2x4s vacum or mechanical secondaries?

[Michael C Henry]

I have two 600cfm 1860 Holley's w/ vacum secondaries .I've seen a pair of 600 cfm Holley's w/mechanical secondaries for a dual four set up. What are the advantages? Recomendations?

[Ron61]

When I had my motors with the two four set ups I really thought the vacuum setup was better for street driving. I like the mechanicals if you are racing or just really getting on it all the time but if you are just driving around town and don't have the RPM up and should open them you are goin to have the mother of all bogs. Just my opinion and I know there are others who have more experience with them than I have so watch for their posts.

Ron61

[SCOBRAC]

So why would you want to wonder when and if the secondaries are going to open? It takes a lot to draw open Both of the 600 cfm's.. Got an hydraulic cam FE?.. Forget it by the time they start opening you are out of revs and you may never get them open all the way.. If you want to try and use the secondary respring kit and go with the lightest springs and work your way back.. The bog desciribed in the previous post is just further evidence of 1200 cfm being too much carb. Do you think it bogged at the mid point of the secondaries opening? no.. The beginning.

Holley does a wonderful 450 cfm as well as 550 cfm mechanical secondary carbs that with practice open when you tell them to.. I can get 18 mpg or 10 mpg depending how I choose to use what I have.. and I have never had a bog..

[Ron61]

Scobrac is correct about my post. The cars that I had back then both had solid lifters and were race built engines. The car I was actually referring to in my post belongs to a friend of mine. I now have a SB and BB and just use a single 850 double pumper built to 890 on both of them and they run great. That is why I said to look for posts from people that have a lot more knowledge about engines than I do.

Ron61

[SCOBRAC]

Ron is right..

But he also has a stroker motor.. His "351" really probably bored and stroked to 383 will turn 6000-6500 rpm.. Most FE's go to 5300.. 5500 and quit making power.. There are some that will go to 6000 and above but not with a juice cam and you may never get the secondaries to fully open most of the time.

Us FE guys would need a really hot solid cam and a really light rotating assembly to get that kind of revs..

[Michael C Henry]

I have the FE that can see 6,000 plus it's a side oiler w/aluminum heads 11.5 comp .Ive twisted a 406 to 85'00 and lost a bearing which shouldn't happen on a side oiler.
Question the 50 cc accelerator pumps have the same volume as the 30cc if your using the cam?They appear to accomadate more travel thus more fuel if a different cam is used.Correct or wrong?

[Ron61]

I can't answer your question at all about the springs but I would like to see that 406 that you twisted to 85,000 RPM and only lost a bearing. Scobac seems to have the ideal setup for what you are trying to do and you should contact him as he has been there and done that and knows what he is talking about. Hope you get it all sorted out soon.

Ron61

[bmalone]

Mike:

I have (2) 750s using mechanical secondaries, and it work great. Vacuum is iffy to get them to open right now.

Bill

[Mr.Fixit]

Vacuum secondaries all the way unless you leave every stoplight by dumping the clutch at 7500 rpm. Get the vacuum pot covers that have the nipple sticking out so that you can link the two vacuum pots together with a length of vacuum hose a la factory ford dual quad setups. If you can feel the secondaries "open" what you really are feeling is when your motor finally has enough airflow to overcome the bog that it was operating in due to the secondaries already being open and the motor being overcarbed. You can play with the secondary vacum pot springs, but absolutely do not remove the check ball from the vacuum pot. The secondaries will just flop open instead of making the smooth transition. Also, put a tiny bit of progressive into the linkage, so that one carb starts to open the primary throttle blades a wee bit before the other carb. Dual quads really liketo have downleg boosters in them instead of the straight ones. Good luck

[Michael C Henry]

That 406 was in 68 in Des MoinesIA. I had to race the same car three times untill I beat them so bad. It knocked from that point .It had a thing with bending Crane push rods and I was sure it was the valvetrain .Turned out to be a rod bearing and I continued to drive it untill it finally scattered the next day.That is where I learned the important difference Between a center oiler and a sideoiler.Ford cam # C8AX-6250-D .600" lift 330° duration 115° overlap Shell lifters, 600CFM Holley.BW T85 three speed w/OD wired out.55Ford 2dr post (' Ford 4.56 gears any doubts?

[SCOBRAC]

I'm pretty sure you meant 8,500 rpm.. 427 so or not.. turn an FE to 8,500 and you will need a much bigger trunk.

[SCOBRAC]

Bill's motor is the 5% I referred to earlier.. Tunnel port motors are the exception as are roots style eaton blowers. They NEED fuel like others wish they needed fuel.. Is he over carburated at 1500 cfm.. You betcha.. But the mechanical secondaries are opening up.. Is there a ferrari (or anything else) made this side of a Winston Cup car that will stay with him.. nope.

As a precaution I looked up the secondary opening specs for a SINGLE Holley 4150/60 on a big block.. With ONE CARB you need to turn 4,960 rpm to open your secondaries fully with the LIGHTEST spring .. With the standard spring you will need to turn 7,130 rpm.. Now.. With two carbs those values are increased by at least 25% for a grand total of.... 6188 rpm with the lightest spring (past redline for a 427 so) And.. Ready.. 8,912 rpm for a standard spring.. Way past sweeping up your FE from the street.. Here is Holley's carb size formula..


RPMxCID
---------- x N where N = Volumetric efficiency is .9
2 x 1728

This means IF you can turn your FE at 6,000 rpm and say the 427 is bored .030 over and is really 440 CI you NEED 764 cfm...


Good Luck..

[bmalone]

Well, thanks for the compliment Michael. I knew it was overkill going in, but that's what came with the car and I think I'll keep it for awhile.

You are right about the fuel. It disappears in a hurry. I'm getting a whopping 4 - 8 mpg, depending how much I've got my foot into it.

Bill

[Sharkey]

I've found this sites info helpful and I'm using their Carb (Eliminator - under new products) -- great unit. www.aedperformance.com Try it - you'll like it.

[chopper]

My experience was a little different from those listed above. My car had a set of race-prepped carbs on it when I bought it. The carbs were mechanical secondary and had been worked for drag racing, and as such were unsuitable for the street. Rather than spend a lot of time trying to recalibrate them for the street, I replaced them with a pair of 1860 vacuum secondary units. I spent a good bit of time fiddling with them to get them to run acceptably on the 484 inch stroker motor. I ended up with one size larger than stock primary jets, installed 8.5 power valves and the heaviest secondary diaphragm springs to slow down the secondary opening. I found that with the lighter springs, the secondaries were opening too quickly and causing a major bog. The car runs much smoother now but I do note that it has lost a little bit of punch at the top end in fourth gear. For example, with the mechanical secondary units, I could break the rear tires loose at 3500 rpm just by flooring the pedal; with the vacuum units, it won't do that in fourth gear any more. It does bring in the power more smoothly, however, in the lower gears. Just my experience.

BTW, I've tried to find a pair of the vacuum tops with the nipples and have been unable to do so to date. Does anyone know where I can find a set of these lids? Thanks.

[ST]

Chopper
I found a set of those tops here in Vegas at National Perfomance Center.(702) 384-9080. Try a couple of speed shops localy and then give them a call.