(2)450 DP's vs (2)750 DP's

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MomsDuster

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Here's what i'm planning, I have my 71 Duster, 440 Dodge, 10.0 CR, Stealth as cast aluminum heads. 231/237 @ .050 110 lobe separation 106 ICL, Weiand 1987 Hi-Ram Tunnel Ram w/2 ProForm 750 DP, 3000 stall and 3.23 gears. I picked up a pair of Quick fuel HR 450 double pumper carbs. ( https://www.holley.com/products/discontinued_product/parts/HR-450 ) I'm going to swap them for the 750's to see the difference in performance. I'm curious if the smaller venturi's will have better low end velocity and give better low end performance. What does the FABO brain trust think?
 
You’ll lose power.

A better option (IMO) would be to call Lightning Racing Carbs and order boosters from him.

Swapping to good booster will do way more for drivability, fuel distribution and the engines ability to RPM.

A smaller carb just kills power.
 
I was always told the twin 450’s for milder street applications.
I’ve road in. 440 Charger with twin tunnel ram 450. I thought it was pretty darn good.

Likewise, twin 750’s for racing, unless it’s a super beast. Back then, a good 30 years ago, I couldn’t understand how a regular 440 could be happy with 1500 cfm worth of carb. It’s a good bit to fully make use of. But I didn’t know anything of this arena and of performance.
 
I've got twin 450s on my 440. Ran the same mph as my 750 mighty demon on an M1 open plenum. 114 mph 3880 weight for my 68 Roadrunner

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You HAVE the 450s, so try um. You may lose some all out power, but the response might be better too. Only one way to find out.
 
Pulled the 750’s off today and set them up to be stored for a bit. Over the last week I took the 450’s and pinned everything. Moved the IFR’s to the bottom, tapped the main bodies for transition slots restrictors and cut the choke horns off (mostly for easier air bleed replacement). Installed them and got them idling good. Big lean hesitation off idle, put blue pump cams in and it’s super responsive. Lean at low speed cruise and at cruise. Got too hot in the garage so I’ll be back at it tomorrow morning. I’ll look at IFR’s first. Stay tuned if you’re interested.

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Good idea to pin the carbs before you start. How much emulsion did the blocks have? Let us know what it takes to make run right.
 
How much emulsion did the blocks have?
(3) .028 in each well. Originally I thought that might be a tad excessive. Maybe I’ll tap and plug the middle one and retest before I change any jets…. lol r will that actually do the reverse of what I’m thinking?
Thoughts?

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Pulled the 750’s off today and set them up to be stored for a bit. Over the last week I took the 450’s and pinned everything. Moved the IFR’s to the bottom, tapped the main bodies for transition slots restrictors and cut the choke horns off (mostly for easier air bleed replacement). Installed them and got them idling good. Big lean hesitation off idle, put blue pump cams in and it’s super responsive. Lean at low speed cruise and at cruise. Got too hot in the garage so I’ll be back at it tomorrow morning. I’ll look at IFR’s first. Stay tuned if you’re interested.

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What do you consider lean down low as far as AFR numbers. I’m curious.
 
(3) .028 in each well. Originally I thought that might be a tad excessive. Maybe I’ll tap and plug the middle one and retest before I change any jets…. lol r will that actually do the reverse of what I’m thinking?
Thoughts?

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I honestly wouldn’t do anything until you run them. Then make changes.
 
Update: installed larger IFR’s from .031 to .033. Fattened up the low speed cruise a bit. I upped the IAB’s from 70 to 72. Now it’s real nice and smooth from idle up to cruise where it gets pretty lean. Real responsive. I don’t have a big selection of smaller jets (below 70) so I took the secondary jets a put them in the primary side. From 55 to 60. Threw a set of 71’s in the secondary and went for a cruise. It’s a bit fat at cruise now and pig rich WOT. I have some jets on the way. Hopefully be able to get back at it this weekend.
 
Yeah, if it’s surging it’s lean.

Just remember the leaner you can run without surge is where you want to be, regardless of what the AFR gauge says.

If it will run without surging at 15.5 or 15.0 or whatever, the leanest you can run the better.
Be really good at reading plugs I'd add in that.
I agree with the afr statement you said, as if you recall my readings vs my plugs and exhaust ports. We averaged about 12.5 afr with lows in the 11s & nice clean plugs... just a little color.
 
Yeah, if it’s surging it’s lean.

Just remember the leaner you can run without surge is where you want to be, regardless of what the AFR gauge says.

If it will run without surging at 15.5 or 15.0 or whatever, the leanest you can run the better.
I’ve found over the time I’ve spent tuning the 750’s is 14.5 (by the gauge) at cruise is where she’s happiest. Anything much more than that she feels flat. Having the gauge is nice for seeing the outcome of a change. I use my butt dyno to gauge whether it was a positive or negative change.
 
I’ve found over the time I’ve spent tuning the 750’s is 14.5 (by the gauge) at cruise is where she’s happiest. Anything much more than that she feels flat. Having the gauge is nice for seeing the outcome of a change. I use my butt dyno to gauge whether it was a positive or negative change.
Nice to see you posting and working on the Duster, Mark. Are you looking for better mileage or response? What MPG are you getting with the 750s? I am super happy with my 660's with a similar car set up and get 11mpg. Mind you no vacuum advance, no power valves, etc..
 
@71GSSDemon hey Eric! Not exactly sure what I was looking for in this swap. I got the carbs awhile back. Been kicking around the idea and finally pulled the trigger. Probably most interested in the differences in response. Plus I enjoy playing with different tunes and ideas.
As for mileage, the best I’ve gotten is 12mpg on the highway. That was on a decent road trip with the wife. I guarantee that it’s not that good when I’m by myself! LOL
 
I agree with what newbomb Turk said also about installing annular boosters on your 750s, I would go with 16 hole. I use 12 hole on my 470 with 2-850s and my fuel consumption numbers on the dyno are outstanding.
You can even jet down smaller with the annular boosters too which might even pick up mpg with your 750s.
 
My 750’s have annular boosters. Just to clarify, I wasn’t having any trouble with my 750’s. I’ve gotten them tuned pretty darn good IMO. In fact I’ve been holding off on putting the 450’s on because the 750’s do work very well. I’m playing with the 450’s because I have them and I enjoy the process.

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My 750’s have annular boosters. Just to clarify, I wasn’t having any trouble with my 750’s. I’ve gotten them tuned pretty darn good IMO. In fact I’ve been holding off on putting the 450’s on because the 750’s do work very well. I’m playing with the 450’s because I have them and I enjoy the process.

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Oh, duh....****, I totally forgot you have had annular boosters... sorry, carry on brother
 
Ok, a couple of jet changes and float adjustments and she’s pretty dang close! Super responsive! Super smooth, and feels pretty good. Going to leave the tune as is for awhile and get some miles on it. Really looking forward to putting the Mickey’s back on and seeing what she’s capable of. If she still has traction, my butt dyno says the 60’ and 1/8 should pick up. The 1/4 time and mph is up in the air. Stay tuned.
 
Ok, a couple of jet changes and float adjustments and she’s pretty dang close! Super responsive! Super smooth, and feels pretty good. Going to leave the tune as is for awhile and get some miles on it. Really looking forward to putting the Mickey’s back on and seeing what she’s capable of. If she still has traction, my butt dyno says the 60’ and 1/8 should pick up. The 1/4 time and mph is up in the air. Stay tuned.
Good job, hoping it makes ya happy
 
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