Manifold pressure ?

[N184JG]

While out flying last night my max MP was 28.3 on take off local baro was 29.51 on ATIS we are at 720ft . I have a cold air inducton with an intake filter in the upper lift side of cowl. Could I do better or is this normal?

[asav8tor]

Forget the altimeter setting. Look at the MP gauge before engine start and note the precise reading.

Then note the WOT MP reading on takeoff roll

Accelerate to max speed at 1000' above the takeoff alt at WOT add 1 to that number.

Compare those 3 readings all off the same gauge.

[Kahuna]

Originally Posted by N184JG

While out flying last night my max MP was 28.3 on take off local baro was 29.51 on ATIS we are at 720ft . I have a cold air inducton with an intake filter in the upper lift side of cowl. Could I do better or is this normal?

Remember the ATIS altimeter setting is for the sea level pressure to be set in the Kollsman window.
Lets assume your static MP (just sitting there not running) was 28.3.
(29.51-28.3)*1000ft/in = 1210' should be read on your altimeter.

Doing this in reverse, assuming 29.51 atis and your actual 720', I would expect 28.79" on MP static.


Can you tell us what the static MP was vs full throttle MP? That would help us.

edit. Hey I poked around and found this nice site for you for reference. Its well laid out. Simple read and explanations of this.

[Kevin Horton]

Originally Posted by N184JG

While out flying last night my max MP was 28.3 on take off local baro was 29.51 on ATIS we are at 720ft . I have a cold air inducton with an intake filter in the upper lift side of cowl. Could I do better or is this normal?

With that elevation and altimeter setting, the pressure altitude was 1100 ft, and the ambient pressure was 28.75. But, we don't know how accurate your MP gauge is, so it is hard to know whether you really had 0.45 in. Hg pressure loss or not. Next time, you could record the MP gauge reading with the engine stopped, and then compare that to what you see during take-off. That comparison will take MP gauge error out of the equation.

As a point of reference, on my RV-8 with IO-360-A aeries engine, and Van's "snorkel" taking air from the cowl inlet, the MP during take-off is 0.2 in. Hg higher than the MP with engine stopped. This suggests that the pressure increase from the prop blast is greater than the pressure losses in the induction tract.

[db1yg]

If you have an electronic mp gauge turn on the master and note the mp on your gauge prior to start up. Do a normal preflight and take off. Note the mp on the same gauge just as you lift off. Vans FAB looses an average (on 4 test planes) about .8 of an inch of mp (comparing static to t/o mp) through the induction under these conditions. Anything less than 1" loss is considered good.

I did a LOT of induction system testing with my carbed James cowl system back in 06 due to induction restrictions (1.8" mp loss)---ended up making my own carb box--I now loose .6" mp under the above conditions.

BTW, Will James redesigned his carb box to reduce the turbulance and associated induction loss.

Hope this helps,

Cheers,

db

[N184JG]

Update,out flying a few minutes ago with engine not running M.P. was 29.1 on VM1000c on take off M.P. was 28.3 , .8 less. I think I am ok.