I'm planning to run the original Carter 3921 with a 4035 on a stock dual quad intake. As these are the original carbs '65 & '66 for a 401. What modifications should be made? I've talked to a guy at a carb shop and he says there not recommended without "major" mods. Anyone know the cfm rating for the original dual set-up? Thanks Ian
No documented evidence, but was told 525 cfm each for original carbs. But. WOW. that is still 1050cfm! I am going to use 2-600cfm edelbrocks on my 425. Everyone tells me Nailheads love big carbs.
Ian, Carb size was 625cfm. Try searching earlier posts for more carb tips and answers. There is a lot of info to consider. Check this for example;http://www.v8buick.com/showthread.php?t=11390&highlight=tune+dual+carbs A good carb shop could probably help you more if you have one in your area. Good luck, John :TU:
I did a swap on a '66 Riv 425 with a Poston cam and we were running a pair of Rochesters off of my 455's, I believe they were 730 cfm and that nailhead loved them. :3gears:
What confuses me is they say a Nailhead can't breathe at higher RPM's.........but one will handle 2 Quadrajets without a problem.......that's almost 1500 cfm! That doesn't look like a breathing problem to me but I've never built up a motor either. :bglasses:
The nailhead breathes well enough, just not as well as wedge head designs. The Riv we built used the two 730 Rochesters on had a stock bottom end and heads with the next to largest Poston cam at the time, NH400 rings a bell. It was running a built switch pitch and 3.42 posi, He could nearly stay with my '69 lark. I never actually saw the tach but he claimed to shift at 6000 or so. Also keep in mind this was the '66 engine with the largest valves and ports.
69gs400 wrote: "Also keep in mind this was the '66 engine with the largest valves and ports." Glad to know this. So what exactly did Buick do different to the 66's?
I think the heads were all the same........the only difference on the '66 heads was the casting had provision for AIR injection tubes for the new Calif. emissions. Bigger valves in '66 heads from the factory is news to me......I know Carmen can do it special but it places the valves very close together. :bglasses:
Thanks Joe for busting my bubble :laugh: . I have a couple of 66 Riv heads laying around and hoped I had something a little better. :3gears:
I have no idea how to identify them other than the red paint used in '66. I would have to do some looking but I remember reading in some of my Buick literature that the '66 had the best flow of all the nailheads, They may have been reffering to all nailheads including the 401 so maybe they meant that the 425 heads in general had the best flow. I will see if I can find the article. Regardless, a cammed 425 will take a LOT of fuel and air.
I wonder if it was that the Q-Jet flowed better thatn the AFB's???? That is the only change that I am aware of unless the new 66 body style had a different exhaust configuration that also helped. Larry
Makes sense to me. I do know the Q-Jet intake flows better than the AFB. The article probably was written because of the carb. difference.
1st.off, the original dual AFB's were rated at 625 CFM's each. So that's 1250 CFM's total capacity. Do they actually flow that much CFM's, is anybody's guess. Has anyone ever done any testing on them to prove this??? It seems that all Buick's, 455's, NailHeads, & even the Straight Eight's all loved more CFM's. It seemed the more we added to that ole staright eight the better it ran. Same with the NailHeads & the 400-455's. What I do know is that after trying different set-ups on my friends NailHead the engine could only flow so much CFM's & that was it. Example's are as follows. All this is on an engine dyno, unlike a chassis dyno where there can & usually are many varibles. On an engine dyno once you have established a base, most of the time readings are repeatable within a few HP & Torque. With my stock AFB manifold, only cleaned up with an adapter plate that has been on it for 30+ years & port matched to the manifold & 750 CFM double pumper Holley carb. would flow 580-590 CFM's. This was the best combination for the broadest overall torque & HP with very little difference at where the full advance on the timing was set, between 28-38*. This timing goes against all previous thoughts about timing & a NailHead. It was able to maintain a flat power curve past 6800 RPM's in all tests. But this is just with this set-up. With a Q-jet manifold & an 850 Q-jet carb. would flow 590-605 CFM's. With the dual Quad Edelbrock intake with two 750 CFM Edelbrocks would flow 610-620 CFM's. This was also the best performing as far as HP & Torque were concerned. The next was with a home made tunnel ram set-up with two Holley 600 CFM vacumm secondary carbs. flowed about 630-640 CFM's. Even though it flowed more air it was not able to utilize the extra air flow & HP & Torque numbers were down a significant amount as thought they would be. But the air flow was up. All the carbs. were properly set-up & the ignition timing was left the same for comparision purposes. So why is that??? The NailHead can only flow so much air, but the more CFM's you give it the better the numbers, why is anybody's guess. More would have to be experimented with cams & such which can lead into lots of money. Why would Buick use two 625's instead of 600's or even 500's. Obviously more power was to be made with more CFM's than less. Who are we to second guess Buick's engineering!!! Seems they got it pretty damn close to "RIGHT ON" to me. One thing I can add is that any time MY prototype set of Roller Tip Rockers were added in any of the tests over the stock rockers they were always good for an additional 20 HP. The head & valve sizes remained basically the same from '57-'66 with '66 having vlave stem seals installed on the intakes only & having provisions for AIR for CA. It has been said that the '66 exhaust ports flow slightly better than the eailier heads, but you can't prove that by me on the flow bench. Tom T.
Great information Tom. How are the Roller Rockers? I keep bugging TA since I heard they were involved. I would like to do a nailhead Roller cam and roller rocker engine when they are available. Any problems hooking up the Nailheads to an engine dyno? Ted
No problem at all Ted. As we all know, to find someone to work on our beloved NailHeads is sometimes a seemingly impossible feat. Many engine dyno facilities were called. Most all said the same, "can't do a NailHead, only do fxxfs, cezzxx, etc. The one guy found was interested & always thought about it but never had the opportunity of one to do. So now I know someone who will & is willing. In fact it was no more of a problem than any other engine, in fact in some ways was way eaisier. He's in upstate N.Y. & is a very knowledgable A/Stock record holder for some years now. His pricing was, as far as I'm concerned, very reasonable in comparision to others in my area. On average they were charging $600.00+ a day & they only operate the numbers & don't get involved in the mechanics. This guy was great & helped & offered suggestions & cam profiles, etc. For anyone interested you can PM me & I will forward the details. PM me for more rocker info also. Tom Telesco
