Ed Schurke questions my cam/carb combo...

Ed asks...

Ed, I have two answers for this question. The technical answer and the REAL answer.

The technical answer is that generally speaking, many people think that 650 is too much cfm for a 302 and I was encouraged to stick with a 600 cfm.

The REAL answer is that the cam was given to me, and so was the carb, and this car uses all the free parts it can get.

Not being a cam genious, I don't know all the numbers on the cams and what they do to the running of the car. So feel free to enlighten me on what the "off set" is and what it does. I am assuming you are referring to the 224/232 portion of my numbers...

 

I would say he is talking of the split duration / lift. Ford heads are notorious for having poor exhaust, So the reason for the split. When i was N/A i ran several different Holley's. I found the 650 dp to be the best running and easiest to tune out of the bunch. The carb is only a fuel mixer therefore if there is little air there will be little fuel. 650 doesn't mean the carb will try to flow 650 cfm to the motor at all times, It means its capable of 650 cfm if needed. There is no book or recommendation anywhere that recommends a 1000+ cfm to run on a small block but we have 2 cars that run them on the street with an average of 15 mpg. In my opinion cfm has been much overrated figure of how to pick a carb. I think cfm should be more of a guideline than a law. Sure you can go to big on the throttle bores, But contrary to popular belief this wont make it run rich it will actually be lean. I say err on the high side of cfm and tune it to your combination.

 

So if ford heads have poor exhaust, then when using after market heads u can use a cam with non staggered numbers(split) and get good results. I'm just asking as I didn't know why some cams have the split numbers either. Enlighten me bro

 

Yes and no, most aftermarket heads have better ex ports but they are still in the stock location. And Ford more or less built the heads to fit the cars. In order to make the port flow better it needs to be straighter (aka high port, Air hates to turn corners) But on our cars with the header choices and shock towers this is ALMOST impossible. So on N/A cars the best choice is a split cam. As always this is just my opinion.

 

I also run a bigger than recommended carb. My motor is a 308 and I use a Holley 750HP. I tried a 600 and then a 650, but the 750 really seemed to wake the engine up. Course, it's not a street car and usually is at WOT.

 

i had a 69 mustang once that had a 351 with performer rpm heads and a vic jr. intake. id ran several cams and carbs. a 750 db, and a 650 dp. the 650 ran alot beter than the 750. the bigest problem i had with the 750 was the idle mixture screws on the secondarys. i would have all for screws just off there seats to get it to idle best with out any stumbles. it was rich at idle with them just off the seats. i do belive in geting the correct size cfm carb for the demands the motor has based on its top rpm and displacement. there is a formula that helps you figure it out.

 

That is where the billet metering blocks come into play, You can reduce the idle feed restrictor and cure these kind of problems with the removable jets, Otherwise you have to solder up the hole and re drill or create some kind of other restrictor.

 

It recommends a 750 cfm for my car, which would put the vaccum under the carb at wide open throttle at about 3 inches. Which means the carb is too small.

 

Yes and as i said before this needs to be more of a guideline other than the law. The throttle bore needs the most consideration.

 

A carb is only too big when it can no longer meter fuel for the amount of air pulled through. That's where knowing how to tune comes in. Remember Shelby putting 715 or so cfm carbs on little 289s with crappy iron heads. As I recall they ran pretty good...

 

I wondered the same thing....

 

Oh Heck,,, he's the guy who asked about the split cam pattern!!!