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  #31 (permalink)  
Old 05-02-2012, 07:06 PM
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Originally Posted by lg1969
The Raming effect is only 2% to 5% increase. not 50% increase. Where a typical 350 flow 608 cfm to 1200cfm is pure bunck. If you have an article that will back you proof, Show it.
The ram effect increases with variables, scavenging of cylinders, size of valves, porting, anti reversion , design of plenum and runners, cam overlap, temperature, rpm, we used to pack our intakes with dry ice before a pass, used cool cans, By your explanation a 350 running a single 500 cfm carb by formulated max cfm at specific rpm say 5000 when going to dual 4 barrels you would run 2 250 4 barrels. ( just an example)

The 2 to 5% increase in ram effect does not directly correlate to cfm in a 2 to 5% increase. The vaporization of fuel in the air charge is a big factor the better the vaporization the better the cylinder efficiency the more cfm your cylinders can use and still burn efficiently as rpm increases! your limiting the flow and efficiency of the tunnel ram with small carbs. I dont see your argument in this.

We could pick articles all week off the internet I could post the "anvil articles" and you could post the "t bucket alliance" article or any others on the internet it would never end! Im not attacking you for your views. Im stating my views so take them or leave them!

Maybe Smokie or Grumpy could have used your advice and their cars performance would have been enhanced by your opinion on ram induction and cfm. Your running 2 390 carbs and your happy as honey on a roll, my wifes running 2 600's on a 302 and im running 2 750's on a 302, shes tickled and I want to go bigger with more rpm. I wouldn't be happy with 390's. It would bother me so much I wouldn't sleep at night!!! But there are degrees in performance wanted by hotrodders from the guy thats satisfied with: "it will leave the stop light with any other car and keep up", and a guy like me who stays awake at night wondering how to squeeze a little more out of my set up!!


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Old 05-02-2012, 08:13 PM
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4 barrel carbs are rated @1.5". 1.5" at wide open throttle is not the point that allows the most engine power.

A larger carb(s) that your oversimplified carb selection formula suggests allows less
air flow restrtiction and results in a lower presssure drop at WOT.
The engine works less hard to draw in the air to make the power.

say this engine with 1200 cfm of carb(s) shows a .75" manifold vacuum at WOT max rpm instead of your holy grail standard of 1.5"

now those 1200 cfm carb(s) are flowing 848cfm , not 1200 cfm.

Measured air flow is all relative to the pressure drop applyied.
(manifold vacuum)

Suddenly those too big carbs are not so big.

The fact that they are vac secondary adds a whole lot of flexability as
to wether they will over carb the motor at any rpm point. (bog)
Thats why the larger carbs can be used on this motor and will make more power..

Throttle response and driivability is a matter of the carb(s) being able to meter the fuel at the intended rpm range. its a matter of how active the venturii are
and the secondary opening rate control, not the cfm rating of the carb(s) @1.5".

The cfm rating standard of 1.5" is just a standard to compare the air flow of
1 carb to another. it is not eh absolute optimum standard for max WOT performance on a high performance motor.

The pair of 600cfm carbs will need to be correctly jetted etc to run well.
Then it will be "fun".

LG: You eninge may only be seeing a 2% ram effect and 80% VE but don't lay that on other peoples builds.

A N/A tuned induction and exhaust engine (even a street engine can exceed 100% VE They are not just simple air pumps like an air compressor.

Small carbs, incorrect cam timing, poor cylinder heads and small restrictive exhaust systems will restrict the potential of a tunnel ram induction system.

You can build a very street friendly tunnel rammed motor that will make very good power, with the right combo of supporting parts, (carbs, cam heads exhaust system) but the small 390 carbs will definatily put a limit on the power it can make.

When done right these manifolds make the most power of any V8 intake manifold.
The engine must be able to breath deep and exhaust freely to allow the induction ram effect to work. You want as big a carb(s) and exhaust system as possible.
The induction and exhaust systems must work together (induction and exhaust system tuning and cam profile.) The more air the cylinder heads can flow the better.
The 390's are basicly for the T bucket set.

Last edited by F-BIRD'88; 05-02-2012 at 08:39 PM.
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  #33 (permalink)  
Old 05-04-2012, 03:43 PM
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F-bird: your answer should be included with every tunnel ram sold, its technical and very understandable I cant remember everything like I used to and you filled in pretty much all the blanks!

Its funny as I age I forget the technical of why I do things! Ive been doing them for so long its just a natural thing to do! I tried to explain to a customer one time why I was stagger jetting his carbs with a larger jet on one side of his front metering block (Because he was told by another shop that no one does that) and I could explain because he had a right front cylinder running a little lean but it took forever and I still dont think he understood why with a plenum I had to do that ! That cfm formula is all over the internet and people keep referring to it in threads and articles and use it as the gospel on a lot of the forums to choose carburetors no matter how the engine is built! Like the progressive linkage on the t bucket article posted earlier and applying it to a tunnel ram? How many people read that and are running that set up Ive never seen progressive linkage on a tunnel ram. Or is that excepted practice in some circles ?

Any way thanks for explaining to gamecock 66 what I may have confused him with a lot better then I could have

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Old 05-04-2012, 05:24 PM
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When it comes to tunnel rams it's better toerror on the large side with carbs, not the small side. I've run tunnel rams on more than one car(all SBC engines) and I've tried various size carbs. My present car has a 327 bored .040" and I'm running a Holley Pro Dominator tunnel ram with twin 450 cfm carbs.
The Holley Pro Dominator is not as forgiving as those tunnel rams with longer runners and larger chambers. The short runners and chamber on the Holley make it tougher to get decent lowend performance. I just installed a Weiand tunnel ram on a 350 Chevy with twin 500 Edelbrocks and it took almost no fiddling out of the box to get them working. Idle was a little rich, so had to drop the metering rods to clean that up. I had rich idle on my setup also due to low vacuum, and I changed to 3.5 power valves and it cleared right up.
The Holleys will work better with larger nozzles and a larger 50cc pump. This will get more fuel into the large air mix and help get rid of the off idle stumble that most tunnel rammed cars have. Start with a set of numbered drill bits and see what size fits your nozzles, then bore them out a couple sizes larger. Drive the car and if it still stumbles you can open them up a little more until it clears up. Then go get nozzles for in that drill size, or just run what you modified.
I hope your car is running some low gears also! A heavy car with a SBC will probably work better with at least 3.90-4.10 gears. My little Austin is only about 2500 lbs. and I can run 3.73 gears and work well with my setup.
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Old 05-04-2012, 08:14 PM
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Thanks guys!!! I have thoroughly enjoyed reading these posts. I have definately learned a lot. Not all of it I completely understand but I get the idea. Tomorrow , hopefully I go and swap for all the parts. I am trading a Rockwell 50-017 drill press, BB cam, lifters , timing chain, 2 oil pumps and an edelbrock performer intake for a 351C-2v. Yes I had a Ford Mach 1 at one time... In return I am getting the tunnel ram+carbs and a 4blt 454 GEN IV block with ARP main studs and a fresh +30 over bore.
Tunnel ram will go on the 327 and the block will go on the stand!!! Stand by for more stupid questions on rebuilding, re jetting and over all setup of the carbs and tunnel ram!!!

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