Shag's 3rd carb already? - WTF?

[cmayna]

What brand feed line are you guys using? It seems like there are a few choices and some don't fit right if i recall correctly...

I tried a Mr. Gasket and the fitment into the inlets of the carb sucked big time. I found a Holley back when I started out with the 750 and have kepted it. The one I'm using is 3/8" since my entire fuel line is of that diameter. I assume Holley makes a 5/16 for those that require it.

 

[tarafied1]

I might be stirring up a hornet's nest here... and I do agree that it is a bad thing to over carb an engine... :char

But I have a pet peeve with the combination of the carb sizing equations and the math of marketing.

If you use the supplied equations, and adjust for volumetric efficiency you will get an airflow requirement - but here is the catch - that is airflow at wide open throttle where ideally you should have close to zero manifold vacuum. Keep this in mind...

Now for the marketing math... every carb manufacturer I know measures CFM for 4 barrel carbs at 1.5 inches of vacuum for historical reasons. If your car has 1.5 inches of vacuum at wide open throttle your carburetor is acting as a restriction! If you do the math, you will find that a 800 CFM carb measured at 1.5 inches of vacuum is really about a 630 CFM carb at 0 inches of vacuum.

If you go by the supplied equations above and do not account for the "marketing" flow numbers, I guarantee you are leaving power on the table. :hs

I'm not saying that it is good to over carb an engine and I admit most street cars don't see prolonged wide open throttle operation; but if you are going to use equations, be sure all the terms match...

Good luck,
-Rory

very good info, thanks!

 

[Opentracker]

From a guy with a lot of carbs and a lot of cars.

A small carb will pull the car across the intersection quick.

A large carb will pull the car down the highway quick.

Small carbs have better throttle response and idler better.

Large carbs don't idle as well and use more gas.

I've put carbs from 390cfm up to 950cfm on the same motor. The small carbs work great around town but don't pull hard on the highway. The larger carbs suck around town but work great on the highway. Something around 600-700 works the best overall IMO on a Cleveland for a street driven car.

I have a NASCAR racing 390 on the Mule and it works great for doing burnouts and getting around the villiage here. It doesn't pull worth a darn in 4th gear though.

 

[2ndgen]

From a guy with a lot of carbs and a lot of cars.

A small carb will pull the car across the intersection quick.

A large carb will pull the car down the highway quick.

Small carbs have better throttle response and idler better.

Large carbs don't idle as well and use more gas.

I've put carbs from 390cfm up to 950cfm on the same motor. The small carbs work great around town but don't pull hard on the highway. The larger carbs suck around town but work great on the highway. Something around 600-700 works the best overall IMO on a Cleveland for a street driven car.

I have a NASCAR racing 390 on the Mule and it works great for doing burnouts and getting around the villiage here. It doesn't pull worth a darn in 4th gear though.

I agree 100%.

And for street driven cars, erring on the small side is the right answer (obviously to a point - you can definitely go too small too).

I just wanted to point out part of the reason why the carb sizing equations give "small" numbers compared to what the dyno/track proves to make the most power.

Finally, as with just about anything else on a modified car, if you just slap a carb on and do not tune it you learn little about the true requirements...

If a untuned 600 runs better than an untuned 750 on your car, it does not mean that a tuned 600 will run better than a tuned 750 on the exact same car.

Good luck,
-Rory

 

[180 Out]

The '70 Cougar 351C with FMX auto, and 3.00 9", when I sold it late last year, I had owned it for 12 years. One of the first things I did was to put a 600 cfm, electric choke Edelbrock Performer on it and had no regrets. I used an "RV" level cam (112 LDA, .484"/.512" lift, 204/214 duration at .050"). This combo would pull hard to 5000 rpm and top out at 5500. It might have pulled another 500 rpm with more carb, but at 5500 rpm it was doing 130 mph, which was fast enough for me. Because of its weight problem it also wouldn't win any drag races, but it was a fun and reliable daily driver.

One thing I wanted to mention, if you want good cold start performance make sure the heat riser is functional. The heat riser is the hole in the middle of the intake side of the heads, which corresponds to a passage that runs from one side of the intake to the other, directly underneath the carb. You can see the heat risers on each of the heads standing vertically in this pic:

The function of this system is to heat up the carb as quickly as possible. The whole purpose of the choke system is to deal with the inability of cold fuel to atomize. If you have no choke and your heat riser is also not functional, cold start driveability suffers a double-whammy. Ask me how I know.

 

[SELLERSRODSHOP]

i'm kinda late to the discussion here, but on my 351c & even a built 289 or two the best carb setup i found was the holley 750 vacuum sec. i removed the rear metering plate & installed a block so i could change jets like a DP. i also added the quick change secondary spring kit & spring assortment to fine tune when the secondaries would open. the best dual feed line assembly i found was from summit racing (their brand). it was adjustable, has a built in filter & a fitting for a pressure guage. never had an issue with it.