I'm gonna be getting a Speed Demon 750 carb and RPM air gap intake to replace the Qjet and performer manifold, but i'm a bit confused on the mechanical vs vacuum secondary issue. This is one a 10.2 comp 355 with trick flow heads, looking at about 400~hp (flywheel). It has a manual tranny too.

As of right now i'm leaning more towards the vacuum secondary because it is cheaper and has a choke. The mechanical on the hand is more expensive to start with, and then i have to buy a choke kit for it if i want a choke.

Is it gonna make a difference at all which one i get? I'm not too up do date on carbs so i'm a little lost as to what i should be getting..

Thanks in advance for any help,
Eric

 

If it's a daily driver, get the VS, pocket book will thank you in the long run. But if out and out performance is what you want, mechanical-there is NO substitute

 

Sounds good, but for the sake of argument, lets just say that hypothetically, i wanted to take the car on an 8 hour road trip up the east coast, mostly highway driving, mosting between 70-80mph. With the 3.42 gears, is my mileage gonna be REALLY bad? The car gets a solid 22~mpg now. Obviously the mileage is gonna go down, the question is, how much? are we talking like 15-17, or like 10-11?

I don't care about city mileage, cause although i drive this car about 5 days out of the week, i romp on it everywhere and expect to get no more than 8-9mpg... so thats no biggie.

I'm beginning to lean more towards the mech. now. Please everyone give your opinion on this...

Thanks again
Eric

 

Mechanical secondaries versus a VS carb for the most part isn't a street vs. race debate. Or even a mileage vs. power debate.

Some combos can get away fine with a VS carb with only a small power loss compared to a DP....maybe even perform a little better. Others will not and NEED mechanical.

You need a mechanial.

 

When I had my Holley 750VS and did my romps around town and hwy roddin I'd go all week on a tank of fuel. Don't ask me for MPG cause with mine I don't know and don't care so never checked. Now though, with the new Holley race 830DP, I can literally drive for an hour to hour 1/2 and easily eat up half a tank. Same driving style as before-hard I guess you could say, but with the DP I got, I'd guess I'd be LUCKY to get into the double digit mpg, but then again I don't have overdrive anymore either so I tach about 3 grand going down the hwy, with my 3.42's. Anyone that's ever owned one (mechanical) will tell you the same thing, and if you drive this 5 days a week, with fuel prices the way they are, your pocket book will decide. It'll make your car a bad som-bitch from stoplight-top end, but it'll cost ya. I picked up .3 by switching carbs only if that tells ya anything, plus went from a 1.90-95 60's to consistent 1.80-1.82's. So performance wise it's the only way and if you go that route you'll love the **** out of it, but make sure you have all your gas stations plotted out for a coast drive, or keep a jug in back just in case. My last 2 cents.

 

i think i can live with that. Thanks for your help guys....

Eric<------picking up phone to order intake and carb now.... (good *** i'm about to spend 600+ dollars )

 

I have a 750 DP and get horrible mileage. Probly about 9-11 mpg highway. My RPMS are at 3500 ish or mpre cruisin the highway. I also have 4.11s which doesnt help. In all I'd go with the mechanical. You spend so much on mods, if you want a car with good gas mileage get a VW, if you can sacrifice gas for performance then gas $ shouldnt be a factor. Just my $.02

 

I cruise at 65 right around 2000 rpms with a 24" tire, so i imagine my mileage would be just a bit better than yours. I'm not really concerned at all with mileage, i was more or less just curious about a trip i'm gonna be making to upstate new york in the spring. I have a Jeep Grand Cherokee available to me, but the roads up there are so incredible that i feel like i'm wasting them with a jeep. It's nothing but uphills, downhills, and tight s curves for about 3 hours once i'm in the mountains. And most of it just got repaved so it's all smooth This is something i REALLy wanna experience in my F-body

Thanks for everyones input, i'm gonna order the stuff on Monday.

Eric