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Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

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Walter R. Malik
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Walter R. Malik » Tue Sep 10, 2019 7:22 pm

Simply some opinions ...

1. Cam is slightly to big.

2. Wide Open Throttle torque & horsepower mean absolutely nothing at part throttle cruising.

3. Your O2 sensor(s) are merely reading an average ... fuel and air distribution are most important at these RPM levels.
When it is reading a little rich you are helping that leanest cylinder to perform OK.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by superpursuit » Tue Sep 10, 2019 11:06 pm

Does your RPM Airgap manifold have the step in the divider? If it does then this makes it act a little more like a single plane inlet along with the reversion problems at low speed associated with that. Try filling that step and keep your 4 hole spacer. This will make the inlet behave more as a 180 degree manifold and will give you less reversion and should help your low speed driveability. The cylinders that you say are running leaner could be doing so because they are breathing exhaust fumes from reversion.

Good luck.

Allan.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Tue Sep 10, 2019 11:16 pm

superpursuit wrote:
Tue Sep 10, 2019 11:06 pm
Does your RPM Airgap manifold have the step in the divider? If it does then this makes it act a little more like a single plane inlet along with the reversion problems at low speed associated with that. Try filling that step and keep your 4 hole spacer. This will make the inlet behave more as a 180 degree manifold and will give you less reversion and should help your low speed driveability. The cylinders that you say are running leaner could be doing so because they are breathing exhaust fumes from reversion.

Good luck.

Allan.
It does have the partial cutout.

I’ll give that a try. I tried an open spacer tonight after work, along with some new plugs with an increased gap (from 0.043 to 0.055). Made no difference other than having to riches the mixture screws about an eighth of a turn. It’s running so damn good in every single other scenario. Just need to get this little bug fixed and I’ll be set.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Tue Sep 10, 2019 11:17 pm

Walter R. Malik wrote:
Tue Sep 10, 2019 7:22 pm
Simply some opinions ...

1. Cam is slightly to big.

2. Wide Open Throttle torque & horsepower mean absolutely nothing at part throttle cruising.

3. Your O2 sensor(s) are merely reading an average ... fuel and air distribution are most important at these RPM levels.
When it is reading a little rich you are helping that leanest cylinder to perform OK.
Agreed on all accounts.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by exhaustgases » Tue Sep 10, 2019 11:56 pm

Float level? Lean it some and close the plug gaps.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Wed Sep 11, 2019 3:25 am

Fill in that partial plenum divider to isolate the dual plane plenums. Change diz curve for 24-26 deg at idle 38 total.
Use ported vac adv not full manifold.
Primary idle air bleeds will need slight fine tuning to correct off idle transition afr shift where when throttle is slightly opened... Slow cruise.

The 107 lsa 235 deg cam is not helping.
This is why most catalog cams are wider lsa. 112 ish.
Simply better drivability thru less overlap.

Tighten the intake bolts potential man gasket vac leak ;n 1 or more cylinders. Try to find the offending weak cylinder.
Leakink ex header gasket...

Verify correct diz rotor/cap phasing.
Adjust vac adv mounting plate if nessessary.
Vac adv (mounting point) effects cap/rotor phasing

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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Wed Sep 11, 2019 3:43 am

A 750 holley is always more responsive on the street than the big venturi big cfm 1000+cfm 4150 holleys.
More than 500 hp capable. Where drivability is a priority it is usually a better choice overall.

Tweeking the pcv valve flow rate +/- can have a nice effect right where you are having drivability issues.
The pcv valve may now be just now not the right pcv valve for this motor. Ya it matters and getting the pcv valve right will save you messing needlessly with the carb.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Wed Sep 11, 2019 3:48 am

This is why ALL oem carbed intake manifolds have heated plenums. Really helps fuel vapourization at idle and low rpm cruise rpms better drivability.
A cold intake plenum will always be more troublsome to tune for drivability.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Wed Sep 11, 2019 5:15 am

How much does the manifold vacuum increase from curb idle to the level road 1500-1800 RPM?

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Geoff2 » Wed Sep 11, 2019 5:20 am

It is the 'big cam behaving badly syndrome'.

There are some things that should be checked. Popping exhaust is NOT normal; could be valve adjustment, wiped lobe, etc.

Definitely do NOT use an open spacer. That will only make the surging worse. Keep the plenums divided for max vacuum.

Try backing out the mixture screws 1/2 a turn as an experiment. Might idle rough, but less surging at cruise. This would be a sign that the IFRs need enlarging.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 8:04 am

exhaustgases wrote:
Tue Sep 10, 2019 11:56 pm
Float level? Lean it some and close the plug gaps.
Already played with that. Float level is in the bottom of the site glass on both bowls, as recommended. I've had it up and down from there 1/4 inch. Makes a difference in the mixture, but the issue persists.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 8:05 am

F-BIRD'88 wrote:
Wed Sep 11, 2019 3:25 am
Fill in that partial plenum divider to isolate the dual plane plenums. Change diz curve for 24-26 deg at idle 38 total.
Use ported vac adv not full manifold.
Primary idle air bleeds will need slight fine tuning to correct off idle transition afr shift where when throttle is slightly opened... Slow cruise.

The 107 lsa 235 deg cam is not helping.
This is why most catalog cams are wider lsa. 112 ish.
Simply better drivability thru less overlap.

Tighten the intake bolts potential man gasket vac leak ;n 1 or more cylinders. Try to find the offending weak cylinder.
Leakink ex header gasket...

Verify correct diz rotor/cap phasing.
Adjust vac adv mounting plate if nessessary.
Vac adv (mounting point) effects cap/rotor phasing
I've done all of that except filling the divider. I will try that next.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 8:06 am

Tuner wrote:
Wed Sep 11, 2019 5:15 am
How much does the manifold vacuum increase from curb idle to the level road 1500-1800 RPM?
I haven't measured this, I will in the next day or two.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 8:13 am

Geoff2 wrote:
Wed Sep 11, 2019 5:20 am
It is the 'big cam behaving badly syndrome'.

There are some things that should be checked. Popping exhaust is NOT normal; could be valve adjustment, wiped lobe, etc.

Definitely do NOT use an open spacer. That will only make the surging worse. Keep the plenums divided for max vacuum.

Try backing out the mixture screws 1/2 a turn as an experiment. Might idle rough, but less surging at cruise. This would be a sign that the IFRs need enlarging.
Popping is the wrong word. It's like when an older motorcycle with a dedicated carb for each cylinder has the carbs out of sync. When you are at a slow cruise, some of the carbs are closed and those cylinders aren't firing, they are being "dragged" along by the ones that are firing. With my big diameter side exit exhaust, there is a certain deep sound on deceleration (sounds cool). The "popping" I described is an intermittent version of this sound coming from the exhaust, that I interpret as a the offending cylinder(s) being in "decel" mode.

I plan to fill the divide in the intake, and use a divided spacer (or the 4 hole) to see if that will clean things up.

bigger IFR's (or smaller IAB's) help, and basically eliminate the problem.. it's just so rich that it fouls the plugs on the richer cylinders in a few days. This is leading me to believe that the issue is fuel distribution. I only have an o2 sensor on one bank, but as a few people have said, that is an average of those cylinders. I suspect that during low RPM cruise, I could have some cylinders at 16:1 and others at 12:1, which would average at 14:1.

Also, it is worth reiterating... this issue in not bad. It is barely noticeable, but everything else is working so well that It is annoying me, especially since I am fixated on it now.


I really appreciate all the advice and suggestions.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Kevin Johnson » Wed Sep 11, 2019 9:36 am

Just a thought:
https://www.gsparkplug.com/shop/spark-plug-heat-range wrote:It was common before the modern era of computerized fuel injection to specify at least a couple of different heat ranges for plugs for an automobile engine; a hotter plug for cars which were mostly driven mildly around the city, and a colder plug for sustained high speed highway use. This practice has, however, largely become obsolete now that cars' fuel/air mixtures and cylinder temperatures are maintained within a narrow range, for purposes of limiting emissions. Racing engines, however, still benefit from picking a proper plug heat range. Very old racing engines will sometimes have two sets of plugs, one just for starting and another to be installed once the engine is warmed up, for actually driving the car.
Perhaps use this strategy to stop the plugs on the richer cylinders from fouling in a few days. ???

There is a lot of experience on the board -- is this strategy too risky if the heat range change is minimal?

To be clear: I am suggesting using more than one heat range simultaneously.

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