CAM's and Shunting.

[DaveG]

I have been trying to resolve a shunting issue on my 500 for a couple of years now. (prior post: http://www.pistonheads.com/gassing/topic.asp?h=0&a... After extensive testing on 14CUX and MS2Extra I would like to explore the correlation (if any) between CAM profile and shunting - in particular the "Idle vacuum". I think this particular CAM characteristic might direct us to a CAM's tendency to shunt?
For example my CAM has an idle vacuum of 51-53 Kpa (15 In.Hg) for an idle of 800-900rpm. This could be considered a comparatively 'poor' vacuum and a more usual vacuum for an engine in good condition might be 35-40 Kpa (17-20 In.Hg). After much tuning, I still have a light shunt between 1200-2000rpm on both 14CUX / MS2 and I am thinking that my vacuum (CAM profile) just happens to conflict with the physical design of the RV8 (ie. The idle [datum] vacuum is reflected across the load/rpm range and just does not suit the induction design?) There is no doubt the higher the vacuum (wilder CAM) the greater the problems with idle and possibly shunting? I have not been able to tune it out.
Can you help with providing data for the following table:



For those of you who have the vacuum data, you can assist me in creating a more complete table based on your own experience - it could be useful and of general interest in CAM selection.
So let's keep it simple - if you have a shunt, what is the idle vacuum and what CAM?.
Cheers
David

Edited by DaveG on Sunday 24th July 00:43

[DaveG]

Hi Mark. I think you mis-interpreted my expression 'high vacuum'. Depending on whether you are expressing in pressure terms of Hg or Kpa? (Numerically 'High vacuum' = high Hg = low Kpa).
I agree with your description and I am trying to see if there might be a vacuum threshold which correlate to a tendency to shunt. With a std Cam it can be tuned out, but as the duration and Ica change (as does the idle vacuum) is there a point at which it cannot be properly tuned out?

[DaveG]

Hi Dnb, I was interested in your comments and your previous mapping response - which is hard to transcribe: ( http://www.pistonheads.com/gassing/topic.asp?h=0&a... ). No where do you seem to say what ECU you are using?
I take your point on the sampling rate, currently on 8 but also been through 12, 24, 32. I think my maps are pretty good, but still need to take a dyno run.
If you have cured your shunting with a H404 I would be very interested to see your maps (VE, Spk, AFR) - maybe email me.

[DaveG]

Thanks for putting together your comprehensive response. My experience seems to support your hypotheses in general. I appear to be seeing some positive feedback in the control system, but cannot seem to identify the main contributors because of the spectrum of occurring conditions. (rpm 1250-199)(MAP 30-45Kpa)(AFR 14-15)(TPS 9-13)etc.
It is difficult to know what the internal sensor sampling rates might be. For data logging I think all channels are 20 per sec. I think I see your point concerning aliases. The rpm oscillation I see which is superimposed on the steady state rpm during a shunt is 60-100rpm at about 1 cycle/sec. Just out of interest here are a couple of random snapshots:-






In relation to AFR's, I use a Tech Edge LSU4 which has a pretty linear characteristic and for the most part I have kept my Accel Enrichment (AE) fairly constant, only varying the MAP/TPS percentages (Now using Speed Density, previously TPS=100%, now MAP/TPS=75%, however previous AlphaN was 100%TPS)

It really is a conundrum when there are so many variables, very time consuming changing one parameter at a time and trying to determine the effect, especially when it is probably going to be a combination of changes to resolve. I need a fellow tuner and a private road !!
Thanks for your input, it all helps.