Fanning the flames of the 650 vs 750 carburetor issue
04-24-2004, 08:48 AM
#1
Fanning the flames of the 650 vs 750 carburetor issue
I have seen many posts discussing the merits or requirements of a 650 versus 750 cfm carburetor on this Forum. Let me tell you guys what I saw yesterday on the engine dyno.
My buddy took his 1967 Pontiac 400 in to our engine builder's dyno. This is a 10:1, 400 ci, fresh rebuild with less than 4000 miles, but with a new cam. I don't have the specs but it's a Comp XE274 for Pontiac so a little under .500" lift. HEI, Edelbrock Performer, roller rockers.
We tried three different carburetors.
A 650 Speed Demon vacuum secondary is the carb he has been running, which was recommended by Demon with his old Comp XE262 cam.
A 750 Speed Demon vacuum seconday is the carb that was recommended by Demon for my '76 Suburban with 454 and a Comp XE 256 cam (plus headers, intake, etc).
A 750 Holley mechanical secondary was available through the engine builder.
After I broke one of my con rods last week he decided not rev past 5000 RPM.
Long story short...the 650 VS made the most torque and HP up to about 4500 RPM, at least 10 more ft-lbs.
The 750 VS made the least.
The 750 DP made about 9 more HP at 5000 RPM. Had he revved any higher he might have picked up a little more but the intake would probably have limited that above 5500 RPM.
The most area under the curve, or highest average TQ and HP was made with the 650 VS. The other thing is that the dyno operator, with about 30 years at this stuff, said the vacuum secondary carbs work better on cars than on dynos due to the way the engine is loaded up in a vehicle vs on the dyno, so the 650 may have even more advantage on the street or strip.
So now I am left wondering why Demon recommended a 750 VS for my 454, when it has only 54 more cubic inches and a smaller cam???
Feel free to discuss amongst your selves.
Jeff
My buddy took his 1967 Pontiac 400 in to our engine builder's dyno. This is a 10:1, 400 ci, fresh rebuild with less than 4000 miles, but with a new cam. I don't have the specs but it's a Comp XE274 for Pontiac so a little under .500" lift. HEI, Edelbrock Performer, roller rockers.
We tried three different carburetors.
A 650 Speed Demon vacuum secondary is the carb he has been running, which was recommended by Demon with his old Comp XE262 cam.
A 750 Speed Demon vacuum seconday is the carb that was recommended by Demon for my '76 Suburban with 454 and a Comp XE 256 cam (plus headers, intake, etc).
A 750 Holley mechanical secondary was available through the engine builder.
After I broke one of my con rods last week he decided not rev past 5000 RPM.
Long story short...the 650 VS made the most torque and HP up to about 4500 RPM, at least 10 more ft-lbs.
The 750 VS made the least.
The 750 DP made about 9 more HP at 5000 RPM. Had he revved any higher he might have picked up a little more but the intake would probably have limited that above 5500 RPM.
The most area under the curve, or highest average TQ and HP was made with the 650 VS. The other thing is that the dyno operator, with about 30 years at this stuff, said the vacuum secondary carbs work better on cars than on dynos due to the way the engine is loaded up in a vehicle vs on the dyno, so the 650 may have even more advantage on the street or strip.
So now I am left wondering why Demon recommended a 750 VS for my 454, when it has only 54 more cubic inches and a smaller cam???
Feel free to discuss amongst your selves.
Jeff
