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Carb Size Street Car - Advantage of QF 780 cfm over 735 cfm?

68 Sport Satellite

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Hey guys,
let me preface by saying I don't have much carburetor experience and am trying to make an educated guess with recommendations from others. I know every engine / car combo is unique and lots of variables come into play.

I've been running a Quick Fuel Vacuum Secondary SS series 735 CFM carb on my mild cam 451 since new (currently 6500 miles on the motor). The carb performs pretty well with no big issues and I had it jetted and tuned at a dyno shop. The car ran a best 1/4 mile ETA out of 10 track runs of 12.80 sec. Rear wheel numbers are 391 HP / 523 lb-ft torque. Not bad, but would a carb swap make it any better? I know the online calculators often show a smaller carb than what real world results are and I've often wondered if I'm leaving power on the table and have read up quite a bit on using 750 vs 850 carbs. I don't need maximum dyno numbers, but what I am looking for is a noticeable power increase on the street and a better 1/4 mile time would be nice!

The way I understand it, the Quick Fuel Holley type 750 carbs include the 735 and 780 Vac Secondary SS carbs offered. If I don't care about fuel economy, for street driving and a once a year track visit, is there any noticeable driving advantage of a 780 or 850 cfm Quick Fuel Vacuum Secondary carb over my current 735 cfm for my combo?

I've had numerous mopar friends advise me to get away from the Vacuum Secondary carbs and switch to Mechanical for better responsiveness. However, my car is an Auto Trans 727 with 2200 stall converter so I'm hesitant to go with mechanical secondaries. One thing I came across in a separate thread that many don't seem to talk about is the intake manifold effect on vacuum vs mechanical secondaries. I don't know enough on that to comment.

Here's my car info:
451 ci
727 Auto trans
Lunati Voodoo 60302 Cam (.475" lift, 220/224 at .050")
3.55 gears
28" tall rear tires
10.2:1 compression
Eddy RPM Heads
Eddy RPM dual plane intake w/1/2" phenolic spacer
Schumacher Tri-Y ceramic coated headers
2.5" exhaust w/H-pipe/Dynomax Ultraflow Welded (straight through) mufflers

Thanks in advance.
 
I like your combo, for your stated purposes, just as it is. And NO, I would not go to mechanical secondaries.
 
Small cam, tall tires for the gears. Good intake and heads. I’d say it’s a pretty decent combo as it is, and dyno tuned by a shop is good.

It may get a bit better with a bigger carb, but not a lot.

go to 3.91 or 4.10 gears will help.

might be a bit slow for what you’ve got.
 
One thing the dyno guy did not do is try different squirter sizes or carb cam sizes. Are those something I should do?
 
You can play with the secondary dashpot and spring on a Quick Fuel. It can make quite a difference.
You have a mild cam so top end is not going to be were the engine is happy to run.
Stick to your street gears and the street torque set up.
 
Hey guys,
let me preface by saying I don't have much carburetor experience and am trying to make an educated guess with recommendations from others. I know every engine / car combo is unique and lots of variables come into play.

I've been running a Quick Fuel Vacuum Secondary SS series 735 CFM carb on my mild cam 451 since new (currently 6500 miles on the motor). The carb performs pretty well with no big issues and I had it jetted and tuned at a dyno shop. The car ran a best 1/4 mile ETA out of 10 track runs of 12.80 sec. Rear wheel numbers are 391 HP / 523 lb-ft torque. Not bad, but would a carb swap make it any better? I know the online calculators often show a smaller carb than what real world results are and I've often wondered if I'm leaving power on the table and have read up quite a bit on using 750 vs 850 carbs. I don't need maximum dyno numbers, but what I am looking for is a noticeable power increase on the street and a better 1/4 mile time would be nice!

The way I understand it, the Quick Fuel Holley type 750 carbs include the 735 and 780 Vac Secondary SS carbs offered. If I don't care about fuel economy, for street driving and a once a year track visit, is there any noticeable driving advantage of a 780 or 850 cfm Quick Fuel Vacuum Secondary carb over my current 735 cfm for my combo?

I've had numerous mopar friends advise me to get away from the Vacuum Secondary carbs and switch to Mechanical for better responsiveness. However, my car is an Auto Trans 727 with 2200 stall converter so I'm hesitant to go with mechanical secondaries. One thing I came across in a separate thread that many don't seem to talk about is the intake manifold effect on vacuum vs mechanical secondaries. I don't know enough on that to comment.

Here's my car info:
451 ci
727 Auto trans
Lunati Voodoo 60302 Cam (.475" lift, 220/224 at .050")
3.55 gears
28" tall rear tires
10.2:1 compression
Eddy RPM Heads
Eddy RPM dual plane intake w/1/2" phenolic spacer
Schumacher Tri-Y ceramic coated headers
2.5" exhaust w/H-pipe/Dynomax Ultraflow Welded (straight through) mufflers

Thanks in advance.
Nice.
What is the car weight?
What was the trap speed?
What track and what day?, or what was the DA on your best et day?

I would guess that the car would pick up in both seat of the pants feel and et/mph with 850 cfm and DP. My first pick would be a small Venturi 950
 
Nice.
What is the car weight?
What was the trap speed?
What track and what day?, or what was the DA on your best et day?

I would guess that the car would pick up in both seat of the pants feel and et/mph with 850 cfm and DP. My first pick would be a small Venturi 950
3700 lbs
best of 12.80 sec @ 108 mph
other times 12.91, 12.87, 13.0, 13.1
Sacramento Raceway, May, 90F outside temp

That's what I'm looking for is seat of pants difference, but DP on Auto Trans car with dual plane intake and mild cam? Don't want to run rich where it's burning my eyes either. (I'm open to a 950, but not at that expense, however, maybe with it being a 451 it can handle it?)
 
90 F at Sac, DA was probably pretty bad, like 3500' maybe.
I'd bet it would pick up at night or early morning.
Seems like a nice street combo; I'd be tempted to throw some 1.6 rocker arms at it. Also make sure it's as light as can be at the track.
I'd vote against the DP on a combo that mild (cam/converter/gear); if the throttle response is immediate, the pump cam & shooters are probably fine.
 
Your et is slow for the mph. At 108 , you should be in the 12.20-12.30s. Are you running regular hard street tires or slicks/stickies? I doubt you would gain much from just a carb swap. Your stall speed is low, your cam is small, and your headers are small. I bet it's a nice mild driver. I imagine a 240ish at .050 cam, 9.5 or 10 inch 3500 converter and 1/78 headers could get you in the high 11s. I prefer DP over VS. BTW, I love 68 Satellites (I own one).
 
Your et is slow for the mph. At 108 , you should be in the 12.20-12.30s. Are you running regular hard street tires or slicks/stickies? I doubt you would gain much from just a carb swap. Your stall speed is low, your cam is small, and your headers are small. I bet it's a nice mild driver. I imagine a 240ish at .050 cam, 9.5 or 10 inch 3500 converter and 1/78 headers could get you in the high 11s. I prefer DP over VS. BTW, I love 68 Satellites (I own one).
Depends on what the OP is looking for in his car. His 60 ft is probably 1.95 - 2.05. That is about all you are going to get in "true" street trim. Sure, add a converter, gear, and tire and its a 12.3 car. But it might not be what the OP wants.
 
3700 lbs
best of 12.80 sec @ 108 mph
other times 12.91, 12.87, 13.0, 13.1
Sacramento Raceway, May, 90F outside temp

That's what I'm looking for is seat of pants difference, but DP on Auto Trans car with dual plane intake and mild cam? Don't want to run rich where it's burning my eyes either. (I'm open to a 950, but not at that expense, however, maybe with it being a 451 it can handle it?)
You have a nice running car. Glad you're going to the track. Keep trying different things and going to the track. You'll find out what does, and what does not work for your car, and you will get it to faster. Trust me on this.

My 4250 lb Charger with the MP 272/.455 cam, factory heads, 3.23 and 1900 stall ran best with a 800 DP on a dual plane intake. When you dropped the hammer from a 25 roll, it would go sideways instantly. None of the VS carbs would do that.

Please don't take my word for it. Try it.
 
90 F at Sac, DA was probably pretty bad, like 3500' maybe.
I'd bet it would pick up at night or early morning.
Seems like a nice street combo; I'd be tempted to throw some 1.6 rocker arms at it. Also make sure it's as light as can be at the track.
I'd vote against the DP on a combo that mild (cam/converter/gear); if the throttle response is immediate, the pump cam & shooters are probably fine.
Sacramento is at sea level. On a 90° F day & 30" hg it will be between 2000-2500.
 
Thanks for everyone's feedback!

Those Sac raceway runs were in August around 7pm just before sunset. It gets very hot there, but it's a dry heat. I'm running Nitto 555R drag radials.

I ran the car 2 yrs later at Infinion (Sears Point) near Fairfield (south of Sac 1 hr and closer to cooler weather near the SF Bay delta) in October at 6pm, outside air temp 75F, hoping for quicker times in cooler air. Only difference was I now had a 20-lb heavier AGM size 27 battery up front. The car ran slower at 13.1 for each of 2 runs. I guess weight transfer really is a big deal. I wish the nearest track wasn't a 2 hr drive in traffic on Wednesdays after work or I would go more often.

The comment from BSB67 - that sounds awesome on the 2nd gear getting sideways. Not sure how I would do with that on the occasional track visit, but I bet seat of pants the difference would be noticeable. On my track runs, I've usually rolled out from the line at around 2000 rpm with 2/3 throttle and then at about 20 mph I nail it and things stay straight with no loss of traction.

Before the Sacramento runs I had only run down a track ever with any car on one other occasion at Willow Springs for heads up untimed noon time drag racing the week of Spring Fling. Won 4 of 5 runs against cars with lopier sounding cams that I had thought would smoke me. Guess I got lucky and I'm still figuring things out.

The goal with the car has always been a reliable long lasting hot street car that handles well with an occasional track visit. I've got bolt on Firm Feel and Hotchkis suspension, subframe connectors, etc. For my combo I do know I have some corks in it. I wouldn't mind some TTI long tube headers for one. I think my intake is decent. Throttle response is ok, but not as sharp and crisp as I'd like, which is the main thing that has me thinking some carb work or swapping may be in order. If I had to do it over, I would choose different pistons as any more than 32 degrees total and the motor pings. I'm at 10.2:1 and really need to be at 9.5:1. Overall, the motor runs exceptional. Just seeing if it could be better. 1.6:1 rocker arms could be something. I would want iron adjustables like the 1.5's I've got.

So far, most of you are saying my 735 QuickFuel carb is adequate, while 2 are saying there's power to be had with an 850/950 with mechanical secondaries. I guess it can't hurt to try another carb and hang onto my current carb in case I need to swap it back. I was assuming Auto Trans with low stall converter wouldn't like a DP carb, but who knows?
 
The most important factor has not been mentioned! Could the engine use a carb with more airflow?
Looking at the cam, which is quite mild, I would say NO. Using a bigger carb that provides airflow to 6500 rpm on an engine where the cam peaks at 6000 will not add power but might be sluggish in low end response because of larger throttle bores.

If you are using an open carb spacer, I would try using a 4 or 2 hole spacer.
 
Nice.
What is the car weight?
What was the trap speed?
What track and what day?, or what was the DA on your best et day?

I would guess that the car would pick up in both seat of the pants feel and et/mph with 850 cfm and DP. My first pick would be a small Venturi 950
Advice on which small venturi 950 DP you would try? Quickfuel or Proform or something else? I'm just recently learning about small venturi size for increased air flow speed, but I don't yet know what venturi size would be considered small.
 
The most important factor has not been mentioned! Could the engine use a carb with more airflow?
Looking at the cam, which is quite mild, I would say NO. Using a bigger carb that provides airflow to 6500 rpm on an engine where the cam peaks at 6000 will not add power but might be sluggish in low end response because of larger throttle bores.

If you are using an open carb spacer, I would try using a 4 or 2 hole spacer.
Thank-you Geoff! That's good advice. Yes, my cam RPM range is 1800-5800 rpm. Yes, I am using an open hole spacer. I've been wanting to try one of the Wilson spacers. My current 1/2" spacer with the tall Eddy RPM and carb choke horn puts me at max height right now. Do you think a 1/2" spacer is doing much? Some have told me to remove the spacer altogether. I had added it not for performance, but to combat heat soak fuel boiling after engine shut off/re-start.
 
I would try a 4-hole spacer. A half-inch isn't all that tall, but a 4-hole increases velocity, which helps with vac signal & low-RPM response. Open spacers add volume, which tends to benefit upper-RPM use. You can help the heat soak using a GM Z-28-style aluminum baffle & insulator, it works. You're making some strong cylinder pressure for 91-octane with the small cam, that's why 32 degrees max on that fuel. Another fun experiment would be mixing in some 110, bump it to 36, and make a few runs. I like your build theme.
 
Sounds like it runs good like it is. If it ain't broke, don't fix it.. You might gain at the upper RPMs, but lose response down low. You probably wont even get to the upper RPMs with those gears. That would make it less in the "seat of the pants" acceleration. Just my opinion.
 
I would try a 4-hole spacer. A half-inch isn't all that tall, but a 4-hole increases velocity, which helps with vac signal & low-RPM response. Open spacers add volume, which tends to benefit upper-RPM use. You can help the heat soak using a GM Z-28-style aluminum baffle & insulator, it works. You're making some strong cylinder pressure for 91-octane with the small cam, that's why 32 degrees max on that fuel. Another fun experiment would be mixing in some 110, bump it to 36, and make a few runs. I like your build theme.
Would you mind sharing a photo of the baffle/insulator you're talking about. I didn't find it on a google search.

Last year I bought an aluminum sheet deflector from Quick Fuel for this very reason, but haven't installed it yet due to required modification to clear my throttle linkage. I've since read that the one I bought is fiber laminated to Aluminum and some of have it disintegrate and transfer particles into the motor, so the one I bought won't be going on. If there's an all aluminum one that works well I'm interested.
 
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