well?

 

650 CFM will support 400 horsepower or more if that answers your question.

Get the list number off the choke horn of the carb (0-4778 etc) to find out what you really have. You call it a 650 DP, which I assume to mean double pumper. It will not have vacuum secondaries if it does in fact have dual accelerator pumps.

 

well im building a 350.030 over.... with a xe 268 cam... performer rpm intake.... and im stumped on whether to go 650 mechanical secondaries or 750 vaccuum secondaries with a choke....cuz this car will be driven everyday and hes tight on money.

 

the motor is wanted for performance/reliability. so its hard descision

 

What heads? exhaust? etc.

If the rest of the motor is done by 5800 RPM or so, then the 650 will be enough. If he wants to shift any higher than that and actually nake the car go faster by doing so, he'll need lots of flow, and the 750.

 

manuel go with the double pump automatic go with vacuum secondaries. The XE 268 is recommended to have a stall converter for everyday use

 

I was recommended to get a 650 DP or a 750 VS carb for my setup.

 

If money is tight and this is a daily driver you should consider a milder cam. The 268Xe will not be that good on gas.

Either carb can be tuned to be fairly good on gas. But not with that cam.

One of the most common mistakes in building a performance daily driver is over camming it.

Better to put more attention into head/ induction/ exhaust flow
(porting etc) and cam the motor for torque and efficiency
in the rpm range where it will usually be used.
Something in the range of 210 to 218 @.050" duration usually
is a better choice for a daily driver overall.
Especially with a stock converter and modest gearing.

The extra top end peak horsepower that the XE268 potentially can make, will be rarely used and will not make up for the loss in low end torque and overall flexability of a better matched cam.

Consider a cam with less duration and use 1.6 rockers on the intake side.

For an example Check out my car in the reader's ride section.
the car is good on gas and as you can see is no slouch.
ran 13.4 with the stock converter and th700r4.
That cam is a winner.

Either the 650DP or the 750Vac sec carbs are calibrated a little
on the rich side as they come "out of the box"
Mileage can be improved by leaning out the primary jetting a bit.
The primary idle and off idle calibration can be leaned out a bit too. (idle feed restriction) Get a book on holley carbs to find out how.

 

this car will be raced..... we decided on the 268 with 1.6 rockers and the 750 vs........ with 72 cc heads... with the speed pro 9.5:1 with 76 cc. 010 2 bolt nickel block... performer rpm intake.. hedman long tubes into dumps

 

IMO a 600 CFM Double Pumper should be enough to feed pretty much any 305, a 650 cfm at the most. I'd say a 600 CFM double pumper for your setup.

 

He is not using it on his car. He said it is going on a 355 he is building for a friend.

Ben

 

Oh, then my bad.

 

yea well my friend drives around everday with his 357 and his elgin cam with 292 adv duration and 480 lift....he is alright... gas sucks but if you go easy on it it gets like 10 mpg. the 268 cam is milder than that cam... should get much better mileage. Plus my buddy im building the motor for is kind of a sucker for that rump rump that more duration can give you.

 

Yes but does your buddy have gearing and stall that match the cam that gives him that rump rump that he is a sucker for. If not he is liable to learn the hard way.

Ben

 

I run the same cam in a 355 in my S10. I used a 600 vs for break in and a couple of runs down the strip. It ran a low 13 with that carb on top of a Weiand xcellerator single plane. I now have a 750 vs and I would'nt trade anything for it, except maybe a demon. If he is going to drive it everyday I would stick with the 750, just my opinion though. Oh yea, the gas mileage sux with my setup.

 

yea hes got a 700r4 and 3.42 gears.... should be fine.... the overdrive is good for mileage and the low first gear in the 700 is kool.... that cam can be run with the stock converter but he is going to buy a 2500 rpm stall i beilive.

 

With a big cam like the 268XE running the car in over drive will not save you any gas.

Think about it. The motor will not be spinning high enough to be running efficently .

Over drive needs a short(er) duration cam to see any benefit
in gas mileage.

Use a smaller more flexable cam and focus on improving
head, induction and exhaust flow and tuning.
Only then will you have the flexable , wide power band to
have your cake and eat it too.
The 268xe is only sligtly milder than the for mentioned 292/.480"
cam.
still going to be hard on gas.

 

heh o well that's what he wants... i told him to buy a 300 dollar junker and ride that around till it dies so he could save some gas money.... if it's too harsh we'll just change the cam.... if he can handle the money for gas then it's ok..... wouldnt the overdrive help at higher speeds.... cuz highway driving with a th-350 and those gears would kill you wouldnt it?

 

o yea and the heads are ported truck heads with 76 or 72 cc chambers..... the ports look pretty nice.... the pistons yeild 9.5:1 with 76 ccs so the compression is ok..... but the intake is a performer rpm and the exhaust is hedman 1 5/8 long tubes with 3" collectors going into 40 series flows with turndowns on them.