Holley 4150. 650, 750

[ArizonaGuy]

What’s the difference between a 650 and a 750?
They both have 1 11/16” TB bores.
Are the main bodies and/or boosters different, or is it just calibration?

[1980RS]

The ventures' are smaller on the 650 than the 750, also the 650 has straight leg booster and the 750 has down leg booster. Other than that main the bodies they are the same.

[HQM383]

Both same throttle plates as mentioned.

Venturi
750cfm - around 1.375" minimum diameter. Some brands are a bit different and as much as 1.400" venturi like a demon but generally around that 1.375"

650cfm - again varies with brand and model but can be between 1.25" to 1.300" venturi.

The next question would be "how can they flow all the same". short answer is they would not and the flow rating number is as much a product name as absolute flow capability figure.

Two to consider:
Holley Classic HP 950cfm - 1.750" throttle bore and 1.375"venturi (isn't that the same as a 750?)
Holley 4871 850cfm - 1.750" throttle bore, 1.561" venturi (isn't that larger than 950 above?)
Holley Street HP 850 - 1.688" throttle bore, 1.563" venturi (how could that flow the same as above?)
Holley classic 1000HP - 1.750" throttle bore, 1.561" venturi (haven't we seen this combo with another name?)
Plenty more when you get into Demon and Quickfuel dimensions and their 'names'.

It's all in a name.

[BOOT]

The ventures' are smaller on the 650 than the 750, also the 650 has straight leg booster and the 750 has down leg booster. Other than that main the bodies they are the same.

The Holley/QF HP mech secondary mainbody's have downleg 650 & 750

[frnkeore]

It's hard to understand Holley's CFM ratings.

These are 600, 650 & 660 carbs, with the same throtte and venturi size:

Carb# Mod # CFM Kit Needle Pri - Sec Pri-Sec PV Accel Sec Venturi Throttes
Seat Jet Jet/PT Block pump Sprg Fuel Bowls
R50469 4160 600 703-47 N/A 6-511 122-65 N/S N/S N/S 125-25 .025 BROWN 108-13-2 1-1/4, 1/516, / 1-9/16 1-9/16

R50470 4160 650 703-33 N/A N/A 122-74 N/S N/S N/S 125-65 .040 PINK 108-13-2 1-1/4, 1-5/16 / 1-9/16 1-9/16

Kit N&S Pri jet Sec jet PV Noz Spg Pri Bowl
R2818 4150 600 37-1537 6-506 122-65 122-76 125-65 .025 Purple 134-101 1-1/4, 1-5/16, / 1-9/16, 1-9/16

R4777 4150 650 37-485 6-504 122-71 122-76 125-65 .025 .025 134-103 134-104 1-1/4, 1-5/16, / 1-11/16, 1-11/16

R4224 4160 660 37-1537 6-508 122-76 34R9716-12 N/R .025 .025 134-101 1-1/4, 1-5/16, / 1-11/16, 1-11/16

Then, I found the following CFM ratings listed for 1 3/8, 1 7/16, / 1 11/16, 1 11/16 carbs:

715, 750, 780, 800

For carbs with 1 3/8, 1 3/8, / 1 11/16, 1 11/16 combo's:

Only two, 750 & 770

[levisnteeshirt]

A 650 has a taper on the throttle bore that opens to the throttle plate diameter at the bottom too ,, easy to spot

I have saw a 4779 750 with a straight leg booster . only 1 ,, maybe a mistake at the factory ?? I felt for sure it was a 700 but labeled a 4779 on the choke horn ,, I should've taken pics

[levisnteeshirt]

Both same throttle plates as mentioned.

Venturi
750cfm - around 1.375" minimum diameter. Some brands are a bit different and as much as 1.400" venturi like a demon but generally around that 1.375"

650cfm - again varies with brand and model but can be between 1.25" to 1.300" venturi.

The next question would be "how can they flow all the same". short answer is they would not and the flow rating number is as much a product name as absolute flow capability figure.

Two to consider:
Holley Classic HP 950cfm - 1.750" throttle bore and 1.375"venturi (isn't that the same as a 750?)
Holley 4871 850cfm - 1.750" throttle bore, 1.561" venturi (isn't that larger than 950 above?)
Holley Street HP 850 - 1.688" throttle bore, 1.563" venturi (how could that flow the same as above?)
Holley classic 1000HP - 1.750" throttle bore, 1.561" venturi (haven't we seen this combo with another name?)
Plenty more when you get into Demon and Quickfuel dimensions and their 'names'.

It's all in a name.

From what I gather QF and others started rating some of their carbs at 2.00" HG ( Mercury ) drop , instead of the industry standard 1.5 " HG ,, soo ,, to not feel left out in the non sense Holley rated a 950 how they did ( it is a 750 with a 850 plate basically )

This has caused allot of confusion ,, tell a guy with a 950 he might run better with an 850 he looks at ya like your nuts

A 1.450 bore 950 seems to be popular , CNC'd main body to 1.450 ,, Bo Laws does it

I like a 830, 1.560 body with a 1.750 throttle plate ,and no it isn't a 750 plate , it's a 830 plate ,, gives a open wheel mod a lil more throttle control IMO but will still pull hard on top

[HQM383]

Both same throttle plates as mentioned.

Venturi
750cfm - around 1.375" minimum diameter. Some brands are a bit different and as much as 1.400" venturi like a demon but generally around that 1.375"

650cfm - again varies with brand and model but can be between 1.25" to 1.300" venturi.

The next question would be "how can they flow all the same". short answer is they would not and the flow rating number is as much a product name as absolute flow capability figure.

Something to consider:
Holley Classic HP 950cfm - 1.750" throttle bore and 1.375"venturi (isn't that the same as a 750?)
Holley 4871 850cfm - 1.750" throttle bore, 1.561" venturi (isn't that larger than 950 above?)
Holley Street HP 850 - 1.688" throttle bore, 1.563" venturi (how could that flow the same as above?)
Holley classic 1000HP - 1.750" throttle bore, 1.561" venturi (haven't we seen this combo with another name?)
Plenty more when you get into Demon and Quickfuel dimensions and their 'names'.

It's all in a name.

From what I gather QF and others started rating some of their carbs at 2.00" HG ( Mercury ) drop , instead of the industry standard 1.5 " HG ,, soo ,, to not feel left out in the non sense Holley rated a 950 how they did ( it is a 750 with a 850 plate basically )

This has caused allot of confusion ,, tell a guy with a 950 he might run better with an 850 he looks at ya like your nuts

A 1.450 bore 950 seems to be popular , CNC'd main body to 1.450 ,, Bo Laws does it

I like a 830, 1.560 body with a 1.750 throttle plate ,and no it isn't a 750 plate , it's a 830 plate ,, gives a open wheel mod a lil more throttle control IMO but will still pull hard on top

I wasn’t aware of the 2.00” HG thing, I thought they were more accurate and honest than other brands at the time but the 2.00” makes sense.
The QF dimensions
650cfm - 1.250v x 1.687t
750cfm - 1.390v x 1.687t
850cfm - 1.390v x 1.750t
950cfm - 1.450v x 1.750t
1000cfm - 1.520v x 1.750t
Makes more sense than Holley’s
650cfm - 1.250v x 1.688t
750cfm - 1.375v x 1.688t
850cfm - 1.563v x 1.750t or 1.563v x 1.688t (Street HP)
950cfm - 1.376v x 1.750t or 1.563v x 1.750t (Street HP) 1.600v x 1.75t (XP)
1000cfm - 1.561v x 1.750t
And then there is older Barry Grant owned Demon
650cfm - 1.280v x 1.688t
675cfm - 1.280v x 1.688t (green sleeves)
750cfm - 1.400v x 1.688t
775cfm - 1.400v x 1.688t (red sleeves)
825cfm - 1.425v x 1.750t
850cfm - 1.562 x 1.750t
975cfm - 1.500v x 1750t (gold sleeves)
1000cfm - 1.562v 1.750t

This is why I have a chuckle to myself when people say their Holley 950HP Classic is too big so they ‘drop down’ to an 850, and also the strange looks and arguments with the situation you mentioned, or that an 850 Demon is the right size for their engine but a 1000 way too big. There are also cases where guys have tried 3 different brands of carbies of the same cfm rating on the dyno and concluded one brand is better than the other not realizing they were not apples for apples comparisons as venturi varied from 1.45” to 1.600”

As you know this all makes carb selection more of an art than punching some numbers into a calculator using the old formula and then picking your favorite brand close to that number. It would be good to know some mathemagician that can come up with a formula for minimum or maximum required venturi and another for venturi/throttle bore % or ratio for a more informed decision that avoids the marketing department derived names.

[levisnteeshirt]

A 950 usually has a stepped 830 type booster ,, that is an improvement over a 750 type booster

I like to put that .156 stepped booster in any 4150 ,,, never done it to a 600 , i don't have any dyno data but the owners like it after they try it ,, i did it to one 750 and didn't tell the owner i did it ,, he just called me and asked me what i changed that it felt a ton better

[ArizonaGuy]

Good stuff fellas, thanks