Too much carburetor?
Thread Starter
Le Mans Master
Joined: Sep 2003
Posts: 7,776
Likes: 0
From: Everywhere
Too much carburetor?
Several folks have told me to use a 750 Speed Demon Mechanical Secondaries on this engine combination:
350 - bored .030
9.0 compression
Crane Power Max Cam,.454/.480 lift, 272/284
Performer RPM intake
AFR 190 heads
B&M Holeshot 2000
Dynomax 1-5/8 Headers
3.70 gearing, TH350
dual exhaust
However, I thought that a speed demon 650, vacuum secondaries would suffice, for an engine making less than 400hp and reving to 6K. The folks that recommended this to me know much more than I do, so I wanted to get everyone elses opinion.
Future mods will include MSD ignition, and flowmaster 4-series, roller tip 1.52 rockers.
350 - bored .030
9.0 compression
Crane Power Max Cam,.454/.480 lift, 272/284
Performer RPM intake
AFR 190 heads
B&M Holeshot 2000
Dynomax 1-5/8 Headers
3.70 gearing, TH350
dual exhaust
However, I thought that a speed demon 650, vacuum secondaries would suffice, for an engine making less than 400hp and reving to 6K. The folks that recommended this to me know much more than I do, so I wanted to get everyone elses opinion.
Future mods will include MSD ignition, and flowmaster 4-series, roller tip 1.52 rockers.
Racer
Joined: Dec 2003
Posts: 257
Likes: 0
From: Livermore CA
Re: Too much carburetor? (Sling Blade)
I agree with you, a 750 with mechanical secondaries seems a little big for a 350 that probably will not see over 6000 rpms. From the factory a LT-1 came with a 780 vacuum secondary stock, and they reved way past 6K rpms. Either carb would work (if properly set up) well on your car, but just like most performance parts there are trade offs. The smaller carb will have more throttle responce and give you more lower end torque, but the larger carb will give you more horespower at the extreme high end of your engines rpm range. It's all about trade offs, and what you will be doing with your car. If you were to race it a lot the larger carb (set up properly) would be my choice, but if it were to be a daily/weekend pleasure cruiser I would go with the smaller vacuum secondary carb.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,370
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Too much carburetor? (Sling Blade)
Run the 750 mech sec carb. I've run several dyno tests clearly showing that anything bigger than a 302 will easily support a 750 with outstanding throttle response, idle, and WOT performance. I'm running a BG 750 mech sec Speed Demon on a bone-stock 350, and it runs fantastic. I just ran a 650 vs. 750 comparison on a 302 Ford on a dyno, and the 750 outperformed the 650 at every point on the rpm curve (both carbs were jetted and optimized to the engine). We've even run 850's on small block 406 engines with exceptional results. Don't skimp on the carb - big ain't bad in spite of other opinions you're bound to hear (remember - the Q-Jet is a 750cfm mechanical carb, and it was used on engines as small as 250 cid).
Burning Brakes
Joined: Aug 2001
Posts: 1,002
Likes: 0
From: Bay City MI.
Re: Too much carburetor? (lars)
Its confusing for sure on the carb question, I have been told that a 750Dp would be better for my 467 rat motor for all around driving. My current setup is a 850DP Holley, and other than bad gas mileage I love the way it drives. I would think that your motor should support a 750 with no problem.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,370
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Too much carburetor? (467-Ratman)
Here's the "musical carb" game we (the CCC - the Colorado Corvette CraZies - the Ultimate Corvette Tuning and Beer Drinking Fraternity) have been playing out here in Denver:
I yanked the 650 carb off my 350-equipped '64 Roadster and donated it to a Forum member's truck. 650's make good truck carbs.
Forum member "blkblt" yanked the 750 BG Speed Demon Mech Sec carb off his 427 '66 and donated it to my 350. My 350 has never run so nice. We then installed a new 850 mech sec Speed Demon on the 427. The 427 has never run as well.
Don't skimp on the carb...
I yanked the 650 carb off my 350-equipped '64 Roadster and donated it to a Forum member's truck. 650's make good truck carbs.
Forum member "blkblt" yanked the 750 BG Speed Demon Mech Sec carb off his 427 '66 and donated it to my 350. My 350 has never run so nice. We then installed a new 850 mech sec Speed Demon on the 427. The 427 has never run as well.
Don't skimp on the carb...
Thread Starter
Le Mans Master
Joined: Sep 2003
Posts: 7,776
Likes: 0
From: Everywhere
Re: Too much carburetor? (lars)
Thanks for the information :cheers:
The only other question is regarding the mechanical secondaries. Would I lose much power by using vacuum secondaries? The only reason I mention this is because the vacuum carb comes with an electric choke. In winter here in SC, the temp gets to 20 quite often. Do I really need the choke, or just go with mechanical secondaries.
Are mechanicals harder to tune than vacuum secondaries?
The only other question is regarding the mechanical secondaries. Would I lose much power by using vacuum secondaries? The only reason I mention this is because the vacuum carb comes with an electric choke. In winter here in SC, the temp gets to 20 quite often. Do I really need the choke, or just go with mechanical secondaries.
Are mechanicals harder to tune than vacuum secondaries?
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Melting Slicks
Joined: Dec 2000
Posts: 2,041
Likes: 0
From: Wichita, KS
Re: Too much carburetor? (lars)
Not to get into a pissing contest over this but;
Your C2's must weigh 500 pounds less than my '79.
Would these cars be Standard transmission cars also vs. my 3000 stall converter T350?
You are operating your engines in skinny air at 5000 feet while I am at less than half of that.
And, most importantly, you can undoubtably tune a carb closer to perfect than most of us.
I'm still not sold on the mechanical secondary 750 carb. for a 350 automatic.
Your C2's must weigh 500 pounds less than my '79.
Would these cars be Standard transmission cars also vs. my 3000 stall converter T350?
You are operating your engines in skinny air at 5000 feet while I am at less than half of that.
And, most importantly, you can undoubtably tune a carb closer to perfect than most of us.
I'm still not sold on the mechanical secondary 750 carb. for a 350 automatic.
Le Mans Master
Joined: Jun 2003
Posts: 5,446
Likes: 2
From: Not Correctly Restored Stingray Fla.
Re: Too much carburetor? (1979toy)
You need to find out the spec of the 750 Demon. NOT all 750's are the same. A BG 750 believe it or not is enormous. I agree with a couple of the other replies about a 650 :nono: I personally think a Holley 750hp should be fine for you. Yes it has mech. secondaries (which is what you should have) but not major size jetting. You take a Holley 750hp, 750 Speed Demon, & BG 750rt and ALL three have different specs. It's the "other stuff" that makes a big difference (power valves, squirters, accelerator pump diaphragm & cam) all the internal "stuff"
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Re: Too much carburetor? (Sling Blade)
I love the way my over-carbed big block runs!! Go with the 750 DP and don't look back. Tune it by reading Lars' carb papers and get an education in the process.
Brett :thumbs:
[Modified by Brettmc, 1:16 PM 2/2/2004]
Brett :thumbs:
[Modified by Brettmc, 1:16 PM 2/2/2004]
Racer
Joined: Apr 2003
Posts: 349
Likes: 0
From: Katy TX
Re: Too much carburetor? (Sling Blade)
A 750 Demon is too much carb for your setup. Call BG, they will be able to assist you, but a 750 Demon flows more than a 750 Holley. BG will also tell you the same thing. For your setup you will be much happier with a 650 Demon. Im affraid you wouldnt be able to jet the 750 down enough, you would run way to rich.
Racer
Joined: Dec 2003
Posts: 304
Likes: 0
From: Pasadena Maryland
Re: Too much carburetor? (Sling Blade)
Of course everyone has their own opinion on which Carb to use. I'm sure your engine would take to the 750 with no problem. I have a similar set up with my 78 and I'm using the new Holley street avenger 670 cfm. It's good for engines up to 400 hp and you can tweak it for another five percent capacity if you want. They also sell a 770 cfm for engines over 400 hp. It's a vacuum secondary with electric choke. I've been tinkering with cars since I was a teenager and this is one of the best carbs I've owned with the exception of my Holley Three Barrel. Any of you guys old enough to remember that carb.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Re: Too much carburetor? (djgrelli)
Sure. They show up on eBay all the time. A transitional carb between the 4150s and the 4500s. But with the "small" one being 950cfm, you better need the airflow.
Racer
Joined: Apr 2003
Posts: 349
Likes: 0
From: Katy TX
Re: Too much carburetor? (Sling Blade)
I run a 385 stroker bored .040 and the best of everything that can be added. I also went with a 750 Demon and had problems getting it dialed in right I have the jets set at 74/84 right now, and Im still on the rich side. Trust me, or simply call Barry Grant, they will tell you same thing.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,370
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Too much carburetor? (bhmyers)
:rolleyes:
I give up.
Have any of you guys spent any time on an engine dyno and seen the real numbers...?
...and big carbs don't run any richer than small carbs.
E-mail me if you need help with your setups.
I give up.
Have any of you guys spent any time on an engine dyno and seen the real numbers...?
...and big carbs don't run any richer than small carbs.
E-mail me if you need help with your setups.
Drifting
Joined: Nov 2003
Posts: 1,874
Likes: 7
From: l In
Re: Too much carburetor? (Sling Blade)
750 cfm is no problem on the engine you are building, and mechanical secondaries are great( if you are going to be coming out of the hole at 4500 rpm), but with only a 2000 stall converter you will fall flat on your face. You can run as many cfm as you want with a vacum secondaries carb. CFM doesn't mean dittle as long as you have it jetted correctly, and those heads will need some air.
Just my 2 cents worth :chevy
Just my 2 cents worth :chevy
Drifting
Joined: Nov 2003
Posts: 1,874
Likes: 7
From: l In
Re: Too much carburetor? (Sling Blade)
What is the power range on your cam?
Basically all I was saying is that unless you are going to race the car only, then use the vacum secondaries carb. Better all around performance.
My last project was a 68 Vert with a 327, blue printed, balanced, the works with good heads, 4 spd car with 355 rear gears. The car ran great with a 750 double pump, mec. secondaries, but it only ran the best at around 4500 out of the gate. If I would just let out on the clutch and stand on it it just didn't have the power tu use that much fuel all at once. This motor had power to 7000 rpm.
In your case ( just my opinion ) with the automatic you will be better off with the vacum secondaries. My last project was a 79, with a built 350/automatic, there is where I learned my first lesson with the carb situation.
Hope this helps.
750/vac. sec. :chevy
Basically all I was saying is that unless you are going to race the car only, then use the vacum secondaries carb. Better all around performance.
My last project was a 68 Vert with a 327, blue printed, balanced, the works with good heads, 4 spd car with 355 rear gears. The car ran great with a 750 double pump, mec. secondaries, but it only ran the best at around 4500 out of the gate. If I would just let out on the clutch and stand on it it just didn't have the power tu use that much fuel all at once. This motor had power to 7000 rpm.
In your case ( just my opinion ) with the automatic you will be better off with the vacum secondaries. My last project was a 79, with a built 350/automatic, there is where I learned my first lesson with the carb situation.
Hope this helps.
750/vac. sec. :chevy