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  1. #1
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    Vacuum or mechanical?

    A thread elsewhere about carbs reminded me that I've been meaning to ask someone who knows more about these things than me (that's you lot) a question..

    Vacuum or mechanical secondaries?

    To be more specific, on a built 428FE (that might end up stroked, depending on the state of the rotating assembly on inspection), intended to end up in a Cobra, with a T56 transmission.

    Your thoughts, kind gentlemen?
    My DeDion build diary..
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  2. #2
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    Hi Aaron

    Loads of power, lightweight and short(ish) gearing, I'd go for mech secondaries. I started with Eddy carbs on my car which aren't quite true vacuum secondaries like a holley, but are sort of close enough and have since changed to mechanical secondaries and now run a holley clone quick fuel double pumper.

    More experienced guys may be able to refine my words but what I've learned, very loosely speaking just to get the general idea across... vacuum secondaries mean you can just generally boot the throttle whenever you like and the carb will only open by as much as the engine needs. With mech secondaries you can bog the engine by giving it too much but from what I've found in something like a cobra is that's only a problem if you are flooring it in 5th at 1000rpm. What's your final drive ratio? That might have a bearing if you have built your car for snapping necks or for cruising at idle revs. Mine is 3.54:1 and the revs tend to rise so quickly its not a problem, or with good thumping cobra torque the wheels just spin and again.. engine bogging is not a problem, use the wheelspin as a second clutch

    Vacuum secondaries make your engine/carb combo more tolerant of an oversized carb (in terms of CFM). If the carb is way to big then the vacuum secondaries just wont open fully. With mech secondaries as mentioned above you whack the throttle open all the way and the air flow almost falls to zero, and carbs rely on the air flowing in order to meter fuel properly so you get a nasty bog and you spend your time chasing your tail with accelerator pump tuning when the real problem is your carb is too big. This problem can be reduced by correct carb sizing for mech secondaries. I've seen this first hand for myself with the difference between 650cfm and 750cfm on my 383 SBC. The general rule of thumb is to select the carb size you think you want, then go down a step. Same thing as with cams, try to resist picking huge number for bragging rights.

    If you had a massive heavy, tall geared land yacht and an auto gearbox then vac would definitely be the way to go.

    Don't let people tell you mech secondaries get terrible economy and there's nothing you can do about it. My old Eddy carb got around 18-20mpg on my 383. Out of the box my double pumper mech secondary got 12mpg but with a bit of tuning I now normally get 17-18mpg and I probably drive more enthusiastically with this carb as the throttle response and pickup is sharper.

    Another point is mech secondary just looks cooler, "double pumper" always gets a snigger and the look on peoples faces when you show them the carb and press both pumps to get four waterfalls of fuel never gets old

    regards

    Dave A
    Last edited by hifihedgehog; 08-03-18 at 12:31 PM.

  3. #3
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    Thanks Dave - that confirms what I thought, and why I was looking at mech secondary carbs. The engine builder, however, was recommending vac secondary (bore wash and over-fuelling being the rationale).

    FWIW my final drive is 2.88:1 and the T56 has a fairly long sixth (0.63) so the top end is definitely geared for cruising rather than hurtling, but even then I was thinking mech secondary.

    But.. I'll get the engine built the way the engine builder desires, I imagine, so there's a chance of a warranty Can always change later..
    My DeDion build diary..
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  4. #4
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    Dave has put up a good summary there.
    I'd go vacuum myself.
    This is where I'd be with your engine.
    https://www.holley.com/products/fuel.../parts/0-85770
    Don't be tempted to go any bigger.
    My CCX 445ci FE had a 2.88 and .63 final drive too, this was just the ticket.
    I removed the 850 double pumper I had on previously.
    Along with fitting a milder cam.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
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  5. #5
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    my 5th gear is 0.8 so coupled with my 3.54 final drive its a lot shorter than your combo. It sounds like you might want to consider Kev's advice if the character of your car is a low rev cruising torque monster that's going to be pulling from 1000-1200rpm. Mine is a different character and I tend not do drive below about 1500-1800 because I didn't heed my own advice when choosing my cam

    I may be completely wrong but the way I understood it was if you open mech secondaries too far then air velocity drops, fuel metering then also stops so you have a lean condition. Bore wash is caused by overly rich condition. Over fuelling can be solved (as I did myself) by swapping jets and air bleeds.I understand what bore wash is and how bad it can be but citing that and over-fuelling as a reason seems like a cop out for just not bothering to set it up properly... although I can't compare my own knowledge and experience to a pro engine builder...
    Last edited by hifihedgehog; 08-03-18 at 04:14 PM.

  6. #6
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    The general consensus is that mechanical secondary carbs are slightly better for lighter, manual transmission vehicles with the big proviso that you size them correctly for the car as they will be less forgiving than a vac sec carb.

    Why the vehicle weight is important, well I'm assuming here, but you'd think the lighter car doesn't allow the engine to build as much vacuum and so is slow to open the secondaries? Given that Holley secondaries are tuned by changing the spring, I'm not really sure why you can't tune that out? I'm sure others (Dave?) will know more about this.

    Either way, getting the engine dyno tuned is a must. Holleys can be very lean out of the box.
    Lloyd B
    Current: Crendon #54 in build - 427 Side Oiler/Cobrajet Heads/Dual 600cfm Holleys/4 speed toploader/Vintage cast knock on wheels
    Dedion Dax/Clarkson 383 Chevy with roller 4/7 swap cam, AFR195 heads - SOLD

  7. #7
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    I think a heavy car is more likely to take a little while to get moving so with mech secondaries if you blindly floor it at low revs there's more chance the engine could be spending a little time chugging away secondaries wide open and completely useless air/fuel ratio doing damage until it picks up. Lighter car it will zip up a lot quicker to higher revs where it actually needs the secondaries open. Even though I have mech secondaries, on the road in "real" driving vac sec all the way. If your racing and the only reason your not 5-7krpm and foot flat is because there's a yellow flag then yeah you should be on mech secondaries. The reasons why I run mech secondaries is mainly just because I thought it was cooler, and it was something new and interesting to learn, and if I really try hard to convince myself I think it feels like it gets back on the power harder and sharper after a gear change, but I could be imagining that. I can tell if I'm too much in the throttle at too low rpm by keeping an eye on my vac gauge and AFR meter. I like that sort of thing, getting the optimum myself and trying to understand whats going on and why.

    On another note my holley clone was outrageously rich out of the box, a fair few holley users in the US say on a few forums they are like that because although ultimately not brilliant, too rich is safer than too lean so they are calibrated on the side of caution for that reason however something about modern fuel blends mean nowadays the mixture ends up leaner for the same calibration? It's something I'm still learning about and don't fully understand tbh.
    Last edited by hifihedgehog; 09-03-18 at 07:47 PM.

  8. #8
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    Anyone tried running Quick Fuel adjustable vacuum secondary units? Aftermarket and fit Holley carbs. Just put them on my dual quad 427 (2 x 600cfm Holleys) and they are amazing. They come with the purple rated spring but you have a little screw to advance the opening of the secondaries. No messing with changing springs and very effective. Easy access under the air cleaner and you can adjust on the roadside with a screwdriver. Leaflet says you can tune to suit road conditions- not gone that far!!
    Got mine from Peter Knight at Knight Racing Services, Daventry

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