I'm finding idle kpa of 55 at 7000ft.....406, cam 226/222 at 50, 494 lift with 1.6's and 114lsa. superram. idle 850rpm.
anyone else try tuning at altitude???? What idle kpa are you finding at idle?? It just seems as if I'm chasin my tail on a few things...
Also the day before was a lot colder and the idle kpa was then 40??? It couldn't change that much from temp???
Otherwise I'll start hunting for an intermittant vac leak! (Already ruled out all the rubber hose-next is the fun superram....)
Oh yea...this can be frustrating!!!

 

A common vaccuum leak (i.e. to all cylinders equally) is not going to change MAP (think about the IAC, it's a common vaccuum leak, and it increases or decreases airflow). But if the RPMs are the same and the load is the same, then the airflow is the same, under normal conditions. If the MAP is higher, and the RPMs are the same, then more air is getting in, but there is also either more load or less power produced to maintain that RPM.
I would look for something causing a misfire, something adding load, something changing the ignition timing, or something leaning you out a lot (along with checking for vacuum leaks). You've already done the rubber hoses, but it wouldn't hurt to spray some stuff around the manifold to see if it changes the idle speed briefly.

 

You kpa will drop as you increase in altitude.

 

I live at 7000 feet any my idle kPa's are in the 30-35kPa range. It's on the high end of that range when cold, but I've never encountered a 15kPa difference.

FWIW, I did need to adjust the MAP calc. in my ALDL def file. When I first started tuning the 749, I found that the reported MAP value at key-on did not match the barometric pressure value.

 

I pull around 50 kpa of vacuum... at sea level (mistimed cam)...

I would say that if the MAP is lower when the engines cold it may be indicitave of a possible fueling problem. Not enough/too much fuel, or the increct ammount of timing, for that matter, can throw off the idle vacuum.

 

Thanks for all the above...
I hooked up my cas engine analyzer-no misfire, the only thing showing was high secondary kv; so I'll change plugs wires etc. first then pull the lid on the superam and snug everything up there.....oh well....just another evening spent in the garage....

 

did you have a good burn time ??? post up a snap shot of your secondary firing voltage patterns.

 

No surprise, there.

If your not, your not trying enough....

Yep, no surprise there. As air gets colder it gets denser, so it alot less willing to bend around the butterflies, and it's got alot of O2. Not to mention when it's cold there's usually some extra advance which'll drive the K/Pa down a lil.. I've seen 40 when it's been really cold, on an engine that's normally 55 when warm.

 

Only problem is, denser air results in a higher MAP and thinner air results in a lower MAP.

 

As for the reason a colder temp may result in a lower MAP (which is contrary to the way denser air should behave); what was your idle speed at the time? Colder temps usually result in a higher idle speed (until the engine if fully warmed up). The higher idle speed will make a long duration cam have better vacuum which will result in the lower MAP value.

Once the engine is fully warmed up, the variation due to temperature alone will not be that great. I live in an area where the temperature routinely goes below 0*F in the winter and 100*F in the summer. I have done a ton of data logging as I was making a correction to the MAT tables due to ambient air temp.

My results tend to show that I have a slightly higher MAP value once the engine is totally warmed up on a colder day than a hotter day. Also, if you monitor it enough, you can even notice the barometric effects of the air temp.