Initial jetting recommendations wanted
#2148313
09/05/16 09:42 PM
09/05/16 09:42 PM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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I may actually be firing up my 451 soon and would like to know what primary and secondary jet sizes (and power valve) I should start with. 10.3 CR, ported iron heads, .654" Mini-Express cam (316 advertised, don't know the .050 but prob around 270), 850 DP on Edelbrock RPM, 18 initial/34 max advance, adjustable vac advance can, 1-3/4" headers, dual 3" exhaust with x-pipe. Probably idles at 1000-1100 rpm with 7" vacuum. I have an O2 sensor bung and will be putting in a wideband sensor/gauge for dialing it in, just want to fire up something approximate for easy break-in and shakedown cruise. Thanks for any recommendations. -Charles
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2148473
09/06/16 12:50 AM
09/06/16 12:50 AM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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The carb isn't anywhere near new (although I treated it to a rebuild kit), but if I can look it up on Holley's site that will be a good starting point. I figured I'd start with a more modest initial to avoid starter kick-back problems. I can always put the FBO plate in a different position if it wants that much and can still start hot. Thanks for the tips. About the mismatched combo: Search for "mushroom" or "Mini-Express" here on Moparts and you will find several guys who run mid-10's with iron heads in heavier cars than my A-body, for example: https://board.moparts.org/ubbthreads/ubbthreads.php/topics/646923/2.htmlEdit: your own .sig says it all: "Do what you can, with what you have, where you are." - Theodore Roosevelt And the RPM intake seems to really perform well in every "manifold shoot-out" I've been able to find. Probably gives up some hp over a single-plane at very high RPM's but I'm not brave enough to spin it over 6500 or so, and it will be on the street more than the strip so the corresponding loss of low-end from a single-plane is a factor.
Last edited by DrCharles; 09/06/16 12:52 AM.
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2148496
09/06/16 01:24 AM
09/06/16 01:24 AM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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You don't mention the car weight, gearing, trans type, converter stall (if applicable), or performance goal. But from what I've tested over the years, either you have way too much cam for the intake and headers, or you have the wrong intake and too-small headers for the cam.
As far as using ported OEM iron heads, sure, they'll work. I squeaked into the 10.9s with some on an RB 451 pump-gas engine in a 3700# E-body.
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2148708
09/06/16 12:23 PM
09/06/16 12:23 PM
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Joined: Oct 2015
Posts: 570 UK
rb446
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mopar
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Here's my old set up for my '69 Cuda just for reference>>>
Stock 440SP bottom end, BV 906's around 9.8:1, old list 4781-1 850DP, 80/80 was stock jets, I had 86/90 w/no rearPV, 50cc fr pump, 2.5pv, Team G, 324-290@.050/.650/.650sft. 2" f/wells, 14 intitial 38 all in on 98 oct leaded, HS r/rockers, tickover was a smooth count the cylinders firing 900rpm. footbraked to 4800 flash launch, ran superb back in 1990 good 60's just not very quick in the ET dept.
Last edited by rb446; 09/06/16 12:29 PM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990 1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Initial jetting recommendations wanted
[Re: SportF]
#2149226
09/06/16 10:56 PM
09/06/16 10:56 PM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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74's in the front, 84's in the back. Block the power valve. I assume you mean only the secondary (rear) power valve. Just curious, why would you recommend blocking that off... I could install one that opens as low as 2.5" of vac. I don't know yet (until I get it on the road) what my cruise vac is going to be, and at what point I start getting into the secondaries. I imagine it'll take some tuning with a wideband O2 sensor... RB446, I'm impressed that you got that "Open 24-7" flat tappet cam (290@.050) to idle at 900 rpm. Now THAT's a lot of cam for almost anybody Edit: Came across this very detailed article on 850 DP tuning. I'm a bit intimidated... but will tackle it after the engine is broken in and the car is street-legal driveable. http://www.corvetteactioncenter.com/tech/bbfh/big-block-from-hell-part-17.html
Last edited by DrCharles; 09/07/16 12:35 AM.
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150298
09/08/16 12:42 PM
09/08/16 12:42 PM
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Joined: Dec 2014
Posts: 304 Florida
Mark Whitener
enthusiast
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More like 82/88 with a 4781 850 DP, no rear PV.
Mark Whitener [url=www.racingfuelsystems.com[/url]
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150587
09/08/16 08:05 PM
09/08/16 08:05 PM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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List 4780-2 and under that, 1276 (date code?) Turns out that's actually an 800 DP, not an 850! It's been in my parts stash a long time... wonder if I was "misinformed" by a long-ago seller I'll probably have to get a bigger carb eventually but the 800 should work fine for break-in and cruising. Meanwhile I found this handy Holley reference table and it shows stock jetting as 70/85 with a 6.5 power valve. Obviously the power valve needs changing to something like a 3.5 since idle vac will be 6-7 inches (from people who have run similar sized engines with this cam). There's only one PV number listed for the 4780-2 so it may not even have a secondary PV - I will check the next time I have a chance to work on it!
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150632
09/08/16 09:13 PM
09/08/16 09:13 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
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I Live Here
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I would put a p/v in there a couple of "`s below CRUISE rpm`s not idle as it has no influence. That`s a wide jet spread but try it as is I guess and what size ifr`s, idle and high bleeds does it have cos that will influence the curve and overall characteristics of the car..........Then there`s the t-slot holes............
Last edited by Thumperdart; 09/08/16 09:14 PM.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150641
09/08/16 09:25 PM
09/08/16 09:25 PM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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Thanks. There seems to be some disagreement as to the effect of power valves at idle... Holley says it can enrich the mixture at idle (although I don't think there is a large effect). In your experience it doesn't make any difference? I have no idea what cruise vacuum might be with such a large cam. Guess I will have to try it and find out!
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150721
09/08/16 11:14 PM
09/08/16 11:14 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
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I Live Here
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Thanks. There seems to be some disagreement as to the effect of power valves at idle... Holley says it can enrich the mixture at idle (although I don't think there is a large effect). In your experience it doesn't make any difference? I have no idea what cruise vacuum might be with such a large cam. Guess I will have to try it and find out! Drive it w/a vac gauge you can read on a steady cruise state, then light load etc and get a feel for your richening/lean needs.......
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150734
09/08/16 11:28 PM
09/08/16 11:28 PM
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Posts: 4,220 West Plains, MO
DrCharles
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Thanks Cab. I agree, idle circuits are for idling (and if the transition slot is too exposed I wouldn't be able to adjust the idle mixture with the screws either). Yes, it has a setscrew underneath to open the secondaries (that the carb has to come off to reach). As a ballpark guess I have just barely cracked the secondaries a few thousandths (a previous owner has already drilled tiny holes in the primary throttle blades). The primaries are set with the idle speed screw so the transition slot is "square" appearing (around .020"). For initial break-in I want 2000+ rpm anyway, so if the idle speed is in that range with these settings, it won't be a problem. Guess I will put in the stock jets for now (70/85, 6.5" PV). I'm certainly seeing a range of recommendations here
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Re: Initial jetting recommendations wanted
[Re: krautrock]
#2150794
09/09/16 12:53 AM
09/09/16 12:53 AM
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Joined: Apr 2006
Posts: 4,220 West Plains, MO
DrCharles
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the carb i got, which is probably not perfect for my app but seems to work fine so far, has a screw on the secondary throttle shaft so i can adjust it from the top, pretty nice. I could convert mine (with the Holley 26-137 kit). But that little bracket and screw costs $43 Probably worth it if I have to take the carb off more than once, though!
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Re: Initial jetting recommendations wanted
[Re: DrCharles]
#2150953
09/09/16 12:26 PM
09/09/16 12:26 PM
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Joined: Jan 2003
Posts: 14,505 So. Burlington, Vt.
fast68plymouth
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To the best of my knowledge, 4780's all came with only two corner idle. If yours hasn't been converted to 4 corner idle, I think you're going to find it a chore to get it to idle well when in gear, and/or you'll have to have it idling at a fairly high rpm, with a cam with as much duration as what you're running.
Part of the reason for so much spread in the jetting is the 800dp's have larger Venturi in the secondaries, so the signal is a little weaker and more of the total air flow at wot is going through that half of the carb. The "if it were me" answer is, I'd start out at 73/85 jetting, 5.0pv...... And convert it to 4 corner idle. Quick fuel makes an adjustable secondary idle speed bracket, Jegs has them for about $18.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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