Idle Vacuum - Noticed Something
Thread Starter
Joined: Aug 2004
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From: Pace, FL
Idle Vacuum - Noticed Something I'm working on a 5.3L truck with a 235/242 111 Cam in it... and it idles around 55kPa @ 950. That's a lot of vacuum for such a large cam in a low compression, small motor. I didn't do the install, so I can only assume it's installed properly.
But with timing set @ 22 degrees and it leaned out well enough that it doesn't surge with adaptive spark off, it idles at 55kPa.
Now, to get vacuum up (less kPa), I typically raise the rpm or use more spark and less airflow.
But the vacuum of this size cam with this much overlap has surprised me.
What do you think could be contributing to the good vacuum reading?
One theory I have is the TB needs to be drilled out. It struggles in drive to idle but does fine in park. And it's set at .69V. So I'm thinking the low airflow is contributing to the better vacuum as it's encountering an inherent restriction. But with the timing as low as it is and the RPM set to a reasonable area... it has prompted me to start this thread.
Thoughts?
But with timing set @ 22 degrees and it leaned out well enough that it doesn't surge with adaptive spark off, it idles at 55kPa.
Now, to get vacuum up (less kPa), I typically raise the rpm or use more spark and less airflow.
But the vacuum of this size cam with this much overlap has surprised me.
What do you think could be contributing to the good vacuum reading?
One theory I have is the TB needs to be drilled out. It struggles in drive to idle but does fine in park. And it's set at .69V. So I'm thinking the low airflow is contributing to the better vacuum as it's encountering an inherent restriction. But with the timing as low as it is and the RPM set to a reasonable area... it has prompted me to start this thread.
Thoughts?
MAP at Idle Hi Jake, I complement you on your good tune.
I observe even lower MAP 43-44 KPA at best with my ECU-882C often, not always.
I run a LOT of 236/242 cams with 116 C/L which would help with IDLE KPA over your 111 C/L.
When you INCREASE that orifice in the TB, you could see manifold pressure waves, NOT A GOOD REPORT of MAP.
We do a great deal of work to SMOOTH the MAP reading, the average read.
I would guess that you have a quality exhaust system with high flow.
Lance
I observe even lower MAP 43-44 KPA at best with my ECU-882C often, not always.
I run a LOT of 236/242 cams with 116 C/L which would help with IDLE KPA over your 111 C/L.
When you INCREASE that orifice in the TB, you could see manifold pressure waves, NOT A GOOD REPORT of MAP.
We do a great deal of work to SMOOTH the MAP reading, the average read.
I would guess that you have a quality exhaust system with high flow.
Lance
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
It's supposedly installed dot-to-dot... so it's on a 108 ICL with the 3 degrees advance ground in.
And it's 1-3/4" headers.
I had a 234/242 111+3 installed in my car for about 2 years. It idled around 69kPa @ 950 with 25 degrees spark. But it was a very solid idle.
And it's 1-3/4" headers.
I had a 234/242 111+3 installed in my car for about 2 years. It idled around 69kPa @ 950 with 25 degrees spark. But it was a very solid idle.
I don't think thats very high considering the idle speed of 950. The stock 5.3L idles at 550 and pulls 40kpa iirc. 55kpa would be just under 15" of Hg. If it pulled that at 550rpm I would be concerned. but almost 1k rpm? that seems like an unnecessarily high idle to me. That cam should pull maybe 10" of Hg at 750-800rpm, which is where I would try to idle it, and maybe even slightly less if it produced a reasonable, predictable lope. Lower idle speed will save fuel. Keep a/f 14.7 to 15.2, timing could be 17 to 24. Right off idle at 1200rpm into 80kpa+ region yank it back to like 5*~ though, while the engine is moving slow, until you get the rpm up a bit. I usually try a few different (5, 8, 12, 15) and if I detect no difference and the wideband logs dont change between them I use the lowest number.
If it has trouble in gear, there are like 9999 settings for idle air control in gear in hptuners, why not play with that?
If it has trouble in gear, there are like 9999 settings for idle air control in gear in hptuners, why not play with that?
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Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
I tried messing with the idle air. When it's idling at 200 counts and still bogs, there isn't enough physical air getting into the engine. Or the converter is trash.
Especially when it idles at 950 with 30-40 IAC counts in park.
But there's no way I'd expect a cam with 16.5 degrees of overlap to idle at 55kpa at 800 rpm in a 5.3L. 55kPa is a lot of vacuum given the size of the cam.
I mean, this cam is nearly as big as an MS4 in terms of overlap.
The cam I have in my car now pulls 60kPa at 850. And it only has 5.5 degrees of overlap. And that's at 1.08 lambda (actually leaner than that) with 20 degrees timing. It can idle down more. But it's stable there without a lot of NOx smell given the lower timing. If I push it up to 24-25 base it drops down to 55-56kPa as expected. But the NOx smell is too acrid. So I typically err on the side of a little less timing to get that smell out without cats. Which makes the kPa read a little higher for less vacuum.
Also did an MS4 recently that pulled around 70kPa at 900 with 22 degrees leaned out 1.08 lambda on my wideband. Again that has 18.5 degrees of overlap. But the MS4 has really aggressive lobes. So it generates a little more vacuum than my old EPS cam did with milder lobes.
Especially when it idles at 950 with 30-40 IAC counts in park.
But there's no way I'd expect a cam with 16.5 degrees of overlap to idle at 55kpa at 800 rpm in a 5.3L. 55kPa is a lot of vacuum given the size of the cam.
I mean, this cam is nearly as big as an MS4 in terms of overlap.
The cam I have in my car now pulls 60kPa at 850. And it only has 5.5 degrees of overlap. And that's at 1.08 lambda (actually leaner than that) with 20 degrees timing. It can idle down more. But it's stable there without a lot of NOx smell given the lower timing. If I push it up to 24-25 base it drops down to 55-56kPa as expected. But the NOx smell is too acrid. So I typically err on the side of a little less timing to get that smell out without cats. Which makes the kPa read a little higher for less vacuum.
Also did an MS4 recently that pulled around 70kPa at 900 with 22 degrees leaned out 1.08 lambda on my wideband. Again that has 18.5 degrees of overlap. But the MS4 has really aggressive lobes. So it generates a little more vacuum than my old EPS cam did with milder lobes.
Last edited by JakeFusion; May 24, 2017 at 10:30 PM.
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Verdad Gallardo I tried messing with the idle air. When it's idling at 200 counts and still bogs, there isn't enough physical air getting into the engine. Or the converter is trash.
Especially when it idles at 950 with 30-40 IAC counts in park.
But there's no way I'd expect a cam with 16.5 degrees of overlap to idle at 55kpa at 800 rpm in a 5.3L. 55kPa is a lot of vacuum given the size of the cam.
I mean, this cam is nearly as big as an MS4 in terms of overlap.
The cam I have in my car now pulls 60kPa at 850. And it only has 5.5 degrees of overlap. And that's at 1.08 lambda (actually leaner than that) with 20 degrees timing. It can idle down more. But it's stable there without a lot of NOx smell given the lower timing. If I push it up to 24-25 base it drops down to 55-56kPa as expected. But the NOx smell is too acrid. So I typically err on the side of a little less timing to get that smell out without cats. Which makes the kPa read a little higher for less vacuum.
Also did an MS4 recently that pulled around 70kPa at 900 with 22 degrees leaned out 1.08 lambda on my wideband. Again that has 18.5 degrees of overlap. But the MS4 has really aggressive lobes. So it generates a little more vacuum than my old EPS cam did with milder lobes.
Especially when it idles at 950 with 30-40 IAC counts in park.
But there's no way I'd expect a cam with 16.5 degrees of overlap to idle at 55kpa at 800 rpm in a 5.3L. 55kPa is a lot of vacuum given the size of the cam.
I mean, this cam is nearly as big as an MS4 in terms of overlap.
The cam I have in my car now pulls 60kPa at 850. And it only has 5.5 degrees of overlap. And that's at 1.08 lambda (actually leaner than that) with 20 degrees timing. It can idle down more. But it's stable there without a lot of NOx smell given the lower timing. If I push it up to 24-25 base it drops down to 55-56kPa as expected. But the NOx smell is too acrid. So I typically err on the side of a little less timing to get that smell out without cats. Which makes the kPa read a little higher for less vacuum.
Also did an MS4 recently that pulled around 70kPa at 900 with 22 degrees leaned out 1.08 lambda on my wideband. Again that has 18.5 degrees of overlap. But the MS4 has really aggressive lobes. So it generates a little more vacuum than my old EPS cam did with milder lobes.
https://www.hptuners.com/forum/showt...l=1#post410822
well fine then sour pants! 
Only god knows why the ramp does what it does.
Two things you can do are:
1. double check accuracy of map sensor reading with aux gauge
2. put a dial indicator on a rocker and degree the cam
actual reading + good cam position = not a problem? too much vacuum is a sin?
Only god knows why the ramp does what it does.
Two things you can do are:
1. double check accuracy of map sensor reading with aux gauge
2. put a dial indicator on a rocker and degree the cam
actual reading + good cam position = not a problem? too much vacuum is a sin?
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
Lol. I'd love to see 55kPa with an MS4 cam @ 800rpm.
But I may have to check the MAP sensor to see. It otherwise behaves correctly.
When it fired up and I saw that... I was like is that really the cam that was installed?
But I may have to check the MAP sensor to see. It otherwise behaves correctly.
When it fired up and I saw that... I was like is that really the cam that was installed?
Jake I can only think of two reasons. That map is off or it's not the cam you think it is. I've re re reread your post and it's not making sense.
If the map is off it would def explain some off idle stuff too
Might need a hole in the blade but that's a different topic.
If the map is off it would def explain some off idle stuff too
Might need a hole in the blade but that's a different topic.
Injection Instant at LOW PW Hi Jake, sure you know tuning though others here could benefit.
So I ask the degree of your Injection Instant at IDLE with LOW PW of ???
The inventor of the engine rated "0" degrees at firing as do I using a 720 cycle time.
My method with the ECU-882C is to fire the injector at 390-420.
I then advance the Injection Instant as the RPM increases.
I then delay the Injector Close as the PW is increased.
This method allows for the fuel to be placed into HIGH SPEED AIR, the time when the piston has its fastest speed AND most valve lift.
Thus MY luck with KPA.
Lance
So I ask the degree of your Injection Instant at IDLE with LOW PW of ???
The inventor of the engine rated "0" degrees at firing as do I using a 720 cycle time.
My method with the ECU-882C is to fire the injector at 390-420.
I then advance the Injection Instant as the RPM increases.
I then delay the Injector Close as the PW is increased.
This method allows for the fuel to be placed into HIGH SPEED AIR, the time when the piston has its fastest speed AND most valve lift.
Thus MY luck with KPA.
Lance
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
Jake I can only think of two reasons. That map is off or it's not the cam you think it is. I've re re reread your post and it's not making sense.
If the map is off it would def explain some off idle stuff too
Might need a hole in the blade but that's a different topic.
If the map is off it would def explain some off idle stuff too
Might need a hole in the blade but that's a different topic.
But the rest of this has me going huh?
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
Hi Jake, sure you know tuning though others here could benefit.
So I ask the degree of your Injection Instant at IDLE with LOW PW of ???
The inventor of the engine rated "0" degrees at firing as do I using a 720 cycle time.
My method with the ECU-882C is to fire the injector at 390-420.
I then advance the Injection Instant as the RPM increases.
I then delay the Injector Close as the PW is increased.
This method allows for the fuel to be placed into HIGH SPEED AIR, the time when the piston has its fastest speed AND most valve lift.
Thus MY luck with KPA.
Lance
So I ask the degree of your Injection Instant at IDLE with LOW PW of ???
The inventor of the engine rated "0" degrees at firing as do I using a 720 cycle time.
My method with the ECU-882C is to fire the injector at 390-420.
I then advance the Injection Instant as the RPM increases.
I then delay the Injector Close as the PW is increased.
This method allows for the fuel to be placed into HIGH SPEED AIR, the time when the piston has its fastest speed AND most valve lift.
Thus MY luck with KPA.
Lance
Either way, it's much closer than stock 5.55 normal reference scale. SOIT is 285 degrees there...
Originally Posted by JakeFusion
The hole drilling will probably be today to test that out.
But the rest of this has me going huh?
But the rest of this has me going huh?
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
Drilled a hole. Still bogs.
Raised the idle to 1050 in gear and 950 park. When it goes from park to gear, it goes from 55kPa to 65kPa... and recall, that's from 950 to 1050rpm. That's nutty!
The cheap *** converter is bogging the **** out of the engine.
Raised the idle to 1050 in gear and 950 park. When it goes from park to gear, it goes from 55kPa to 65kPa... and recall, that's from 950 to 1050rpm. That's nutty!
The cheap *** converter is bogging the **** out of the engine.