Borrowed 750 Speed Demon

Discussion in 'Other' started by sootie007, Mar 22, 2009.

  1. sootie007

    sootie007 65 Skylark -455 - T350

    A buddy is letting me borrow a n.i.b. Barry Grant Speed Demon 750. D.P. he has. I took off my Holley 850 D.P. and installed it today.

    First impressions:
    right out of the box it started .....I simply prefilled the bowls then after 4 pumps it idled damx near perfect out of the box at 1000 rpm. I dropped it to 875 in Neutral. I couldnt believe how well it idles (EVEN STONE COLD) -first startup- NO CHOKE. My newer Holley 850 dp would never do that....I had to hold my 850 DP at ~1200 for about a minute then let it go and it would hold its own after that period of warming up.

    That idle eaze adjuster baseplate on the speed Demon is a great idea. It prevents you from exposing too much of the transfer slots trying to get more air in at idle.

    Supposedly a 750 BG flows better than a Holley 750 as the BG is "wet flow tested" vs Holleys which are dry tested. This one has the contoured venturi-openings like the newer Holley Hp's do.

    It has the float bowl windows so you can actually see your float bowl levels quite clearly.

    It rained so I didnt even attempt to get on it from a stop but from a roll it feels "cleaner" for lack of a better term when hitting it about 1/2 throttle.

    Everyone here kind of advocates bigger is better cfm with the BBB (including myself on Chevys) but we just had this smaller one laying around and were curious what the difference might be E.T wise with a 100 cfm smaller carb, a stronger vac signal running through it,contoured venturi vs old style , a lousy rear gear, tight converter etc.

    The only thing that is different is the Holley fuel bowl lines are threaded different than the Barry Grants so I had to pickup a fuel line just to try it (ouch $) oh and the air cleaner stud is bigger in the Grant.

    Both have downleg boosters.

    Heres a pic showing the 850 Holley and the 750 Speed Demon side by side just to compare looks......

    Butterflies are 169 for the Grant Speed Demon and 180 for the 850 Holley D.P.

    I will post e.t.'s after Wednesday night..... should be interesting....J

    Attached Files:

  2. online170

    online170 Well-Known Member

    I had the same impression going from a holley 650 to a holley 750. Both DP and both mech secondaries.

    The reason was because the 750 has a 4 corner idle, and i think something in the idle circuit has been drilled out to allow more fuel in from a off idle start.

    It has that HUGE amount of low end torque, easy revving, and explosive feel, which the 650 lacks.

    My freind has also had both these carbs on his olds 350 (fairly stout engine). He put on a demon 850 after the holley 750. Right out of the box it was set pretty good, except the accelerator pump wasnt strong enough, and he couldnt find an agressive enough profiled cam, so he turned the existing one upside down, and now hes happywith it.

    Impressions, the650 and 750 were NIGHT AND DAY DIFFERENCE! With a big cam you can really notice the 4 corner idle, and how it makes a mean cammed car fairly tame on the street. The 750 and 850 were pretty close though, but the 850 was smoother and felt a bit torquey. The extra cfm could attribute to that.
  3. sootie007

    sootie007 65 Skylark -455 - T350

    I had some time tonight to hit an abandoned road and practice launching a few times with this smaller 750 Speed Demon mech sec. Right off the bat I noticed when I foot braked to 1200 then mat it - it doesnt suffer from the lean bog like the Holley 850 did- which incidently had the same size squirters and pink accelerator pump cams at pos 2- so its cleaner leaving the line. As you may recall I couldnt get rid of the bog 100% with the 850 (2:56 + tight converter I guess). Next practice I took it to 1500 and got some mild wheel hop and shudder then it took off. So I think I will leave at ~1300 tomorrow and see what happens. Tonight seat of the pants- pulling in the higher rpms I hate to say but my 850 felt better "G" wise at those higher rpms. BUT I think leaving a little cleaner with a good strong charge pull all the way down with the smaller 750 "may" average out better for me tomorrow night. I am really curious what my 8th to 1/4 mph increase will be ...for some reason my car only gained 19 mph from 1/8 to 1/4 last time out ...I have noticed Street - Strip guys that run great on here usually pickup 22 mph in that range. I am not touching anything else on purpose so I can strictly see the results of the carb change. J
  4. online170

    online170 Well-Known Member

    It doesnt mean a whole lot, and you cant determine what felt better until you tune them both in for your car.

    For my freind 350 olds. We tuned the 750 to within 1 jet size, and matched squirters and what not perfectly. When he mashed it fast or slow, there was no bog, there was no cough, and we absolutely got the max performance out of it.

    We slapped on the 850 (untuned and out of box) and the car was noticeably more explosive off the line. It was very hard not to spin.

    In any case, if you wanna sell your demon, let me know :)
  5. sootie007

    sootie007 65 Skylark -455 - T350

    Interesting article on carb vs injection on the dyno...on their 455 sd Pontiac they jetted a 750 Speed Demon 76, 83 and it produced 520 hp and 547 torque.

    "The Demon line of carburetors flows slightly more air than its competitors (the 750 actually flows around 875 cfm), so it was close in size to the amount of air the original Q-Jet would move."

    455 Pontiac SD With 750-CFM Speed Demon Carburetor
    RPM Torque
    (lb-ft) HP BSFC
    3,333 514.4 326.4 0.42
    3,400 518.4 335.6 0.40
    3,500 521.4 347.5 0.40
    3,600 523.9 359.1 0.39
    3,700 524.6 369.6 0.38
    3,800 527.1 381.3 0.37
    3,900 529.0 392.8 0.35
    4,000 535.9 408.2 0.30
    4,100 538.6 420.4 0.35
    4,200 541.9 433.3 0.30
    4,300 545.3 446.5 0.35
    4,400 547.1 458.4 0.30
    4,500 545.8 467.6 0.34
    4,600 546.2 478.4 0.33
    4,700 542.5 485.5 0.34
    4,800 540.0 493.6 0.30
    4,900 536.3 500.4 0.35
    5,000 533.9 508.3 0.35
    5,100 527.9 512.6 0.30
    5,200 522.3 517.2 0.35
    5,300 515.4 520.1 0.35
    5,400 502.1 516.3 0.36
    5,500 488.1 511.2 0.38
    5,600 476.5 508.1 0.39
    5,691 468.4 507.5 0.40

    Last edited: Apr 4, 2009
  6. sootie007

    sootie007 65 Skylark -455 - T350

    Car ran better with the SMALLER 750 Speed Demon Carb.

    It went
    13.15 at 104.45 besting my old 850 Holley Double Pumper at
    13.51 at 103. improvements of -.36 and +1.45 mph out of the box also went 1 mph better from 1/8th to 1/4 mph gaining 20 mph ...still not that great but getting better as I think I need to pickup 22mph to get into the 12's . I have read a ton of articles on this (too many) and the bottom line appears to be with a combo with a tight converter,lousy rear gear and heavy car you are better off running a smaller carb supposedly to keep the "charge velocity" up as the rpm's dont INITIALLY come up quick enough to keep the fuel properly suspended in the charge when going to full throttle. On "my combo" it appeared to be true and the car is much more tolerant off the line and doesnt suffer from the 850 double pumpers temporary lean condition when dumping the throttle.

    850 Holley is jetted 80*4= 320 The Barry Grant stock is jetted 76*2 + 83*2 = 318
    Last edited: Apr 14, 2009
  7. Cliff R

    Cliff R Well-Known Member

    I've back to back tested at the track hundreds of carburetors. We typically only see a few hundrenths of a second difference and about half a MPH when both test carburetors are well tuned for the application.

    Last year, we tested 9 carburetors on two different vehicles on a track rental. The list included 5 large cfm Q-jets, 1 800 Edelbrock AVS, 1 4781-2 Holley 850, one custom 925cfm Holley, and one 750 cfm older style q-jet.

    The entire spread of ET was .12 seconds and less than 2mph difference between any/all carbs tested on either vehicle.

    The AVS was the slowest of the bunch, both ET and MPH.

    The quickest ET went to the custom Holley by .02 seconds (69 Firebird).

    Highest MPH went to my own 1977 Pontiac q-jet, which was .02 seconds behind the 925cfm Holley on the 69 Firebird, (on my own car the Q-jet ran more mph and quicker ET)

    One vehicle was a 455 powered 69 Firebird, about 460hp running 12.20-12.30's.

    The other was my own 73 Ventura, apprx 514hp running 11.50-11.60's.

    Anytime you see dramatic differences in ET/MPH with carburetors, one of them was pretty far off the mark someplace for that particular application.....FWIW.....Cliff
  8. sootie007

    sootie007 65 Skylark -455 - T350

    Nice insight support what you say also I have chased many articles around the net including one Holley Hp comparison article on the same motor where they put on 7 different cfm holley hps and the bottom line was during that test ( 440 horse mule motor) anything over 650 cfm - 1000cfm was within 5 hp and 5 ftlbs of each other.......... J
  9. Cliff R

    Cliff R Well-Known Member

    I did some dyno testing as well to support that. A couple of years ago we had my own engine on the dyno, and back to back tested a 1970 Pontiac Ram Air carburetor against my 1977 larger cfm unit, the power numbers were identical to apprx 4500rpm's, from there on up the larger carburetor made/picked-up a few more hp at every rpm. At peak power (5600rpm's), the larger carburetor was around 10hp up on the smaller unit.

    I followed up that testing at the track, and the difference was exactly .12 seconds and less than 2 mph improvement with the larger carburetor.

    The most interesting part of the testing is that if you use any currently available "formula's" to determine the cfm required to support a 514hp 455cid engine at 5600rpm's, a 750 cfm carburetor is more than adequate.

    It's my guess that the large cid 455 engine prefers the larger carburetor, as it is fed by somewhat "conservative" cylinder heads/intake manifold without a lot of cross-section in the runners (Pontiac engine). This has the 455 sucking pretty hard on the entire intake tract at high rpm's. It may simply see the smaller carburetor as a slight restriction in the system?......Cliff
  10. sootie007

    sootie007 65 Skylark -455 - T350

    Good info ......they way I looked at it my combo is probably at best 425-450 hp so on my 455 for me any 750 cfm unit that was able to produce 520 hp - 547 tq with box stock jetting on that Pontiac motor above is plenty enough carb for me too. A board member I have talked to on here actually runs a 700 dp on many of his combos and runs 12's no problems......he also consitently runs 11's on the bottle with that carb and a 2 series gear. I think the best thing the smaller 750 carb has gotten me is driveability with my "tight" combo. It just feels "crisper" for lack of a better term and is much much much more tolerant stalling it up on the mild converter with my 2:56 , suffers no bogging , temporary lean - no matter how I launch it. I am not selling my 850 yet by any means but I am happier with this 750 so far and have only just begun to tweak it. I think none of us by human nature would ever think of going down in size on a carb trying to make a car go faster. So far it has helped me with "my combo". I really like to hear from guys like yourself who have done the hard work over the years on dynos and at the track and just dont quote hearsay but have facts - real experiences ...... thanks for your insight it is much appreciated. J
  11. sailbrd

    sailbrd Well-Known Member

    Nice to have you on our board Cliff. My Pontiac friends all swear by your carbs.

    Notice to all. If Cliff gives information on carburetors take it as gospel.

    Also, since J Osborne is no longer doing Q-jets this is the place to go:
  12. Cliff R

    Cliff R Well-Known Member

    Thanks guys. We test everything, street, dyno and at the track, to be able to supply information as accurate as we know it to be.

    The biggest problem in this is getting the car to run the numbers with deadly consistancy. It took me quite a few years to get my Ventura to run right on the number and repeat it every run.

    These days if we don't touch anything, the runs will be within .1 seconds (or less) of each other, if the weather and track conditions hold throughout the day.....Cliff

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