carb size for 550 hp small block

[streetk14]

What would you guys recommend for a street driven 427-434 small block in the 550-570 hp range? The engine is going into a '68 Camaro with a TKO 5 speed and 3.89 gears. American speed uses a 750 Dp on their 550 hp 427, Shafiroff recommended an 850 for their 550 hp 434. The 750 seems a better choice for the street, and it produced some impressive numbers. How much power (if any) do you think the 850 would be worth over the 750? Throttle response and driveability are important to me. I don't want to kill low speed throttle response, but I don't want to choke my new motor either. Any ideas?
-Andy

[rocketman]

i think a well tuned 850 would make 20-30 hp.a 750 on a 434 would be straving i think.response will be better wit a big carb the trick is the tuning.

[Johnny Blaze]

WHat rpm are you going to be turning?

If the car is mostly street driven as it sounds, I think you would be better off with the 750.

[ProdigyCustoms]

Size has nothing to do with streetability of a carb, that is a myth. The guys that believe a big carb will not run on the street, are the same guys that think the car will idle rich if the jets are too big! All a bigger carb means is it can give more air. It does not give more fuel unless you jet it to do so. Now can the carb be to big and give your motor to much air, or too much fuel with a big carb? Yes! But that is a wide open throttle problem, not a drivability problem. I have multiple cars running 950HP and 1000HP carbs, and they are some of the nicest carbs you ever drive.

FYI, Dominators are excluded from all the previous statements as they are built as race only carbs and do not have drivability features built into them. However, we have some of these running the streets also.

I would say a good 830 (750 with the top wacked off) to 870 would be good for your application. I would not run a non modified 750 on anything that big.

[Lowend]

You are getting near the limits of a 750DP, but it would still work pretty well.
Take a look at Pro-Systems carbs, if you want it done right those are the guys to deal with
http://www.pro-system.com/

[streetk14]

WHat rpm are you going to be turning?

If the car is mostly street driven as it sounds, I think you would be better off with the 750.


I'm expecting peak power in the 5800-6000 rpm range. I guess that means shifting at 6200 rpm or so at the track. There aren't any tracks close to where I live, so the car is mainly used for a little fun on the weekend. I haven't been to the track in a few years now.
As far as carb type, has anyone here used the Holley street avengers? They make a 770 and an 870 model. I know they say mechanical secondaries are better for manual trans cars, but these seem like a good street carb. I'd also like a good electric choke this time around. I have the choke removed on my current Edelbrock 750. The poor cold-start driveability is getting old, even here in California. It would be nice to be able to have a nice running car, even when cold. The 870 street avenger might be a good choice. Any suggestions?

[ProdigyCustoms]

Oh please don't ruin that nice motor with a vacum secondary carb. No Demon carbs either. You need a HP style carb for the amount of motor you are wanting. As far as cold starting, I am tweaking a 55 Chevy with a Bill Mitchell 427 as we speak, and it cold starts just fine. Baby sit it for 15 seconds, and she is good to go. Of coarse we are talking about a 50 degree cold start, not a 20 degree snow day.

Your going with someones crate engine, I highly recomend you let the builder worry about the carb.

[streetk14]

Oh please don't ruin that nice motor with a vacum secondary carb. No Demon carbs either. You need a HP style carb for the amount of motor you are wanting. As far as cold starting, I am tweaking a 55 Chevy with a Bill Mitchell 427 as we speak, and it cold starts just fine. Baby sit it for 15 seconds, and she is good to go. Of coarse we are talking about a 50 degree cold start, not a 20 degree snow day.

Your going with someones crate engine, I highly recomend you let the builder worry about the carb.


I plan to buy the new carb from the engine builder and have it tuned on the dyno. I am leaning towards American Speed for one of their 427's. They recommend a 750 dp (which is also what they use on the dyno). So that is probably the way I'll go. I've never run a choke before, and I probably don't need one. My current 360ci motor has a rather large (242/248) roller cam, so that's probably why it is grumpy when cold. It wasn't as bad when the engine used to have a much smaller GMPP Hot cam. I'm in southern California, so it never gets very cold. Thanks for the input.
-Andy

[ProdigyCustoms]

Curious, What is the price of the American Speed motor with carb, distibutor, intake, etc. Curious how it compares price wise to Mitchell?

[LowBuckX]

950HP with annular discharge boosters. learn to tune and it will get you what you need. Vizard had an article about this in a recent PHR

[streetk14]

Curious, What is the price of the American Speed motor with carb, distibutor, intake, etc. Curious how it compares price wise to Mitchell?

PM sent

[streetk14]

950HP with annular discharge boosters. learn to tune and it will get you what you need. Vizard had an article about this in a recent PHR

I have read that annular discharge boosters are great for street use with large cams. I'll have to check out that article.

[LowBuckX]

Booster Science
Understanding Boosters Can Enhance Top-End Output Without Impairing Bottom End.
By David Vizard


We have often said that there is no such thing as a carb that is too big, and we can throw in some examples to back this up. How about a 350 small-block Chevy for a working truck (big torque numbers from idle on up) with a Holley flowing 985 cfm? Or, how about a similar 350 for a Trans Am with a 1,020-cfm Holley? Yes, great power with good drivability has been accomplished successfully with large-cfm carburetors, and you can do it too.
We realize this statement appears to fly in the face of convention and what most carb manufactures preach. But, the reality of the situation is that to simplify life for the typical consumer, carb manufactures and, surprisingly, many magazines (in making that simple statement) don't tell the entire story. Why? Because they are in business to sell--or to help sell--a carb that functions right out of the box that doesn't teach their customer the intricacies of carb design. Given the opportunity to make a fuller statement, a carb manufacturer might say something more like, "Don't use a carb that is too big unless you know how to select or even scratch-design a booster that will still give an appropriate signal."

And to read the rest click here > http://popularhotrodding.com/tech/0511phr_carb