Manifold pressure question

[74657]

Can someone please explain the relationship between manifold pressure, temperature and altitude?  I flew today @ 7500 ft and could only make 22.5".  OAT up there was 72F.  It was 95F on the ground.  Looking at Rocket Eng.'s website the performance chart lists a power setting for 24.5" @ 7500 ft.  Are the 2" due to the high temperature or the fresh engine still being broken in?  (5-6 ours on it now).

It was great to be back in the air after 5 months of downtime while my plane was being fixed.  We still have some bugs to work out of the engine.  It runs rough from 13-18" unless it is leaned out.  WOT still needs a little bit more fuel as my CHT's get hot if I climb too fast.  I absolutely love the "shock cool" indicator on my EDM 830.  I have it set at the very bottom of the screen for quick reference during descent.  The speedbrakes are awesome!!  I came in from MKE over Lake Michigan @ 7500 and held my altitude as long as possible.  Popped the brakes and sunk like a rock right down to BEH which is 1 mile from the shore of the lake. 

 

[smccray]

Check Out John Deakin on Avweb: Pelican's Perch #15: Manifold Pressure Sucks!

http://www.avweb.com/news/pelican/182081-1.html

There's a bunch of different articles about how to operate an engine on AvWeb written by John Deakin. There's a subset of the articles posted on the Advanced Pilot Seminars website as a primer for the course.

[danb35]

I can't speak for Rocket, but you should recall from your Private Pilot ground school that ambient pressure decreases at a rate of approx. 1" per 1000' altitude.  Standard pressure at sea level is 29.92", or 30" if we're rounding off.  Thus, I wouldn't expect to see more than 22.5" at 7500 MSL without some sort of forced induction system.

With the mixture issues, are you sure that your engine has been set up correctly according to SID 97-3E?

[74657]

Quote: danb35

With the mixture issues, are you sure that your engine has been set up correctly according to SID 97-3E?

[carusoam]

74657,

http://www.mooneyspace.com/index.cfm?page=3&mainaction=posts&forumid=3&threadid=2964

You may want to take a look at this particular concurrent thread regarding the operation of an IO-550 in long bodies.  They are discussing Set-up, FF, CHTs and RPM.

Best regards,

-a-

[Parker_Woodruff]

The rule of thumb is you lose about 1" of MP for every 1000 feet you climb.  There is some resistance through the induction system, which is why on takeoff roll my M20J could never get higher than about 28.5" MP.  My M20J would get about 22" MP at 7500'.

25.5 gph sounds much too low for an engine cranking 305 horsepower.  Fuel flow on my M20K is about 24 gph, which is probably a touch on the rich side, but better than being too lean at such power settings.

Rocket's fuel flow number sounds right to me for an engine crankin' 300hp.  25.5 gph sounds much too lean.

[RJBrown]

Missile? not Rocket.

[Parker_Woodruff]

Quote: RJBrown

Missile? not Rocket.

[fantom]

On my soap box....again.

Wouldn't it be much more considerate for everybody if all of us included our location, and year/make of plane when posting, especially when asking a question?????

Why people don't is mind boggling to me.

 

[74657]

Sorry,

92 J with a Missile conversion....thats in the sig.  I flew from KBEH (SW MI) to 57C (East Troy, WI) and back.  35 minute flight each way, direct over Lake Michigan.  6500 ft to and 7500 from.  Engine is a freshly overhauled IO-550A.

[74657]

The 25.5 gph number comes directly from the TCM setup sheet (I can't remember the exact range off the top of my head).  I don't think that the IO-550A was designed to be installed in a tightly cowled, cowl flapless Mooney.......  It flows 28.5 gph after making some adjustments, down from 30gph that resulted in a horrible stumble with any manifold pressures below 18".  The reduction was made to try and clear up the mid range bog but to limited avail.  My mechanic and the outfit that overhauled the fuel system are now in direct communication. 

Is anyone familiar with the autolean feature on this fuel system?  From what I have read it should pull handle about 75% of the leaning that is typically done by hand.  I didnt lean it at all manually the entire time I was at altitude but CHT's were in the 380-385 ish range with CHT's near 1380.  Like I said earlier, the outside air temp at altitude was 72F.  Fuel flow @ 22.5"/2350 rpm/188kts was 16.5 gph and EVERY vent wide open!!