Tuning Question-383 with 4150/HP center - Team Camaro Tech
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post #1 of 12 (permalink) Old Jul 10th, 14, 01:14 PM Thread Starter
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Sean
 
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Tuning Question-383 with 4150/HP center

Hello all,
I have some tuning questions that I hoped you could assist me with. For background, my engine is a 383/6.00 rod/9.9:1/AFR 195cc heads/Perf RPM manifold/1.75 D368 headers/3"pypes exh with x-pipe. Cam is a Lunati Voodoo Hyd roller, 231/239 at .050, .535/.550 lift. 1.52 rockers, MSD digital 6AL/Blaster coil/Summit mech/vacuum billet dist. Autolite 3924 plugs.

I took my Holley 4779-9 carb and swapped in their HP main body when I built the new engine. I have set it up as follows:

-35F/35R squirters
-Floats barely trickling fuel out plugs
-6 psi fuel pressure (Aeromotive bypass reg, stealth tank/pump, -8 lines/fittings). Used a Summit -8 carb line, replaced the "90" with a "T" so fuel comes into rear, flows through line/carb, then to reg on fender which then directs excess back to tank. Basically a fuel log w/o paying aeromotive 300-something...
-70F/86R main jets
-9.5 PV
-std pumps/cams f/r
-PV front/Plug rear
-idle screws 3/4-1 turn out, best vacuum between 12-15 (i.e. thats the range I have gotten, its not bouncing between those)
-11 deg initial timing on the dist, plus 7 vacuum for 18 total, plus 21 mech for 32 deg at 3000, 40 would be the max it would see with all three at cruise.

Along the way I have tried 6.5-9.5 PV, 31F/35R squirters, and started with the Std 72F/84R jets. I also rebuilt all the soft goods on the carb during the swap to the new main body.

My issue is that I have the idle running pretty good (about 13.5-14.0 on the AFR), light cruise at 13.5-ish after going to the 70 front jets (from 74), and no off-idle stumble. My issue is that any decent to full application of throttle results in pegging the AFR at 17 or so, and while it creeps back to a lower number it doesn't do it quickly enough to tell me that I am not going to turn a piston into a paperweight. I thought upping the PV would help as would increasing secondary main jet size. However, the PV didn't change much, and I feel like I am getting a big enough spread between primary and secondary jets that I am failing to address the underlying issue (seems like most folks typically end up around 8-10 sizes between). Is this an issue with the PVCR being too small?

I have also noticed a really nasty bark as it gets into the lean area that I believe is detonation. It seems to pass through it though which seems odd.

Thanks in advance!

1969 Z28 Clone/383 SBC/Holley #4779 750DP w/HP center section/Tremec TKO600/Dutchman 9"/Pypes 3" race pro/Doug's D368/ 17x8, 17x9 VWW/Nitto 555
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post #2 of 12 (permalink) Old Jul 10th, 14, 03:07 PM
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Don
 
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Re: Tuning Question-383 with 4150/HP center

Omalley, your set up is much like mine, cam and all with AFR 195 heads but I run 10.3 compression. I have a 3310 vac secondary tuned with a LM1. Ended up with 70 primary and 80 secondary with a 55 PV in front. Std cams. Like you I cruise 13 or so but nailed I go to 10:1 . I run 17 initial with total of 36 , no vac advance, all manual curved.
I don't run large squirters, #31 in front but vac secondaries work well . My idle with the PV is at about 12" . I do not run a rear PV.

I do have an 850 HP downleg vac secondary carb that tuned with the LM1 and runs better but does not have the crisp throttle as the 3310. I do get over 20 mpg on tours with 3.73 gear and TK0600.

I think you need to get that 9.5 pv out of there first, depending on what your idle circuits are doing, I would go to a 45 or 55 to check things out. Your fuel demands need to come on quicker for sure. Your fuel pressure and volume sound good but what is it doing WOT?
I have run many hi psi pumps, elec pumps and still come back to a fuel log fed by a plain holley #327 style pump.

You need to do something quick or you will have a paper weight. hope this helps..
PS: your cam is identical to mine from UD Harold.

Forgot to mention, My elevation is 3350' so take that into consideration when comparing.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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Last edited by zdld17; Jul 10th, 14 at 03:18 PM. Reason: elevation factor
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post #3 of 12 (permalink) Old Jul 10th, 14, 03:15 PM Thread Starter
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Sean
 
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Re: Tuning Question-383 with 4150/HP center

Thanks for the info-you are right about the paperweight part. I just don't get why it's going so lean from mid throttle up with relatively large 86 rear jets. Checking fuel press at WOT is a good idea I'll need to try that. I thought about the high speed bleeds but if I understand correctly they affect circuit timing more than volume.

1969 Z28 Clone/383 SBC/Holley #4779 750DP w/HP center section/Tremec TKO600/Dutchman 9"/Pypes 3" race pro/Doug's D368/ 17x8, 17x9 VWW/Nitto 555
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post #4 of 12 (permalink) Old Jul 10th, 14, 03:47 PM
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Steps
 
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Re: Tuning Question-383 with 4150/HP center

Quote:
My issue is that any decent to full application of throttle results in pegging the AFR at 17 or so, and while it creeps back to a lower number it doesn't do it quickly enough to tell me
At what rpm range does this lean happen?

Try upping the intial 3 or 4 degs without changing anything... may result in a slight power drop above 3500 rpms... can sort that later by changing the springs and cent...

If the above doesnt remove , the issue is then likely to be fuel or a vaccuum leak....maybe play in butterfly bushes or misaligned butterflies... incorrect/bad PVC valve

My Spelling is not incorrect...it is creative

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post #5 of 12 (permalink) Old Jul 10th, 14, 03:57 PM Thread Starter
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Sean
 
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Re: Tuning Question-383 with 4150/HP center

Thanks Steptoe I will try that as well. It will happen at pretty much any rpm. I tried putting it in 4th at low speed and going WOT, same result as normal rpm (3k or so) in 2nd or 3rd and nailing it. Seems to be throttle position dependant, as opposed to rpm. Seems like it's starting about the time the secondaries are probably starting to open, but that's kind of guessing.

1969 Z28 Clone/383 SBC/Holley #4779 750DP w/HP center section/Tremec TKO600/Dutchman 9"/Pypes 3" race pro/Doug's D368/ 17x8, 17x9 VWW/Nitto 555
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post #6 of 12 (permalink) Old Jul 10th, 14, 05:04 PM
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Al
 
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Re: Tuning Question-383 with 4150/HP center

I have a very similar setup to yours - just a slightly smaller cam (230/236) and a 4778-2. I don't have a wide band to test with, so I just go by what runs well.

I also swapped to a Holley HP main body and I never could get the damn thing to run correctly. I bought a complete air bleed kit and messed with that for awhile - changed squirters, pump cams, pump cam positions, metering blocks, etc. all to no avail. I know that they say just swap your stuff over, use the huge jets they give you with the kit and all should be well. I got to wondering why my 4778 has 4778 stamped on the metering blocks and why my 4777 has 4777 stamped on its metering blocks. Best I could figure is that they are mated or matched to the calibration built into the main body.

My dist is an MSD E-curve, setup w/ 17 initial, 15 mech in @ 3000 and 10 vac.

Anyway - long story short - I switched back to the correct main body and returned the carb to factory specs and the thing runs like a striped ape.

You and I would not be the only 2 people who have had difficulty getting the Holley HP main body swap to work.


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Last edited by BPOS; Jul 10th, 14 at 05:16 PM.
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post #7 of 12 (permalink) Old Jul 10th, 14, 05:16 PM
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Don
 
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Re: Tuning Question-383 with 4150/HP center

Quote:
Originally Posted by omalley1537 View Post
Thanks Steptoe I will try that as well. It will happen at pretty much any rpm. I tried putting it in 4th at low speed and going WOT, same result as normal rpm (3k or so) in 2nd or 3rd and nailing it. Seems to be throttle position dependant, as opposed to rpm. Seems like it's starting about the time the secondaries are probably starting to open, but that's kind of guessing.
Reading your last comment about secondaries,probably opening, you might use a paper clip at the closed position of the diaphragm stem against the bottom the of the diaphragm housing. If that rod moves to open the secondaries, it will move the clip, leaving you with a tell-tell sign that they opened.

You can also connect a simple vac guage with a long hose from it to manifold vacumn and read it as you run WOT, say to 6200 rpm or so where ever it stops pulling, this should tell you at what vac in inches (hg) that all your issues are occurring.
I would guess that if you WOT vac never gets below the current PV setting, your main fuel channels are not opening when needed but are doing so as you state the your AFR is dropping from a high point of 17.
My guess is your new carb body had the down leg boosters but same CFM rating as the 4779?

When also running this WOT , observe what is the lowest vacumn read you see with this guage connected with long hose , held in your hand or where you can read and keep your eye on the road. Need to do this in a 4th gear wot pull. If your reading is below 2" or 1.5", my understanding is , your carb is too small. I am aware of the math to size a carb but this is very fool proof.
These methods are very old school but do work. Careful on the road!

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #8 of 12 (permalink) Old Jul 10th, 14, 05:23 PM
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Al
 
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Re: Tuning Question-383 with 4150/HP center

Don - 4779 is a double pumper - no diaphragm


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post #9 of 12 (permalink) Old Jul 10th, 14, 05:25 PM
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Don
 
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Re: Tuning Question-383 with 4150/HP center

Quote:
Originally Posted by BPOS View Post
Don - 4779 is a double pumper - no diaphragm
My bad, knew I was looking at this too hard, normally its duck soup, just cant mess it up.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #10 of 12 (permalink) Old Jul 10th, 14, 05:49 PM Thread Starter
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Sean
 
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Re: Tuning Question-383 with 4150/HP center

Thanks for all the replies-I was kind of wondering about going back to the old center section as well. Seems like the "just swap whatever you have over" does fly in the face of some logic there. In the back of my head I wondered if this type of swap shouldn't ideally be accompanied by some flow testing to check fuel curve, etc.

Seems like I also read somewhere that the earlier metering blocks were better for some reason.

Hopefully going to get the car out here in the next day or so and will try some of this stuff.

1969 Z28 Clone/383 SBC/Holley #4779 750DP w/HP center section/Tremec TKO600/Dutchman 9"/Pypes 3" race pro/Doug's D368/ 17x8, 17x9 VWW/Nitto 555
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post #11 of 12 (permalink) Old Jul 10th, 14, 05:59 PM
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Don
 
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Re: Tuning Question-383 with 4150/HP center

Quote:
Originally Posted by omalley1537 View Post
Thanks for all the replies-I was kind of wondering about going back to the old center section as well. Seems like the "just swap whatever you have over" does fly in the face of some logic there. In the back of my head I wondered if this type of swap shouldn't ideally be accompanied by some flow testing to check fuel curve, etc.

Seems like I also read somewhere that the earlier metering blocks were better for some reason.

Hopefully going to get the car out here in the next day or so and will try some of this stuff.
Just a thought here, If you know of a friend that has a tried true bench flowed carb say maybe a 750 cfm or so, to try out , this may help you rule out some things. But I would follow comments made my Steptoe, bump up timing, but keep under 38 total. This may require some dist recurving. Good Luck.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #12 of 12 (permalink) Old Jul 10th, 14, 11:18 PM
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Steps
 
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Re: Tuning Question-383 with 4150/HP center

Quote:
same result as normal rpm (3k or so) in 2nd or 3rd and nailing it.
So it still does it over 3500 rpms even over 4K ??

If so that climates the timing curve...

Quote:
Seems to be throttle position dependant, as opposed to rpm. Seems like it's starting about the time the secondaries are probably starting to open, but that's kind of guessing.
I think u are on the right track... yrs ago had a very similar thing on a IMPCO 425 proppane carb running on a mechanical secondary holly base plate....
ended up 3 issues..
1st was the wrong gasket on the bottom... possible
2nd was the custom base plate setup for the secondary return spring mechanism (unlikely here)
3rd the custom actuation timing of the secondarys opening was a smidgen late on throttle postion...
When I opened the secondaries slightly earlier, because of previous adjustment (jetting for a holley) it bogged .. the data logger indicated over rich spike... returned back to basically 'out of the box ' setup and everything fell into place text book.

My Spelling is not incorrect...it is creative

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