Carb recommendations for SBC

OK Gurus, I need a carb choice for a 500+ hp SBC 350, it is planned for stage rally use.

All opinions welcome

EFI FTW. Carbs suck.

actually they mix air and fuel

750 CFM Speed Demon?

Quadruple Webers

Stage rally means a lot of bouncing around and accelerating from fairly low RPM, so I lean toward a Holley vacuum seconday 4 bbl with float bowl stuffers. IMHO, mechanical secondaries on a Holley are only good for drag racing. Opening all 4 venturis at once at low RPM/low vacuum/high torque need situation will result in a big ol' bog.

The stuffers will keep fuel foaming to a minimum.

As far as CFM goes, here's a chart that will help:

http://www.holley.com/data/TechService/Technical/Selecting%20A%20Carburetor.pdf

A 350 at 7000 RPM and 90% VE uses 638 CFM. At 6500 RPM which is a bit more realistic, it uses 592 CFM.

Traditional ham-fisted hotrodders absolutely insist that 750 cfm is required, but it will only give you a few extra HP and the expense of part throttle manners and torque in the mid RPMs.

The Qjet is the single most accurately metering carb ever; aftermarket or factory. They have tiny primaries and huge secondaries which means super snappy throttle response, fuel efficiency, and lots of yummy turbulence. In fact a large number of truck TBI users have documented that retrofitting a Qjet gave them better mileage and about 10 more HP.

The Qjet is also a true vacuum secondary air door operation, which means it only flows as much as the engine needs. The same basic Qjet carb has been installed on GM vehicles since the 60s in displacements from 3.8 V6 all the way up to the 8.2L Caddy 500, including being used in several of them as late as 1990 while still passing emissions. Versatile is an understatement.

There are two versions; a 750cfm and an 800cfm. The difference is with the primary bore size. Either one is a great carb, but starting with the 800 means you have a little more versatility if you ever need more than 750 cfms. Look to GM trucks and vans 76- later, but before they had the little electric plug on the air horn. Those are best left unmodified in their stock application. The 76-later carbs on trucks are almost always 800 cfm, regardless of engine. Junkyard $20, $30 rebuild kit, then find some jets/rods at a carb shop for $1 to fine tune the fuel trim.

The Qjet should be fine at Rally. The strong deflection might be cause for a little modification to prevent the fuel from sloshing away from the jets, but its easy to do with a drill bit and Doug Roe's Qjet book.

My second choice would be a vac secondary Holley spreadbore in 670 or 750 cfm.

Third choice would be a Holley squarebore.

Fourth choice would be a Holley double pumper

Last choice would be anything else. Edelbrock makes a fine carb for a cruiser, but they aren't that accurate. They kinda take stabs at metering and get close enough for most eye-candy cars.

As far as CFM is concerned, that's a tough one. Carb companies over and under-rate their carbs at will. Holleys are pretty accurate, but BG carbs are notoriously underrated so they can brag about the extra 3 hp. When you're selecting a carb, you want to shoot for a certain vacuum reading at WOT/redline. For street cars, I shoot for about 1.5" Hg. For street/strip, I look for around 1" Hg. For race only I shoot for less than .5" Hg. I would probably shoot for the street/strip area since you will be spending a lot of time going between full and part throttle. Race-only (like drag racing or roundy-round) is mostly a full-throttle thing.

Using the following equation: CFM = (CID x RPM x VE) / 3456. That will be a very accurate estimate for the street end of things. I've used that equation for all of my street cars and have almost always been right between 1" and 1.5" Hg at WOT/redline.

One of the nice things about the Qjet is that the secondary air doors operate based on that vacuum differential. They will always provide the same WOT manifold vacuum regardless of the engine configuration. You just put on a Qjet with a potential for 800 cfms and it will adjust the flow based on that vacuum.

What ever carb you choise send it to Dayton Umholtz in NY (kinetic proformance) tell him your combo of pistons bump stick etc he'll set it up right bolt on and go.

I know several happy customers. Sadly i've not used him my self but have installed his work and it was 100% both times.

44

I have no idea of the VE but the engine was built to make 500hp and 500ft lbs, will the quadrajet cope with this.

I take it all Q jets after 76 have serial numbers start with a 170xxxxx

At what RPM?

The Cad motor with a single Q-jet made 500+ lb/ft but not a pantload of HP because it was all low-end torque.

500hp out of a SBC will mean high RPM which = a lot of air.

You should probably get some figures like cam profile and compression ratio for this motor. Then talk to someone like Cliff Ruggles (or send him the carb) for some tuning advice.

I should think 800cfm would be enough. My Turbo Pontiac motor has an 800cfm Q-jet and doesn't use all 800 cfm.

Shawn

I have the Qjet that came with the car that was rebuilt and the 750cfm Barry Grant Speed Demon if you want to talk about it. I would have stuck with the Qjet, but the shop guy said while it would work I'd have to pre-plan my spares purchases, as most shops don't carry the parts. Most speed shops carry Holley stuff on the shelf and most of that is cross compatible while the Barry Grant stuff. I also think that the manifold is predrilled for the Holley/Barry Grant type carb.

Jason

P.S. And the engine should be a 408 (400 bore .060" over)

Ok, is it a 400 or a 350 then?

It will make a difference, you'll want more radiator for sure if its a 400 block.

The 400 has siamesed bores, the 350 doesn't. 400s tend to run hotter because of this.

I've found that Holley carbs only like WOT, q-jets usually have better part-throttle repsonse than an equivalent Holley. I've never used a Demon but hopefully they would have addressed that issue.

Shawn

Quadrajets need the throttle shaft bushings replaced when you rebuild them, which adds a bit to the cost, unless you have a drill press and are able to do it yourself. Seriously, EFI would be better though. I have a 70 chevy truck and if I dont sell it it is getting converted to EFI, and I dont have any plan of owning anything else with carbs.

The main reason I didn't go EFI was that it would take at least $3k to do it for not necessarily any more power. If the Qjet is as good as said then one could just get one of those exhaust gas monitors(?) and do just as well...

ECU
Injectors
Special fuel pump
Extra fuel lines
Computer to tune it with
Special intake manifold
Plus More

That said if you have time and money it might create some extra benefits...

Jason

Wow... love you all, but some of the misinformation here is pretty interesting.

Yes, a Qjet will be enough. More than enough. If you don't want a Qjet (which will size itself) then don't go with an overkill size! 800 is ridiculous. The equation I posted is an industry standard. 800 will kill some low end torque, destroy throttle response, and make tuning it a pain in the arse.

Seriously. This is my business, its my job. I'm a hot rodder. I build American V8s all day. I have a Qjet on my caddy 500, a Qjet on my 4.3 V6, a Holley 600 on my boat, an Edelbrock 600 on a Pontiac 389, and a Carter on a pontiac 421. Going too large is far worse than going too small. If you're trying to chase down the last 3 hp and don't care about part throttle and other manners, then put a 1200 cfm carb on it and good luck.

I would personally put a Qjet on it that is modified as if it were an off-road carb. My second choice would be a Holley 670 spreadbore vacuum secondary..

Period.

I stand by it.

The Qjet is sounding better as the drivability at part throttle is very important for rally use. EFI may be more flexible at part throttle but won't make more power, that money would be better spend in the cage and suspension.

Steve

will kill some low end torque

If he's building a 500 hp 350, what low end torque?

OK the engine is a 400 bored to 408, rpm range is 7000 although cam makes power from 2500 to 6500, it was built by a engine builder to make 500+hp and 500ft lbs in a fairly constant torque curve. I am still learning about it sorry for any confusion.

Jason knows it better than me at this time.

Note I taking most note from those who do include reasons that refer to my intended purpose like you Curtis73 (message sent)

44Dwarf wrote: What ever carb you choise send it to Dayton Umholtz in NY (kinetic proformance) tell him your combo of pistons bump stick etc he'll set it up right bolt on and go. I know several happy customers. Sadly i've not used him my self but have installed his work and it was 100% both times. 44

Where in NY is this? It sounds like a custom Carb shop I visited around Morrisville when I was in the Auto Club and in the Auto Tech program at SUNY Morrisville.

I would go Holley vac secondary mostly because there's a ton of tuning stuff available in just about any town in the US. A Q-jet, not so much from the tuning angle but plenty of repair stuff.

I agree 100% with oversizing a carb being a bad idea. If anything, go a little under if you can't hit the perfect CFM rating. Remember, CFM is rated at WOT under ideal conditions and it's very rare you'll run a carb that way. As noted earlier, you'll spend most of your time at part throttle or under acceleration and that's where sticking your foot in an oversized carb will bring on the bogs, particularly with a mechanical secondary carb. Driveability is more important than WOT operation.

Trans_Maro wrote: You should probably get some figures like cam profile and compression ratio for this motor. Then talk to someone like Cliff Ruggles (or send him the carb) for some tuning advice.

Cliff wrote the book (literally) on the Quadrajet. He'd be a good choice for dialing the carb in. http://www.amazon.com/Rebuild-Modify-Rochester-Quadrajet-Carburetors/dp/1932494189

Travis_K wrote: Quadrajets need the throttle shaft bushings replaced when you rebuild them, which adds a bit to the cost, unless you have a drill press and are able to do it yourself.

They only need to be bushed if there is excessive play. Not all Qjets need this. In fact, Jet Performance (Qjet gods) says that they only need to do about 1 in 3. They also sell a $60 kit that comes with the reamer and bushings, so it can be done carefully at home, or in a neighbor's drill press.

OP... you had mentioned not knowing the VE. We can guess if you have all the specs. I can even run it through a dyno sim and get darn close.

An adendum I'd like to make. The only time I don't like using a Qjet is when the intake is not a spreadbore intake. Adapting a Qjet to a squarebore intake isn't terrible, but it removes most of the benefits. For instance, my marine engine was shipped with a squarebore aluminum intake with the bronze plating in the water passages. In order for me to run a Qjet, I would have had to buy a spreadbore marine intake with those plated passages; a $300 thing. Instead I just went with a vac sec Holley and I've been very happy. Throttle response and fuel efficiency aren't big issues in a boat.

However... A Qjet is a wonderful carb. Let's go nuts on our estimate. Let's say its a 408 that spins to 7000 rpm and has 95% VE. That's not going to happen in your case, but even then it will only require 770 cfm. A Qjet is more than adequate.

That old equation is great. The old hotrodders rule is to take RPM x CID/3456 and then choose the carb size one lower. The more modern equation includes the VE. In that case, you would want to choose the carb that is as close to that number as you can get without being too far under.

But... that brings up another very important point. People often try to compare their large carb choice by comparing the old-school dual-quad setups. The reason for dual quads back in the day was because the largest 4-barrel carb you could realistically get was 390 cfm. For anyone with a serious engine, you HAD to double up on carbs.

Compare these tested numbers from the dyno at our old shop. Engine was a Vortec chevy 355 (30-over). 9.5:1, a custom cam that spec'd out to 220/228 with about .500" lift on a 110 LSA. We did nothing but swap out carbs for each run.

Qjet 800 = 379hp@5000, 442tq@3500 Qjet 750 = same within 1-2 hp Holley 600 = 374hp@5000, 448tq@3500

Of course that's not apples to apples, but it does show how carb sizing is not as important as it might seem. For that matter, you could put a 2bbl carb on that engine and still make better than 350hp. The idea of putting on a carb that is too big is not only counterproductive, but it doesn't make that much more power. Not that you would notice in a rally race anyway. People always want to maximize their power, but they don't realize that in doing so they can actually slow the vehicle down and add tuning headaches and rabidly consume more fuel.

Most Q-jets I have seen needed new bushings. Actually the only one t hat didnt was one I ran on my truck for a while that came off of a boat. BTW, as far as I know the 800 cfm ones came stock on the later 350s in trucks.

Don't forget; the 454 TBI has a Q-jet baseplate.

If you ever decide to go with a Megasquirt later it's an easy swap using your existing manifold.

Shawn

neon4891 wrote: Where in NY is this? It sounds like a custom Carb shop I visited around Morrisville when I was in the Auto Club and in the Auto Tech program at SUNY Morrisville.

KINETIC PERFORMANCE 373 N. Van Dorn Rd. Ithaca, NY 14850 Phone: 607-277-6462