I have been doing a lot of research and reading here and elsewhere to assist me with my carb selection for my 402/4spd. I have read several times that 4 speed cars benefit more from mechanical secondaries, but I can't figure out why. Can anyone shed some light on this? Also from posts that I have read here, I am now leaning toward a Rochester Quad. I have one now that I pulled out of a parts car I just purchased. It decodes as a '79 Chevy w/ manual transmission (17058207). Is this carb any good (ie. worth rebuilding). The car will be primarily road tripped/cruised, with occasionally taking the imports "to school". Thanks for all the good info!

My take on the 4spd=DP thing is that a 4spd engine and trans combo generally sees less engine load(drag,resistance,friction) .Making them respond quicker, and rev up quicker.
Some are geard fairly close so there is less RPM loss after a gear change,meaning they stay closer to peak power.

Originally posted by 7DSS:
The car will be primarily road tripped/cruised, with occasionally taking the imports "to school". Thanks for all the good info! Be careful which import you decide to "take to school" :eek: . Don't get me wrong, I want to show them what a real car is also, but this subject has been discussed quite a bit and some of these rice burners can blow the doors off of a 3800 lb Chevelle unless the Chevy has some real power and gearing. Just be careful. graemlins/thumbsup.gif

Cars that "launch" above 3500 rpms benefit from a DP carb, meaning most 4 speed cars, or cars with loose coverters. Mild Stall auto, or 2.73 geared 4 speed cars prefer vaccum secondaries so that the engine isn't overwhelmed ( i.e. intake charge velocity drops to zero), as you go WOT. In most vac secondary apps, inclucing QJ, the secondaries open up as the engine demands more air, keeping charge velocity up. With a mechanical carb, you go WOT and everything is wide open. At 2000 rpm, with low piston speed, the motor can't use all that air.

I've toyed with building a 496 to look like a 325hp 396, M20, 3.31 getset. DEFINTELY a vac sec application. ( QJ, of course!)

Anyway, its more of a converter/gearing issue than 4 speed v auto. IMHO. If the engine can really wind up at launch, piston speed is sufficient to start pulling all those cfm of WOT down the runners. If not, then it is best to have a slight delay before the carb is fully WOT.

Not to let the cat out of the bag, but mr. 4 speed is going to prove this hypothesis right this spring.

Use the Q-jet for anything that you don't side step the clutch at 6000RPM. I have found that trans brake cars work better with a double pumper but not much else.

Once again, thanks for the info. Since I don't know anything about Rochester Carbs, is the 17059207 Qaudrajet a decent carb to rebuild/modify? Or should I go for a more "period correct" Quadrajet.

Another thing that needs to be taken into consideration is that with a manual transmission (if you aren't power shifting) you need to let off the gas to hit the clutch. This can be a problem with vacuum secondaries...there can be a delay with the top flaps opening. With a mechanical secondary carburetor it's very simple, when you kick the gas to the floor it's wide open immediately. Yoou can keep your foot to the floor with automatic transmissions. I never had much success with vacuum secondary carburetors...especially Quadra-Jets. I love Holley carburetors with mechanical secondaries, much easier to tune and work on IMO.

The reason I brought that up is because my buddy ran into this problem with his 327.

Originally posted by stingray454:
Another thing that needs to be taken into consideration is that with a manual transmission (if you aren't power shifting) you need to let off the gas to hit the clutch. This can be a problem with vacuum secondaries...there can be a delay with the top flaps opening. With a mechanical secondary carburetor it's very simple, when you kick the gas to the floor it's wide open immediately. Yoou can keep your foot to the floor with automatic transmissions. I never had much success with vacuum secondary carburetors...especially Quadra-Jets. I love Holley carburetors with mechanical secondaries, much easier to tune and work on IMO.

The reason I brought that up is because my buddy ran into this problem with his 327. Great point graemlins/thumbsup.gif

Originally posted by stingray454:
(if you aren't power shifting) you need to let off the gas to hit the clutch. Aw come on you didn't leave me any room there tongue.gif

Your supposed to let off when you shift??? :D

One of the other things with DP's that's rarely discussed is all that fuel washing the oil off the cylinder wall. In a taller geared street car, the engine doesn't rev quick enough to use all that pump shot. So it ends up in the oil pan. Both of my BBC's had 3.07's. While I could tune them to work with the DP just fine, the rings wore out quick. This doesn't seem to be a problem with a high stall and lower gears.

The QJ's are basically all the same. Just as long as it doesn't require more than one wire to install it. You should be fine. You'll need to check into the correct jets, rods, hanger for your engine. Being a 79, it might be real lean for your application. I went up 2 jet sizes on my factory QJ on a 454 truck. Usually, you just change rods for a small adjustment. It ran a lot better and still passed CA smog. I'd recommend getting a book on whatever kind of carb you use.
The will teach you little tricks to make your car run better.
Or you can call one of the shops to find out what you'll need to change. It will get you in the ball park so you won't have to worry about hurting anything.

Your supposed to let off when you shift???When your are running a Borg Warner Super T10 behind a 454 big block...yes smile.gif . I'm doing all I can to prolong the life of the T10.

Thanks to everyone for the input. Sounds like there is nothing written in stone on the carb issues, and I will need to really take a look at how the car will be used, and make a decision from there. Everything that everyone says make sense, it seems that the different carbs all excel at different loads/driving styles. So for a middle of the road, I'll probably stick with the Rochestor and send it off to JET for a stage 2. Thanks again. Maybe someday, I will have enough expereience and wisdom to contribute something to this site !

Originally posted by 7DSS:
Maybe someday, I will have enough expereience and wisdom to contribute something to this site ! You already have - I have learned something from this post and I bet others have too!
graemlins/thumbsup.gif