650 or 750
Thread Starter
Instructor
Joined: Aug 2007
Posts: 131
Likes: 0
From: Madison NJ
650 or 750
I have a new 420 HP 383 crate motor with a 6000-rpm red line.
The builder says to run a 750 double pumper.
The Holley Tech says all a 383ci needs is a 650 double pumper. He said that the 750 will hurt bottom end and only help above 6500-rpm.
What do you guys think.
The builder says to run a 750 double pumper.
The Holley Tech says all a 383ci needs is a 650 double pumper. He said that the 750 will hurt bottom end and only help above 6500-rpm.
What do you guys think.
Racer
Joined: Jan 2008
Posts: 288
Likes: 0
From: Schaumburg Illinois
run the 750. it will not hurt the bottom end and might even pick up a few ponies on top. your real issue is vacuum vs mechanical secondaries. if you're running an automatic trans, go vacuum no matter what. i speak from experience. you will need a tall converter and a lot of gear, like 3.73s to make the mechanical secondaries happy. i have a 750 DP holley on my car and i don't have enough converter stall or gear to make it happy off the line. i want to put a manual in the car so i'm living with it till then. if i was staying automatic, i'd swap to vacuum secondaries.
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
Alot of guys like to over carb thier engine thinking bigger is better. I went through this on my 383 (dyno @ 460 crank HP w/ a 750) I ran a 750 Holley HP on it and it roared but was not happy with the low end, right off the top of the pedal. I did have it dyno tuned & it was tweaked. The dyno tech suggested it may pull more velocity with a 650, so I called the Holley tech and he agreed. OK, so I mounted a 650 Holley and viola, just the responsivness I was looking for, pulls hard all the way through the pedal & off the line it's a screamer! I know it has alot to do with your particular set up & your heads. Bring more air in, you must be able to get more air out. This of coaurse is only indicitive to my set up but just dont be eager to over carb.
Burning Brakes
Joined: Dec 2003
Posts: 1,004
Likes: 7
From: Adelaide South Australia
IMHO...
First off I say stick with a 750cfm carb on a 383 with a 6000rpm redline. You will achieve better peak power from your engine.
Sure a 650cfm might provide a bit more torque down low, but if you want performance, its power in upper range that counts.
A 650 cfm will begin to choke at upper rpm and cost you peak horsepower.
However....
Are you running an auto? or a manual trans?
If using an auto, you may be better off with a vac secondary 750.
The transition to secondary venturis is smoother and works better with slush box.
cheers
tom
First off I say stick with a 750cfm carb on a 383 with a 6000rpm redline. You will achieve better peak power from your engine.
Sure a 650cfm might provide a bit more torque down low, but if you want performance, its power in upper range that counts.
A 650 cfm will begin to choke at upper rpm and cost you peak horsepower.
However....
Are you running an auto? or a manual trans?
If using an auto, you may be better off with a vac secondary 750.
The transition to secondary venturis is smoother and works better with slush box.
cheers
tom
Race Director
Joined: Dec 1999
Posts: 11,094
Likes: 990
From: Sierra Foothills CA
Without hesitation I'd go with the 750. You have plenty of cubes & power to utilize that carb. I doubt you'd lose much if any low end at all and you'll pick up at the top end. On a mild 350 the 650 would definitely be the right choice. Another Holley tech would have told you to use the 750- not all those guys are blessed with real world experience. Good luck & enjoy your new motor!
Thread Starter
Instructor
Joined: Aug 2007
Posts: 131
Likes: 0
From: Madison NJ
run the 750. it will not hurt the bottom end and might even pick up a few ponies on top. your real issue is vacuum vs mechanical secondaries. if you're running an automatic trans, go vacuum no matter what. i speak from experience. you will need a tall converter and a lot of gear, like 3.73s to make the mechanical secondaries happy. i have a 750 DP holley on my car and i don't have enough converter stall or gear to make it happy off the line. i want to put a manual in the car so i'm living with it till then. if i was staying automatic, i'd swap to vacuum secondaries.
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Racer
Joined: Jan 2008
Posts: 288
Likes: 0
From: Schaumburg Illinois
I have a manual trans. and I'm running the motor with a 670, vacuum secondaries. The engine has an off idle stumble. I was told that due to cam I have low engine vacuum. The engine builder and Holley Tech said I need a double pumper to get rid of the stumble. But they do not agree on the size.
Drifting
Joined: Apr 2007
Posts: 1,892
Likes: 3
From: Springfield Illinois
You might have to use your seat of the pants dyno.. I would say that about 4000-4500 RPM's you'll notice a huge difference between the 750 and the 650. When I was testing carbs on my my L82 with a slightly larger than stock cam, sure, the 650 had a little more snap, but by 3000 rpms the difference was hardly unnoticable. By 4000, it felt like the 650 was starting to run out of breath and the 750 definitely had more pull up top. I probably would only consider a 650 carb, if I was building a 350 with a redline of 5500-6000. The 383 will appreciate the extra flow.
Burning Brakes
Joined: Oct 2001
Posts: 911
Likes: 5
From: TN
650 double pumper
run the 750. it will not hurt the bottom end and might even pick up a few ponies on top. your real issue is vacuum vs mechanical secondaries. if you're running an automatic trans, go vacuum no matter what. i speak from experience. you will need a tall converter and a lot of gear, like 3.73s to make the mechanical secondaries happy. i have a 750 DP holley on my car and i don't have enough converter stall or gear to make it happy off the line. i want to put a manual in the car so i'm living with it till then. if i was staying automatic, i'd swap to vacuum secondaries.
I have been using a Holley 650 double pumper on my original owner 1970 Corvete 350 turbo 400 since 1974. It has 3:08. I love having absolute full control of the manual secondaries, I would never consider having anything but the double pumper manual secondaries.
My original owner 68 327/350 4-speed 3:70 has had the same type Holley 650 double pumper since 1974, no way would I change from the mechanical control of the secondaries.
Having no mechanical control of the secondaries during the early use of my 68 and 70 was enough for me. Instant starts, with the Holley's.
My operating rpm use does not usually go above 5K, no race track use, just good ole enjoyable driving all these years.
Last edited by 70ZZ3 96LT4; Dec 11, 2009 at 11:38 AM.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
The smaller carb isn't going to give up any power with those CID/HP numbers, and will provide better throttle response in all scenarios except zero-to-WOT.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
I agree that I'd tune what you have before buying something new. A DP carb can cover bad tuning by just pouring gas into the engine - but that's not good for the engine or for your wallet.
Burning Brakes
Joined: May 2006
Posts: 774
Likes: 1
From: Round Rock Texas
Who built the motor and if it was an engine builder what does he say? Mine was a crate motor and dynoed at the plant and all of the documentation they gave me and advice was to go with a 750 double pumper for the most torque and hp. I thought I knew better and fooled around with a couple of smaller carbs, but in the end I have a BG 750 Speed Demon. Like someone said earlier...take it out and you be the judge by stomping on it with both carbs.
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Melting Slicks
Joined: Jul 2006
Posts: 2,275
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From: Salmon Arm, BC
The real issue that has not been addressed is what kind of driving are you really going to do with this car? If its mostly street driving with the odd spirited blast I'd choose the 650. If it's mostly drag strip/autocross/beating on it all the time I'd choose the 750.
The 650 will take your engine right to 7000 rpm no problem....it just won't make the peak power that the 750 will.....guessing at 15-20 HP less.
The 650 will take your engine right to 7000 rpm no problem....it just won't make the peak power that the 750 will.....guessing at 15-20 HP less.
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The real issue that has not been addressed is what kind of driving are you really going to do with this car? If its mostly street driving with the odd spirited blast I'd choose the 650. If it's mostly drag strip/autocross/beating on it all the time I'd choose the 750.
The 650 will take your engine right to 7000 rpm no problem....it just won't make the peak power that the 750 will.....guessing at 15-20 HP less.
The 650 will take your engine right to 7000 rpm no problem....it just won't make the peak power that the 750 will.....guessing at 15-20 HP less.
I have a 383 stroker with approximately 450 hp in my 61. I put a Speed Demon 650 double pump on it. Great throttle response and low and mid range torque. Could it use a 750? Well, if I lived frequently over 6500 RPM's, which I don't, I suppose so. The 650 is plenty of carb for the way I drive. If you are going to race your car or are constantly hammering it at higher RPM's then go with the larger carb.
Pro
Joined: May 2003
Posts: 593
Likes: 0
From: London England
If I've done my maths correctly at 6000rpm a 383 requires 663 CFM.
If the engine is staying as is, I would definitely go for the 650.
Although theoretically it might fractionally strangle the engine at max rpm, depending on the cam you may be dropping off the power curve anyway.
For the potential tiny loss at max rpm the throttle response, smoothness and torque will more than make up for it. How often do you drive at max RPM? You don't, you hit it and change gear.
If you have further plans for the engine, different cam, higher rev limit etc then you should go for the 750 to allow extra flow.
The Holley guy is right, the 650 is the best match for your engine.
If the engine is staying as is, I would definitely go for the 650.
Although theoretically it might fractionally strangle the engine at max rpm, depending on the cam you may be dropping off the power curve anyway.
For the potential tiny loss at max rpm the throttle response, smoothness and torque will more than make up for it. How often do you drive at max RPM? You don't, you hit it and change gear.
If you have further plans for the engine, different cam, higher rev limit etc then you should go for the 750 to allow extra flow.
The Holley guy is right, the 650 is the best match for your engine.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
The good "rule of thumb" formula is: CFM = ((CI x RPM)/3456) x VE
I see VE as right around 85% for an engine making around 1 HP/CID.
So the engine overruns the 650 at around 7000 RPM, the 750 around 8000 RPM.
On a 650, we're into the secondaries at around 3500 RPM.
I'll stand by my comment that tuning the existing 670 is likely to produce the best overall results - even if it does give up 2-5 ponies peak HP.
I see VE as right around 85% for an engine making around 1 HP/CID.
So the engine overruns the 650 at around 7000 RPM, the 750 around 8000 RPM.
On a 650, we're into the secondaries at around 3500 RPM.
I'll stand by my comment that tuning the existing 670 is likely to produce the best overall results - even if it does give up 2-5 ponies peak HP.