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  1. #1
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    Carburettor advice (Ford 302)

    Hi Guys,

    I am replacing the old Holley 4 barrel on my 302, primarily because it doesn't have a choke and struggles starting, but in for a penny in four a pound and now I want to get the most out of my engine.

    I would love to fit Webbers but the £3k price tag is beyond me so I would like some advice on what carbs you have experience with and would recommend?

    (Ford 302, 10.5:1 Pistons, GT40 Heads, Crane fireball cam)

    Thanks,
    Andy

  2. #2
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    This is where i'm at...


    I have been considering:


    Webbers - (If the scratch cards come good)


    EDELBROCK 1405 600CFM - The safe bet


    Edelbrock AVS2 650 - May be too much?


    Then I read the below... and started looking at Holley 0-3310SA 750 CFM 4 bbl


    Here's the deal on carbs. You can take a general V-8 engine and put a 650 double pumper carb (Holley #4777) on it. It will drive OK but probably be a turd out of the hole and get horrible gas mileage. You'd be much better off with a 750 vacuum secondary carb (Holley #3310) instead. Yes, CFM wise it's bigger, and technically it is overall BUT, because it is a vacuum secondary, you are only driving around on the primaries, even if you nail it to the floor off the line you are only opening the primaries. This means you are actually driving around with a 375 cfm 2 barel until the engine pulls enough air speed through it to open the secondaries. So, is driving around with a 375 cfm 2 barrel too big of a carb for your engine? Hardly! Always remember this; when you put a carb that is WAY too big on your engine, it will actually run LEANER than it will with a smaller carb. This is because it takes AIR SPEED to PULL fuel through the boosters. Remember that shop vac analogy I used earlier? Putting too big of a carb on is like removing the tip from your shop vac hose. The engine takes a big gulp of air but it can't siphon fuel through the main jets and past the boosters because tyere is no air speed to pull it through. Carbs don't GIVE the engine fuel with pressure. The engine pulls fuel through the carb via a "venturi effect' and velocity. No velocity = a lazy carb, no fuel delivery, and a sluggish engine, especially at lower RPM's.




    To be honest i'm carb a newbie and finding myself out of my depth and any advice from you knowledgeable people would be greatly appreciated.


    Thanks,
    Andy

  3. #3
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    Longridge, Lancashire
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    Hi Andy,

    I've got an Edelbrock 1406 600cfm on my 347 (stroked 302). Had it dyno tested and setup by Peter Knight racing. Gives great torque and power curves. Really looking forward to driving it but still got a little way to go yet.

    Cheers,
    Stu
    AK427 Gen III, Ford 347, TKO600
    Build Blog:- stu238-ak427.blogspot.co.uk

  4. #4
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    That's going to be an awesome motor when you get it back! I'm a little jealous!

  5. #5
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    Hi Andy

    I've had a few different sized carbs with different secondary styles on my SBC 383. My experience may help you decide.

    I started with an Edelbrock 1407 performer 750cfm vacuum secondaries. This came with the engine back when I knew nothing about carbs. Seemed fine to begin with but I had nothing to compare to, having never driven a carb'd car before the cobra. The secondarys are vacuum (sort of, for simplicity lets assume they are) When I started to learn about carb tuning and had a go tuning myself my experience of these carbs is that they are fine for cruising and probably perfectly fine for a big heavy automatic car but for something like a cobra, it just couldn't keep up with rapid throttle movement when smashing through the gears. I could cover the inevitable bog a little bit with tuning but they are not very flexible on secondary tuning. The secondary opening point is not adjustable unless you start grinding counterweights in the mechanism. Beyond the idle mixture screws, the idle circuit as a whole (i.e. also feeding the transition slot, important for low speed driving) is also not adjustable unless you start drilling. It's perfectly fine for a mild cam but anything racy with lots of overlap the idle circuit soon runs out of headroom so I really struggled to get good throttle response in town driving. Best mpg on a run I got was about 17

    Then changed to Edelbrock AVS Thunder 650. Adjustable secondaries by turning a spring. Not quite the same as the Holley design where you have a range of springs to swap out, but achieves the same thing. The change from the 750 eddy to the 650 was night and day. Smaller primaries meant that fuel metering was much better at small throttle openings and low rpm. Noticeably smoother and far more responsive. My original big carb was ok for top end but took a great deal of work to cover up a bog on acceleration. The 650 was great though. I didn't notice any loss of top end performance on the seat-of-pants dyno. A real dyno confirmed (same dyno used for original 750 carb) peak hp was basically the same. peak torque actually came in at slightly lower rpm, and gained about 10lbft if i remember correctly. Best mpg on a run was about 20

    The 650 AVS did have a tiny tiny bog that I could never quite get rid of but in all honesty it was perfectly fine. It's only because I had an AFR gauge that I convinced myself I needed to tune it out. In reality I could barely feel anything. I drive with the AFR gauge off nowadays unless there's something really wrong because I realised I was wasting my time agonising over the AFR value and not enjoying driving.

    After that, changed to a Quick Fuel (holley clone) SS-750-AN, 750cfm annular boosters, mechanical secondaries.
    I changed to the Quick Fuel 750 mech sec. during my AFR obsession period. Due to the booster design its not really 750cfm, its more like 730. My god this carb is absolutely brilliant. Everything is adjustable. Idle ports, high/low speed air bleeds for fine tuning AFR, accel pump cams, the lot. All of that adjustability, especially of the idle circuit, means I have it really well matched for my fairly big cam (comp cams XE-284h). Adjustability of the air bleeds for AFR fine tuning means that my top end improved as well as I was able to shape the AFR curve exactly how I wanted it. Revisited the same dyno as before and gained 30hp. Annular boosters do some magic with making the fuel flow more even so torque went up again a smidge (another 15lbft or so) and peak torque came in another 500rpm lower, helping low and midrange driveability. The mech secondaries deliver instant floor-it response far better than both Eddy carbs ever did. Best mpg on a run, 19-20 ish.

    My summary is the Edelbrock performer range is probably fine if you have a run of the mill engine that doesn't really push the boundaries in any way, or if you have a defective backside and either can't feel or simply don't care if things aren't performing at their optimum.
    Eddy Thunder AVS range if you've got something a little out of the ordinary and need some better adjustability, slightly hotter cam for instance. Carb too big is way worse than carb slightly too small. 650 will be fine.

    In your case, 650 AVS vs 600 Performer? Not too sure but I think the 650 edges it because the secondary is more easily tunable. You may be lucky and the 600 could be spot on, but if its not its very difficult to adjust. 650 down to 600 on your 302 you might start to lose a little bit of top end if you have good flowing heads and a hot cam. I could be talking rubbish here though as its slightly outside the range of my direct experience so I'm guessing a bit.

    Holley mech secondary (double pumper) if you really want to roll your sleeves up and get involved and want to be able to adjust everything under the sun to get things as good as possible. My quick fuel carb straight out the box was awesome for wide open throttle but absolutely abysmal everywhere else. I mean laughably bad. A few months of tuning and I now get pretty much as good driveability as the 650 eddy and amazingly the same average mpg on a long run due to tuneability of the idle and transition circuit.

    I have no experience of Holley vac sec carbs but I expect they will be absolutely fine, will probably be ok either 650 or 750 however personally I would go 650 for better low speed driveability and may be even better for a quick out-of-the box solution on a road car than a mech secondary double pumper.

    Anyone who says double pumpers get terrible MPG just haven't put enough effort into tuning them properly, or they have a timing problem and are blaming it on the carb

    regards

    Dave

    P.S> Extra bonus info if you haven't fallen asleep yet - related to what you read and shared above, that's also my understanding, exactly why vacuum secondaries are more forgiving. The engine only takes what it needs so a vac secondary carb is also more tolerant to you choosing the "wrong" cfm. Mech secondary carbs will be far more picky about the cfm value and will be difficult to tune if you slightly beyond what they are designed for. Regarding 650 vs 750 what I've learned from reading and experienced first hand is that the smaller primaries of the 650 means that the vacuum is generally higher, and its the vacuum that pulls fuel through so high vacuum = nice steady pull of fuel so the fuel flow is even and precisely metered. Low vacuum pulls the fuel through in uneven blobs and the mixture suffers accordingly giving poor driveability. It also depends where the throttle butterfly is in relation to the idle circuit and transition slot.

    The transition slot works hard at low throttle gentle driving and fills the gap between the idle discharge port and the main circuit. The main circuit needs a fair old bit of air flow to activate. All the time your rumbling around town below 2krpm you're probably nowhere near using the primary jets. On my 750 eddy combined with hot cam I had to have a really rich idle mixture to cope with the cam overlap, and quite a large idle throttle opening. This meant that when driving around town, the transition slot was not properly exposed in its normal range so the fuel was not metered correctly = terrible driveability. I could compensate by turning the idle mixture screws well out to try and get more fuel but it was really just chasing your tail and not solving the problem. I even drilled holes in the throttle plates to allow me to close the idle screw down as far as possible. It helped a little bit but didn't completely cure the problem.

    The smaller primary of the 650 meant the vacuum was higher at idle and the fuel was pulled through smoothly so was a more stable mixture. This also means that with the throttle idle position adjusted to the correct range when cruising on very light throttle you were properly in the transition slot and everything was working nicely.The transition slot would work properly up to when the main jets come in, which depending on your setup may not be until 2500rpm or so, (varies massively but guideline rough figure).

    The quick fuel carb I have now solves all these problems by simply letting you adjust everything. Loads of replaceable brass restrictors all over the place so if you know what you are doing, (and its taken me 5 years of fiddling with various carbs to get anywhere near the level of knowledge required) you can do a good job of it. Also this carb was designed for hotter cams so the idle circuit is nice and fat with fuel as standard so it can cope with something with a hefty dose of overlap. I had to drill my Eddy AVS 650 to increase the idle fuelling. It did work, but it was VERY nerve wracking. The risk of course with the super duper holley/quick fuel carb is that with so much stuff to tweak with, you can easily just completely ruin it, but as everything is screw-in brass bits you can always put it back to how it came out the box, unlike the Eddy carbs where beyond jet sizes you have to start drilling and making permanent changes.

    All of the above is my own words from my own reading and experience and some of it may be overly simplified but is probably generally speaking somewhere approaching sensible information
    Last edited by hifihedgehog; 20-02-19 at 12:31 PM.

  6. #6
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    quick fuel do a 650fcm annular booster model, I am not a sales rep for quick fuel

  7. #7
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    Dave,

    "If you haven't fallen asleep yet" Not a chance... that is an awesome response! People like you make these forums great!

    Your experience with the Edelbrock AVS Thunder 650 sounds very good and the simplicity of theses carbs appeals to me and my novice status. As the Edelbrock AVS2 650 is basically a Thunder 650 with annular boosters, this should eliminate any bog which gives me a lot of confidence that it's the carb for me.

    The Quick Fuel SS-650-AN looks a really impressive carb (especially with its sexy red bits haha), but as I don't have 5 years to build my knowledge and a Matrix style "I know Kung Fu" machine hasn't yet been invented, I think i'll keep it simple with the Edelbrock. It also looks like I would be using a mallet to crack a peanut, as the premium paid for the Quick Fuel seems aimed at people who want to be able to regularly tune from road to track performance, run on alternative fuel and with boost. Basically people with more experience and track focus than me, I'll be hitting the track a couple of times a year with the Cobra, it is far more likely to be on A roads to car shows and the annual Le Mans trip.

    Maybe in a few years i'll have gone on a similar journey to you and be fitting a Quick Fuel!

    For now, i'm off to order my Edelbrock AVS2 650. I'll report back once it's in the car

    Thanks Again,
    Andy

  8. #8
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    what, wait a minute, you're actually going with my advice? brave man!

    I'm sure it will be fine, and when you get it installed it should either be fine or very close to fine, and tuning the jets and rods on eddy carbs is quite intuitive, a few on here have done it with great results, Denis (oglyoodoo) in particular is very knowledgeable.

    I believe the fuel inlet for eddy is on the other side to Holley. If your lucky I think the eddy may have a symmetrical casting and on the newest designs you can swap the hose barb for a blanking plug and have the inlet on either side. PCV hose connection is at the front instead of the rear so that might need some hose re-routing to bear in mind


    Dave A

  9. #9
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    I always take good advice as it's always useful to have someone to blame if it all goes wrong!

    You're right the blanking plugs can be easily swapped around and re-routing the PCV hose is just about within my capabilities.

    Took a gander at your blogspot, very handsome Cobra you have there! Off topic; I noticed you haven't fitted stone guards. I have just got mine back from a the bodyshop with a fresh lick of paint and really don't want the stone guards back on. How are you arches holding up?

    Andy

  10. #10
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    Thanks! I found the interior the hardest bit to do. I'm most proud of the way the dashboard came out but the carpet caused us grief. Wife and I did it ourselves instead of shelling out £££ for Dax carpet kit and some of it I'm not entirely happy with, but its not quite annoying or bad enough to rip it out and start again.

    Regarding the rear arches 5 years and 7000 miles they do have a few chips on them but old english white paint over grey gel coat hides them well

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