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Old 06-26-2010, 12:53 AM
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Wideband tuning holley 750 at the track...

Took my Vega to the track tonight and logged my runs with my wideband. The 60' times weren't so great. I launch at about 1800 rpm. First gear was rich from 2400-3300 and then it went lean from 3300-4800 before finally leveling out. My question is this... How could I adjust the carb to smooth this out a bit. 2nd and 3rd gears were pretty good. See the attached graph, I circled first gear in red. Here is the quick breakdown of the car:

75 Vega
406 sbc
dart heads
mild cam
performer rpm intake
holley 750 vacuum secondary no choke
th350 2400 stall
ford 9" 373 gears
26x9 slicks

Best run tonight in 100 degree heat:
60' 1.798
330' 5.070
1/8th 7.803 89.88mph
1000 10.154
ET 12.139 @ 113.50 mph
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Old 06-26-2010, 12:59 AM
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If it isn't obvious on the graph... Black is rpm, purple is AFR.
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Old 06-26-2010, 10:35 AM
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Anybody have an ideas? I'd like to go back out to the track this afternoon if I can make some changes. Here is what I figure so far:
1. it is too lean while I'm staged to launch at 1800rpm
2. Too much accelerator pump squirt. It looks like enough to counteract the initial lean condition and then go too rich.
3. Goes lean once again while secondaries are opening

Possibly a power valve that opens sooner would remedy the initial lean condition, then I could work on the accelerator pump?

Last edited by iambigperm; 06-26-2010 at 11:00 AM.
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Old 06-26-2010, 01:03 PM
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The converter stall is way too low for drag racing.
If you want to leave hard you have to leave at or very near peak torque.
( even with a "mild cam") Peak torque occurs way above 3000 rpm.
think 3500 stall+ if you want to go fast.
There is .4 sec et in this car with the right converter stall.

The near 16:1 idle afr is way too lean. should be 12.5 to 14:1 at idle and off idle tip in.
Low speed par throttle driving. Get the idle and off idle corrected and you won;t need near as much accelerator shooter and will be able to leave from very near idle speed. Loading up the converter at the line kills the converter's flash stall effect. You want to stage and leave from very near curb idle and shock the converter for max launch torque.
Loading up the rpm against the converter is a crutch for improper throttle response caused by incorrect idle and off idle afr and incorrect idle off idle timing curve.
Use the wide band to fine tune the idle and off idle circuit.
(idle air bleed and idle feed restriction). Very small changes in the idle air bleeds makes a huge/huge difference.
If you are using a holley 3310 consider upgrading to a Proform HP style center body w screwin air bleeds and vac sec.

Power valve:
The power valve has nothing to do with it. It opens as soon as you floor The gas and the manifold vacuum drops. But the car pulls out of the hole on the idle and off idle circuit + accerator squirter as the throttle opens.
The accelerator pump will not make up for incorrect idle/off idle afr.

Then when you get that right, the secondary side opening speed/timing is critical. This requires custom tuning the sec vac pot spring tension and vac sec feed port size. You need to make this adjustable.
What is the timing curve? The idle timing and timing curve are critical.

camshaft specs?

Last edited by F-BIRD'88; 06-26-2010 at 01:16 PM.
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Old 06-26-2010, 01:15 PM
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The idle afr is right at 14.0 but I had started the log when I was staged so you can't see it. It doesn't lean out until I bring it up off idle. I have ran 11.3 in the quarter with this car exactly as it sits(in Seattle@ sea level and 70 degrees). I don't think the converter is the issue I have ran plenty of 1.6's for 60' times with this one. I was told this was a 2800-3000 stall but I think its a little tighter than that. I had this car running great in Washington, but here in Vegas it is a different ball game.
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Old 06-26-2010, 01:25 PM
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Quote:
Originally Posted by iambigperm
The idle afr is right at 14.0 but I had started the log when I was staged so you can't see it. It doesn't lean out until I bring it up off idle. I have ran 11.3 in the quarter with this car exactly as it sits(in Seattle@ sea level and 70 degrees). I don't think the converter is the issue I have ran plenty of 1.6's for 60' times with this one. I was told this was a 2800-3000 stall but I think its a little tighter than that. I had this car running great in Washington, but here in Vegas it is a different ball game.
To tweek the off idle afr:
Reduce the primary idle air bleed. a very small amount. .001" makes a big change.
Get the car to leave at 1000rpm. ( timing+idle circuit)
Get the idle throttle position /idle transfer slow exposure right on both the pri and sec throttles.
Make some logs of you driving the car at idle, off idle low speed driving.
Different throttle inputs.
What is the jets size? primary idle air bleed, sec idle air bleed, idle feed restriction orifice sizes. What are the high speed air bleed sizes.what model carb ( holley list number)? stock metering blocks? other?

details matter. Even the air cleaner matter. More than you think.
Are you running with or without the air cleaner installed.

ya the converter is too low a stall.
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Old 06-26-2010, 01:28 PM
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As far as the timing it is 14 degrees at idle, 36 total, all in by 3000.
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Old 06-26-2010, 01:32 PM
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Quote:
Originally Posted by iambigperm
As far as the timing it is 14 degrees at idle, 36 total, all in by 3000.
What cam is in this motor?????. More idle timing allows adjustment of the throttles at idle ( transfer slot exposure) get this right first before touching the air bleeds. The throttles must be in the sweet spot at idle or the curb idle mix screw adjustment will be off once the throttles are opened a bit ( idle transfer slot) ( idle air bleed, idle feed restriction)
The more initial timing at idle the better it will leave from very near idle.

Like 24deg initial. What cam?
You need to shock the hell out of that converter?
The more you shock it ( quick throttle response from near idle launch) the more torque it makes/multiplies at launch.
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Old 06-26-2010, 01:37 PM
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It was a 3310 and I added a rear metering block. The choke horn is gone so most of the numbers are too... I have the carb info written down in my trailer, but that's at work so I'm not going to try to remember it all... I'll get those numbers later today. I did buy the carb brand new as a factory holley unit so most of it is stock aside from the rear metering block and choke horn machined off. I did change the squirter and secondary spring as well, but I haven't drilled any holes or anything.
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Old 06-26-2010, 01:39 PM
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Would be nice if you could also log intake manifold vacuum in addition to rpm and afr. The intake manifold vacuum indicates throttle position and load.
(you'll also want it to help tune the high speed air bleeds) (track the high rpm afr, rpm and manifold vacuum.

If the rear metering block is from a 600 or 650 it could be a problem (now or as you get deeper into the carb tune. The carb idles off both the primary and sec sides at idle. front back balance is critical.
The (#21) sec metering plate idle orifices an be used as a baseline comparision.
Removing the choke horn will definately effect the off idle afr. (idle air bleeds)

The lower the converter stall speed the more criticial all this tuning is if you want to go fast(er).

Last edited by F-BIRD'88; 06-26-2010 at 01:53 PM.
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Old 06-26-2010, 01:47 PM
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I had a 3310 and also have a old Ford 427 muscle car vac sec 735cfm that is like night and day compared to a 3310. (down leg booster and much more finely tuned fuel circuits) The 3310 is a "universal carb" that needs lots of fine tuning to make it great for your car instead of ok for any car.
I highly recomend the hp body upgrade w tunable air bleeds and down leg boosters if you are serious about getting the carb right.

You can make and fine tune your own air bleeds with some solder and precision pin drill set and a dial caliper to measure. Ya its that critical.
When you get it right, modern efi seems sluggish.
You need to make that wide band meter earn it keep.
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Old 06-26-2010, 02:07 PM
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Some people find disabling the sec throttles while dialing in the primary side helps. Then re- enable the sec side, tune it and retune/fine tune the primary side. The primary and sec side must have correct balance. especially critical as the sec side transitions to open throttle.
Unfortunatly your wide band afr meter only shows you an average afr reading for that cylinder banks ex. Carb spacers can really effect the front back- cylinder/cylinder carb balance. You got to get all 8 cylinders working right.
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Old 06-26-2010, 02:17 PM
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To see the effect of the idle air bleeds on the off idle afr, hold the engine at say different rpm between idle and 2000rpm while holding your finger over the idle air bleeds and watching the afr meter. The more you block off the air bleeds air flow with your big fat fingers the richer the afr will shift on the afr meter. Then get someone you trust/who can drive to repeat it while idling in gear with theor foot on the brake, loading the engine against the converter at different rpms. Watch the tach, afr and manifold vacuum. Fine the best idle spark advance. recurve from there. More inital base spark=better throttle response.

Further from your logs it looks like the sec side needs transition tuning.
do the pri side first.
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Old 06-27-2010, 12:38 AM
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I did a run with the caps on the exhaust and figured out that the oxygen sensor is too close to the dumps to read accurately at low rpm. When I ran the car through the muffler I saw different readings when staged. It was actually about 14:1 @ 1800 rpm, the rest of the run was the same as the log. It did still have the very rich dip right off the line accompanied with a hesitation. I'm going to try a different accelerator pump cam. It has a red cam in it and I'm going to try a pink one. Didn't have any with me though. Only a couple more race days in vegas before we take the summer break for the heat.... After next weekend there isn't any racing until the end of August

As far as your earlier question... I have a Comp XE268 in it. Performer RPM intake.

Thanks for all your help F-Bird!
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Old 06-27-2010, 08:37 PM
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Ya its important to have accurate data.
I found that two when I tried a test pipe on the end of my exhaust.
Would read lean at idle.

14:1 may not be ideal. Some engines need to be a litle richer at idle.
Remember you are only reading a average of 4 cylinders.
You can still experiment with the air fuel ratio in the idle/offidle transition circuit to see gains.
XE268:
recurve the distributor to allow up to 24deg base timing at idle yet 34-36deg total advance. Ya that involves modifying the distributor.
But will allow low rpm near idle launching for max converter response.

With the 268 cam you engine is not really in the program till 3000 rpm.
Peak torque occurs between 4000 and 4700rpm
A 3500-4000 stall will make you go faster.
A "2400" stall is bare minimum if you like launching real hard and going fast(er).

The break from track time will allow you to make changes.
Might try a adjustable vac sec housing (QFT). You can make your own custom built, if your savey.

Last edited by F-BIRD'88; 06-27-2010 at 08:42 PM.
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