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Author Topic: 302 jetting  (Read 8873 times)
JohnZ
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« Reply #15 on: December 05, 2007, 11:35:04 AM »

3.) What about the power valve sizes?  Stay with the stock 105 on primary and 85 on the secondary?

If you run a 105 (10.5" Hg.) power valve on the primary, you'll be in it almost all the time with a 30-30 cam, which only pulls 9"-10" Hg. vacuum at idle; the stock primary power valve is a #65 (6.5" Hg.), and the secondary is a #85.
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'69 Z/28
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Dave69x33
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« Reply #16 on: December 05, 2007, 09:16:57 PM »

Jerry,

I understand your position.  Proven knowledge does not come cheep - in time or money.

JohnZ,

Youíre probably right on the power valving.  While restoring my Z back in Sept. '01, I experimented with jets and power valves with sequence listed below base on spark plug readings, vacuum readings, etc.  In short, less seamed to be better.  With my engine set up, I get about 12" - 14" Hg, so experimenting with 105 appeared reasonable.

Primary - Jets: 68 to 72 back to 68, Power valves: 85 to 105 back to 85.
Seconday - Jets: 72 to 76 back to 72, Power valves: 65 to 85 back to 65.

In the book, Holley Carburetor Manual by Haynes, Ch. 9 Reference and Specification, for the Model 4150 780, it list 72 jets for primary, 76ís for secondary, and power valves 105 primary, 85 secondary.   Yet the 1969 Chevrolet Chassis Service Manual, p. 11 in specifications for the Holley carb (for 302's) list jets 68 primary, 76 secondary.

All spec's are in the same ball park but must assume they are set more for emission compliance rather than optimum performance?

As other have noted, the 72P/76S jet combination appears to be the ticket but again, I wonder what power valve combinations, etc. are being used?

I suppose that I try the 72P/76S jets with the 85P/65S power valve combonation next.   

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AFONEFE
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« Reply #17 on: December 05, 2007, 10:07:54 PM »

72 and 76s are the ticket for sea level or close to it.  6.5 power valves are what works best in the 302, just like John pointed out.  I will say that we also sometimes block the rear power valve. 

Gary Sommers 69 Z28 has been a test bed car for us and it runs in the mid 13s in 90 degree weather, granny shifting with street tires and through the mufflers.  I did a Stock Eliminator carb set up on this car.  With a good driver and traction this car would run in the mid 12s.  He was on the chassis dyno this summer and it made 350 hp at the rear wheels.  A good Stock Eliminator 302 engine will make 460-470hp at the flywheel.       

Jerry
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69 Z/28 X-77
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GaryL
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« Reply #18 on: December 05, 2007, 11:03:01 PM »

I have mine tuned with the 72/76 jets and it runs very nice. Off idle on up. I run Thorley headers and have the full vacuum advance modification JohnZ has recommended. However, my dist curve is a little different in that I use 18deg initial and 18 deg mechanical. Yes it flat works with the 30-30. Not a hint of pinging with 93 octane.

Hey AFONEFE, what cam, SCR is the Gary Sommers engine running?
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Gary

Lemans Blue X33. DZ, M20, manual steering. Only BU code rear end is original.
Jerry@CHP
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« Reply #19 on: December 07, 2007, 07:33:08 PM »

Stock Eliminator profile camshaft, .485 lift.

I was at AFONEFE's place when writing the post.

Jerry
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GaryL
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« Reply #20 on: December 07, 2007, 09:08:37 PM »

Stock Eliminator profile camshaft, .485 lift.

I was at AFONEFE's place when writing the post.

Jerry

OH. Thought he was Jerry also. Embarrassed
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Gary

Lemans Blue X33. DZ, M20, manual steering. Only BU code rear end is original.
Dave69x33
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« Reply #21 on: December 08, 2007, 01:45:03 PM »

AFONEFE  & GaryL,

Thanks for the input.

Sounds like 72 primary & 76 secondary jets are a good baseline for a stock 302, with stock exhaust manifolds and the 30-30 cam, yielding 9" - 10" Hg  manifold vacuum.

The power valve tuning tip that I read recommends picking a PV 1" - 1.5" below manifold vacuum at idle.  Thus, for a typical stock 302 yielding idle vacuum of 9" - 10" Hg, then a #65 (opens at 6.5" Hg) or #75 would be appropriate.

Again, my 302 block was bored to .060" over, and running with a 327 crank, flat top pistons, and the GM P/N-3972178, 70-71 Z28 mechanical cam (and 3:55 rear gears).  My engine combination yields about 12" Hg. of manifold vacuum.

Staggered Power Valves:

For reader clarification, was purpose to stagger the original PV #ís (with original PV #ís noted in this discussion on the original 69Z 302 with the 780 carb), to optimize fuel delivery during the transition between primary and secondary 4-barrel opening, yet provide good power balanced with good emissions and economy?

Given that I get 12" Hg manifold vacuum, is it recommend that I select #85 power valves in both the primary and secondary metering blocks?

AFONEFE.....you mentioned that #65 PV's work best in a 302. I assume you mean in both the primary and secondary metering blocks?  Or do stagger the PV's with a #65 primary and #85 secondary as JohnZ recommends?

Or, is it wise to match the PV's in both primary and secondary metering circuits so that both PV's are pulling open together and enrich fuel supply to all four barrels simultaneously?  Then when power demand increases, further dropping manifold vacuum and pulling the secondary throttle plates open (via the vacuum secondary diaphragm spring), the carb is set for optimal performance rather than optimal emissions and economy?







 
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GaryL
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« Reply #22 on: December 08, 2007, 05:34:28 PM »

I guess I missed the 8.5 PV on the secondaries. I thought the 4053 carb had 6.5 on both sides.  Huh
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Gary

Lemans Blue X33. DZ, M20, manual steering. Only BU code rear end is original.
Jerry@CHP
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« Reply #23 on: December 09, 2007, 09:57:49 AM »

Use 6.5s in both ends.  You can also block the rear power valve and richen the rear jets four steps to comenpsate.  Works well too.

Jerry
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