Jester

I usually say if you have a manual, or an auto with over 3.73 gears, go with a DP. Other than that, stick with a vacuum secondaries. What you y'all think?

ede

i guess i'd have to agree, but i prefer mechanical secondaries, but not many carbs have them.

AlkyIROC

Both carbs are good when used. Although vacuum secondary carbs are rarely used on the dragstrip, they still are ok to use. Vacuum sec carbs are best used on the street when partial throttle use will open or close the secondaries depending on engine demand. They can still be used in drag racing with tuning kits. These kits allow control over when the secondaries open with different spring rates. Super class cars can even use them for adjusting their ETs by fine tuning the spring rates.

DP carbs provide better throttle response and provide better fuel demands however when used on the street can be big gas guzzlers. Even if the carb has progressive linkage, the extra fuel from the secondary squirter will still provide poor mileage.



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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!

Member of the Calgary Drag Racing Association

[This message has been edited by Stephen 87 IROC (edited September 10, 2000).]

Biochem

Are all DPs mechanical secondaries?

My question is that my current q-jet has the small primaries and large secondaries, so it gets OK gas milage. Can a similar thing be done to a mechanical secondary carb too?

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock... all the suspension stuff... 9-bolt posi disk is in... K&N filter... 93 octane...

I got the GO... I need the SHOW... Interior is next!

Jester

I have a spreadbore DP, and it does pretty well in the mileage department IF you're easy in the pedal though. if you start laying into the secondaries every stoplight forget it.

ede

the little holley i bough for the 266 isn't a DP but is a mechanical secondary. i wish i'd of gotten a 500 2bbl or tried to cut back a 600DP.

AlkyIROC

The Q-jet isn't really a DP because it only uses one accellerator pump. It does use progressive linkage so part throttle operation only uses the smaller primaries.

DP refers to the accellerator pumps. Vaccum secondary carbs don't have a second accellerator pump because the secondary butterflies are opened by a vaccum signal and don't open as aggressivly.

novadude

Well, normally I'd agree with you, but I have seen DP carbs work very well in application where they shouldn't.

My friend has a '70 Vette. The engine is a 454, 9:1 Compression, 216 deg cam. Trans is a TH400 w/ stock converter. Gears are 3:36. A very mild set-up. Car is heavy as hell... 3700#, because it has A/C, & power everything. He is running a 800 CFM DP on a Holley Single Plane intake. On paper it sounds like a horrible choice, but I have been in this car many times, and can say that the throttle response is sick. It launches hard on slicks w/ no hint of a bog.

What I am saying is that carb selection is an individual thing. Sometimes ya just gotta try something to see if it works. The textbook guidelines only get you so far.

As for me, I'll stick with my Junkyard Q-jets, 'cause I am cheap! lol

AlkyIROC

Nothing wrong with a Q-jet when they're set up properly. Look at a Stock Eliminator car. F/SA camaro with a cast iron intake, heads and a Q-jet running low 12's or high 11's. If it's built properly it can out perform a holley.

Disco Man

DPs where first introduced for racing applications, and are a little bit harder to tune for normal street driving (however it can be done). The vacuum secondaries however are more suited towards street driving. So my advice is if you are looking for top end power the DP is for you. If you want good performance, better fuel economy, and better driveability in normal driving conditions the 4bbl with the vacuum secondaries is what you want.

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Disco Man

1982-83 Cross-Fire Trans Am Site: http://members.xoom.com/the305cfi/

1979 (L78/W72) 10th Anniversary Trans Am
1982 (LU5) Trans Am

Damon

Not all carbs with mechanical secondaries are double pumpers. The QJet has mechanical secondaries but with an airflow sensing secondary air valve.

Confused yet?

OK, let's put the QJet aside. Every OTHER non-Rochester carb I have ever worked on has a secondary set of accelerator squirters on it IF the secondaries are mechanically operated. i.e. Holley.

There is no reason why a DP should be any worse mileage wise than an equivalent Vac Sec carb IF YOU KEEP YOUR FOOT OUT OF IT! Go for a sub 6 second 0-60 at every light and you will waste a lot of gas no matter what, but generally more with a DP.


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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.

"One of the last remaining QJet tuners on Planet Earth!"