I'm a little torn on which I should get for the 400....The shop is recommending a mechanical secondary Speed Demon carb, and a full mechanical advance distributor.

What are the advantages/disadvantages of both mech/vacuum types??

Can you mix/match the two??

Car is mostly driven on the street, not my daily driver and no long trips with an occasional run on the strip for fun.

Engine is set up for an engine friendly 6 grand max.

- Thanks for any input in advance.

 

How big of a cam are you going to use?

 

Sounds like the shop you're working with likes race parts.

The mech. secondaries is fine with a manual trans, but there's really no reason not to take advantage of a vacuum adv. dist.

Race cars usually don't run them because they're not needed. The car never sees part throttle, or a cruise RPM. They're WOT 95% of the time. But since your car will be mainly street, you can easily benefit from it.

 

With your setup, mechanical would work well. Obviously vacuum will work good as well, but since you are using a manual transmission, there is no harm in going the mechanical route. As for an automatic, unless it is making insane HP, or if it has a 3000 RPM stall converter or higher, I would go with a vacuum sec.

 

I would go with mech. secondaries also.

 

I would consider using a vacuum secondary carb if one of the following conditions was true:

1) The engine is in a tow vehicle.

2) You've got a mis-matched combo with too little stall speed (which you avoid by using the Holley website charts for mechanical carb selection and/or picking the proper stall speed for your cam).

3) You want to slow down your ET (as in going from high altitude to lower altitude in a sportsman class, and you can't dial quicker than 13.00 sec - ask me why I might even think of that...); or off-the-line torque in a class that limits traction improvements.

As for vacuum advance, as AJ said, there's no reason not to use it - unless you've got a trailer queen.

 

Yes, he does by the way AJ92!!....

He also asked me if I needed a choke on the carb - because the Speed Demon 750 he recommended doesn't come with one!!

Do you think the vacuum advance distributor would lose anything on a mechanical advance off the line??

Yes Bandit, I'm running a 5 speed..... I gather you think I should go mechanical sec carb, and mechanical advance distributor? Would I lose anything or gain any streetability using a vacuum advance distributor?? Or does it really not make a diff when you have a manual??

five7kid - The guy at the shop talked me out of using a Holley, he says a Demon carb will "outflow a Holley silly".... he did mention he's been racing and building engines for 30 plus years, so I should probably listen to him.

Sounds like you're saying I may lose "off the line torque" - if I don't really lose any streetability - I'll go with the mechanical.

Thx for the replies so far!

 

You can get a kit that'll put a choke on the speed demon. I really liked how my 750 DP performed on the (gasp) 305 hehehe freakin great carb. Sold my Holley Street Avenger 670 cause it was sitting on the shelf for months after putting the Demon on. The 750 is finding it's new home on top of a 350 later this week

 

Use the Mech carb, but a vacuum advance distributer if you run the car on the street. A vacuum advance wont change how the car runs at WOT, but will help street manners. The Mech secondary carb will help responsiveness, and will do better after shifts w/ the 5 spd because the secondaries open as soon as you push the pedal down instead of having to be controlled by vacuum. Oh, btw, I cant really see the T-5 lasting to long behind a motor like that.

Ben

 

Evil - That's good news! I thought the street version only had a choke. Looks like I'll get a choke kit with it then!!

Momar - So, a vacuum advance distributer is the way to go? Looks like I have 2 votes for mechanical and two votes for vacuum advance distributor.....I'll have to do a little research it seems..

Everyone seems agrees with a mechanical secondary carb, so I guess that part is a done deal!

I wouldn't throw a stock T-5 behind a 400 and expect it to last. That's why I dropped the $$ for a complete upgraded trans from G Force. They claim it should handle it with no problem now. I intend to find out!

 

I didn’t see anyone that said anything about going w/ a mechanical only dist. Even if you have vacuum advance you will have the same mechanical advance as you would without it. Vacuum advance is not present during WOT. It just helps the car run good while at part throttle. So basically vacuum advance makes no difference at the track, but can make all the difference in the world driving it around on the street.

Ben

 

Ill ditto that. a 400 T5 combo isnt gonna last long enough to even tell if Vac or Mech is better. I know a fellow who had the same combo and that car never moved longer than a week or so. He blew to many trannies to count and it wasnt 6-8 months and he sold the car off.

You can find people on this board with bilt 305's and some good running 350's with a T5 but I think the 400 has way to much A$$ for the T5. Espically 5th gear. never use 5th while racing on it hard.


When I replaced my T5 at 204000 miles the guys at the yard said if I wanted it to last, run straight 30w oil in it. Instead of the auto ATF fluid GM puts in. Its will makes the bearing last longer and give it better cushion when the gear slap together. But still thin enough to protect.



Matt

 

Well here ya go. ]
Vacuum advance distributor, YES!!!!
Mechanical secondary carb, YES!!!!!
Mechanical advance distributor, NO!!!!
Vacuum advance carb, your choice but Id go mec.

I run gear oil in my WC T5, not supposed to but I havent had any problems, if it goes I have an excuse to upgrade parts. If you want to follow GM's reccomendations for a thin ATF like fluid use Syncromesh, it's what GM would have reccomend if it was avalible at the time.

 

SSC - Thanks! I believe that's the way I'll go then- Mechanical secondary 750 cfm Demon, and a vacuum advance distributor...
I tried gear oil on my old trans, and it was a b*tch to shift for the first 30 minutes of driving, and it didn't mesh well.

Matt - I doubt he was running the same setup as I am....unless he also had his trans built by G Force....as I said above, it was built to handle this 400 and more. They (G Force) claim It'll handle up to around 600 HP and TQ....even with this engine, I shouldn't get near that rating. And yes, I've heard all about the casing flex thing. I run an additive in it from them called GF3000 - it kinda looks like red assembly lube, and it's some slick stuff!!

Thanks for the input guys!!

 

Oops I missed that. sound neat, you got a link for Gforce???
Id like to read what they do/ did to it.


Matt

 

Let's be clear about something:

Distributors have basically 3 modes of advance: 1) computer controlled (which isn't what we're talking about here); 2) mechanical advance; and 3) vacuum advance.

To the best of my knowledge, there is no such thing as a vacuum-advance-only distributor out there. There are distributors that only have mechanical advance, but they are (as stated) race-only items. The typical non-computer-controlled street distributor, such as most any HEI you see out there, has mechanical advance weights inside the cap (on the rotating shaft, and typically under the rotor), and a vacuum advance servo on the outside.

When we say "vacuum advance distributor", what we're really saying is "vacuum/mechanical advance distributor". It's been such a source of confusion in the past that I typically say, when someone is going from computer-controlled to non-computer carb, "You also need a vacuum/mechanical advance distributor."

I sure hope that clears up what appears to me to be a truck-load of confusion...

 

My post was directed only towards the secondaries. As for the advance on the distributor, there is no advantage for you to use the mechanical one, that is for stip only use, I'd go with the vacuum for street use. With your setup, the vacuum secondary's streetability difference from the mechanical secondary is next to nill. The gearing of the 5-speed tranny will be able to use the extra fuel without bogging fomr a rich condition like the auto would.

 

Ill ditto that. Good point.

 

I think I know what you mean Bandit - I've always known that vacuum secondaries, especially on an auto will save gas, because unless you wind out it enough to create enough vacuum to open the secondaries, it simply doesn't allow them to open.....Every manual car I've ridden in with a DP carb, seems to have more of a "kick" on acceleration. Gearing helps too!!

five7kid - I know all about distributors -


The vacuum advance ones only allow advance when load dictates - by using a vacuum motor pull arm to advance the timing to a certain extent, then a mechanical (centrifugal) type takes over.

Yeah, I took vocational in high school for auto mechanics, and spent almost two years in college for auto/diesel....wayyyy back in the day.....old skool

 

I'm going to throw a wrench into this and recommend a Edelbrock Performer 750cfm carb.

Like Old Vic Jr. says on TV. It'll run "right out o' the box".

 

So did my Demon 750 Took it out of the box, hooked up everything, fired it up and drove it.

 

if yoiu're looking for a milage carb, that'd be a great choice. he seems to be geared towards performance, so i'd skip the Eddy.

 

My Demon 850DP ran fine right out of the box. Could have bolted it on set the idle and gone on a trip.

 

Yup, my demon 750 dp ran fin right out of the box too. It was a bit rich, but nothing awful. I set the idle mixture and it drove great. I havent got a chance to tune it for performance yet because my cam died though.

Ben

 

I used to have a Performer 600 cfm version on my old Charger years ago when they first came out....been there done that.

The secondaries NEVER opened up correctly. I took the car to get tuned/dynoed because I just couldn't get my car to run right with it no matter what I tried....and the engine was a complete 360 mopar rebuild.

I just don't care for the design itself - "counterweighted" secondary air valves... the shop ended up having to cut "slots" in both air plates in order to delay them opening too soon and causing it to bog. Some people have good luck with them, or at least I've heard. Not meant to be a bash F-BIRD'88 - I know you know your s*it.......As for me - NEVER AGAIN!!! I'll buy thier intakes and headers though.

 

Have you seen the new ones??? The have two round holes and two squares punched out of the secondary air valve.

Matt

 

<img src="http://v8buick.com/attachment.php?s=&postid=176696">

Had a nice thread about it over a V8Buick.com

Considering moding the Q-jet air valve the same way or get a replacement one from Eddy and stick it in a Qjet.

The holes were the right way to go.

An Eddy is just a better built Qjet in some respects.
But done right a Qjet can run just as well.

http://v8buick.com/showthread.php?s=...ght=step+drill


Matt

 

Uh oh, I think ya failed the class

Vac advance carbs open secondaris when vacuum falls below the PV setting (Holley's anyway).

Vac adv distributors give most advance at highest vacuum...i/e cruise RPMs, then as you accelerate to WOT, vacuum adv drops out, and mechnical takes over...in effect you could be cruising at 42*BTDC, say (vac adv on top of mech adv) nail it, vac adv "goes away" and Mech Adv runs you up to 34/35 (or where ever you have it "all in") for the WOT run.

 

 

Uh, yeah, he's got me too. lol :lala:

 

How the vacuum part of the advance operates depends on the setup. If ported above the throttle plates, vacuum increases as the throttle plates are opened, and timing advances. If ported below the plates, or in the intake, it's the opposite..I've seen them set up both ways on diff makes. The basics still apply.

I thought I was trying to convey the basic operation, not a detailed description....I didn't flunk, so I must've known the details at one point or another!!

 

I'm sorry...I don't know why I had the PV equating to the secondaries.

Looks like I failed too

 

I'd have to think about that for a moment...but I don't think it's accurate.

If you have vacuum adavance set to intake vacuum, then you are pulling in some vacuum advance all the time while te engine is idling...if hooked to ported/timed, then only when throttle blades are opened and you have low-load on the engine.

In either case, once at cruise RPMs, vacuum advance should be the same, then react the same at WOT..i/e fall out and have mechanical take over.

Maybe we're saying the same thing, but I'm just not understanding it...my wife can vouch for this....

 

They both do the same thing other than ported is not functioning at idle. It is above the throttle blades and they have to be open slightly to get vacuum above them.

Ben

 

That is correct - I'll add that the ported vacuum is above the throttle blades at idle.

As manifold vacuum drops (such as in WOT application), ported vacuum will drop as well.

 

I couldn't have said it better myself......really!!

 

I have only one more question -

Since I've never personally owned a DP carb before, I assume there's a way to adjust when the secondaries open at a certain throttle position to prevent bog etc??? Something like the spring tension adjustment on a Q-jet air valve??

Yes, I'll admit I don't know it all!! That's the only reason I frequent here...