Don Terrill’s Speed-Talk

another question guys i have a 650 holley double pumper thats been around for years last time it was used was on my buddies midly build 400 sbc i want ito put it on my built 400sbc which is around high 400-low 500hp range with 11:1:1 compression will this carb still be good enough or should i go with and have to buy a 750 double pumper

If it's a 500hp engine, you need the 750

What are you using it for?

get another 650 and a tunnel ram.

400 cubes are hungry for A/F. Let they eat.

I would definitely use the 750. A 650 will be too small for the combination you have. The 650 will act like a restrictor plate on that engine. I've had a combination like that before and the 750 DP worked great.

Just put a 750 HP main body in the 650 you have and turn it into a 750. All other parts are the same, including the baseplate. A little tuning and you're good to go.

the 750 hp plate may work then cause i ws gunna have it rebuilt any way do you have a part number for that thanks for the help fellas oh its a street once in a blue moon strip car

Ericnova wrote:Just put a 750 HP main body in the 650 you have and turn it into a 750. All other parts are the same, including the baseplate. A little tuning and you're good to go.

Easier said than done if you want it to be anything nice. If he has an unaltered 4777 650, it's not a four corner idle carb. The metering blocks are calibrated differently too, and I'm not referring to jets.

yea its unaltered i jus sell it and get a 750

I'll play devi's advocate because I'm traveling and i'm bored. the 650 you have will work great as a street carb for your engine, will the 750 make more top end power? Sure it will, maybe 10 on a 500 hp engine if both are tuned optimally, but you have to ask how much time will you spend at WOT on the street? 500 hp gets you well above most speed limits pretty quickly. 650's are very under-rated carbs, both the 4777 and the newer HP's, they are capable of very respectable power numbers when tuned properly and the throttle response is very, very good, which is most important in most of the impromtu, short little street skirmishes you might encounter on cruise night. Since you already have it, I'd try it first before spending money on a new carb. Just my .02.

if a 650 was plumbed w/cold air
and a 750 was drawing in hot under hood air
it would be real close.

What???????????

400 cid making high 400's to low 500's horsepower should have nothing less than a decently well designed 1000 cfm carb! A 750 will only strangle the engine combination and not allow you to run a good balance between main jet and high speed air bleed.



Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

The 650 is way small for that motor.

My choice would be an HP 950.

Guys, this is a street car, it's not an all out race car, when was the last time you used the trans brake and 2 step at a red light? What are you gonna do when some punk in his Dad's corvette jumps on you while you're doing 35 in high gear? 500 hp isn't that much, a WELL built 650 would surprise a lot of you. I'm not saying it would win any dyno contests, I'm just saying that the carb that achieves the highest peak number isn't always going to be the fastest in all situations, now I know Troy could probably set him up with a pair of 1250's that would knock 2 seconds of his 1/4 time but 500 hp is 500 hp, the power an engine can make is based on the parts inside of it, a larger carburetor can't make any of those parts do their job any better than they already are, unless of course it really is too small, if you know what you're doing a 650 is not too small for 500 hp. Using Dave Braswell's formula (I'm sure you guys may have heard of him) of 1.3-1.4 cfm per HP you only need a max of 700 cfm to support that, getting 700 out of a 650 would take me all of about 15 minutes, without increasing venturi size, 800 isn't really a problem, more than that is possible but then it would be cheaper to buy the 750.

I will say this, a very common thought about carbs is to run the biggest you can get away with, without very expensive fuel metering circuitry (like Troy's Webber plates) what you are going to end up with is slow moving air at the boosters which will result in slow engine acceleration, my philosophy has always been to run the smallest carb that doesn't hurt the power, higher air speed=faster acceleration. In any type of racing the amount of time you spend at max. power is only a fraction of the time you are running, to take advantage of max. power you have to get there first.

I actually prefer slower air speeds past the booster so as not to "shoot" the poorly atomized fuel droplets at the plenum chamber floor.

Larger venturi / slower air speeds allows me to better atomize the fuel exiting the booster so the combination of smaller fuel droplets and slower air speed allows a greater portion of the air / fuel mixture to stay in suspension improving fuel distribution, homogeneity therefore combustion efficiency. All of this equals better throttle response, more torque and more horsepower from the engine parts you already have.

A side note: operating with less engine vacuum @ WOT equals = more pressure filling the cylinders If there is more pressure (less vacuum) in the intake track it's like having more duration and lift (without the downside of drive-ability, loose of low end) as a result of increased cylinder filing

The shear displacement of that engine demands more cfm. 1000 would be the starting point in my opinion - even with Holley style metering blocks, just not set up like a 650 would be.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44