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Manifold Pressure on Take-off

Darren Kerns

Darren Kerns

Well Known Member
The other night on take-off I pulled 27.1 in MP and & 2690 RPM on takeoff. I was showing 92% on takeoff. Engine was running smooth but seemed like I should be getting more MP.

When I shut it down, it runs up to 29.1 MP. I have checked for induction leaks but have not found any. All EGT and CHT were good.

Should I be looking at something else or just fly it.

Thanks,
Darren Kerns
RV7 N599DT
 
Any chance you carb heat or filter bypass door was open? Hot air will lower you MP.
 
Darren Kerns said:
The other night on take-off I pulled 27.1 in MP and & 2690 RPM on takeoff. I was showing 92% on takeoff. Engine was running smooth but seemed like I should be getting more MP.

When I shut it down, it runs up to 29.1 MP. I have checked for induction leaks but have not found any. All EGT and CHT were good.

Should I be looking at something else or just fly it.
Click to expand...

You have the opposite of an induction leak.

Think of it this way...the throttle plate is a pilot-controlled induction leak. When it leaks a lot, without restriction (throttle plate parallel to inlet airflow), the indicated manifold pressure is very close to the same as outside pressure. If it doesn't leak much (throttle plate closed), the indicated manifold pressure is less than outside pressure.

Describe induction system? Photos?
 
I am taking it in for inspection June 17. The air filter could be dirty. I have IO-360 180 hp. I have a K&N air filter in cowl. I have 31 hours on it since last inspection/cleaned. It looks clean though.

Thanks
Darren
 
Dan
I have forward facing FI. Smooth cowl on bottom. The K&N air filter is in the cowl inlet like other FI engines.

Thanks
Darren
 
I had occasional readings 3 to 5" low on takeoff for a year, and occasionally at lower throttle settings as well. I did not believe them as the fuel flow and perceived power was typical. I changed the MAP sensor and it went away. If you've verified that the throttle is indeed opening fully, and the air cleaner is clean, I'd look at the MAP sensor. Vibration might be causing it to read low, as mine seemed to do.
 
Those smooth cowl, in cheek snorkle setups are pretty awful for pulling decent MP. I have the same thing in the -8 and don't pull the MP it should. I'd pull the filter completely out and make a quick trip around the pattern just to verify or discount the filter's contibution to the issue.
 
Alex
I had to replace my AFS Manifold Pressure transducer last year. I have checked my FF on takeoff and it is normal.

Maybe I am just over-analyzing.

Thanks Guys for all the help and advice. I appreciate it.
Darren
RV7 N599DT
 
Toobuilder said:
Those smooth cowl, in cheek snorkle setups are pretty awful for pulling decent MP. I have the same thing in the -8 and don't pull the MP it should. I'd pull the filter completely out and make a quick trip around the pattern just to verify or discount the filter's contibution to the issue.
Click to expand...

+1 , but limit ground time w/o filter. There are a couple of K&N conical filters that are used, one is pretty restrictive. If you take it out tell us what the number or dimensions are. The E-0995 is the larger one.

EDIT: I see you already stated a rectangular filter.
 
Last edited:
Dan
Long time. It just seems like I notice it more and other times I don't. I know my friend's have watched me take off and they said my engine sounds normal and great. Loud and powerful.

It will set me back in the seat on takeoff so maybe I am just over analyzing. Maybe my K&N filter could be dirty.

Thanks everyone for the help and advice. I really appreciate it.

Darren Kerns
RV7 N599DT
 
Indicator

What Indicating system are you using? VM1000. G3, EI, etc, etc..........sounds like a VM1000 issue. Please confirm equipment.
 
Darren Kerns said:
Dan
It will set me back in the seat on takeoff so maybe I am just over analyzing. Maybe my K&N filter could be dirty.


Darren Kerns
RV7 N599DT
Click to expand...

More of a possibility than a dirty filter, is over-oiling the filter during the filter service. Over-oiling can be quite restrictive.
 
Mitch
It is AFS. I had to replace the transducer a couple months ago because it quit. I use the K&N Recharge kit to clean and oil. I always let it dry good before re-installing.

Thanks
Darren
 
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