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Old 05-01-2006, 12:59 PM   #1
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Blow-through Holley carb wideband tuning

I'm in the process of tuning my blow-through setup that I bought off a guy who had to get rid of it. I'm finding out that he didn't do any tuning after the procharger was installed (only 90 miles were on it, 400 on engine). So I'm going to post where I'm at now and how I got there so that maybe I can tune this thing right. =) I'm new at carbs but I bought some books and reading a lot here.

Specs:
350 crate motor
cam, High Energy Comp Cam, not sure which model, don't have specs
Vortec Heads
Edelbrock RPM Air Gap
4150 750cfm Holley carb with Mechanical secondary and Pro Form main body
carb came with 73 primary jets, 83 secondary jets, pink cams both #1 hole
0.31 shooters,6.5 power valve front, plug in rear
ProCharger P-1SC at 8lbs of boost
700r4 with Moser 12 bolt with 4.11 gears
MSD 6AL with 85551 mech advance distributor

Fuel pressure set to 5psi at idle, boost referenced 1-1
13-14 psi edelbrock mech fuel pump
Idle 600-700rpm, set float levels
idle screws are all 1/2 turn out
base timing at 12 degrees, 33 total around 3200 rpm

With the initial setup the car would go lean on acceleration and sometimes pop/backfire through carb. After reading up on some posts here I learned that the pink cams are more for racing than the street so I went with blue cams in the # 2 hole. The blue cams would still pop and the wideband would go way lean. Next I tried green cams and the pop/backfire went away but the wideband still had lean spikes so I changed the shooters from 0.31 to 0.35 and seems to have worked well.

Now with the pops/backfires gone I did some logging for cruise at 60mph (~2500rpm, 1-2bls boost) and WOT runs. At cruise I get an AFR around 11 but I know something between 14-15 would be better so I tried jetting the mains down but the AFR remains around 11. I went down 2 sizes twice so now I have 69 main jets. I thought maybe the power valve could have been blown by the backfires so I put a new 6.5 power valve in but the AFR is still around 11. I'm not sure what to do next but I'm going to cruise some with the vacuum gauge and see where it's at. I use the little port on the front of the base of the carb under the float bowl, is that the right one?

Since I couldn't get the AFR to drop at cruise I tried taking a look at what happens at WOT. Basically it was going lean, like 16-20 so I took out the power valve plug and put in a 6.5 power valve in the rear. It looks like it dropped the AFR a little bit but not much. I guess my next plan of action here is to jet up in the rear, check fuel psi at WOT, and/or get jet extensions? I'll try jetting up in the rear and if that doesn't work I guess I have a fuel problem, right?

So my current setup is:
69 primary jets, 83 secondary jets
green cams #2 holes, 0.35 shooters
6.5 power valve front and rear

current AFR
rich idle ~12.5
rich cruise ~11
lean WOT 14-17

Tonight I'm going to check the vacuum at cruise and maybe try jetting up the rear.
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Old 05-02-2006, 09:39 AM   #2
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Pulled the carb off last night and noticed the secondary blades were not open at all so I adjusted them to be open just a hair. After reinstalling the carb I was able to get the idle AFR anywhere I wanted it so I adjusted idle screws to get an AFR of 14 to 15. Took the car for a drive to see if this would help the cruise AFR and it seemed to be between 11 and 12. Maybe a slight improvement, will have to check other logs to confirm. My part throttle acceleration was right at 12.5.

Should I open up the secondary blades a little more to help with my rich cruise condition?
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Old 05-02-2006, 11:46 PM   #3
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1. check float level/fuel pressure

if that is ok, your idle feed restriction (feeds transition circuit also) is too large since changing the main jet made no difference in cruise, common on double pumpers. the books that you have should show you where the idle feed restriction is on your metering block, without a picture I cannot tell you where it is unless I was holding the metering block in my hands

opening the secondaries further will not help your condition, the reason it helped your idle was because it allowed you to close your primarys to expose less of the transition slot so your idle mixture screws responded. I did the same thing on my 800 double pumper.

I was lucky not to have to modifiy the idle feed restrictions on mine. I cruise in the low 14's AFR's with (71 jet with .094" PVCR) primary and no jets on the secondaries. At WOT with 12 lbs boost I have high 10 AFR's, I will work that leaner but am concentrating on the part throttle first

DO NOT go WOT with your AFR's like that you will melt the engine down I would put 99's in the secondarys and work from there.

Is your mechanical pump boost refrenced?

what carb hat do you have?

make sure you have the nitrophil (sp) floats not the hollow brass. the hollow ones will collapse under boost around 10lbs.

I would also suggest a single plane intake for forced induction because of distribution.

BTW, if you have a secondary power valve that carb is not all a 750
the smallest carbs that got secondary power valves are 800's (if I remember right). so you might have a 750 main body and a 800 secondary metering block or an entirely unknown carb.
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Last edited by myrideisyourmom; 05-03-2006 at 11:03 AM.
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Old 05-03-2006, 11:26 AM   #4
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1. Float level is at bottom of sight hole threads, fuel pressure at idle is 5psi, fuel pump is boost referenced to low profile carb hat

Idle feed restrictions!? Thanks, it would have taken me forever to find that. Too bad these are not easy to lean out. I've searched for info and found articles where they've put wire in the restriction to lean it out but I think that's just a temp fix to see if it helps. I guess I could try that and if it works I might look into a ProForm billet metering block with emulsion jets. I'm assuming emulsion jets are the idle feed restrictions.

Yeah, I have the solid floats.

Single plane intake huh, might give me more hood clearance too, =). Not sure, will have to research.

Well the secondary metering block had a plug in it so I put a PV in to help out with the lean condition.

I'll be putting in larger jets in the secondary ASAP. When I did WOT I only did it for a few seconds to see where the AFR went. Can those few seconds still cause damage?

Thanks a ton, I feel I have somewhere to go now. I was at a dead end for the cruise rich condition.
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Old 05-03-2006, 11:47 AM   #5
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I would not waste money on new metering blocks

I have made my own adjustable ones by tapping the block for brass set screws for the desired metering orrifice, also can be done for the air bleeds. then get a numbered drill bit set(not fractional) and you can make about any sized orrifice you want.

the emusion jets ARE NOT the same as the idle feed restrictions. the emusion jets go into the main well and are used to tune out lean/rich spots in the WOT fuel curve.

if you where only at WOT for a few seconds you are probably ok. I hope your engine has forged pistons and has proper ring gaps for boost.
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Old 05-03-2006, 01:41 PM   #6
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also, make sure your supercharger bypass valve is opening at cruise, if it is not it would cause excessive pressure on top of the carb and change the pressure differiential causing the carb to meter rich at smaller throttle angles
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Old 05-03-2006, 02:55 PM   #7
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Quote:
Originally Posted by myrideisyourmom
also, make sure your supercharger bypass valve is opening at cruise, if it is not it would cause excessive pressure on top of the carb and change the pressure differiential causing the carb to meter rich at smaller throttle angles

I don't have one of those. So the 1-2lbs of boost I get at cruise can be the cause of my rich condition? I'm off to look up bypass valves!
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Old 05-03-2006, 07:08 PM   #8
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bypass valve is also known as a blow off valve or BOV

consider it mandatory so you dont get compressor surge
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Old 05-03-2006, 08:04 PM   #9
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I think I found something. Summit lists the proform main body with the follow air bleeds:

Primary Outer Air Bleed Sizes Included: 73
Primary Inner Air Bleed Sizes Included: 36
Secondary Outer Air Bleed Sizes Included: 73
Secondary Inner Air Bleed Sizes Included: 36

Notes: Includes two 36 air bleeds for the secondary outer air bleeds.

I took the carb off and changed the secondary jets and looked at the primary metering block to see what the IFRs look like. While I was looking I noticed that all 4 of the primary air bleeds were the same size!? They are all size 36. The secondary has 73 outer and 36 inner. I think the previous owner put the extra 36 air bleeds in the primary. Could this cause my rich cruise condition? If so, can I just swap the primary and secondary outer air bleeds so that I have 73s up front and 36s in the back? Or should I leave the secondary air bleeds alone and buy new ones?

Thanks
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Old 05-03-2006, 08:16 PM   #10
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the inner bleeds are for the main circuit and the outers are for the idle/transition circuit.

so yes that would make the carb ultra rich in the idle and transition range.

I would reccomend getting a set of air bleeds (or brass set screws of the same thread) and putting it back to proform specs and tune from there.

you should be able to find a table somewhere that cross refrences the air bleed number to an orrifice size if you want to use drilled set screws, thats what I do (im cheap though)
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Old 05-04-2006, 01:34 AM   #11
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Get a real fuel pump before u melt the engine... go with an Aeromotive A1000 0r A2000 with the Aeromotive reg. What type of carb hat are using ??? The right pump, reg, and carb hat will make tuning so much easier. Also make sure the carb is modded 4 boost..
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Old 05-04-2006, 12:31 PM   #12
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If you are using 2-point adjustable idle, you need those 36's in the secondary idle spots. If 4-corner adjustable idle, you need 73's in all four corners.

Of course, I bought my mainbody before they started putting in those extra 36's. It wasn't a problem until I went with a bigger cam, and now I've got "issues". I'm picking up a pair of 36's after work today.

As said, having the 36's in the primary idle air bleeds would cause a rich condition. Swap the primary and secondary, and you should be in better shape (assuming you have 2-point adjustable idle).
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Old 05-04-2006, 12:41 PM   #13
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Quote:
Originally Posted by five7kid
If you are using 2-point adjustable idle, you need those 36's in the secondary idle spots. If 4-corner adjustable idle, you need 73's in all four corners.

Of course, I bought my mainbody before they started putting in those extra 36's. It wasn't a problem until I went with a bigger cam, and now I've got "issues". I'm picking up a pair of 36's after work today.

As said, having the 36's in the primary idle air bleeds would cause a rich condition. Swap the primary and secondary, and you should be in better shape (assuming you have 2-point adjustable idle).
Yeah, I have the 4-corner idle. I ordered some Barry Grant bleed blanks off of Ebay this morning. Car part stores around here don't know what I'm talking about.

Need 36's huh, wanna trade?
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Old 05-07-2006, 10:59 PM   #14
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Drilled out my idle air bleeds and my cruise AFR on level road is high 13's or low 14's. I used a 1/16 bit since it was the biggest one that I could fit inside the 73 air bleeds. The 73's are a little larger then that but it worked.

Since the air bleeds fixed my rich cruise condition I put the primary jets back to 73's, PV plug back in the secondary and 99 secondary jets. WOT now starts in the 12's and creeps into the low 13's around 4200 RPM and ends up in the mid to upper 13's from 4800 to 5200 rpms.

I need to work on my part throttle when I get more time.

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Old 05-08-2006, 01:27 PM   #15
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I would get the wide open throttle a little richer yet. My final WOT AFR goal for mine is 11.5 ,running leaner would make more power but Im lazy and dont like changing pistons.

Remeber a forced induction engine should run a little rich to be safe. You high end lean out could be emulsion circuit related or your just starting to run out of fuel pump. Force induction takes ALOT of fuel at higher pressures than some fuel pumps will put out. Usually pumps(aimed at carbs) in the aftermarket are rated at free flow and drop dramatically at just 5psi of fuel pressure. It would probably be a good idea for you to get larger needle and seats also
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Old 05-10-2006, 09:43 AM   #16
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My one Holley book suggests disconnecting the secondary and tune the primary only so I did just that. I ended up with 79 jets to get 12-12.5 AFR at WOT with just the primaries. Reconnected the secondaries and the WOT AFR stayed in the 12's but high 12's on the top end. My jet kit only goes up to 99 so I'll need to order a few in the 100 range.

Now I think I have too much pump shot since my AFR will hit 9's when moving around a parking lot.
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Old 05-10-2006, 10:47 AM   #17
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You could reduce the secondary main bleeds as well.
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Old 05-10-2006, 01:34 PM   #18
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sounds like your getting under control
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Old 05-11-2006, 09:11 PM   #19
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I think you are going about this wrong. There is no way you need more than a 99 jet. Have you drilled your PVCR's yet? Does it just lean out at top end (for ex- 1,2 gear good 3 lean)- if so you are probably draining the bowls. In that case raise float level, bigger needle & seat (~.110), better fuel pump (A1000). Also be careful with your total timing.

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Old 05-23-2006, 01:37 PM   #20
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Ok, since I jetted my mains up to 79 I'm back with the rich cruise so my next plan of attack is too jet the mains back down for a good cruise AFR and then drill out my PVCR's for WOT. I'll probably do this again with the secondaries disconnected.

If my WOT AFR is still high with the secondaries connected with 99 jets I'm going to try smaller secondary high speed air bleeds. I wonder how sensitive these are, hmmm...

Sound like a plan?
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Old 05-23-2006, 04:58 PM   #21
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Quote:
Originally Posted by wwnf
Ok, since I jetted my mains up to 79 I'm back with the rich cruise so my next plan of attack is too jet the mains back down for a good cruise AFR and then drill out my PVCR's for WOT. I'll probably do this again with the secondaries disconnected.

If my WOT AFR is still high with the secondaries connected with 99 jets I'm going to try smaller secondary high speed air bleeds. I wonder how sensitive these are, hmmm...

Sound like a plan?
Are you still running with a Power Valve plug in the Secondaries? If so...put a Power Valve back in ....PRONTO!!

Most people are under the impression that you tune a Holley with the main jet for WOT A\F ratio. If you read any really GOOD books ( such as those by David Vizard and Dave Emanula ) you will find that is incorrect.

WOT mixture is GREATLY influenced by the Power Valve channel restriction. The proper way to tune a Holley ( for the most efficient street use and High Performace ) is to adjust the main jet size to get the correct A\F at cruise and then adjust the Power Valve channel restiction size to adjust WOT.

This is a bit of a tricky procedure ( since increasing a PVCR by .002" is a significant change ) and most people screw it up. Much easier to just put bigger main jets in to get the desired result. This will work of course...but you get a pig rich cruise as a result.

Now a blow through carb is a different animal. You need ALL of the fuel that you can possibly get at WOT....but still have a reasonable cruise A\F mix. [b]You will never achieve this with a PV plug in the Secondaries[\b] without running HUGE secondary main jets. A PV plug requires a main jet 6 to 8 numbers larger ( Sometimes 10 sizes ) to achieve the same mixture at WOT.

So ...put the Power valve back in the Secondary side. Tune the car for good A\F at cruise and then tune the WOT with the Power Valve Channel restrictions.

Buy some number drills and use a Pin Vise to enlarge the PVCR holes. Do not go more than .002" at a time without retesting. When you get close to the desired WOT A\F mixture you can trim the top end with the high speed air bleeds. High Speed Air bleeds mainly affect the mid range to top of the fuel curve. Main fuel jets ( on a Holley ) are more general. They affect the entire curve, but mostly mid ramge to high mid-range.


BTW....have a close look at that Pro-Form body and see if you have any of the issues that I mentioned in this thread:

http://www.thirdgen.org/techboard/ca...ml#post2901498

Edit: while I was typing my rather lenghty reply...it looks like you came to the same conclusion. Remember to use that Secondary Power Valve. You need the enrichment it provides.

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Old 05-24-2006, 11:55 PM   #22
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Try reading over at turbomustangs.com. They have a very good section on blow-thru carbs; including how to mod carb.

Not trying to start an arguement, but myself and many other blow thru cars plug the rear pv. I agree- keep the primary one and drill pvcr in small steps.

Matt
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Old 05-25-2006, 03:46 AM   #23
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Quote:
Originally Posted by blown3rdgen
Try reading over at turbomustangs.com. They have a very good section on blow-thru carbs; including how to mod carb.

Not trying to start an arguement, but myself and many other blow thru cars plug the rear pv. I agree- keep the primary one and drill pvcr in small steps.

Matt
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Yeah...I've read some of those articles. Good stuff.

http://www.hangar18fabrication.com/blowthru.html

Most seem to be from a Drag Racing perspective. Seems the Drag Racers prefer a bit different setup than what us Road Racers do.

The only reason for using a rear PV plug is that HARD launches while Drag Racing can uncover the rear Power Valve and cause a lean spot.

Road racing and a Street setup simply do not have hard enough acceleration to uncover the rear PV, and it can be advantagous to use a Secondary PV to get the additional enrichment under these conditions. But I do see the point of plugging the Secondary PV if Drag Racing.

You can get the necessary WOT mixture both ways, but using dual Power Valves gives a more precise curve. But it is a very good point about Drag Race Launches uncovering the rear PV.

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Old 05-30-2006, 04:06 PM   #24
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Installed a MSD BTM 8762 over the weekend. The retard with boost now allows my engine to go to 5500 rpms with no problems.

Before I do anymore tuning I want to install a BOV or surge valve. I'm not sure if those are both the same thing or not. I want whichever will only give me boost when I'm at WOT. I don't see the need to have boost all the time. My understanding is they work off of a vacuum reference and stay open until you hit a certain vaccuum level. Plus I'm not sure if it would affect my part throttle rich condition or not but want to get it done. I'm getting fuel out of my throttle shafts. If this is mostly from compressor surge when I let off the throttle after a WOT run, I don't know. I've seen some throttle plates with a milled channel so that you can boost reference the throttle shafts but I don't want to go that far yet.

There's a ton of blow off valves on ebay, are these any good or should I stay away?
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Old 05-30-2006, 07:56 PM   #25
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get a surge valve. You need something that bleeds off at idle/ cruise- not just a generic blow off valve. Stock procharger ones are available on ebay every now and then.

Matt
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Old 06-20-2006, 01:11 PM   #26
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Installed a 9inch K&N filter and a procharger pro-flo surge system over the weekend. I'll have to post a picture later but it's really cool when you let off the gas, ppppssssshhhh, =)

As suggested, I ordered a pair of 0.110" needle and seats, and a 2 bar MAP sensor so that I can hook that up to my RPM converter.

Now I'm going back and jet my mains for cruise and then open up my pvcr for WOT. I'll do the pvcr drilling in small steps. I did calculate that a pvcr size of 0.076" and 73 jets would give me about the same total area of 0.059" pvcr and 79 jets but I'll take it in steps since I have 7 drill bits in between. Plus I might be changing jets when I jet for cruise again.

#53 0.0595
1/16 0.0625
#52 0.0635
#51 0.0670
#50 0.0700
#49 0.0730
#48 0.0760

Link to numbered drill bit chart I'm using
Drill Bit Size Chart
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Old 06-22-2006, 09:22 AM   #27
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Removed the power valve protector check ball last night. Maybe this will help with the WOT tuning when I get back to it.
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Old 06-22-2006, 09:37 AM   #28
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I'm lost. How would that help with tuning?
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Old 06-22-2006, 12:36 PM   #29
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I read here, Blowthrough Carb How-To, that you have to remove the protector check ball for the power valve to function properly under boost.
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Old 06-23-2006, 07:24 AM   #30
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Quote:
Originally Posted by wwnf
I read here, Blowthrough Carb How-To, that you have to remove the protector check ball for the power valve to function properly under boost.

How much boost are you maxing out at here? It would take a huge amount of constant pressure to keep that check ball up blocking the passage to the PV. If you were at those levels now your going to risk rupturing the PV and might as well block it off and jet up or switc to a different style metering system IE carb.
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Old 06-23-2006, 12:05 PM   #31
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I max out around 8 lbs of boost
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Old 06-28-2006, 04:16 PM   #32
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Quote:
Originally Posted by blown3rdgen
I think you are going about this wrong. There is no way you need more than a 99 jet. Have you drilled your PVCR's yet? Does it just lean out at top end (for ex- 1,2 gear good 3 lean)- if so you are probably draining the bowls. In that case raise float level, bigger needle & seat (~.110), better fuel pump (A1000). Also be careful with your total timing.

Matt
383 blow thru

I think Matt is right, I've been going over some logs and my top end leans out like I'm running out of fuel. I've looked into a complete new fuel system and don't have the cash right now but will be saving for an A1000.

My current setup is all -6AN lines and was wondering if I could hook up an inline electric pump near the tank to help out the mechanical pump. I guess this could be the A1000 or cheaper pump. Would this work or is it just a waste of time? Can -6AN lines provide enough fuel in the first place? I guess my options are the following:

1. keep existing setup and add a cheap inline helper pump
2. keep existing -6AN lines, bypass mech pump, add A1000 pump
3. Install A1000 pump with new -8AN lines straight to my regulator (AEI-13301)


I appreciate all the help guys, I really enjoy working on the car, just wish that I started it from scratch but we're getting there. Thanks

Last edited by wwnf; 06-28-2006 at 04:24 PM.
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Old 07-07-2006, 04:18 PM   #33
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I want to upgrade my fuel system and I'm leaning towards an A1000 pump. Can I use my existing -6AN feed and 3/8 return or do I need to go bigger?
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Old 07-08-2006, 08:28 AM   #34
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The A1000 will be fine by itself. You won't need a helper pump or anything else. As for lines, I would run at least -8 feed and return. I too started out with a little holley black and -6 lines, but quickly maxed those out and had to upgrade. That stuff gets expensive and wish I had done it right the first time. In this case going to big will not be a problem. If trying to save money, you can buy a roll of 1/2" aluminum for like $20 and run that the length of the car. Then just use tube nuts to adapt -an lines as needed.

Matt
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Old 07-10-2007, 09:24 PM   #35
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Re: Blow-through Holley carb wideband tuning

Hey guys I see that this thread is about a year old but Im in the process of helping a buddy do a set up on his car (non third gen but its all the same). Im just wondering how everything turned out and what you had to do to get it set up? got any pics? Im trying to get all the info I can before we start attempting to do any mod's.

Thanks
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