Mechanical secondaires

[76bonanza]

What is the advantage of running mech as apposed to vacuum secondaries? Never had mech people tell me I should try it.

[WorkinLonghorn]

I heard they are good for racing,gives you more control. I'm sure some will comment.

[cdowns]

they're better for more aggressive cams where the gas is dumped in relation to your foot/vaccumes secondaries have a bit of lag for the engine to catchup b-4 dumping the fuel in so that the engine doesnt flood

[cableguy0]

vac secondaries on a street truck and about anything heavy at that. mech secondaries usually when your doing mostly strip or racing.vac secondaries will stay closed more and yield somewhat better mileage as opposed to a mech secondary carb where it just wants to empty the tank in a hurry

[Schralper]

I've got 650DP on my 355/ Weiand Xcelerator/ 10.3:1/ hedman torq step headers full 2.5" and it works GREAT. Sea level to 7K'. Worked fine when engine w/ 8.8:1, also. BTW it's a daily driver. Mileage was the same w/ 750 vac. sec. Throttle response is better fer sure. DP's offer more tunability.
Though you can replace the secondary fuel plate w/ metering block w/jets on vac sec., it helps. Got a 750 like this on my Burban's 400.
If it's tuned right, DP's work better.
Especially if you like to get on it, cuz I sure do
Opinions may vary.

[mnunn454]

As I see and understand it, and as the Holley Website explains it, the mech secondary/double pumper carb works best for engines that rev up quickly (e.g. light cars, stick shifts, low (numerically) rear end ratios). Vacuum secondaries carbs are usually (but not always) better for the other 90% of us. The back 2 barrels of a vac secondary carb opens only when the engine's airflow has come up enough to demand it. Both types usually require some decent tuning to get optimal performance.

I had a Mech secondary 750 CFM Holley on my 454 "El Camona" that has 3.55 gears and a TH700 tanny. Nailing the gas from a stop would cause it to pop back through the carb and stall briefly due to getting too much air and leaning out. Swapping to a vac secondary Holley carb fixed the problem instantly.

I've learned since then that I could have increased the accel pumps and the squirters on the mech secondary carb and that probably would have eliminated the bog, but I'm still very happy with the vac secondary carb's performance.

[cory321]

I'm not sure but aren't edelbrock carbs mech secondarys.

[Schralper]

Originally Posted by mnunn454

As I see and understand it, and as the Holley Website explains it, the mech secondary/double pumper carb works best for engines that rev up quickly.

Like say a performance engine?
Not much point in a hi performance carb on lo performance engine, eh.

Originally Posted by mnunn454

Nailing the gas from a stop would cause it to pop back through the carb and stall briefly due to getting too much air and leaning out. Swapping to a vac secondary Holley carb fixed the problem instantly.

But spending some time tuning it woulda solved the problem WAY cheaper.
Are you sure it was leaning out?
Cuz generally lean will cuz it to lay flat on it face then go.
Maybe timing, too?

[piecesparts]

There will be comments that the mechanical secondaries are NOT for a street truck. That is a choice for each person. I am running an Edelbrock AFB design carb on my truck and also on my Son's truck, with no issues with the fact that they are mechanical. What is noticed the most in this type of carb is the changeover to the secondaries being a large surge on a mechanical vice a vacuum carb. That is if you don't adjust the carb to be moderated in it's functions. My 383 stroker (agressive cam) responds well to the mechanical secondaries and my Son's 350 (slightly agressive cam) works just as well, so it is not a problem to us. I have run vacuum secondaries and would agree that the transition is smoother, but it is just what you are used to.

[mnunn454]

Originally Posted by cory321

I'm not sure but aren't edelbrock carbs mech secondarys.

I've never seen one that is. But let me clarify though what I mean by vacuum secondary.

All the Edelbrock carbs I've ever seen have a vacuum controlled air horn (air valve) atop the mechanically opened secondaries. While the back barrels (down on the throttle plate) might indeed open mechanically, the air doesn't start flowing until the top plate opens. And that only happens when the engine CFM (demand) increases and the vacuum level starts to increase. This prevents the leanout "bog" that's typical with carbs that have no acellerator pump for the secondaries. Also, it operates (functions) just like a Holley which uses vacuum to physically open the secondaries. That's what I meant by vacuum secondaries on an Edelbrock.

Edelbrocks are kind of a hybrid carb similar in function and design to a Thermoquad or a Quadrajet.

The advantages of a vac secondary carb are you don't squirt a 35CC shot of gas every time you open the back 2 barrels. Plus, the engine vacuum never goes to zero even if you stomp it. So there's rarely if ever any "bog" when flooring it, even from an idle with a heavy vehicle and a long 1st gear. The main disadvantage of a vacuum secondary carb is because it isn't squirting in the extra gas when the engine needs it most, it's a lower performance alternative that's better adapted for 90% of us that do mostly normal or street driving.

Will performance be better with a well tuned double pumper/mech secondary carb? Absolutely (but some will argue). Will economy and driveability be better with a well tuned Edelbrock or Holley or similar vacuum secondary carb? Absolutely.

'Guess it all boils down to your needs and wants.

Hope that helps.

[mnunn454]

Originally Posted by Schralper

Like say a performance engine?
But spending some time tuning it woulda solved the problem WAY cheaper.
Are you sure it was leaning out?
Cuz generally lean will cuz it to lay flat on it face then go.
Maybe timing, too?

You're right. It would have been cheaper ($400 cheaper in fact) to fix and tune the double pumper.

However, there comes a time and an aggravation level for each of us when our "limit" is reached. I had been into the DP on 3 different occasions and spent $75 on various parts and gaskets and hadn't yet solved the problem. I had reached my "limit". So, like you have on various occasions, I threw down the 4 plastic numbers with an expiration date and viola', the problem disappeared. I don't regret it.

After I cooled off a little, I pulled the DP carb apart for a 4th time and threw in larger squirters, larger acellerator pumps adjusted all the springs and rebuilt it again. I now have it on my C-10 and she screams. Stomp the gas anytime and no bog at all, just tire smoke and squeel.

However, though I love the performance, my vac secondary Elky gets two to four (20-40%) more MPG than the mech secondary truck does

There's always a price to be paid for performance.

[Schralper]

Some fuel for this fire
http://www.classictrucks.com/tech/cl...ng_carburetor/

"my vac secondary Elky gets two to four (20-40%) more MPG than the mech secondary truck does"
BUT from the looks of your sig, the elky's got OD? and truck doesn't?