SBC tunnel ram carb change , READ... MORE CFM

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SBC tunnel ram carb change , READ... MORE CFM

Post by joespanova » Sat Sep 09, 2017 10:39 pm

Currently running 2 4224's ....never a problem. But , these carbs still have the 660 base plates and except for the secondary metering block conversion and the choke horns milled off , are box stock. The intake is the Victor Ram.
So , I call Patrick James and he's adamant that the 660's are "dated" and in 2017 a pair of 750s set up for my application will perform better.
I'm not convinced yet as I've seen some stout 434s make big power with a single Dominator. So why would my 377 want more CFM?
Any changes to improve performance DO seem to have hit a "wall" though. Larry Meaux seems to think I have a restriction in the induction somewhere...............and since the only things never changed are the 660s I'm beginning to wonder.
The cars runs high 9.30s @ 141ish. 2975 lbs.
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by tjs44 » Sat Sep 09, 2017 11:21 pm

Talk to Big Joe!Tom

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by F-BIRD'88 » Sun Sep 10, 2017 3:26 am

stout 434s make big power with a single Dominator. So why would my 377 want more CFM?

It does not work like that. and this kind of false rational will limit you.
Not enough information to say what the real limiting factor is.

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by naukkis79 » Sun Sep 10, 2017 4:08 am

Tunnel ram has long tuned intake paths. Air will not flow whole time, signals in air will go at speed of sound and with longer intake tract air flow will be less time, higher velocity than with shorter intake path. It's pretty easy calculate. So the longer intake tract you have the more cfm your carb has to flow or it will be counterproductive to what you want - it you are carb cfm limited you probably want as short intake path as possible.

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by mag2555 » Sun Sep 10, 2017 6:18 am

Let the motor tell you what it wants!
Hook up a vacuum gauge for a pass and if at the rpm of peak torque your reading 1/2" or greater, then yes you could use more Carb cfm!

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by F-BIRD'88 » Sun Sep 10, 2017 6:52 am

The peak torque rpm is not the point where the engine
puts the most air draw demand on the carbs.

It will be at or higher up than the peak horsepower rpm.
Mostly higher up.
This is only one indicator as to if largers carbs are
going to make the car faster.....quicker...

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by joespanova » Sun Sep 10, 2017 7:34 am

Interesting points , please continue.................. :mrgreen:
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by joespanova » Sun Sep 10, 2017 7:35 am

F-BIRD'88 wrote:stout 434s make big power with a single Dominator. So why would my 377 want more CFM?

It does not work like that. and this kind of false rational will limit you.
Not enough information to say what the real limiting factor is.
So , what do you need to know?
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by steve316 » Sun Sep 10, 2017 9:26 am

Adding cfm doesn't make the car faster. #-o

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by steve316 » Sun Sep 10, 2017 9:30 am

I should have posted that I have added larger carbs on cars that made more hp on the dyno with them, but was slower at the track. :(

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by woody b » Sun Sep 10, 2017 10:49 am

To say 660's are dated is true, but carbs are dated. (I'm not recommending efi, just stating the obvious) Could you borrow a pair of 750's from someone to try? I know, they wouldn't be the magical "set up for your application" carbs, but it might give you an idea before spending a couple grand.
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by Coloradoracer » Sun Sep 10, 2017 11:13 am

You'd be surprised how big of a carb or carbs an engine might want to perform best. I'll agree, dyno numbers will differ from track numbers, but the fact remains that many engines out there are under carbed for what they are. When you can measure vacuum under the carb, it's power lost on the table. Less than 1/2" under full load is best, and the only way you can check that is with a data logger while going down track. There is a common misconception that a bigger carb will slow a car down. Not always the case. For something that's street driven, smaller is usually better, but only to a point. For race applications, run the biggest carb or carbs that you can without losing et or mph. I know some will disagree with this, but pumping loss is pumping loss. Tunnel rams will create different requirements that a single or dual plane intake will as well. I know of several small blocks out there under 400" running dual dominators on tunnel rams....and they aren't rpm monsters either. As to what's best for your combo, you've got nothing to loose by trying a set of 750's. Depending on rpm and the setup of your engine, I'd even venture to say a pair of 1050's would work well too, provided they are properly calibrated.....
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by Old School » Sun Sep 10, 2017 12:04 pm

Thirty some years ago I replaced a very good pair of 4224's with two 4779 750's. I picked up a solid tenth with a glide on a 3400 car. This was on a 468 shifting at 7000. Your air requirements at 8500 on a 377 is very close to what mine was at 7000 with the larger engine plus the stick shift should keep you in a smaller rpm range. Much less shift recovery compared to a glide.

How much did you gain by increasing the bore and making the engine a 377. From what I can gather by re-reading the "increasing the bore" thread maybe 5 or 6 hundredths? Which of the thee cams do you have in the engine now?

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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by joespanova » Sun Sep 10, 2017 3:05 pm

Thanks guys.
The bore increase as well as compression netted about a tenth , but I haven't run in the best conditions yet either.
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Re: SBC tunnel ram carb change , READ... MORE CFM

Post by DaveMcLain » Sun Sep 10, 2017 4:49 pm

You have to remember too that with a dual quad tunnel ram setup you are sort of headed toward an IR configuration with one carburetor venturi over each intake runner. In any intake with a plenum the carburetor's air flow is shared between each cylinder but the closer you get to IR the less that tends to happen and the larger the carburetor can/has to be to produce the best performance.

I think something like a 750 with a down leg stepped booster and maybe even a 1 3/4 inch throttle would probably help performance on the track vs the 660's but you never really know until you try. In my experience it takes a pretty big change in carburetor air flow to make a significant difference in performance but if the 750's produce a better fuel curve it might be a win win situation.

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