Where did 5 kts of airspeed go?

[Christer]

Here is a puzzling question? I have a new 0-360 engine with a MA4-5 carb. I have a Garmin G3X system that records every imaginable parameter once per second.

On my initial phase 1 test flights, I had the following performance indications:
TAS 170 kts, MAP- 23.5, RPM 2410, density alt 3047, OAT 12 deg C.

I had an issue with the carburetor and it was exchanged for a new one of the same exact P/N (5-4164-1). After installation here are my performance numbers:

TAS 165 kts, MAP-23.6, RPM 2390, density alt 2914, OAT 10 deg C

Somewhere I lost 5 kts. Fuel load was very close to the same so aircraft weight was the same. Both flights were at full rich due to new engine still. The engine seems to be producing less power only by the numbers, it runs and feels the same.

There is one foot note to add. During the carb change I noticed that the hole in the Van's throttle cable bracket that sits between the carb and the sump was slightly smaller in diameter than the sump opening by about 1/8 in. I opened it up to match the sump which should have improved performance if anything.

Any good thoughts on where the 5 kts of airspeed went?

[RV6_flyer]

Where was the mixture set and what was the fuel flow?

On my O-320 RV-6, I can see a 10-Kt change in speed by only changing the mixture is setting. The mixture setting involves 1.5 gallons of fuel flow that corresponds with the speed change.

[Steve Melton]

what is your EGT spread now? same as before?

[AlexPeterson]

I'd not worry about speed at full rich - the fuel air mixture is towards the absurd end of things with those settings. Your new carb is probably putting just a little more fuel through, which further reduces power. Do you have the two fuel flow numbers to go with your two sets of data?

During break-in, you can lean it provided the cht's stay behaved.

[turbo]

a disconnected pilot static could cause a difference. happen to me on the encoder connection a couple years ago.

[Christer]

Alex, you bring up a good point. My fuel flow with old carb was 13 GPH and with the new 11.3 GPH. This also confirms what Gary is seeing with a lower FF getting a 10 kts speed change. The spilt between min and max EGT on old carb was 85 deg and with new carb 130 deg. I am new at analyzing all of this stuff (but is fun to do), but it looks like maybe a leaner mixture with the new carb as indicated by less fuel flow. This may be part of the explanation. This begs the question, why is there less fuel flow? Float level?

[AlexPeterson]

Christer - I would have expected your new full rich fuel flow, which is lower than the old, to actually run you a bit faster, if anything. From a proper full rich mixture, one should see a small increase in speed as one leans to peak power, then a fairly sharp drop-off in speed as one continues to go leaner.

What is your fuel flow at takeoff? It should be in neighborhood of 16 gph at your 900' elevation. If it isn't, there are threads here that discuss jet size, fuel flows, etc.

[lr172]

Originally Posted by Christer

The spilt between min and max EGT on old carb was 85 deg and with new carb 130 deg. I am new at analyzing all of this stuff (but is fun to do), but it looks like maybe a leaner mixture with the new carb as indicated by less fuel flow. This may be part of the explanation. This begs the question, why is there less fuel flow? Float level?

The EGT spread could easily be caused by reducing the airflow interference when you removed the small lip from the bracket. Little things like this seem to make a noticeable difference on the Lyc intake system.

Float level does effect fuel flow, however, I would suspect that your new carb has a much smaller jet than than the old one and this is driving the lower fuel flow. I would hope that a new carb would have the float adjusted properly. I would guess that the 11.2 GPH is on the lean side of max power and this is why you see the performance reduction. While this shouldn't be an issue for cruise, I would be concerned about fuel flow at T/O power levels. You should observe your fuel flow at T/O and report the results here. You can also lean a bit from full rich at the settings outlined above. If speed increases a bit then begins to fall off, you were on the rich side of Max power. If speed drops off slowly with leaning, you were on the lean side of max power and your 5 MPH was likely lost from reduced full flow.

Assuming the old carb was not slobbering rich (and it may have been), you'll want to address this issue to insure you have good fuel flow for T/O and climb operations.

Remember, we're talking about Peak Power production and NOT Peak EGT. Peak power is on the rich side of Peak EGT. I vaguely remember seeing a Lycoming chart for a 360 that showed peak power in the 12 GPH range at 75% power. You can find a chart for your engine and it will tell you what the fuel flow should be for your %age power setting.
Larry

[blueflyer]

Originally Posted by Christer

......My fuel flow with old carb was 13 GPH and with the new 11.3 GPH. .......but it looks like maybe a leaner mixture with the new carb as indicated by less fuel flow. ...........why is there less fuel flow? Float level?

another obvious and unlikely thing to check is that you are still getting the full travel of the mixture arm on the carb after installation. Maybe that's the reason for your reduced fuel flow.

[Christer]

I looked at fuel flow during take-off. For the old carb it maxed at 18.1, new carb maxed at 18, so identical and obviously a big enough jet. I looked at the engine data a little closer and it looks like I did lean the engine some (can't remember for sure they all start to run together) with the new carb as I had a rise in EGT to a peak and then ran on the rich side of peak. This should have given me more power if I understand correct, but still had the 5 kts loss. I might have to go back and try more specific testing. The bottom line still remains the I lost 5 kts with a crab change. I did fly a GPS box and verified that my TAS is off by 1 kt. I am still very happy with my RV-8 and it runs great, these are just things that make you go hmmmm.