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Old 12-18-2019, 01:21 PM   #1
KyFarm
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carb sizing

I am about to pull the plug on a Chevy HT383.

Some literature from Chevy says to go with 770 cfm carb
Other lit from Chevy say to go with 670cfm

Called Holley and they said Chevy always oversizes their recommendations for crate motors and they don't know why.

Holley ran math and said a 570 is what is perfect for the HT383.

Any thoughts on what size to go with?

Motor will never see the top of the rpm band, 69 K10, just want to drive it around the country roads around my farm.
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Old 12-18-2019, 01:44 PM   #2
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Re: carb sizing

I'd go with what Holley recommends...a 600cfm should be fine
Although I've ran everything from a 390 to a 750 on a 350sb
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Old 12-18-2019, 02:28 PM   #3
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Re: carb sizing

Street Carb CFM : 470.99
Racing Carb CFM : 609.52
(using 5,000 RPM)
https://www.summitracing.com/newsand...cfm-calculator
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Old 12-18-2019, 04:55 PM   #4
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Re: carb sizing

I'd go with a Quadrajet. It will give you the best performance for anything from a little 283 to a great big 455, and probably beyond that.
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Old 12-19-2019, 02:26 PM   #5
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Re: carb sizing

I have a Holley 670 street avenger on my L31 crate engine with a comp cam and a super victor. Really runs well.
Jimmy
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Old 12-19-2019, 08:08 PM   #6
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Re: carb sizing

I have always ran 3310's (750 cfm) on my worked over street strip 350 engines. For funs I built an 1850 (600 cfm) with center hung bowls and a 4 corner idle circuit for my current street/strip 350 in my 1972 GMC. I gotta say the 600 cfm is a lot more fun on the street... When I hit the drag strip I'll take the 750 with me and switch if I need to.

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Old 12-19-2019, 08:16 PM   #7
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Re: carb sizing

Back when I used to read every hot rod magazine out there, they would do engine builds and calculate the cfm requirements. They would come up with those same low (theoretical) cfm numbers and then try a 600 carb for the first dyno runs. Then they would slap on the trusty Holly 750 and the dyno numbers would go up. I read the same story many times. In the end they seemed puzzled and theorized that "this particular engine just works better with the 750 carb". I'd say buy a 750 and forget it.

Then again we don't build engines just for the dyno. I still think a 600 carb is too small for a high performance 383. The 600 could be nice and peppy in every day driving and may be the hot ticket in a street truck. I guess it is a tough call.
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Old 12-20-2019, 08:08 PM   #8
Tom
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Re: carb sizing

The engine will use 600cfm. A 600cfm carb running 600cfm has to be creating some vaccum, which reduces power. A 750 is breathing easier at 600cfm hence the power increase.

OP- For a stock ht383, that won't be upper rpm, a 600cfm carb is plenty. So go with the 670 out of your two choices.
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Old 12-26-2019, 08:37 PM   #9
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Re: carb sizing

thanks for all the info
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Old 12-30-2019, 12:05 PM   #10
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Re: carb sizing

'Throttle shaft bushing repair kit' are the words you want to enter into Amazon if your throttle bores are worn out. The kit comes with bushings and a reverse cut reamer.

I wish I had this kit before overhauling my Quadrajet, although it does seem to work OK as is.
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Old 01-05-2020, 09:36 PM   #11
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Re: carb sizing

We had a real healthy 350 with a Street Avenger 670.....great carb, good street manners to.

I have a mild built 305 that was my daily for 16 years. 570 Avenger on that one.

My next build will be a VERY healthy vortec headed 350...it will get a Street Avenger as well.
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