Static RPM: Borer + 160hp

[HighRoad]

Just purchased a used Borer 82x44 that the paperwork says was re-pitched to 42" to put on my Pacer. Wondering what static/climb rpm others are getting with their borers and 160hp? I am getting 2400 static and 2500 on climb out @ 65mph. I realize a cub and the pacer are going to climb at different speeds, but what rpm should I shoot for for max takeoff/climb performance? I had a 74x56 pitch before and it seems I gained about 100rpm static/climb with the borer and lost 3-5 mph in cruise (thought I would loose more). The borer makes more thrust for sure, as at low idle I have to be on the brakes to keep the airplane from rolling where before this was not the case. It also accelerates and climbs out better and I have noticed less vibration. The propeller is going in for overhaul soon and I am thinking of having them flatten it out some if the prop will take it. What do you think?

Thanks

[PerryB]

I have an 82-41 on my 150hp PA-12 and am getting speed/rpm numbers just about exactly the same as yours. I don't quite get 2400 static, more like 2350-2375. If I let it run out at full throttle (straight&level) I get 2750. Just one thing, have you verified the accuracy of the tach? Those rpm #'s seem a bit high for a 160 swinging 44 inches, but then you may have a super healthy motor too. Not a bad problem to have! Check the tach, though. Its important to know your data is accurate.

[HighRoad]

Perry,
Thanks for the response. My prop has been re-pitched to 42 according to the paperwork. I have not had this verified. The tach was new a year and a half ago as the old tack was way off. May have them check it next time airplane is in.

[mvivion]

Highroad,

Tachometer accuracy has nothing to do with the age of the instrument. I once bought a rebuilt tach, installed it and checked it, only to find it was 75 rpm off.

Sent the rebuilt back, and exchanged for a NEW tach. Installed it and found out it was 100 off....

At that point, I sent it back and ordered a digital tach.

Many mechanics have a small strobiscopic "tach checker" that you point straight ahead into the prop arc while in flight, and compare the digital readout to your mechanical tach.

Borrow one of these devices, go fly the plane, and figure out what your engine is REALLY doing, before you start bending that prop.

IF your numbers are really what they appear to be, I think you're doing pretty well. Those numbers seem reasonable for a borer at 42 pitch and a 160.

But first find out if your tach is accurate. If it's not, you can simply mark it with a placard as to the amount of error to be legal.

MTV

[skywagon8a]

You can check your tach yourself if your airport has mercury vapor lights. Just face the plane towards the light with it shining through the prop. Run up the engine to a rpm that is divisible by 60, such as 2400. The merc vapor light operates on 60 HZ. The prop will appear to turn either forward or backwards. Adjust the rpm until the prop appears to stop turning. The true RPM will then be a multiple of 60. Note the difference, if any, on your tach. If, for example, the tach is reading 2450 when the prop is stopped, you should set your rpm 50 RPMs higher than you want the engine turning. For example, if you like to cruise at 2500, then set it at 2550 and it will actually be 2500.

[cubdriver2]

You can check your tach yourself if your airport has mercury vapor lights. Just face the plane towards the light with it shining through the prop. Run up the engine to a rpm that is divisible by 60, such as 2400. The merc vapor light operates on 60 HZ. The prop will appear to turn either forward or backwards. Adjust the rpm until the prop appears to stop turning. The true RPM will then be a multiple of 60. Note the difference, if any, on your tach. If, for example, the tach is reading 2450 when the prop is stopped, you should set your rpm 50 RPMs higher than you want the engine turning. For example, if you like to cruise at 2500, then set it at 2550 and it will actually be 2500.

How do you know if 2400rpm is right and not 2460rpm.

Glenn

[skywagon8a]

You can check your tach yourself if your airport has mercury vapor lights. Just face the plane towards the light with it shining through the prop. Run up the engine to a rpm that is divisible by 60, such as 2400. The merc vapor light operates on 60 HZ. The prop will appear to turn either forward or backwards. Adjust the rpm until the prop appears to stop turning. The true RPM will then be a multiple of 60. Note the difference, if any, on your tach. If, for example, the tach is reading 2450 when the prop is stopped, you should set your rpm 50 RPMs higher than you want the engine turning. For example, if you like to cruise at 2500, then set it at 2550 and it will actually be 2500.

How do you know if 2400rpm is right and not 2460rpm.

Glenn

You don't. Start your test at idle (ex: 600rpm) and work your way up. As you pass through each multiple of 60 you will see the difference. It likely will be linear. As the rpm gets higher the error will become greater.

[Steve's Aircraft (Brian)]

Brent,

We have a tru-tack digital strobiscopic device and checked your new tack when we installed it. If memory serves me correctly it was spot on at 2500 RPM, but with us doing aver 100 annuals a year it is hard to remember every little detail. And that also does not mean that the new one has wandered off since it has been installed. Next time you get over we can take it for a quick flight and doubble check.

Brian.

[HighRoad]

Thanks Brian,
I remembered you telling me it was correct when the new tach was installed. When you do the tips, we should check it.

Thanks

[Trimcable]

my 2 cents on prop pitch. I have tried borer props pitched to 41 through 44 on my 160 h.p. pa-12. I have tried 2 different props both stamped 42 inch pitch with 75 to 100 rpm difference. One of them was original pitch and the other was repitched to 42. I currently own a 42" and 44" that i use for different conditions. On the same day as I change props there is 100 rpm difference in these 2 props. With the 44" pitch on floats I get about 2350 to 2400 rpm on spool up and 2450 to 2500 on the step. The 44" prop will just about red line at 2700 in straight and level flight, and the 42" prop will over rev in straight and level flight. If your tach is on and it sounds like it is I would find the prop that works best for what ever kind of flying you do, take off vs cruise and go with it no matter what number is stamped on it. The static rpm's you describe sound common, so if it works on your airplane run it. If you want to repitch I would expect about 50 rpm change in static rpm per inch of pitch.

[StewartB]

I paid attention yesterday in 35* temps. My PA-12 has 160hp and an 8242 prop. Static was 2425 on an analog tach. Full throttle in level cruise was 2650 or slightly higher. The plane has hydraulic wheel skis and Cub gear if that means anything.

SB

[Darrel Starr]

My STC papers from Cubcrafters stated the static RPM for my 82/43 Borer prop on my 160 hp 0-320 should be 2375 to 2475 RPM. I didn't pay much attention to this until Windonhisnose complained that in the cold weather the static RPM for his SC seemed to be low. So I watched the RPM as I powered up on take-off on a 20F day and saw soemthing in the 2300 - 2350 rpm range. I asked the guys at McCauley about this and got the following reply:

Dear Darrel,
You are correct the RPM will be lower in cold dense air, but in respect the engine should produce a little more horse power from that same cold dense air, all this being a small factor in final RPM. From a hot summer day to a cold winter day you may see roughly a 50-75 RPM change. Does Cub Crafter have a temperature compensating RPM chart or they just say 2375 2475 is it?
When we’re this close to limits I need to ask if this RPM was checked with aircraft tach or an independent calibrated hand held tach? Is the carb heat sealing off, or is there some other issue restricting the engine from obtaining full RPM? Are you reaching cruise speeds and is take off performance diminished? With some more information we might be able to come to a better conclusion. Hope this helps, thanks.

Regards,
Gary L. Peak
McCauley Field Service Engineer
Phone: 316-831-3844
Fax : 316-206-8033

I wrote back and thanked Gary. I told him that I thought my engine/prop probably were OK. My tach is an EI electronic tach which I think would be dead on or if broken then reading zero.
Darrel

[HighRoad]

Thanks Guys for all the info. After putting some more hours on the prop, I am very pleased. The takeoff/climb performance is improved and I lost only a few (3-5mph) in cruise.

The prop is very smooth as well. Sounds like it must be close to a 42 pitch and I don't plan to change it as it creeps up close to redline full throttle level.