Help deciding between 750 mech and vac on my 454
 

I've got a 454 in my 70 c10 that I need to find a carb for. The engine came from a 78 suburban and is 30 over with stock heads, edelbrock rpm intake and aggressive cam. Unfortunately I don't know the specs on the cam (PO lost the cam card..and I bought the truck CHEAP..) but it's a very choppy idle. I do know that it's got a 3000 stall and 3.40 gears though. Right now it's got a holley 600 on it which seems undersized for the engine and I was planning on putting a holley 750 on it, only thing is I can't decide if I should be running a mechanical or vacuum secondary carb. I am not racing this truck, just beating around town in it. I've read the pros and cons in the article below, but the truck doesn't really seem to fit into either of those categories. What would best suit my application?

http://www.hotrod.com/pitstop/1306_v...etor_uses.html

Thanks

Re: Help deciding between 750 mech and vac on my 454
 

If you really need the 750CFMs go mechanical. I say this for one giant reason. Vac seconds are rarely fully open and when they are it is at a high RPM. One of the vacuum springs in the tuning kit barely lets the secondaries start to open up until red line. I have a 750 vac on a 400sb and ended up putting a bolt in the secondary linkage to force to be a mech secondary. This is the wrong way to do it and causes a nice lean spot when the secondaries come in. I make more power and drivability went up while milage stayed the same. Funny how that worked out. I need a new carb but that is a $500 I don't have at the moment.

Re: Help deciding between 750 mech and vac on my 454
 

In reality, 600 CFM isn't necessarily wrong for that engine.

Using a CFM calculator like this one:
http://www.4secondsflat.com/Carb_CFM_Calculator.html

It comes out to needing 558 CFM at 5000 RPM using 85% volumetric efficiency.

If you go bigger, perhaps a Holley 670. Measure you vacuum signal at idle, that will tell us for sure if a vacuum secondary will work. If it is too low, then yes, mechanical may be your best choice. Normally I don't recommend a mechanical secondary unit for the street, but with a weak vacuum signal, you don't have much of a choice.

Re: Help deciding between 750 mech and vac on my 454
 

If your running around town there is no need for a mechanical secondaries. Vacuum secondaries is almost always the better choice on the street. Believe it or not your 454 will work good with something like a 670 street avenger. Cfm calculators are fine and great in the dyno room but don't always reflect actual driving etc. If you want a tad more top end power a 770 street avenger would also work well.

Re: Help deciding between 750 mech and vac on my 454
 

I agree with a 600 to 670 cfm carb being sufficient. Quite often guys will go with a bigger carb instead of actually tuning the correct size carb to the engine it is installed on. Sometimes the work just right out of the box, other times they need a little tuning.

Re: Help deciding between 750 mech and vac on my 454
 

Total agree with you gentleman and that is exactly why I started my response with "If you really need the 750CFMs" . I have tuned and modded a 3310 till the cows come home using an AFR. Drilled and tapped the bleeds, idle feed restriction, throttle plates and the fuel blocks. Then milled the choke and finally put a screw in the secondary linkage. Performance picked up each time. I am the exception in needing more than 750cfm.

Re: Help deciding between 750 mech and vac on my 454
 

I'm kinda on the other side of this a bit on a couple of things - just one guy's perspective and experience.

IMHO, if it's got an auto...I use vacuum secondaries, period. And overall I'll pick vacuum over mechanical for anything that's mostly street unless the engine is really hot...making over about 1-1/2 hp per cube.

for a bbc, there's enough flow that a square-bore carb can be a bit bigger than the math would indicate without bogging intake velocity - the primaries get a good demand signal and it works ok. IMHO the 3310 is a good "utility infielder" carb and works well for a bbc if it's tuned right. Not disagreeing that smaller carb will work just fine, and the street avenger is a great pick for a put it on and go carb.

I run a 3310 on my c10 355 making 375 hp and I've swapped in and tuned a few other carbs over the years and never found a bit of difference in throttle response or power...but I did find a very slight fuel economy benefit with a 600 vacuum secondary carb.

I did learn to hate edelbock performer carbs, though :)

Re: Help deciding between 750 mech and vac on my 454
 

750 on a 454? WAY to small if you're goin to rev over 4K, great off the line but choked off real quick.
If you've got a aggressive cam & 3K stall sounds like you might be setup for a slightly higher rpm running.
I'm running a 750vac sec on 400sbc ( in Burban) & 850DP on 383 w/ 3K convertor (in 68 C10), 650DP on last 350 (in 78 K10 w/ 35's)
Vac sec is easier to tune so if you don't wanna play around w/ carb go w/ vac sec.
600cfm's are commonly used on 302's
Volumetric efficiency vs carb size never takes convertor stall rpm & real world driving
Holley lists 454cid total airflow 800 cfm @ 100%VE @ 6K rpm
I'd look into 850 vs 750 carb
Also w/ that cam check your timing, if you're running HEI you're gonna have issues.
HEI sucks w/ non stock cams & getting the timing right.

Re: Help deciding between 750 mech and vac on my 454
 

personally I think the motor is mis-matched to begin with. Adding a fairly large cam with stock heads and 3000 stall is what 90% of people do and it's just not the right way to make power. The motor won't get near it's potential and won't be as streetable as it could be. I'm no expert on BBCs but I think they came with some pretty underwhelming heads from the factory.

As far as the carb, I don't know if you really NEED a 750 holley at all, but it probably wouldn't hurt the driveability as long as you go with a Vacuum Secondary.

EDIT: Also, as far as driveability and economy on the street go, The carb is only half the battle. Ignition timing is just as important. Spend some time dialing in the best idle timing, and mechanical and vacuum advance andthat motor will be fun to drive on the street.

FOR ANY STREET MOTOR: I would only go with a mechanical secondary if you have the knowhow and time to tune it in. If you really don't know what you are doing tuning accelerator pumps, then the vacuum secondary will be faster and better out of the box.

Re: Help deciding between 750 mech and vac on my 454
 

have to done any testing to confirm that putting a bolt in the secondary linkage is actually an improvement?

The vacuum secondaries open on demand from airflow. If they aren't opening then you might not have enough airflow to really need them. Holley's secondary spring kit is more what you need. I'd bet that it's faster with that bolt taken out.

https://www.holley.com/products/fuel...ts/parts/20-13

Re: Help deciding between 750 mech and vac on my 454
 

Its definitely not an improvement. It causes a massive lean spot when you kick into the throttle. Your doing nothing but losing power.

Re: Help deciding between 750 mech and vac on my 454
 

oh yeah no matter what carb you pick you'll probably need to drill a small hole (.0625 -.125") in each primary throttle plate to be able to set your idle speed w/o uncovering the transfer slots. Check your vacuum @ idle in gear, if 12" or less go DP & you had better have a vacuum canister if you've got power brakes.

Re: Help deciding between 750 mech and vac on my 454
 

Wow, that's interesting. I'd really like to see a BBC spinning at 6000 RPM with 100% VE. That would be one heck of an engine indeed! Maybe some sort of new warp drive?

Re: Help deciding between 750 mech and vac on my 454
 

Yeah, it's amazing how the "advice" for a street driven truck morphed into 6k @ 100% ve fire snorting monster.....
Never going to happen with a stock-headed, over-cammed engine.
For a street driven anything stick with a vacuum secondary carburetor and forget the "gimmicks" that do nothing but cost you power and driveability.
The "screw thing" is just ridiculous. It forces the secondaries open with-out the additional fuel needed to cover it...and if your vacuum signal is so low that you think you need a double pumper, you have other issues anyway.
Double pumpers are strickly racecar stuff, for engines that run at wide open throttle

Re: Help deciding between 750 mech and vac on my 454
 

Not that amazing of an engine really
Ever heard of ram air, a turbo or supercharger? 100%+ VE (search youtube for Nelson, Moran, Morris, Fulton, etc. engines)
He stated a 3K stall convertor which indicates, to me, that the engine revs over 5K. Otherwise why have a hi stall? So 6K redline is not reaching
Stock heads are easy enough to swap out for AFR's/ Brodix
I never claimed it was goin to be 100%VE, but carbing it for it.
A Dominator might not be the hot call but 750cfm is 400ci territory.
DP's are strictly for race cars? Sheesh I guess the 850DP on my 383 makes it a race truck :metal:
DP's do take more tuning skill to dial in
I thought the solid roller cam in my 68 w/ 3K convertor was what made my daily (summer) driver a race car.
My Burban is closer to what Harrison. has in his truck, though I've got a 400 w/ 750 vac sec, RPM intake, ported OE heads & 276 hyd roller cam & 2K convertor. Though aftermarket heads would really wake up this engine.
Dang I don't know nuthin :waah:

Re: Help deciding between 750 mech and vac on my 454
 

Hilarious.

Re: Help deciding between 750 mech and vac on my 454
 

I have the kit and ran the lightest spring there is. Pull the graph and look at it. Most of those springs actually limit the carb and the motor never sees the 750 CFM. One actually causes the secondaries to basically never open!

As for testing. Butt dyno and an independant passanger. Most testing is done knowing how the truck runs on a certain section of roads. Forcing the secondaries open does cause a bog/lean spot at their tip in but it does recover from it. This happens because you can't get the blades 100% synced. There is no accelerator pump on the rear of a 3310 (except the original factory carb). As for the after effect acceleration is quicker through the RPMs and speed increased on a given section of road. Oddly fuel milage went up also on the interstate by 2 MPG also. I am not trying to figure that out. AFRs stayed constant but I am sure it has to do with the volume and not the mixture. All this was originally done as a test and it worked. Now I just need to get a new carb because this one is pushed as far as it can be. Life just got in the way of dropping the coin. It works so hot rod parts are last on the budget. Bank note, food and broken parts come first. This month gave way for a broken header that needed replaced. The replacement will be a quick fuel ss 830DP.