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Old 05-01-2014, 04:58 AM
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Carburetor Selection

Hi there,

I have a question concerning carburetor selection that hopefully some of you folks can answer for me. I race a 75 Camaro at the local dirt track. I used to run it on a 1/4 mile asphalt track, but funds dried up and they shut it down. The rules at the asphalt track required a spec Holley 4412 2 barrel carb. The track I run at now allows any 4 barrel, vacuum secondary, 650 cfm max. No double pumpers or dual feed carbs.

I've been running my 2 barrel out on the dirt and it seems to do ok. The local track is a 1/4 mile while the track I sometimes travel to about 150 miles away is a high banked 3/8 mile track. My motor really doesn't seem to be lacking in power compared to all of the guys who think I'm crazy for running my 2 barrel. I'm not running anything fancy. Rules limit us to mostly stock equipment. Our head rules require stock heads with a max 1.94" intake valve (no vortecs either). So I'm running 882 heads that have been mildly ported. Stock cast iron intake. Stock exhaust manifolds. .040 over flat tops. Isky hydraulic cam specs are: .465 lift on intake and exhaust, 270 advertised duration, 221 @ .050 lift, 108 degrees lobe separation.

Do you think it would be worthwhile to drop the money on a four barrel or keep my 2 barrel? I really like my current carb and I've never once had a problem with it. I'd hate to spend $350-$400 on a new carb only to have issues with it.

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Old 05-02-2014, 05:04 PM
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Anyone have anything for me?
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Old 05-02-2014, 08:38 PM
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You answered your own question!
Don't fix it if it ain;t broke!
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Old 05-03-2014, 05:42 AM
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When I was racing dirt, the car was full throttle 98% of the time during the race. If you are getting enough fuel with the 2 barrel, no need to get a 4 barrel. The thing that matters is you are getting enough fuel in the engine, does not matter if it gets there through 2 jets or 4. On top of that, you are going to spend much more than $350-$400 changing carbs, unless you buy used or have stuff laying around.

Kelly
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Old 05-03-2014, 07:36 AM
How fast is fast enough?
 
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A carb would help but so would changing the heads or cam, if you can find a deal on any of that I'd go for it. I'm surprised you can keep up if you're giving up that much. All combined you're probably leaving at least. 50hp on the table.
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Old 05-03-2014, 08:19 AM
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That 4412 Holley, is, if not mistaken, a 500cfm carb. So if you have a 350 engine +.040, you are about at 355cid +/- a couple ci. Using this formula,

CID x RPM x V.E. / 3456 = CFM

355 x 6500 x .90 = ~600 cfm

I used 90% VE (Volumetric Efficiency) which is probably high with that engine combo, but it at least gives you an idea of where you should be for carb. With that said, your 500 cfm is ~15 - 18% smaller then it could be for best performance.
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Old 05-03-2014, 08:39 AM
How fast is fast enough?
 
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"Best performance" would depend on fuel delivery and restriction. If yor fuel delivery quality doesn't suffer the best performing carb is always the biggest one. Bugger than 600 could still show gains.

That being said a better optimized cam may be with more than a new bigger carb and cost half as much.
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Old 05-03-2014, 09:54 AM
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If you go to holley's store on ebay you can get a holley 1850 600 cfm carb for just a little over $200 and they are like new. I got one of these 5 years ago and it was basically a new carb and has been fine ever since.

Holley 0 1850C 600CFM Factory Refurbished 4BBL Carb Manual Choke | eBay
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Old 05-03-2014, 05:49 PM
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Thanks for the input.

I don't think I want to go any bigger on the cam because I would be giving up too much on the bottom end. Some of the guys I run with have cams up in the low .500s and they're real boggy coming off the corners and they're even worse on restarts. We can't run any kind of roller valvetrain equipment so even roller tip stamped rockers are out. I also should've mentioned we're not allowed to have screw in rocker studs, so I'm at the max lift I feel comfortable running with press in studs.

I think .9 VE is pretty high considering the fact that I have to run stock heads, stock intake, and stock 2" exhaust manifolds. With my rpm hitting 6200 before I lift, the math puts me at 509 if I use a VE of .8 (which I think is closer to reality). All this said, I don't believe formulas are the "end all, be all". I was mostly curious if you folks had any first hand experience of something like this.
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Old 05-03-2014, 07:51 PM
How fast is fast enough?
 
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I didn't say you'd need to go larger on the cam, just that te one you have isn't really meant for your application. You'd be surprised what 4 degrees here and there and better lobes can do.
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Old 05-03-2014, 08:28 PM
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I suppose I did jump to conclusions there........

I've already got my engine built for the year and I'm not gonna do a cam swap on it this year, but I'm open to suggestions as far as cam selection goes for in the future. I feel like I did quite a bit of research before choosing it, but I very well could've picked a less than ideal cam. I would love to hear what you think would be a better fit for what I have.
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Old 05-03-2014, 08:36 PM
How fast is fast enough?
 
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If you really want the perfect cam, pick the rest of what you'll be running and then give mike jones a call. A different carb will want a different cam, same fr heads, so make sure you have what you want when picking the perfect cam for your combo.

There is a lot of power in the heads as well, so don't over look that, even the valve job can be worth 20hp.

For a quick fix the rebuild carb idea was a good one, a mechanical secondary would be most appropriate for all out racing, a vacuum carb can work though.

Just think ahead when it comes to spending your engine budget- spend where it counts.
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Old 05-04-2014, 06:53 AM
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Ok.first a few questions. Can you run any head besides vortecs? Is there a compression limit? What octane do you run? What numbers do you see on a compression test? Are you allowed to port your heads? Is there a lift limit? With what you have now I think that little 1850 holley eric32 mentioned should work great. Oh.are you allowed to pin your rocker studs ? If so you could use a little more lift safely. Maybe a little more duration 4-8 degrees on the exhaust to make up for the cast manifolds. And while youre switching intakes you could fill the exhaust crossover.Didnt mean to bother you with all the questions, but a future build down the road could include a few different pieces. Best of luck.
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Old 05-04-2014, 12:35 PM
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I will say the 4412 holley is very popular in the roundy-round curcuit, will you benefit going to a small 4 brl., hard to say, What i would do is send your carb out to "competition carbs" out of Nv.. He tricks out those 4412's for all the dirt track racers. I'd call him & i'll bet he says you'll be better off with a modified 4412 over a small 600vs.

Heres his link.

Competition Carburetion
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