Manual vs. Vacuum Secondaries

[Russ Dickey]

Fellas:
My engine is basically the 362hp Ford Motorsport crate 351W. Has iron GT-40 heads, RPM intake, mild cam, 750 Holley, Mallory distrib, MSD ignition.

A recent trip to the dyno produced very disappointing results. I was hoping for at least 300-310 rwhp, but instead got about 270, even after an hour of adjusting timing and carb jets (down to 68's). The techs at the speed shop said that they thought my car was still way over-carb'd, and suggested I step down to a 650 double-pumper with manual secondaries. They said that it would improve my throttle response and likely produce more HP.

What are your thoughts on this analysis? Beyond that, is the manual secondary carb better than the vacuum? I had always heard that with the vacuum carb, the engine pulls what it needs, giving it an advantage over the manual secondaries. I am in this for performance, not gas mileage, and I appreciate your honest opinions.

Thanks,
Russ

Unique 289FIA - Viking Blue

[jhaynie]

Everything I have heard is that 650-700cfm is the correct size for engines like ours. I talked to the guys at quick fuel technologies the other day and he said he could build a 650cfm carb with a 750cfm throttle body to create a 700cfm carb that would work excellent with a 351W. Around $350.

John

[WBofTN]

Russ:
I'm no expert, but I think vacuum secondary carbs are ment for cruising at
steady speeds like on the highway. Mechanical carbs are for when you want
all hell to break loose.
WB

'93 Unique 427/351W/4sp

[eliminator]

Russ,

On the Carburetor subject the 750 is too much for you engine. The Quick Fuel Carb would be a good deal. On lightweight Cobra's I like the vaccum secondaries, they dont bog down like mechanical secondaries. Give me the vaccum secondary that is set up right and it will outperform the mechanical. Just because it's a mechanical secondary carb it's not going to out perform the vaccum secondary, both are equal at wide open throttle. It's how you get to wide open throttle that makes the difference.


Rick

[Brent]

Russ, The diff. between crank horsepower and rwhp is known as parasitic loss. Your PL is about 25%. I was told the PL for a manual trans and a Jag rear equipped car is around 20%. If this is true you could look for another 18-20 hp with tuning. You will not notice or feel 18 hp by the seat of your pants.
For a street car I would use a vac. secondary carb ILO a mechanical secondary. WOT numbers should not change if the carbs are tuned correctly, but, the vac. carb will have better street manners.

Brent

[Russ Dickey]

Interesting (and quick) feedback! Several of you have mentioned "street manners". Can you be more specific about what that means? I don't want the car to be unreliable or unstreetable, I just want it to be as quick as I can possibly make it with the current powerplant. Don't care about MPG...just E.T.'s! If a carb change will net me another 20rwhp, then I'll probably do it. With a 2500lb car, those 20HP could equal another .3 of a second....just enough to beat some of these kids with their pesky Hondas...

Unique 289FIA - Viking Blue

[gasman]

Russ,

Quick Fuel Technology carburetors are wonderful! Rick speced the one on my car and after several weeks, I'm still very pleased. In my opinion, vacuum secondaries are the only way to go on cars as light as ours.

Steve Foushee

[Brent]

Street "manners" to me is:

a car that starts easily
idles clean
quick throttle response
easy on spark plugs
low maint.

Brent

[eliminator]

Russ,

Are you Drag Racing or driving on the street? Unless you have a "Dragon Snake" replica you need to make it streetable or some young Kid in a Ricer is going to blow your doors off, Terrible Embarrassing when you Bog Down at the Red Light.

Rick

[GeorgiaSnake]

Russ you may want to check and see which spring you have on the secondaries. It is probably the standard plain colored one. If this is the case your secondaries start to open between 1960 - 2240 rpm and don't fully open until you go over 7000 rpm. This is based on a 402 CI motor. I bought the Holley quick change spring kit and the assorted springs ($6 I think) from JEGS .

I have tried several different springs (all color coded) and settled on the yellow standard lenght. It allows the secondaries to start opening around 1420 rpm and fully open at 5020. The difference is astounding - feels like I have added 100 HP. No bog, stutter, skip - the car just hauls ass down the road.

I have also removed all of the choke assembly and have plans to cut the air horn off. I'll then work on jetting the primary (it has 72's) as the car is on the rich side. I'm also experimenting with the power valve and will settle on one after I get some better vacuum readings.

I'm running a Holley 3310 750 on a 396 Windsor with Edlebrock aluminum heads.

Or you could spend $500 for a Barry Grant.

Randy

Unique 289 FIA
Southern Automotive 396
Team III 16's
Brilliant Silver

[GeorgiaSnake]

Up date on the parts and prices from JEGS.

Quick Change Kit 510-20-59 $6.99
Spring Kit 510-20-13 $8.99
Jets (various) 510-JETS $7.99
Power Valves Depends $6.99

Randy


Unique 289 FIA
Southern Automotive 396
Team III 16's
Brilliant Silver

[Silk]

Russ,

I've got a good, solid 351 and initially had it set up with an Edelbrock 750 which I didn't like at all. Talked to a lot of Ford performance people and the Holley tech guys and finally went with a Holley 650 HP Pro double pumper. Mechanical secondaries, no choke. It sets up much better than the older double pumpers (the HP Pro was Holley's answer to the Barry Grant, et al). Much better throttle response, great tuning flexibility. You may lose a couple of HP above 6000 rpm dropping to the 650, but most of us don't spend much time in that realm anyway. Holley guys almost begged my to go smaller. Said that most people are mistakenly convinced that bigger carbs mean better performance. I like the HP Pro both on the street as well as on the track. Whether you go with vac or mech secondaries, I bet you'll like the 650 much better.

Gary
Unique FIA 351W

[Russ Dickey]

You guys are awesome. Thanks for everyone's input.

To answer your question Eliminator, my car is just a street cruiser which I occassionally run down the dragstrip or open track, but I still want it to be as quick as I can possibly make it. Just as you said, losing a stoplight challenge to a ricer is horribly embarrassing, and while I realize that there are plenty of wicked-fast imports out there who will smoke me, I want to do anything I can to prevent it.

I'm trying a little experiment today. A buddy loaned me his 650 Edelbrock Carb with mechanical secondaries. I'm switching it out and heading back to the dyno tomorrow. I'll post the results....

Thanks again to all!

Russ

Unique 289FIA - Viking Blue

[excelguru]

Got some dyno results for us?

Keith :)
Unique Motorcars (http://www.uniquecobrareplicas.com)
Southern Automotive (http://www.southernautomotiveengines.com)
Bruce Bunn (the painter) (http://www.jodecoautobody.com)

My Photo Album (http://members5.clubphoto.com/keith355593/owner-429c-1.phtml)

[spd4me]

My other Cobra had a HI-Po 289 bored to 302 with 302 heads and a Motorsport cam. I put a 600 cfm double pumper and it worked great. If you punch it below 2800 rpm, you might have felt a small hesitation but the best thing I did to boost performance was put a 1" spacer between carb and manifold. This really boosted the power from stop up to 3000 rpm.

[Russ Dickey]

quote:Originally posted by excelguru

Got some dyno results for us?

Yep. Carb was actually an Edelbrock 600CFM with mechanical secondaries. This time the car dyno'd at a dismal 245 rwhp, and did have a slight hesitation when I stood on it and the secondaries opened up. Dyno techs said that now it's not getting enough gas, and that I should really get a 650-670cfm carb. Guess I'll go spend some money....

Russ

Unique 289FIA - Viking Blue

[Brent]

You need some Webers :D

330 cfm EACH. :D

Brent

[clayfoushee]

......AND....something to polish your new velocity stacks with:D

Clay

'98 Unique #9299, 427FE side oiler, top-loader (close), 3.31 rear

Annapolis, MD

[GeorgiaSnake]

Russ you need to read some info on carbs. If the gas flow is correct the cfm has less to do with the equation than most think. The engine is just a big air pump and will only flow a given amount of air. You can get the fuel air mixture right with sevaral different size carbs. http://www.bob2000.com/carb.htm is a good place to start.

Randy

Unique 289 FIA
Southern Automotive 396
Team III 16's
Brilliant Silver

[rdorman]

I think this is always your best bet. Call the manufacturer. Give them all the information required. Buy what they recommend. Have it professionally tuned......... then forget it!
Rick