Jet size increases after adding more power?

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Gotzy
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Jet size increases after adding more power?

Post by Gotzy »

Howdy Folks,

I got a little tunnel rammed 327 SBC with twin holley 450's. I'm about to give it a new short block that should be realizing at least 125 hp more. I'm converting the stock 450's by changing the rear metering plates out for jet plates and just wondering what would be a good starting point for jetting for the new power?

Stock jets are 59's so I was thinking maybe start with square 63 to 65's, any thoughts as I'm just guessing? I don't expect anyone to be able to nail it, I just want a ball park area so I can break the cam in before getting it on the rolling road to tune it.

Cheers
mag2555
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Re: Jet size increases after adding more power?

Post by mag2555 »

Not so much the power valve as you can't really nail down the vacuum rating you need until you check what vacuum you have at idle.
Anyway I would start off with 4.5" valves, but the real question is what you need for the air bleeds and the power valve restriction channel size, as its that channel size which controls the full throttle mixture and the bleeds that do the fine trim , not the main jets !
The main jets provide the mixture between idle and the time when the power valve starts to open.
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turbo2256b
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Re: Jet size increases after adding more power?

Post by turbo2256b »

dont think op is talking about power valves but jetting for the power increase. were is the increase of power coming from higher flowing heads.
any way that much increase might need about a 20 to 30% increase in jetting each holley jet size is a 4.5ish% change
treyrags
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Re: Jet size increases after adding more power?

Post by treyrags »

The new combination will not necessarily want larger jets. Carburetors don't know how much power they are supplying. All they know is the pressure differential below and above them. Your new combination may also be more efficient (bsfc).
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Re: Jet size increases after adding more power?

Post by 1972ho »

On my 660 center squirters when I changed from the plates to QF jet plates I tried the 76/74 jet first and when I went up to a 78 which made it squared I pick up almost 3 tenths.
treyrags
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Re: Jet size increases after adding more power?

Post by treyrags »

There is no set rule. I've seen larger, more powerful engines require smaller jets with the same carburetor. You still need to add fuel while tuning until it slows down then go back for best power.
Gotzy
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Re: Jet size increases after adding more power?

Post by Gotzy »

Turbo... is correct, just trying to get my head around jet sizes when the motor makes more power, with a new 11.85 compression short block, a new custom Jones cam and 7,700 rpm redline I'm just assuming it will need something more?

Carbs are standard with 582 front jets which google to a 58-59 jet. The rear metering plates were #9 which the quick fuel instructions cross refer to 64s which surprised me, though it would have been closer to the fronts.

Any educated guesses as a starting point for cam break in?
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Re: Jet size increases after adding more power?

Post by cv67 »

leave it be, get an 02 kit
Joe was always good for this type of stuff.
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Re: Jet size increases after adding more power?

Post by F-BIRD'88 »

Do not invest further into these small carbs. You seem to have lots of money for other new things but not the (critical) carbs, If you want power with a tunnel ram DO NOT use small cfm carbs in it.
Get bigger carbs 650+++.
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Re: Jet size increases after adding more power?

Post by F-BIRD'88 »

If this is to offy "Turbo thrust" tunnel ram with fully divided plenum that you re working with,
First before carving out the plenum divider try adding 1" and then 2" open carb spacers under the carbs. With the TOO SMALL carbs the added plenum volume and right left sharing and the increased carb height will help and show you the direction to more power.
Yes then it may well require carb jetting adjsutment. But realize your carbs are too small and were never good tunnel ram carbs by design or intent. They will LIMIT max performance.

Once you have tried to the carb spacers ( and rejetted too for new max perf you can decide of removing the plenum dividers is also needed.

Adding manifold plenum volume and height will change the carb metering signal wanting a jet change.

Don't dump money into these 450 carbs.

This old Offy tunnel ram could be made to work even better than it ever did (does) by building a custom plenum top for it ( and better suited 650-750cfm carbs)

Holley now sells blank plenum tops for their Hi RAM series that you could use as a base ( starting point) for a custom Offy tunnel ram plenum top. Don't be afraid to play and develop here.
This will allow you to play with BOTH the intake manifold runner LENGTH and the plenum shape volume and carb HEIGHT above the runners for max performance.
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