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Jetting questions #2856160
12/06/20 05:51 PM
12/06/20 05:51 PM
Joined: Jan 2007
Posts: 8,027
Mt Morris Michigan
mopar dave Offline OP
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mopar dave  Offline OP
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Mt Morris Michigan
Here's something i would like to ask about 2 different combos requiring different jetting: Doms 470 requires more jet than my 511, why? Dom's 470 with RPM's ported, 12:1 compression, solid roller 274/280@50-.440/.427LL on 108- 61* overlap, M1 t ram, same 650 carbs as mine(1.325 venturi) 82 jet squared fenderwell header. My 511 with E MW ported, 12.5:1, SFT 270/276@50-.403/.410LL on 110-53* overlap, Indy T ram, same carbs as Dom 650(1.325 venturi) 78 jet squared , inframe headers.
I would think the bigger cube higher compression motor would want more fuel, not the case. Can anyone explain why?
Dom sees mid 12's on wideband in 66* weather and i see mid 13's in 50* weather WOT. My jetting should be close when the warm summer weather comes back here in Michigan and would expect 13 O range WOT.

Last edited by mopar dave; 12/06/20 06:01 PM.
Re: Jetting questions [Re: mopar dave] #2856176
12/06/20 06:42 PM
12/06/20 06:42 PM
Joined: Nov 2011
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PA
Harry's Taxi 2 Offline
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PA
Maybe your larger engine has a stronger pull on the fuel through the boosters.


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Re: Jetting questions [Re: Harry's Taxi 2] #2856180
12/06/20 06:49 PM
12/06/20 06:49 PM
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Missouri
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jwb123 Offline
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I would agree, it is the velocity of the air going through the boosters that creates the signal to pull fuel from the bowl. I have did a little messing around with putting a brass tube soldered to a jet and putting it on the flow bench and using the velocity probe to measure how hard the booster is pulling on the jet. I have made a little power by polishing and massaging the boosters to get the same flow on all the jets say in a dual 4 barrel application. Several years ago a guy had a formula figured out for holleys on his flow bench, he could build the carb, pull the boosters and then jet them. I remember circle track magazine did an article on it.

Re: Jetting questions [Re: Harry's Taxi 2] #2856188
12/06/20 07:02 PM
12/06/20 07:02 PM
Joined: Jan 2007
Posts: 8,027
Mt Morris Michigan
mopar dave Offline OP
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I'm sure my SFT puts more draw on the boosters than Dom's Solid roller. I cruise with 15" of vacuum.

Re: Jetting questions [Re: jwb123] #2856190
12/06/20 07:03 PM
12/06/20 07:03 PM
Joined: Dec 2007
Posts: 12,587
Great Neck,LI,new york
hemi-itis Offline
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Great Neck,LI,new york
Jet for best MPH at the track up


HEMI-ITIS has no cure.
My condition is fully BLOWN!!
Re: Jetting questions [Re: jwb123] #2856192
12/06/20 07:04 PM
12/06/20 07:04 PM
Joined: Jan 2007
Posts: 8,027
Mt Morris Michigan
mopar dave Offline OP
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That is some technical stuff. I would think flow balancing the boosters would be worth something.

Re: Jetting questions [Re: mopar dave] #2856265
12/06/20 09:36 PM
12/06/20 09:36 PM
Joined: May 2004
Posts: 12,379
Taxes & Virus's R-US, NY
Dragula Offline
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Its all relative....your not even using the same fuels being in different parts of the country....Therefore target WOT AFR's will be different


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Re: Jetting questions [Re: mopar dave] #2856540
12/07/20 01:40 PM
12/07/20 01:40 PM
Joined: Jul 2004
Posts: 19,317
State of confusion
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Thumperdart Offline
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State of confusion
And like we spoke the other day, your manifold is a v-bath all sharing the same open area and my M-1 is a flat base manifold with minimal cross plenum up high for comparison almost making mine like the old IR manifolds acting more like 2 separate carbs requiring a damn near single carb tune x2. My tune now is actually more like two individual 750's as far as ifr's and jetting are concerned as opposed to dropping those down significantly like yours are tuned now however, I'm all ears here and would love other opinions cos the learning game never stops till you close the door......Remember too, I'm at 3500+ feet and run VP-101 Oxy fuel needing a richer mixture than as well and you are damn near sea lever so you would THINK i would need even less jet but not the case and why we always hear, "No two motors are the same even if built with similar components and compression...

Last edited by Thumperdart; 12/07/20 01:43 PM.

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Re: Jetting questions [Re: Thumperdart] #2856557
12/07/20 01:58 PM
12/07/20 01:58 PM
Joined: Jan 2003
Posts: 19,304
Las Vegas
Al_Alguire Offline
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Las Vegas
Many things affect the tune a carb needs, dom touched on a few. One he mentioned that I think often gets overlooked or is misunderstood is elevation, and as far as a carb is concerned it can be a BIG one. The higher up you go in elevation the lower the barometric pressure/air density becomes. For instance Vegas a great barometer is 28.00,(actual readings not the corrected ones used by your local weather man) in the midwest 29.5 is a typical good day. The lower barometer reading will require less jetting to achieve the same AFR's or tune. Not to mention the myriad of internal engine varibale that can effect the engines actual pull on the booster itself, the harder it pulls on the booster the less jet it will need in theory and vice versa.


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Re: Jetting questions [Re: Al_Alguire] #2856626
12/07/20 03:40 PM
12/07/20 03:40 PM
Joined: Jan 2007
Posts: 8,027
Mt Morris Michigan
mopar dave Offline OP
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mopar dave  Offline OP
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Mt Morris Michigan
Yes, I forgot altitude and your oxy fuel. My alt is 500’ and my fuel is pump/race fuel leaded.

Re: Jetting questions [Re: mopar dave] #2856628
12/07/20 03:43 PM
12/07/20 03:43 PM
Joined: Dec 2014
Posts: 304
Florida
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Mark Whitener Offline
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Florida
Distribution and runner velocity are a big part of it. An engine that does both better will atomize, vaporize, and distribute better requiring less overall fuel. Then you add fuel variations, weather variation s in DA, it can be significant. Even a cam change can make a significant difference in jetting needs.


Mark Whitener
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