BBC thoughts

General engine tech -- Drag Racing to Circle Track

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prairiehotrodder
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Re: BBC thoughts

Post by prairiehotrodder »

With the t-ram i understand that each barrel of a carb feeds a intake runner and intake valve. This should mean that the butterflies on the carbs open equally (not progressively) and have the jetting squared. Is this line of reasoning true?

Also - statsystems, the only part of what you said that i didn't agree with is it costing 3 times as much. I'm sure you are right and i don't doubt that you know what you are talking about. My cam may well not be optimum and dedicated t-ram carbs i'm sure would have been better. They couldn't have been worse! This time on this cam i'm gonna try what I think is best and see what the results are. Maybe i'll be disappointed but i'll show the results good or bad.
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Re: BBC thoughts

Post by FC-Pilot »

You may want to down size your IFR as that will make a big difference in you light cruise AFR. There is plenty more that can help, but I am not a carb guru, so you may want to search other posts and stuff.

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Re: BBC thoughts

Post by F-BIRD'88 »

When just sitting in the driveway and running what does the AFR gauge do if when you open the throttle from idle to raise the rpm (1200 up to 2500 rpm) part "throttle transition". ??

Is the afr in this throttle reasonable 14:1 ISH +/- or does i go pig rich 12.5:1 or richer?
or does it go lean 15:1+++ ???

Anywhere in the 15:1 to 13:1 mid afr is good..

Then when opening the throttle more to where mains fuel flow starts what does the afr gauge do?

Is this the point where it goes very rich?
Make sure the air bleeds are clear or there is nothing stuck in the emulsion wells.
Remove each air bleed and blow out with shop air.

if the primary main jet is just way too big then both throttle angle points afr will be too rich.

Swapping the primary jets to 70 should help trim both driving points.

You can do all this while running in the driveway to get it close.

I suggest 70 72 pri and 76-78 sec jets.

( if the IFR's are just too big they may need resizeing smaller but first you want to get the primary main jet roughly in the right zone)
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Re: BBC thoughts

Post by F-BIRD'88 »

Before installing these carbs did you flip them over and look at the throttle opening at idle to see the T-slot edge exposure or all 8 throttles? Or just bolt them on?
You want the T-slot edge exposure about .020" on all 8 barrels.
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Re: BBC thoughts

Post by prairiehotrodder »

I may have to eat a little crow here, but i'm back with my tunnel ram progress. I need to get these carbs sorted out because they are way out in left field. I went for a drive today after making major tuning changes to my carbs over the winter. It wasn't good and i'm starting to think I should just ship them to somebody that knows what they are doing. Engine starts and idles fine at an acceptable AFR. Slow cruise is rich at about 10.9 and as I slowly speed up it leans out. At a fast cruise with more throttle it suddenly leans out a lot. Then at WOT its ragged and showing 12-1 AFR but I think its actually misfiring and very lean. The carbs are holley track warriors 750's. Out of the box they were really rich everywhere. I've posted over on The racingfuelsystems page my whole tuning story and what I've done. Just wanted to bring this thread back to life because i get quicker responses here usually and there are some great guys here. Thanks
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Re: BBC thoughts

Post by prairiehotrodder »

ok quick update with more data.
I changed the jets in both carbs to 76/86. Stock for these carbs is 80/90. I had them at 70/80 which is what most 750 dp carbs are supposed to be at. Went for another drive

Park hot idle is 800 rpm / a bouncy 3 inches of vacuum and 12.0 AFR, need to adjust the idle screws again.
Cruise hot 3000 rpm / 14 inches of vacuum / 11.2 AFR
WOT 10.9 AFR / doesn't sound ragged anymore with the 76/86 jets. I'm not sure i trust the WOT AFR reading. I think the gage is accurate at idle and cruise but not WOT. Don't know why.
At least now i feel i can go to the track and be in the ball park. This highway tuning with a 9 second car is kinda stupid.
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Re: BBC thoughts

Post by MadBill »

I'm too lazy to go searching for what cam you're using, but 800 RPM seems very low for an idle speed, unless you have a very low stall speed converter...

I second F-BIRD'88 re checking the off-idle mixture. With 2 x 4 carbs, the idle circuits will be major mixture contributors up to 50 MPH or more. Open the throttles very slowly* (best via the idle speed screws), working your way up to at least 2500 RPM and see what the mixture does.(*To avoid accel pump input) If it immediately starts going rich you'll need to reduce the idle feed restrictions or increase the idle air bleeds. As a diagnosis for this condition's effect on cruise AFR, try setting the idle speed at say 1200 RPM and then lean out the mixture as much as possible without misfire, preferably 14:1 or leaner, then check the cruise mixture at a steady 45, 55 & 65 MPH.

Here's a useful idle tuning video: https://www.youtube.com/watch?v=JDPwY0LbRVY
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Re: BBC thoughts

Post by cjperformance »

^^ and to add that while doing the no load basic idle circut test its easy to remove the acel pump cams or levers to eliminate acel pump fuel. And while bringing rpm up keep an eye on the boost venturis for fuel pullover with a timing light, the strobe makes it super easy to spot fuel droplets so you will know if you are only on the idle/transfer circuit during tests.
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Re: BBC thoughts

Post by prairiehotrodder »

I warmed up the motor, removed the pump cams and slowly brought the rpm's up. At idle it was trying to go really lean and as the rpms came up it richened up. By 3000 rpm the AFR was 10.9. the funny thing is it sounded like it was struggling to come up but then it smoothed out at that rich AFR of 10.9. I had previously put smaller Idle jets in from stock .035 to .0292. I don't trust my AFR gage anymore so i'm gonna get a new one before I do anything else.
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Re: BBC thoughts

Post by MadBill »

If you can get enough hands in play, you might be able to ease the tips of toothpicks (the machined cylindrical kind) into at least the primary idle air bleeds and see if the meter shows richer as it should or reads suddenly lean as it comes out of lean misfire.
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Re: BBC thoughts

Post by af2 »

Never used a AFR gauge. But lotts of TR and CR 2x4's jets at 70ish and depending on cam the PV has to be inline. The best set up I had for the street on a small block was 2 4412's
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Re: BBC thoughts

Post by prairiehotrodder »

whats a inline PV ?
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Re: BBC thoughts

Post by prairiehotrodder »

So I purchased an AEM failsafe AFR gage that does data logging. It monitors AFR, Vacuum and RPM and logs it all. Hopefully this should be a better tuning aid and also prevent me from hitting the guard rail while watching the gage! Hopefully the plan is to get to the track on May 25 for the test and tune. I'm looking forward to seeing what this new Erson cam and the tunnel ram setup has done for me. As a side note my local track just switched from NHRA to IHRA. Not sure on what that's gonna do to the rules but i'm told I won't need to have my cage certified until I reach 9.0 where NHRA was 9.99.
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Re: BBC thoughts

Post by cjperformance »

prairiehotrodder wrote: Tue May 08, 2018 10:32 am So I purchased an AEM failsafe AFR gage that does data logging. It monitors AFR, Vacuum and RPM and logs it all. Hopefully this should be a better tuning aid and also prevent me from hitting the guard rail while watching the gage! Hopefully the plan is to get to the track on May 25 for the test and tune. I'm looking forward to seeing what this new Erson cam and the tunnel ram setup has done for me. As a side note my local track just switched from NHRA to IHRA. Not sure on what that's gonna do to the rules but i'm told I won't need to have my cage certified until I reach 9.0 where NHRA was 9.99.
Brian
Isnt that the way, often all the extra gauges etc just provide more distraction ! #-o
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Re: BBC thoughts

Post by prairiehotrodder »

prairiehotrodder wrote: Fri Oct 20, 2017 12:30 pm With the t-ram i understand that each barrel of a carb feeds a intake runner and intake valve. This should mean that the butterflies on the carbs open equally (not progressively) and have the jetting squared. Is this line of reasoning true?

Also - statsystems, the only part of what you said that i didn't agree with is it costing 3 times as much. I'm sure you are right and i don't doubt that you know what you are talking about. My cam may well not be optimum and dedicated t-ram carbs i'm sure would have been better. They couldn't have been worse! This time on this cam i'm gonna try what I think is best and see what the results are. Maybe i'll be disappointed but i'll show the results good or bad.
Well here is the truth on this deal for anyone that cares. I went way slower. Down on power. Should have listened. May swap my old Lunati cam back in. Don't know what else to do. The tunnel ram runs and drives ok and seems to be responsive to my tuning at the track.
Brian
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