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I was wondering why different engine manufactures recommend different throttle openings before engine start? On many Pipers 1/4 Inch is recommended while the Cessna 172N recommends 1/8 Inch and the Cessna 172S again recommends 1/4 Inch.

Why do we open the throttle before starting the engine? What difference does it make opening the throttle more or less, has it to do with the fact that it isn't good practice to have high engine power when the engine is still cold and oil pressure is low? And why should we open the throttle differently when the engine is flooded or we start the engine when it is still hot?

Starting Engine Checklist Cessna 172S

Starting Engine Checklist Cessna 172N

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    $\begingroup$ The actual distance of movement isn't important, you don't want too much as you say, but it's also hard to start with a closed throttle. Typically I have one hand on the throttle and advance, retard as needed during cranking. $\endgroup$
    – Ron Beyer
    Dec 22, 2020 at 14:41

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It's just a ballpark value that someone decided on during testing, that provides enough air to the engine to let it start and run with a rich primed mixture, without causing too much initial RPM. A quarter inch here, and eighth inch there, it's all good as long as it doesn't cause the engine to burst into life at high power, and nobody expects you to sit with a measuring stick to get it exactly right.

It's not a good idea to start an engine with a very high throttle setting, because high rpm before there is sufficient oil flow = bad in the long run. Crank and rod bearings need to be fully pressurized before any significant load is put on them. Cam followers need to be well coated in oil before they spin at more than minimum speed (especially with Lycomings).

Most engine wear happens in the first 15 seconds of starting, when the oil flow is inadequate and bearings, piston rings, cam lobes and gears etc are running on only a residual oil film. Once a plain bearing is fully pressurized with oil, there is no metal to metal contact and the life of the bearing, in theory, is infinite. If you ran a 2000 hour TBO engine with just one start and let it go without ever shutting down and just kept it supplied with fuel and oil 24/7, it would run far far beyond its normal TBO (although it might need cylinders at some point as valves wear out).

My personal practice is the use the absolute minimum throttle opening that will let the engine start, and let the engine tick over at minimum speed until I see normal oil pressure, before I go to a warm-up RPM. When an engine is warm, that usually allows starting with the throttle completely closed. Oil gets to where it needs to pretty fast; it's just that initial 10-15 seconds that's critical.

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    $\begingroup$ The record is 1559 hours in one flight, which only ended because they had lost so much power that they could no longer climb out after refueling. The engine started with 450 hours, so it barely made TBO despite continuous operation. $\endgroup$
    – StephenS
    Dec 22, 2020 at 17:06
  • $\begingroup$ I made an edit to add that things like valve seats will tend to wear regardless so it may not apply to the entire engine. That being said, Lyc O-320s regularly go to 3000 hrs plus, even the cylinders. The engine in question may have been abused or mis-operated at some point - the exhaust valves are the biggest Achilles heel, and on Lycomings, cam lobes and followers if the engine has sat outside for extended periods. $\endgroup$
    – John K
    Dec 22, 2020 at 19:08
  • $\begingroup$ @StephenS Would you have the source of this ? I would love to read about it. $\endgroup$ Dec 24, 2020 at 12:53
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    $\begingroup$ @StandaaReinstateMonica AOPA article on it $\endgroup$
    – StephenS
    Dec 24, 2020 at 13:25
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As Ron notes in the comments the actual distance is not as important as the fact the throttle is "a bit open".

First off when comparing Pipers and Cessnas remember that they have different throttle actuators. Cessna uses the push-pull/vernier style throttle and Piper uses the throw style both of which are adjusted differently so 1/4 inch on either is not really the same thing. There is also room on the back side to adjust it, Ive flown Pipers where the first inch or so of throttle did nothing, or everything....

Opening the throttle ~1/4 inch is generally an ISO Atmosphere idea and warmer/colder days may call for different operations. The reality is if you poke around you will find advice in all directions. Pilot Workshops notes a 0-Throttle start this informative article notes basically "do what works" and some hot start procedures call for full throttle.

Ultimately most GA, carbureted aircraft are set up to idle nicely with the throttle against the stop when the engine is warm so when starting, with cylinders full of primed fuel, you may need a bit more air.

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