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Is my my takeoff fuel flow high enough?

moespeeds

moespeeds

Well Known Member
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I noticed that my takeoff Fuel Flow is down to 14-15 GPH on my Mattituck IO-360 in my RV8.

I went back and looked through all the data for my flights from when I got the airplane in 2020. It seems that in the colder months I'll get 16-17 GPH, and during the warmer months 14-15.

Is this high enough? I'm thinking it's on the low side. From what I've read the rule of thumb is 9% of max HP. My data plate says 185 hp so I should get a min of 17 GPH. How can I adjust it higher? I did lower my idle mixture last annual because it was too high.

I'm using an RSA-5 Throttle Body. I do plan on doing a GAMI test in the next couple of weeks and change out to GAMI injectors.

I have no idea why my Mattituck data plate says 185 and not 180. I have the logbooks from this engine, but have never been able to find any kind of build sheet or other guidance on what they put in these things. They are long out of business.
 
Minimum or recommended fuel flow requires MAP and RPM. We can assume 29" of MAP for TO, but without providing RPM data (FP and CS props have different T/O RPM ranges) you are going to get guesses and hand me down answers repeated from others as opposed to real answers.
 
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lr172 said:
Minimum or recommended fuel flow requires MAP and RPM. We can assume 29" of MAP for TO, but without providing RPM data (FP and CS props have different T/O RPM ranges) you are going to get guesses and hand me down answers repeated from others as opposed to real answers.
Click to expand...

2670 RPM
28.9" MP
14.5 GPH
 
If we assume .6 BSFC, 14.5 GPH is a bit low for a PV 360 near SL. Your old 16-17 gph number would be a lot closer at 29".

I'd verify that the FF indication is correct if the engine still seems to perform as it always did.

You could cross check historic EGT data as well under WOT, similar DA conditions. If those are close to being the same, the FF may be wrong.
 
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I'm looking in the RSA-5 directions, and don't see a way to adjust WOT fuel flow? I did see that there's a finger screen in there that I never knew about.
 
Max fuel flow is 16 USG on a 180 hp engine, full power.
Here is the manual for RSA5 fuel servo.

Good luck
 

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Lyc IO-360 manual says the lean limit for the A series @ 100% power is 88 Lb/Hr. That equates to 14.7 gallons per hour. However, that rating is at 2900 RPM, so a bit conservative for 2700 RPM. The suggested high limit is 105 lb/Hr, which is roughly 17.5 gallons per hour. Not sure I would recommend you to be bumping up against the extreme high end limit. Just wasting fuel and eating into performance. However, you seem too close to the lean limit and steps should be taken to address that.

I would first check the accuracy of the FF instrument nad then clean the strainer and be sure that your cables are allowing full travel on the mixture arm.

Larry
 
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Tom023 said:
I've also noticed fuel flow is higher in colder temperatures and less during the heat of summer. I believe this is due to lower air density in higher temperatures, resulting in less air pressure in the fuel servo, therefore less fuel mixed with the less dense air.
Click to expand...

This is key. Fuel flow is a ballpark way of estimating AFR (air/fuel ratio).

FF will be lower if leaner, higher if richer at the same MAP, RPM and OAT. If any of these factors change, don't look for X number to be correct under all conditions.

Under Sea Level/ 15C/ 29.92/ 2700 rpm/ WOT conditions the FF should be the same. Don't be alarmed that FF is lower at 4000 MSL and 30C- that's normal.
 
moespeeds said:
2670 RPM
28.9" MP
14.5 GPH
Click to expand...

I have an YIO-360-M1B (180 HP) with AvStar FI. Below is a screenshot just after breaking ground from a Field Elevation of 140' MSL, DA = -1104', OAT = 33°.

2690 RPM
28.8" MP
16.5 GPH

i-2zWVkC8-L.png
 
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rv6ejguy said:
...
FF will be lower if leaner, higher if richer at the same MAP, RPM and OAT. If any of these factors change, don't look for X number to be correct under all conditions.
...
Click to expand...

Exactly what I see, which is about 16.3 GPH at my home field of 1400 ft and about 17.5 GPH at sea level, even with the higher temperature. That's at 2500 RPM, AFP FM200, dual pmags, Hartzell metal, Mattituck IO-360, xxx HP.
 
moespeeds said:
I noticed that my takeoff Fuel Flow is down to 14-15 GPH on my Mattituck IO-360 in my RV8.

I went back and looked through all the data for my flights from when I got the airplane in 2020. It seems that in the colder months I'll get 16-17 GPH, and during the warmer months 14-15.

Is this high enough? I'm thinking it's on the low side. From what I've read the rule of thumb is 9% of max HP. My data plate says 185 hp so I should get a min of 17 GPH. How can I adjust it higher? I did lower my idle mixture last annual because it was too high.

I'm using an RSA-5 Throttle Body. I do plan on doing a GAMI test in the next couple of weeks and change out to GAMI injectors.

I have no idea why my Mattituck data plate says 185 and not 180. I have the logbooks from this engine, but have never been able to find any kind of build sheet or other guidance on what they put in these things. They are long out of business.
Click to expand...
That's too low. Should be 16+ mph.

Quick way to test: run a sweep with the mixture at 75% power. Go from full rich to peak. You should have 175-250 degree F temp spread. If not, change the jet in the fuel servo. It's an easy fix.

Vic
 
I sent my data logs to Savvy and they say it's ok, though on the lower end.

"This flight recorded a max HP of 96% HP for a short period where max FF hit 14.6 GPH. If you were able to get the maximum 2700 RPM, that should get you to 100% HP and max FF should be a little higher. We estimate max FF as (max HP * .09), which would be 180 * .09 = 16.2 GPH if at max RPM/HP, so your current FF does seem to be a little on the low side.

Induction system inefficiencies reduce MAP, which also causes FF to be reduced, so achieving 30" of MAP and 16.2 GPH and is not likely going to happen. Your FF may be a little on the low side but CHTs are good during climb. If you are able to maintain 150-200° ROP during climb, you are getting sufficient fuel for cooling and detonation margin."

In the manual I see references to a takeoff power fuel valve, but nothing about where it is or how to adjust it.

I'm going to do the research this weekend. Thanks all.
 
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IO-360 straight valve with a fixed pitch Sensenich
Performance Airflow Fuel Servo
Take-off roll - 63 litres/hr (16.64 gal/hr) at 29.2" MAP
740 feet ASL strip
After wheels leave the ground and as RPM increases WOT fuel flow increases to a maximum of 68.2 litres/hr (18 gal/hr) - which is what I would have to assume is what I would get on the take-off roll if I had a CSU and was able to achieve 2700 RPM.

My figures seem a bit higher than those already listed here. That said, I seem to be getting good economy in the air. Yesterday we did 2.9 hours off a number of strips in the region and at the end of the day I did the math to find I had a total fuel burn of 30 litres/hr (7.9 gal/hr).
 
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