Thread: Figuring out which carb to get

Tweet

Hey guys, I have a two parter today. The 429 has a Rochester on it right now, my uncle doesn't like Rochester and would like to put a holley on it. I don't have all the details from the previous owner yet, but know that he said it was a 925 (if I remember right) and that he put bigger jets in it.

So questions

1. How do I figure out which carb to get? (edelbrock would probably make him happy too). I found this formula on here

Engine Size (CID) X Maximum RPM / 3456 = CFM @100% Volumetric Efficiency

Where do I get the RPM number from?? (the tach on this thing CAN'T be right since it shows idle ad about 400 and never gets much over 2000, but we'll tackle that another time).

2. I am going to install a power brake kit, which I am guessing will need vacuum. Do I have to get this off the manifold or can I get it off the carb? I saw one on the HOlley site that said it had 4 vacuum ports for those sort of things (I know I need to run the PCV line up to one).

thanks!

Red

what controls the vacuum secondaries? (Denny you are great help and I am sure have realized how bad a learning curve I'm dealing with here, at least I am a quick study). And if it is a model with additional vacuum ports would that take care of the power brakes?

I see on their site lots of options, they have a street performer 750, a double barrel 750 and an Avenger 770. any big difference, or reason to choose one over another?

What RPM did you figure on it? My grandma's boyfriend (I know, sounds funny) has built cars for racing dirt track and he said 750 HOlley tonight when I took him for a ride and we talked about it. I just didn't ask how he knew that one.

Thanks for the help. Got big plans for thsi weekend. (I spent 4 hours last night ripping out the bed, wood and trim pieces). taking out the rear end to have it shortened and get shorter axles, so I can mini-tub and get 10" wide wheels and bigger tires. good stuff.

Red

Check your manifold, I'll bet it's a spread bore because of the Rochester. The Holley is going to be a square bore and won't mate to the manifold with out an adapter (I don't recommend doing that). Look for a good aftermarket manifold that is a square bore such as a Performer from Edelbrock. Holley has a good one along with Wiend and others. Brake booster to manifold. PCV to base of carb. Simple enough. By the way the secondaries are fed off of the primary venturis. All internal so you don't have to plumb anything.

Vacuum secondaries are controlled by engine demand and Holleys have a spring kit that allows you to tailor when they start to come in. I'm a little conservative on CFM selection as I think a 700 is big enough for your application.
Manifold selection is a toss up in my opinion, but I've had good luck using an Offenhauser 360.

Thanks Denny! First to answer Nitro, I know it has an Edelbrock Performer manifold on it now, just not sure which exact model as guy hasn't given me all info yet (could be in mail now).

I looked at the standard and Avenger more carefully, both have vacuum secondaries. the avenger is tuned for 400-500hp engines, has lifetime warranty and 2 3/8" vacuum ports, I sent links to my uncle to make the call, seems to me better to get the Avenger and then have the power brakes right to that rather than drilling the manifold or getting a carb spacer.

any thoughts on that? He'll make the decision but I can give him my input. about a hundred bucks difference. almost all money going into this would be his but I can help a bit here and there and if 100 bucks difference gets me into a more appropriate/better part then I can cover that.

I can't tell you how much I appreciate the help. I'm really enjoying this, have always wanted to build a hotrod and am finally getting the chance, while there is still gasoline available!!

Red

Denny, I agree with everything you posted, I'm just a little conservative on carbs. I've seen too many who think that if a 750 will work good an 800 will work better. Not true.

Well Red, You're more than half way home with this. Keep Rockin' on it.