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  1. #1

    Default 650dp or 750dp on 347?

    I've bought a couple of Ford books and from what I've read, I'm right on the edge of 650dp to 750dp. When you read the mag articles they use 750 and in the books they say 650. Even when trying to sell my holley I have a couple guys asking me if the 650 would work on the 350.

    I know that mods play a role in choosing the right carb, so I've got a new 347 on the stand with heavy ported Edel Perf RPM heads, Dual plane Edel Perf RPM intake, 10.2CR, 1.6 rr, 218/228 545/545 cam, all MSD and equal shorties.

    If I do stick with the 650, I'd have to rejet, right now I've got 70 mains and 78 second, I don't recall what squirts I have but I'd think have to go bigger.

    I think maybe 74-75 main and 86-87 seconds?
    85 GT built 347, Steeda t5, 3.55s, full susp, real Saleen body kit, Saleen rims...for go but need paint to show!
    99 TA ram air Daytona Pace car, 6spd 520rhp/497tq, full featured car Pont High Perf. 10.89@128 and 172mph in the mile run.
    00 TA boltons auto, the driver.
    01 TA ram air 6spd vert, just boltons, the cruiser.

  2. #2
    FEP Super Member SVT Rob's Avatar
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    650 double pumper should be more than enough carb for a 347. You could downjet a 750 double to do the same thing, but you're still going to run rich at idle.

  3. #3
    FEP Power Member wgt500's Avatar
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    650dp or 750dp on 347?
    Neither! Running "double pumper" carburetors on a street engine is kind of like running "locked" centrifugal ignition advance on the street.
    '67 Shelby GT500 428 4spd. (owned for 32yrs. SOLD 11/26/2011. A sad day $$$$. Pics in my album!

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  4. #4

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    ^^^ Agreed. Seems a nice vacuum secondary 700 would split the difference here, and
    be much more streetable than any double-pumper.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
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  5. #5
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    If its gonna mainly be a street car, I also prefer a Holley vacuum secondary carb, but I`m not sure if Holley has anything between a 600 and 750/780. For what its worth, my 85 Mustang has gone 11.6s in the 1/4 mile with a iron World Windsor Jr. headed 306 using a 1850 Holley 600 vac on an AirGap intake.
    The current 331 has AFR 185 heads, and a solid flat tappet cam a fair big larger than the cam listed in the OP, and ran mid 10s all last year, with a best of 10.29, using a stock 650 DP 4777 Holley. So a 600 vac would likely be adequate.
    I even use a Holley vac carb on my 427 FE BB Fairmont, and its gone 9.9s with a 3310 780 Holley. I think a lot of guys THINK they need a bigger carb than they actually do. A typical street car is not revving 7000 RPM with 100% volumetric efficency, so those charts showing carb size applications need to be studied realistically.
    1978 Fairmont 2 door sedan, 428CJ 4speed. 9.972ET@132.54mph. 1.29 60 foot
    Replaced the FE big block with my 331/4 speed in my Fairmont, best 10.24ET @128 MPH.
    1985 Mustang LX hatchback NHRA Stock Eliminator 302 4 speed best in legal trim 12.31@107 mph, but has gone 11.42@115 with aftermarket intake, carb, and iron Windsor Jr. heads.New for 2012! 331 cube SB Ford, AFR 185 heads, solid flat tappet cam, pump gas; 10.296ET@128.71 mph, 1.37 60 foot.
    1979 Zephyr Z7, all original 302 auto, 2nd owner.

  6. #6
    FEP Super Member FM2NOTCH's Avatar
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    your cam is kinda small for a 347, probably won't need to spin it past 5500 rpm, so a 600 or 650 should be enough carb.

  7. #7

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    I'll be the first to say I don't know carbs very well.

    So whats wrong with a DP? Thats all I've ever seen on most modded old school V8s?

    How does a vac second work? Where does it pull the vac from? Intake?
    85 GT built 347, Steeda t5, 3.55s, full susp, real Saleen body kit, Saleen rims...for go but need paint to show!
    99 TA ram air Daytona Pace car, 6spd 520rhp/497tq, full featured car Pont High Perf. 10.89@128 and 172mph in the mile run.
    00 TA boltons auto, the driver.
    01 TA ram air 6spd vert, just boltons, the cruiser.

  8. #8

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    You will see better mileage with a vacuum secondary & nowadays enough parts exist to make one perform just as well as a Dp. The secondaries operate off of manifold vacuum. I would play with the carburetor you have as I agree with FM2NOTCH in it being adequate for your combo. If you would happen to go to a more aggressive cam a larger carb might be needed. I used a 3310 750 vacuum secondary with good results for years on my 10.5 compression 306 but my cam was much larger.

  9. #9

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    For Mustangs I thought that cam was a decent size. I'm running a decent CR at 10.2.
    85 GT built 347, Steeda t5, 3.55s, full susp, real Saleen body kit, Saleen rims...for go but need paint to show!
    99 TA ram air Daytona Pace car, 6spd 520rhp/497tq, full featured car Pont High Perf. 10.89@128 and 172mph in the mile run.
    00 TA boltons auto, the driver.
    01 TA ram air 6spd vert, just boltons, the cruiser.

  10. #10
    FEP Power Member wgt500's Avatar
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    With a double pumper carburetor, the secondary throttle plates open starting when the primary throttle plates are about 2/3's of the way open. When the primaries are fully open, the secondaries are fully open regardless of the engine's demand for air. When the secondaries are forced open at low rpm by mechanical linkage on a street engine, there will be a hesitation followed by slow acceleration. The hesitation takes place because the engine isn't pulling enough air volume at low speeds to cause the secondary venturi to flow fuel. The addition of a second accelerator pump, (double pumper) on the secondary side of the carburetor is used to "inject" additional fuel to mix with the big gulp of air diverted to the secondaries from the primary side of the carb.when they were forced open before the engine demanded the extra air. This secondary accelerator pump shot of fuel is the only fuel the air flowing through the secondaries will get until air flow rises to the point the venturi will flow fuel. This takes place after a lean condition which actually slows acceleration. Yes, you will feel the secondaries "kick in", but what you are feeling is a hesitation. On the other hand, with a light drag car and very low,(high numerical) gearing that launches at 3000rpm, the engine will have the demand for secondary air flow almost instantly after leaving the line, long before a vacuum diaphram could open the secondaries. With this car the secondaries need to be forced open very quickly by mechanical actuation. The secondary accelerator pump is simply preventing a lean condition until the secondary venturi start to flow, which is almost instantly. As you can see, the mechanical secondary carb,(double pumper) is not designed for low rpm use, such as seen by a street car.
    A vacuum secondary carb. uses air flow through the primary venturi to open the secondaries. As air flow increases in the primary venturi to where the engine needs additional air, a vacuum diaphram opens the secondary throttle plates. Since the engine is demanding more air and the secondaries open slowly there is no hesitation, just a smooth transition to all four venturi operation. This type of "on demand" air/fuel flow can take place at 2500rpm or 4000rpm whenever the engine needs the additional volume of air. You can mash the pedal to the floor and the secondaries will not "kick in" until the engine actually needs the additional air/fuel flow, regardless of rpm.
    Remember, the engine is just a big air pump and it determines how much air it needs; the driver doesn't. On the street with a mechanical secondary, (double pumper) carb. all the driver can do by mashing the pedal to the floor at low rpm, is slow down the air flow through the carb to the point of inefficient fuel metering and hesitation.
    Hope this helps!
    '67 Shelby GT500 428 4spd. (owned for 32yrs. SOLD 11/26/2011. A sad day $$$$. Pics in my album!

    '83 Capri 5.0 4spd. ASC McLaren Sport Pkg.(1983 Indy 500 Speed Week Motorcraft sponsored parade car) Original paint. Pics in my album! (SOLD 11/11/2017)

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  11. #11

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    Wow that was perfect! I really understand. Well the only thing I can say is the car is geared for something like this. For me though I don't like it. I have a steeda T5 with close ratio gears and 3.55 gears. Just to give you an example, the RPMs are 2800@70mph. With this 347 it should rip the tires off.

    Any thoughts on where I should start on the jets?
    85 GT built 347, Steeda t5, 3.55s, full susp, real Saleen body kit, Saleen rims...for go but need paint to show!
    99 TA ram air Daytona Pace car, 6spd 520rhp/497tq, full featured car Pont High Perf. 10.89@128 and 172mph in the mile run.
    00 TA boltons auto, the driver.
    01 TA ram air 6spd vert, just boltons, the cruiser.

  12. #12
    FEP Power Member horsepowerjunkie's Avatar
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    My personal preferance with a performance V8 & a manual transmission would be a double pumper. When I'm banging gears, I don't want to wait for the vacuum secondaries to open. I have the 650 Quick Fuel HR series on my 306 Capri and it works great, street or track! Just bolted it on and drove away, it ran great from the get go. The jets were almost spot on. I had a VS Demon carb before & this carb is a night & day difference. I'd keep the carb you've got & jet it accordingly!
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  13. #13
    FEP Power Member wgt500's Avatar
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    Quote Originally Posted by ryanws6 View Post
    Wow that was perfect! I really understand. Well the only thing I can say is the car is geared for something like this. For me though I don't like it. I have a steeda T5 with close ratio gears and 3.55 gears. Just to give you an example, the RPMs are 2800@70mph. With this 347 it should rip the tires off.

    Any thoughts on where I should start on the jets?
    Yes! Buy a wide band air/fuel gauge and let your engine decide what it wants. Anything else is just an uneducated guess.
    '67 Shelby GT500 428 4spd. (owned for 32yrs. SOLD 11/26/2011. A sad day $$$$. Pics in my album!

    '83 Capri 5.0 4spd. ASC McLaren Sport Pkg.(1983 Indy 500 Speed Week Motorcraft sponsored parade car) Original paint. Pics in my album! (SOLD 11/11/2017)

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  14. #14
    FEP Power Member Ethyl Cat's Avatar
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    Quote Originally Posted by ryanws6 View Post
    I've bought a couple of Ford books and from what I've read, I'm right on the edge of 650dp to 750dp. When you read the mag articles they use 750 and in the books they say 650. Even when trying to sell my holley I have a couple guys asking me if the 650 would work on the 350.

    I know that mods play a role in choosing the right carb, so I've got a new 347 on the stand with heavy ported Edel Perf RPM heads, Dual plane Edel Perf RPM intake, 10.2CR, 1.6 rr, 218/228 545/545 cam, all MSD and equal shorties.

    If I do stick with the 650, I'd have to rejet, right now I've got 70 mains and 78 second, I don't recall what squirts I have but I'd think have to go bigger.

    I think maybe 74-75 main and 86-87 seconds?
    If those heads are done right, that engine will make peak hp near 6K and yes depending on VE% you are going to push the limits of a 650.

    The good thing about a dual plane is the lack of plenum volume, which transfers more of the piston demand to the carb(increases signal strength).

    I usually will run a bigger carb on a dual plane as opposed to a single plane intake for this reason.

    I tend to respectfully disagree with what wgt500 is saying that "the engine determines the air it needs, the driver doesn't".

    I say the engine wants all of its air all of the time and the driver tells it how much it is going to get. ANYTHING less than max air is creating negative work(pumping loss) and decreasing HP. This is how we control our speed.

    Proper engine design and tuning of a double pumper along with ignition timing yields violent, instantanous power without hesitation.

    I would run a 750 on your combo, but you will be happy with either.

  15. #15
    FEP Power Member Ethyl Cat's Avatar
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    I do not think you are too far off on jet size for up to about 6K

    Your engine @90% ve with a WOT AFR of 12.6(safe) @6K rpm would need about 74s square.(2176cc/min)

    You have a power valve in the front so you can jet it down for efficient cruise mpg.

    My "uneducated guess" would be you are close

    2 70 jets=890cc/min
    2 78 jets=1290cc/min

    890+1290=2180cc/min

    So I do not think you are but a couple of jet sizes off of target.

    If you get cruise right and it is lean on top, drill out the PVRC channels for additional WOT fuel.

    Getting a wideband is truly the key to success.

    Glad to see you are moving along with your car.

    Best of luck!

  16. #16
    FEP Power Member wgt500's Avatar
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    Quote Originally Posted by Ethyl Cat View Post

    I tend to respectfully disagree with what wgt500 is saying that "the engine determines the air it needs, the driver doesn't".

    I say the engine wants all of its air all of the time and the driver tells it how much it is going to get. ANYTHING less than max air is creating negative work(pumping loss) and decreasing HP. This is how we control our speed.
    Just had to reply! What you are saying, I totally agree with. The engine wants to operate at atmospheric pressure which would be wide open throttle. The Otto cycle P.V. diagram illustrates the pumping loss.
    What I was trying to state in simple terms to a person who doesn't know much about carburetion, is you can't give the engine more air than it demands by simply opening secondaries. If it is running at atmospheric pressure,(WOT) and not flowing the primaries to a pressure drop, (low speed operation) all you do is lower velocity through the venturi by opening the secondaries. Depending on the engine speed and air flow this can create a lean hesitation. The engine will not flow anymore air at atmospheric pressure until speed increases.
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  17. #17

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    Quote Originally Posted by ryanws6 View Post
    For Mustangs I thought that cam was a decent size. I'm running a decent CR at 10.2.
    Your engine will surely run very well with the chosen cam but if you want it to really run it needs more . Those heads work very well ported & will easily make 400+ hp on a 347... With the right bumpstick. Check out Comp Cams Extreme Energy lineup. In my opinion, either the XE 274 hr or XE 282 hr could be used with very good results. These cams are not huge by any means & can be run on the street with no problems. I would use a 750 if you do consider a cam swap.

  18. #18
    FEP Super Member SVT Rob's Avatar
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    Quote Originally Posted by FM2NOTCH View Post
    your cam is kinda small for a 347, probably won't need to spin it past 5500 rpm, so a 600 or 650 should be enough carb.
    With the Edelbrock RPM heads, peak RPM will be determined in how the cam is degreed. Even with worked over GT40P's, I'm making peak power at 6k rpms.
    Quote Originally Posted by Farmer83 View Post
    You will see better mileage with a vacuum secondary & nowadays enough parts exist to make one perform just as well as a Dp. The secondaries operate off of manifold vacuum. I would play with the carburetor you have as I agree with FM2NOTCH in it being adequate for your combo. If you would happen to go to a more aggressive cam a larger carb might be needed. I used a 3310 750 vacuum secondary with good results for years on my 10.5 compression 306 but my cam was much larger.
    I've always had better luck with a mechanical secondary carb, be it a double pumper or not. I'm currently running a 650 DP on my 306, with 10.0:1 compression and the XE274HR cam. Whoever did the carb tuning, did a great job, because it always had instantaneous throttle response, even flooring it at 1500rpm. With the right tools and parts available, you can definitely make a vacuum secondary carb perform just as well as a mechanical secondary carb.
    Quote Originally Posted by Farmer83 View Post
    Your engine will surely run very well with the chosen cam but if you want it to really run it needs more . Those heads work very well ported & will easily make 400+ hp on a 347... With the right bumpstick. Check out Comp Cams Extreme Energy lineup. In my opinion, either the XE 274 hr or XE 282 hr could be used with very good results. These cams are not huge by any means & can be run on the street with no problems. I would use a 750 if you do consider a cam swap.
    I agree with the above. OP, you have a decent amount of lift on that cam but not much in the way of duration. Running split lift/split duration profile will generally make you more power. The XE274HR is a 224/232 duration, .555/.565 lift cam.

  19. #19

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    This isn't just about airflow.

    The engine wants all the air/fuel mixture it wants, which doesn't necessarily mean more
    open throttle bores. Opening the secondaries before the engine is ready for them does not
    make the engine produce more horsepower, any more than putting 400CFM intake ports
    on a 6000 RPM 347 would. Ask anyone who's driven the original Boss 302s on the street
    how that worked out. Everything needs to be sized to promote flow velocity, from the
    carburetor to the exhaust system exit.

    Slamming the secondaries open at 1500 RPM (or even 3000 RPM) does not do that. The
    secondary accelerator pump helps cover up the resultant hole (at the expense of fuel
    economy), but it can only go so far, and at lower RPM, that initial rush is followed by a
    lean sag until the engine catches up with the carburetor.

    Everything is a tradeoff, though, and some are willing to give up more driveability and fuel
    economy in the name of top-end performance bragging rights, than others are. That's not
    necessarily wrong if that's your thing, you just need to understand where each person is
    coming from, and how that fits with how you want your car to perform. There is no getting
    around the tradeoffs, the only decision is which set of tradeoffs appeal more to you. This
    is no different than discussions about how loud your exhaust should be, what springs are
    best, or what rear gear to run. What one person thinks is ideal, another person might not.

    Obviously, I come down on the side of making a street/strip car enjoyable to drive on the
    street, in everyday traffic. That is, after all, where my car spends 99% of it's time. I like
    being able to pull out on the guy next to me without sounding like I'm actually racing. When
    I get stuck in traffic, I like that I can creep along at idle without any bucking, then when
    things start to move, just feed it a bit of throttle without any stumble. But I understand that
    others are willing to give up more of that than I am. I used to be one of them.

    FWIW, Holley has a couple 670 CFM vacuum secondary carbs in their Street Avenger line,
    and Quick Fuel has a 680 CFM vacuum secondary "HR" series carb. Either of these might
    give up some power at redline, but I submit that both of these will give you a broader
    torque band and better street manners on a 347 than a 750DP will. If that's your thing...
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
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  20. #20

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    The Holley Street Avenger 670 is a great vacuum secondary carb for a street car of any weight. I wish I had hung on to mine for future builds. It almost flowed enough for my 408 with a weiland stealth, and had far better cold start manners. The fuel enonomy argument I can't subscribe to, as I lost a negligible amount of mpg with the switch to a 750dp, less than 1mpg... Although a 408 isnt going to get "good" mileage no matter what you top it with. Go with a good vacuum sec carb.
    408W/AFR205/Vic Jr/Prosystems 4150HP XC/solid roller 242/248, .381/.385 with 1.6/ T5(hang in there little buddy)

  21. #21
    FEP Member bmf347's Avatar
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    Lots of great responses here... Although I feel the need to put in my two cents as I went through the exact contemplation that you are with my 347. Your 347 sounds like it is more stout that mine though. I had about 9.5:1 compression a 380hp/410tq... Was originally told to use a 750 as mine would be starved for fuel with the 650. After deciding to go with the holley 750dp and running it for about a month under very spirited street driving.... I was absolutely disappointed with my engine... did some reading and installed a wide band O2/gauge as suggested above and found my afr was sky high. Changed to the smallest jets I had and was still around 20 if I am remembering right.... At any rate I ended up going with an edelbrock 600cfm with mechanical secondaires and wow... Right outta the box with no adjustments I was almost right on 16 for my afr... Never adjusted a thing and was amazed with the power I had under my foot now. Id suggest as many have to get yourself a wideband O2/gauge and go from there. If you start with the 650 I think youll find yourself pretty happy with it. If your not drag racing or track racing it all the time then I really see no reason to go with a 750dp.

  22. #22
    FEP Power Member Ethyl Cat's Avatar
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    I think to simplify it even further....It all boils down to this.

    If you DO NOT feel like placing your car in the correct gear for the acceleration that you desire, put a vacuum secondary on it. Then you can go WOT in 5th gear going 30 mph without a worry.

    If you tend to be an active shifter and downshift when you want to accelerate , you will be able to run the DBL pumper on the street.

    These are the end results of all the tech that is going on in this thread.

    Find out how you drive and act accordingly.

    I want driving impression of this set up.

    Hurry up and get it done already!

  23. #23
    FEP Power Member Ethyl Cat's Avatar
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    Quote Originally Posted by JungleJetDriver View Post
    The Holley Street Avenger 670 is a great vacuum secondary carb for a street car of any weight. I wish I had hung on to mine for future builds. It almost flowed enough for my 408 with a weiland stealth, and had far better cold start manners. The fuel enonomy argument I can't subscribe to, as I lost a negligible amount of mpg with the switch to a 750dp, less than 1mpg... Although a 408 isnt going to get "good" mileage no matter what you top it with. Go with a good vacuum sec carb.
    I have seen 40hp improvement with an out of the box conventional 750DP over a 670 avenger on a mild 393W.

    This was with the afr corrected on both!!

    Same day same dyno tuning session.

  24. #24
    FEP Power Member Ethyl Cat's Avatar
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    Quote Originally Posted by bmf347 View Post
    Lots of great responses here... Although I feel the need to put in my two cents as I went through the exact contemplation that you are with my 347. Your 347 sounds like it is more stout that mine though. I had about 9.5:1 compression a 380hp/410tq... Was originally told to use a 750 as mine would be starved for fuel with the 650. After deciding to go with the holley 750dp and running it for about a month under very spirited street driving.... I was absolutely disappointed with my engine... did some reading and installed a wide band O2/gauge as suggested above and found my afr was sky high. Changed to the smallest jets I had and was still around 20 if I am remembering right.... At any rate I ended up going with an edelbrock 600cfm with mechanical secondaires and wow... Right outta the box with no adjustments I was almost right on 16 for my afr... Never adjusted a thing and was amazed with the power I had under my foot now. Id suggest as many have to get yourself a wideband O2/gauge and go from there. If you start with the 650 I think youll find yourself pretty happy with it. If your not drag racing or track racing it all the time then I really see no reason to go with a 750dp.
    An AFR on 20 requires BIGGER jets to richen the mixture. 16:1 is ok for cruise but is still rather lean for these old engines/ignitions.

    WOT AFR should be around a 13:1 plus or minus depending on the engine.

  25. #25

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    Well, for now no one has bought my 650 that I put on CL...I decided if I don't sell it, its going back on the car rejeted. I spoke to quick fuel and they said the Q 750 is perfect, my machine shop guy said the same thing AND he has a engine dyno there, so I tend to agree on his word. Still though, the cam XE274HR had me thinking. I'd rather spend the 280 on that cam and decide on the 650 or 750 when running. I just found out my buddy has a 750 dp still sitting in his garage. I agree a AFR guage is what I need, I have one, just the guage but still need the wide band. I should be getting my block back from my shop next week. Then the fun begins with putting the engine together. This will be my first Ford build.

    I tend to nail the car when its moving so, I don't see a DP issue plus the high gears it has.

    On another note, my engine & bay will be show quality. I bought all the scott rod tins, grommets + some extra custom covers I've made. I got everything powdercoated black and even the headers are ceramic black. ALL the bolts are polished ARPs. I wanted a dark theme. So the eng bay is matte black and the eng is 90% gloss black. This will look killer. Plus the guy who shot my Pace car I hear is now doing work for fast fords so I may have an in once the car is painted grabber blue.
    85 GT built 347, Steeda t5, 3.55s, full susp, real Saleen body kit, Saleen rims...for go but need paint to show!
    99 TA ram air Daytona Pace car, 6spd 520rhp/497tq, full featured car Pont High Perf. 10.89@128 and 172mph in the mile run.
    00 TA boltons auto, the driver.
    01 TA ram air 6spd vert, just boltons, the cruiser.

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