guy was very happy with the performance of the car,now the car saw about three different people work on it over the first few years. Each time ,things were modified or changed,I dont really know what was done,however one of the guys that previously worked on the car changed out the carburetor, he put a Quadra Jet with an adapter plate on it and after that the car really performed poorly. Now I come into the picture,and suggest that we go back to a Holley because the intake,Elderbrock Scorpion, is designed for that type of base.Using the carb size calculator,I take the engine size and max RPM which is 6500 and determine that a 650 cfm Holley would be the best choice. After changing jets acc.pump nozzles and spark plugs
the car starts running really good,unfortunately no et slips,just seat of
the pants. Well long story short the guy says he wants to go back to a
780 cfm carb. I have explained to him that the carb size recomendation
was within the correct limits. What do you guys think? Will this car
run better if we go to a 780 cfm carb? Thanks for reading such a long post. ed

Most of the "carb size calculators" are intended to yield a carburetor size that will run fairly well and still be driveable on the street for the average basic tuner on the average car with a less than perfect combination. So they tend to suggest a carburetor around 100CFM smaller than could be run by a good tuner on a fairly well matched combination.

The average tuner is going to make 1-2 jet changes, might change a power valve, and maybe even a squirter. A real serious tuner will go so far as to change or modify metering blocks, sort through accelerator pump cams, and change diaphragm springs on the vacuum secondaries.

A smaller carburetor (relative to the biggest that can be run effectively) will create pumping losses, as it will create enough restriction at WOT at the RPM where peak HP is made to generate well over 1.5" of vacuum. Most often, the carburetor the "calculators" will suggest will make 2.0" or more. It does hurt HP. You can get down to around 1.0" of vacuum on a properly tuned carburetor on a good combination.

If a car is not over cammed, and has good gearing, you can run a carburetor 100CFM larger than those calculators suggest. So, yes, you can run a 780 vacuum secondary on it. Remember, the Holley 780 vacuum secondary was the carburetor used on the 302 Z-28 engines, and the 350 LT-1.

If I were going to buy one, it would be the Quick Fuel 780 just like we use on the Stock and Super Stock cars. It is really easy to tune, you can wring a ton out of it if you're willing to work on it. It will normally beat an out of the box Holley with relative ease, as it comes out of the box. You'll get great support from the guys at Quick Fuel, too.

By the way, make absolutely certain that the bottom of the carburetor throttle lever has the correct geometry to make the throttle valve cable on the 700 R4 work absolutely right. Most often, a 4.10 is too much gear for a car with a 700 R4, especially if it is relatively light. You can't shift it fast enough, and the car is going too slow when it goes to second gear. A 3.73 works better for 7-8 out of every 10 cars. I know, I had a 4.10 behind a 700 R4 in a car that would pull to 6500.

[Geerhead55]

Alan,, Do you think he should stay with the Scorpion? As I was reading this, and seeing a max RPM of 6500, I was thinking use an Air-Gap. We're not given any info on cylinder heads being used, so if they're nothing outrageous, perhaps the dual-plane would work better with this combo,,,just thinking out loud.
Paul,, I've loved following your Chevy II build, as I had one myself years ago. Yours looks great, and I hope you have a lot of fun with it.
Danny Durham

[Alan Roehrich]

Originally Posted by Geerhead55

Alan,, Do you think he should stay with the Scorpion? As I was reading this, and seeing a max RPM of 6500, I was thinking use an Air-Gap. We're not given any info on cylinder heads being used, so if they're nothing outrageous, perhaps the dual-plane would work better with this combo,,,just thinking out loud.
Paul,, I've loved following your Chevy II build, as I had one myself years ago. Yours looks great, and I hope you have a lot of fun with it.
Danny Durham

Danny, I don't think the intake is a really big deal here. It's a light car, with a ton of gear. A 3.06 low gear with a 4.10 rear, in a car that weighs maybe 3300 or so. The only problem the car has is the drop between 1st and 2nd with a low vehicle/wheel speed.

Certainly, if I were choosing an intake because it did not have one, I'd lean more toward a Performer RPM Air Gap. Or maybe a Holley 300-25, if I were wanting to move the power band up by running a single plane. It might be different if the car were heavier (say 3600), or had more cam (big), or less gear (3.08 or 2.93). If one of those were the case, then I'd say an intake change would be a good idea. Even more so if two of those were the case.

We also have to realize it is probably a fun street car, and he's probably not hunting that last hundredth.

[Paul Merolla]

Absolutely right on the 4.10 / 700R4 combo, Alan. Just cruising around town, mine shifts into second in about 20 feet from a stop, and there's way too much rpm drop.
Danny - thanks for the comment on the Nova...it's close!