Enough carb, or do I need bigger?

The original engine in my Challenge car came with a 600-cfm Edelbrock 1405 carb, and it's what I've been planning on using on the engine I'm building for it- a 30-over 350 SBC.

Edelbrock's page on the different carbs says that the 600cfm one is matched well for a 350, and I've found largely the same answer from using several of the online calculators for finding how much airflow an engine will need/be able to use when using a reasonable VE (around 90%).

However, I've got a chance to trade the remaining chrome dress-up parts I have for a Holley 750... so I'm wondering if that's worth picking up to use instead and I'm looking for some real-world experience with the different flow-rating carbs to see whether my 600 should be good enough or if I should be going bigger. Thanks!

What heads, cam and exhaust are you using, and what is your max RPM? These all factor in to carb sizing, although I'd say that unless your 350 is pretty radical it's likely that the 600 is fine. I'm running the same carb on a 454 in my truck. It's a mild motor with an rv cam, and I don't rev it over 5000 RPM, but on this engine it works well.

Going too big with the carb is a very common mistake.

In reply to bravenrace:

Vortec heads, for the Challenge will likely be running a ZZ4 cam but if there's a bit extra in my budget I have a GMPP Hot Cam that I'd love to run, current exhaust (and isn't likely to change) is headers to what look to be decently free-flowing mufflers about halfway back under the car. I'm guessing max RPM is going to be around 6000, maybe 6500. I will hopefully have the budget to run nitrous on the drag strip.

Woody wrote: Going too big with the carb is a very common mistake.

Yup, I've heard that said a lot- which is why I wanted to check since there's doubtless LOTS of real-world experience with different size carbs with different engines here. :)

Tuning the Edelbrock will handle it fine, let me know if you need it set up, Sean my mechanic is a specialist

On my 302 in the RX7, the 600 cfm Edelbrock was too small. It could barely handle WOT no matter what jets we used, and if you could get it as close to good for WOT, it sucked for off idle, and in no case did it like cornering forces. Put on a modded 750 Holley (bores flowed for 800CFM) and proper jets and it woke the engine right up. Could do both WOT and idle around at partial throttle like stock. The engine won't suck any more air than it needs, so don't over jet it. But having enough airflow is key. Hot rodders said it was overcarbed, but it ran perfect without black smoke or stumbling anywhere.

Same reason the BOSS 302 used nearly 800 cfm carb in stock form.

Also, the double pumper liked cornering at 1+G where the Edelbrock hated it. Had the same thing happen on an old Mustang autocross car.

yet all my cars run Edelbrocks without issue. Sounds to me like you went the wrong way, too much fuel.

I thought that you "couldn't" go with too big of a carb*? That so long as it was set correctly, the worst you'd have is a carb that you weren't using the full potential of.

Asterisk - unless you go extreme, like a 1000cfm carb on a stock 20R.

I dunno, but a 650DP sure ran well on my wife's 302 in her GT... I've seen a few tuner 650DP's go mid 9's in the 1/4....

Chris_V wrote: On my 302 in the RX7, the 600 cfm Edelbrock was too small. It could barely handle WOT no matter what jets we used, and if you could get it as close to good for WOT, it sucked for off idle, and in no case did it like cornering forces. Put on a modded 750 Holley (bores flowed for 800CFM) and proper jets and it woke the engine right up. Could do both WOT and idle around at partial throttle like stock. The engine won't suck any more air than it needs, so don't over jet it. But having enough airflow is key. Hot rodders said it was overcarbed, but it ran perfect without black smoke or stumbling anywhere. Same reason the BOSS 302 used nearly 800 cfm carb in stock form. Also, the double pumper liked cornering at 1+G where the Edelbrock hated it. Had the same thing happen on an old Mustang autocross car.

I've have to say that you most likely had some other problem with your 600. I've never seen a 302 that wouldn't run just fine with a 600 anything. There are a lot of variables when going from one carb to another, not just the CFM size, and it's not surprising it runs okay with a 750. But a 600 is much more appropriate to a 302. The hipo 289's ran a 600cfm carb.

In reply to aussiesmg:

Tried every direction on it, from smallest to largest. Small would run at idle and part throttle, big woudl almost make it to ful throttle, but nothing liked the full range from idle to 7500 rpm.

Simply put, the Holley worked out of the box.

And it cornered great at high side loading, wheras the Carter/Edelbrock hated anything close to 1 G, stumbling heavily in the corners no matter what jets were used.

The engine was originally a drag race engine with a high rise manifold and 900 cfm Holley, running 9s in a Pinto drag car. I had switched to a low rise manifold and a Performer RPM cam (which is why I thought the Edelbrock carb would work for me) but it still liked to rev. With the Edelbrock it would start to stumble every time it went over about 6500 rpm, regardless of jetting. the Holley pulled to 7500 or so.

In reply to Ashyukun:

You have the 600, try it. If you can get the Holley 750 for trade on parts you aren't using, get it too. It's pretty straight forward to swap them and do some back to back testing.

Now with that said, I prefer the Edelbrock/Carter carbs. The Ed carbs do not like more than 5.5 psi for fuel pressure. I have the regulator on mine set for 5. I've gotten deals on used carbs where the seller complains they could never get it to run right. At 8 psi you won't...

The 600 you have should be the Performer, the Ed version of the Carter AFB. There is a weighted door inside the carb over the secondaries. This is to control the airflow through the secondaries when you mash it. A quick test to see if you are under-carbed is to wire open that air door and take the car for a ride. If the car boggs when you jump it, it needs the air door. If it doesn't, you might be a good candidate for the bigger carb.

If it does bog, you can take the air door out and trim the weight down a little at a time to see how the car responds.

I'm running an 800 Ed on my Barracuda. A buddy is running the same 800 on his 496" big block Charger. Both cars run strong, and are street cars. Each carb is tuned differently for the combo.

On the Holley, is it a vacuum secondary, or a mechanical double-pumper? The 750 double-pumper is the universal drag carb. It could work well on the Challenge autocross too. More tuning options than you can shake a stick at.

Rob_Mopar wrote: On the Holley, is it a vacuum secondary, or a mechanical double-pumper? The 750 double-pumper is the universal drag carb. It could work well on the Challenge autocross too. More tuning options than you can shake a stick at.

Sounds like the 750 isn't a double-pumper, but I may still trade to get it- having the option, essentially for free in my budget, wouldn't be bad if it turns out the 600 just isn't enough for some reason...

When I was running a big block dirt track modified, we ran a 750 double pumper as recommended by the engine builder. Driver want to try a 1010, I think. So he bought it. I put in on and set ti up After warm ups, he said to put the 750 back on. He said the big carb would work OK if there was no traffic.

Carb sizing isn't rocket science but it does spark some interesting debate.

I undersize carbs, but 90% of my builds are street-only. 600 is more than enough. The 750 might squeeze out a few more ponies, but it also may have an adverse affect on throttle tip-in, throttle response, and mid-RPM torque numbers.

A true, vacuum-operated secondary will reduce the potential for oversizing a carb. For instance, the Qjet (which only came in 750 and 800 cfm sizes) was used on everything from the Buick V6 up to the Caddy 500, and was still emissions-compliant as late as 1989.

Having said that, I'm not a big fan of the Edelbrock 1405/1406. Their metering has basically three "stabs" - idle, primary, and secondary. The metering isn't very sophisticated. Holleys are a little more accurate in their metering across wide spectrums of loads and they are pretty simple/easy to use. Qjets are a bit more complex, but are by far the most accurately-metering carb on the street. If you can tune it right, it will offer massive WOT power, tiny primaries for throttle response and street torque, and better MPG than the others.

I go to great lengths to put Qjets on everything I build with a carb. I have a Holley on my boat, but that's basically because I had a Marine Holley sitting on the shelf.

By the way.... a 30-over 350 (355ci) spinning at 6000 rpms and 90% VE ingests 555 cfm.

600 is more than enough.

GM recommends a 750 cfm carb on the ZZ4- and the vortec heads flow better than the aluminum heads that they put on the ZZ4. i d say to go for the 750 to give you a little headroom.. and if it's a vacuum secondary carb (like a 3310), then it will only give the engine as much air as it wants once you get it dialed in.

all that being said, i'd run a stock rebuilt quadrajet from a late 70's pickup before i ran any Holley carb..