Holly Truck Avenger: 470 vs 670

[Eoth]

Just putting my experiences out there for others that may be scratching their heads... When I bought my EB it had a 600CFM Holly that ran like crap. After taking it off road, I decided that instead of sorting out the existing carb, I would just buy a "Truck Avenger" and spend the time and effort getting it dialed-in. According to most everyone (Holly Technical Support included), because I was running a stock 302 I needed the "670CFM". I put it on and it ran even worse. I spend countless hours consulting people and trying different setups and combinations... Things improved but never to where it should be.. My biggest symptoms was hesitation (need to "feather" the gas) and hard engagement of the secondaries. Finally, I called it a duck and decided that the 670 was just too large and purchased a 470. Right out of the box everything was ideal. No hesitation and exceleration is nice and smooth. I belive that the culprit is the 1965 289 Hipo 4v intake manifold that I am using. I know that there are things that I can do to facilitate the 670 and would also increase HP and performance, but most of them result in using more fuel (I am not racing anyone and I dont need high HP for they type of off road that I do)....
When doing my research I found a lot of people using one size or another but I couldnt find anyone that had actually tried both and published it.
So here it is....

 

[DJs74]

Thanks for the information Eoth!

I'm with ya on the whole experimenting of carburetors and at $400 to $600 a pop, it is some expensive learning.

I did the same thing on my EB build - started off with the 670 CFM on my 302 bored 0.030" over, 9.5:1 CR, Comp XE268 camshaft, stock heads with minimal porting and an entire MSD system for fire. It was OK but not the happiest so I reverted back to the 570 CFM.

Not sure if you've ever run the numbers on CFM requirements, but I was amazed of what is actually required vs. what we try. I think my 302 / RPM needs was in the high 300s to low 400s of necessary CFM. If you go to Summit Racing's website and navigate to the carburetor section, most of the Holley carburetor pages have the "CFM calculator tool at the bottom. Simply input your cubic inches and Max RPMs and it will recommend what CFM you are shooting for. Of course, it is not an end all to what you end up with but more of a "for reference tool"

I did the same learning procedure last year on a BB Ford 466 cu. in. - purchased and compared three different carburetors: 750 CFM, 770 CFM and 850 CFM, the winner in that case was the 850 CFM.

Another consideration is mechanical secondaries vs. vacuum secondaries. A light vehicle responds well to mechanical and usually heavy vehicles act better with vacuum secondaries - in my experiences anyway.


thanks for the info

DJs74

 

[PaveBronco]

"According to most everyone (Holly Technical Support included)"
Not sure what these guys were smoking, or maybe they thought you meant drag racing..

 

[smokinjoe]

It's amazing how often most people over-carb. Like you said, 302s don't need or even like a lot of CFMs.

 

[Jdgephar]

It's amazing how often most people over-carb. Like you said, 302s don't need or even like a lot of CFMs.

My stockish 302 runs very good with the 480 CFM Autolite 4bbl. I've never tried a larger carb to compare, and I have no reason to try right now.

 

[73azbronco]

Sounded like you needed to increase the rate on the secondary's to get them to come on later using the 670. And yes, a stock-ish 302 will work "better" out of the box with a 470, but a dialed in 670 will work better with a bigger cam, more torq motor.

 

[bmc69]

"According to most everyone (Holly Technical Support included)"
Not sure what these guys were smoking, or maybe they thought you meant drag racing..

No, there was a stretch of time when Holley actually went on record that they recommended against using the 470TA on V-8 engines, recommending them only for 6 cylinder applications. And there were plenty of "couldn't get the 470 to work on my 302 and finally gave up and put a 670 on it" stories around too.

As both an engineer and a long-time engine and carb builder, the whole issue drove me freaking nuts; how could a carburetor possibly "know" that it was sitting atop a 300 6-cylinder engine versus a 302 8-cylinder engine?? Never did get to the bottom of the "myth versus fact" part of that...

Glad to hear the OPs story though. ;D

 

[Eoth]

Well to be honest, it was kind of fun feeling the secondaries kick in... It felt like was downshifting... I'm sure that I could have taken care of that part but the larger problem was the hesitation.
I wont profess to be a a carb expert (not even close), I just wanted to share my experience and results. I did a lot of "lurking" and research (amazing how much conflicting "data" is out there) and I could not find a single person that posted results when they went backwards from a 670 to a 470 (saw a few the other direction).
Just my two cents....

 

[smokinjoe]

My stockish 302 runs very good with the 480 CFM Autolite 4bbl. I've never tried a larger carb to compare, and I have no reason to try right now.

Exactly, my midly built 302 in my Stang runs a 550 or 600 Edlebrock (I can never remember which) and I've even considered running larger rods in it.

 

[thegreatjustino]

Funny that Holley Technical Support told you to use the 670. If you use the "what carb should I use" tool on Holley's website, it recommends the 470 for every combination of 302 or 351 I've ever run in my Broncos.

 

[Eoth]

Funny that Holley Technical Support told you to use the 670. If you use the "what carb should I use" tool on Holley's website, it recommends the 470 for every combination of 302 or 351 I've ever run in my Broncos.

EXACTLY.... Then they go ahead and print this..
The 470 CFM Off–Road Truck Avenger is the Perfect Carburetor for 4 Cylinder, V–6, and Inline 6 Cylinder.

The 670 CFM Off–Road Truck Avenger is the Perfect Carburetor for Small Blocks or stock Big Block engines.

As already mentioned by bmc69: how does the carb know or care about how many cylinders?

I'm not here to trash Weber and I am sure that more people are using the 670 than the 470. Heck, they even took it back and rebuilt it and added some "voodoo update to it. To their defense how many stock 1969 302 running 1965 289 manifolds are actually on the road?

 

[FlogginHarvey]

I have a TA470 now swapped from an Edelbrock 600 and it starts easier and runs well right out of the box. I want to get it to a dyno eventually to check the numbers and jets but its been great!

I was thinking the 670 but after doing tons of thread searches here it seemed like a no-brainier on the the 470 and my stock 302...glad I did it so far.

 

[broncnaz]

If you look at the chart in holleys catalog they list the 470 suitable for a 350ci engine running no more than 4500 RPM. They also list the 470 good on 300Ci engines up to 5500 RPM.
Most smaller CFM carbs are easier to tune for the lower RPM's and power levels we tend to run at.
I've run mostly 600cfm holleys and they run pretty good. But I have found I like the 525cfm barry grant road demon jr even better. (holley knockoff) It has better lowend throttle response than the 600's and gets slightly better mileage. Of course on the top end it can stuggle although admittatly I really have tuned the carb for topend.

Overall any of the carbs will work when tuned properly. Holley has had lots of issues with there TA carbs. So I think they mostly cover there a$$ anymore and just suggest 670's.

 

[matts460]

The vacuum secondarys will only ever open as much as needed when floored. That's the point. Also the carb calculator is more like a "this is the smallest carb that would work well for this motor" scenario. 670-600 is not really all that over carbed. Peak Rpms are also key too. The 670 in question probly just needed the heavy spring installed in secondary diafram. The inline 6 is two less mouths to feed too so 400cfm would be better suited.. unless it spun up to 8grand or more. Just lots of senarios.