Is a Holley 750 DP too much carb?

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Now that's the kind of mileage I'm looking for. Your right I need to learn about this machine and how to fine tune it.
 
AJ/FormS said:
Yeah those tires are for slantys. But the height is close at about 26.4 high; which calculates to 83.4 in circumference. All my calculations were based on a 27.0 inch tire at 85.0 circumference.
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What is a slanty?
 
I agree with everyone else it's in the tuning.

I just want to make a general statement about carb size. The cfm rating is the restrictiveness of the carb not how much fuel and air it will pass. If you go to small your just killing top end and too large start to hurt fuel atomization and part throttle drivablity. Stop following that terrible formula that goes around. we all know what works, stock to mild 650, hot street 750, race 850 plus.

P.S. if you want to know if it's to big measure the vacuum at a full throttle run.
 
crackedback said:
Had similar builds with a 750dp and got 18+mg on the highway and mid teens around town.
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I had no problem getting 16 mpg with a 750 DP Holley on a 360 .030 over and 3.91's with a .484 lift / 284 advertised MP cam.
Gotta keep your foot out of it to do that, though
 
Crackedback, explain the IFR, IAB thing.

A vacuum gauge would be of great help here.
 
The OP is way too green to be tuning air bleeds and emulsion, and that's if his carb is adjustable.

Certainly he can drill and tap all that stuff and make it tuneable, but he's better off paying someone like Thumperdart on here and over at moparts to do it for him.

I'd leave the gears you have in there. A bit taller tire. And let it go. If it's not you daily driver, what's the difference between a well tuned 12-13 MPG with what you have, or 16-18 if you go to taller gears. You'll give up a butt load of performance with less gear.

BTW, higher numerically is a LOWER ratio. For example, a 4.10 is a LOWER gear than a 3.55. It is higher numerically but the ratio is lower. You are always better off specifying ratio than numeric value. Sometime in the 1990's the numeric value started getting tossed around and has become a bad habit.
 
yellow rose said:
The OP is way too green to be tuning air bleeds and emulsion, and that's if his carb is adjustable.

Certainly he can drill and tap all that stuff and make it tuneable, but he's better off paying someone like Thumperdart on here and over at moparts to do it for him.

I'd leave the gears you have in there. A bit taller tire. And let it go. If it's not you daily driver, what's the difference between a well tuned 12-13 MPG with what you have, or 16-18 if you go to taller gears. You'll give up a butt load of performance with less gear.

BTW, higher numerically is a LOWER ratio. For example, a 4.10 is a LOWER gear than a 3.55. It is higher numerically but the ratio is lower. You are always better off specifying ratio than numeric value. Sometime in the 1990's the numeric value started getting tossed around and has become a bad habit.
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I would be happy with 12 mpg. What is the largest size tires that would fit? Right now it has 215/70r14 up front and 225/70r14 on the rear.
 
In a 70 Dart, in the rear,nothing big enough to survive a stout 360.
I think it's been said here on FABO many times; 255/60-15. These are about 27 tall and 10inches across the sidewalls when mounted on 7 inch rims which are much to small; that's just how they rate them. That tire needs an 8 at the least. You will need a custom wheel with a custom backspace.You may need to move your springs inboard IDK.
I had one of these cars back in 1970. After killing the original E70-14s and several more sets of various tires, I finally bit the bullet an sprung for new wheels and get this Gs! Hyup G60-14s. G's are 8.25 inches across the tread. So they fit on an 8 inch rim. But those puppys were a different style of tire, a belted polyglass so the ratio of tread-width to sidewall-width was different than it is for radials. The sidewall was not much bigger than the treadwidth, perhaps an inch, on an 8 inch rim. So that makes the sidewall dimension maybe 9ish inches. As I remember it, it was still a very tight fit.But I did not have to move the springs. Those Gs still sucked for traction.
On the front on the 4" bc, you are limited to a couple of sizes.
The 235/60-14s being about the biggest on commonly available 7 inch rims.These are a tic over 25" tall.
Depending on your ride-height,you may have to trim the wheel opening for full-lock steering. The most common interference is at the front lower corner. The wheel will have to have an offset to move the rubber inboard , but not so far that it rubs on the frame on the backside. Most 7inch wheels come with a 3.375 bs. This is not enough. I found a set with 3.75" bs. These barely fit and I did have to trim the metal.
The other commonly used tire is I think,a 225/65-14. this tire is about 25.5" tall, but with it's slightly narrower section width is a little easier to fit. This will also fit on a 7"rim. Trimming may still be required, depending on your ride height.
If you look closely, your front 215s are currently probably just avoiding trouble. If your rear wheels are the same as the front, you can try those 225/70s up front and see where you are at. They are almost a full inch taller than the 225/65s. Before you get too excited,you will have to roll the car at least 6ft, (or 3ft in each direction), with all the car weight down,to settle the suspension.
 
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AJ/FormS said:
In a 70 Dart, in the rear,nothing big enough to survive a stout 360.
I think it's been said here on FABO many times; 255/60-15. These are about 27 tall and 10inches across the sidewalls when mounted on 7 inch rims which are much to small; that's just how they rate them. That tire needs an 8 at the least. You will need a custom wheel with a custom backspace.You may need to move your springs inboard IDK.
I had one of these cars back in 1970. After killing the original E70-14s and several more sets of various tires, I finally bit the bullet an sprung for new wheels and get this Gs! Hyup G60-14s. G's are 8.25 inches across the tread. So they fit on an 8 inch rim. But those puppys were a different style of tire, a belted polyglass so the ratio of tread-width to sidewall-width was different than it is for radials. The sidewall was not much bigger than the treadwidth, perhaps an inch, on an 8 inch rim. So that makes the sidewall dimension maybe 9ish inches. As I remember it, it was still a very tight fit.But I did not have to move the springs. Those Gs still sucked for traction.
On the front on the 4" bc, you are limited to a couple of sizes.
The 235/60-14s being about the biggest on commonly available 7 inch rims.These are a tic over 25" tall.
Depending on your ride-height,you may have to trim the wheel opening for full-lock steering. The most common interference is at the front lower corner. The wheel will have to have an offset to move the rubber inboard , but not so far that it rubs on the frame on the backside. Most 7inch wheels come with a 3.375 bs. This is not enough. I found a set with 3.75" bs. These barely fit and I did have to trim the metal.
The other commonly used tire is I think,a 225/65-14. this tire is about 25.5" tall, but with it's slightly narrower section width is a little easier to fit. This will also fit on a 7"rim. Trimming may still be required, depending on your ride height.
If you look closely, your front 215s are currently probably just avoiding trouble. If your rear wheels are the same as the front, you can try those 225/70s up front and see where you are at. They are almost a full inch taller than the 225/65s. Before you get too excited,you will have to roll the car at least 6ft, (or 3ft in each direction), with all the car weight down,to settle the suspension.
Click to expand...
 
AJ thanks for your detailed reply. I'll probably just go up to a 245/70 to avoid any rubbing or modifications to the body.
 
yellow rose said:
The OP is way too green to be tuning air bleeds and emulsion, and that's if his carb is adjustable.

Certainly he can drill and tap all that stuff and make it tuneable, but he's better off paying someone like Thumperdart on here and over at moparts to do it for him.

I'd leave the gears you have in there. A bit taller tire. And let it go. If it's not you daily driver, what's the difference between a well tuned 12-13 MPG with what you have, or 16-18 if you go to taller gears. You'll give up a butt load of performance with less gear.

BTW, higher numerically is a LOWER ratio. For example, a 4.10 is a LOWER gear than a 3.55. It is higher numerically but the ratio is lower. You are always better off specifying ratio than numeric value. Sometime in the 1990's the numeric value started getting tossed around and has become a bad habit.
Click to expand...
 
Thank you for your advice and knowledge. I think I'll just go to a 245/70 tire in the rear. I now understand what your saying about he gear ratio numbers, thanks for that. I do like the way the car launches with the 3.91. I just need to find someone local who can reset and teach me about the timing. Meanwhile I'll start researching that myself.
 
Having a friend who is a long time gear head tweak the timing today, weather permitting. Here's a few pics.

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rumblefish360 said:
I would try more Initial and less total.
Bump it to 16* initial and see how she likes it.
Limit total to 36 or less.

What heads? Is the distributor a MP part? Or a stock unit?
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We looked at the carb today and my friend agreed that the 750 is too big for this motor. jets in it are 72 and 80. Pouring fuel in. I just ordered a new Holley 650 double pumper and gasket set. He's gonna help me set it up and dial it in. Also on your recommendation we are gonna change the timing. He's been racing for years and definitely know what he's doing. I'll let ya know how it shakes out. Later.
 
Gregor said:
We looked at the carb today and my friend agreed that the 750 is too big for this motor. jets in it are 72 and 80. Pouring fuel in. I just ordered a new Holley 650 double pumper and gasket set. He's gonna help me set it up and dial it in. Also on your recommendation we are gonna change the timing. He's been racing for years and definitely know what he's doing. I'll let ya know how it shakes out. Later.
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Just so you know, a 3.91 gear is a good gear. You can use a much bigger carb with that gear, verses, say a 3.23 gear.


Second, CFM rating is based on air flow, not fuel flow. Before you junk a good carb, spend a bit of time and learn how carbs are flowed, and how they meter fuel.

There is no way that carb is too big.


Good luck.
 
yellow rose said:
There is no way that carb is too big.
Good luck.
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Agreed.

What happens when and if the 650 runs rich? If it's a DP carb, you can almost plan on it.

I know plenty of people that have been racing and into cars for a long time that can't tune a carb to save their life. Ignition timing is A number 1 on the list to get correct, then work on the carb.

NO WAY is a 750 too large for what you have. The tune up is a mile off.
 
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AJ/FormS said:
Hey, that formula works on Chebbies!
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I'm just using your post as a jumping board to finish my thought .

That formula is the volume of the cylinders per minute. Its not drirectly related to carb cfm. We don't use that formula to find needed head flow. A short cut for that formula for a 350 (340/360) size engine is to knock a zero off the rpm so a 6000 rpm engine passes about 600 cfm. So if we used the formula to find needed head flow we'd divide the 600 cfm by the 8 intake ports of a v8 so 75 cfm each port. Obviously that don't match up to the way we rate ports cause that engine would need high 200's.

Same with carbs the rating ain't equal to that number. Carb rating is how much cfm it can past at a certain vacuum (restriction). But each engine will be at a different vacuum level at full throttle. If your engine needs 600 cfm of air it will basically pull that much through any carb with in reason don't matter if it's 300 cfm or 1200 cfm just neither will be optimal probably something around 850 cfm give or take would be right.
 
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crackedback said:
Agreed.

What happens when and if the 650 runs rich? If it's a DP carb, you can almost plan on it.

I know plenty of people that have been racing and into cars for a long time that can't tune a carb to save their life. Ignition timing is A number 1 on the list to get correct, then work on the carb.

NO WAY is a 750 too large for what you have. The tune up is a mile off.
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Just to put in perspective a 500 cfm 2 BBL is just the front half of a 750 no one would have a problem running that on almost any engine. A good tune up is all you need.
 
AJ/FormS said:
Hey, that formula works on Chebbies! LOL, fixed it
It was supposed to be a poke at Chebbies not being able to use as much carb as Mopes, on account of our superior design.Purely posturing
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No worries I got ya :)

350 Chev is a terrible performance engine. Look at any similar build ups between 318/340/360 to a 350 the mopars always seem to make 50 plus hp more than the 350. Thats until the aftermarket started making great heads for them.
 
750 is not too big. Not at all. That's a great all around carb. And with proper tuning can be made to run excellently at all throttle positions.
 
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