Jim;

 

I think I can relate to your story. I HAVE 200 HRS ON MY TSIO 5550, live in hot and humid Puerto Rico and don’t fly the ivp much. Now my cylinders are low on compression. I have been flying the engine one hour a day for 10 days as a prescription from my mechanic before testing my compression again.

 

Charlie Kohler blames the problem on low flying time and humidity.

 

What were your readings in those three cylindrers?

 

Thanks in advance, Noel

Hi Noel,

I really don't think the humidity has much to do with it, even thought I respect Charlie very much. This is why; The exhaust valves and the piston rings. First the valves, I don't think that TCM does anything to insure that the exhaust valve have a full wide seat. They just grind the seat and the valve and expect them to match. I had the new cylinders disassembled and Lapped the valves and two of them wouldn't lap in and needed to be recut or ground. When I looked at the old exhaust valve seat they weren't the same all around and to my mind that is what caused them to start burning and that caused the low compression. 2 at 44 and 1 at 38. This was using the calibrated orifice which allows 11 cfm of air at 80 psi. What I found out about the piston rings is that in the mid 1990's, I was told 92, TCM changed the pistons from all Aluminum to a steel belted piston. This is where the rings sit into the ring lands. On the Alu. pistons they got a lot of ware and they required more choke in the cylinders because of expansion. Someone thought that the steel belted Pistons would ware better in the ring lands and have less expansion so they could go down to about .003 in. of choke. As it happens the old pistons seal better with the 80 psi test than the steel belted. So they came up with this spec. to control the cfm input and allow compression down to 40 psi. If below 40 you are grounded in the US. After installing the new cylinders with lapped valves and ground running, the compression tested at 67/80 on all cylinders. This is the best it gets with the new steel belted pistons. As a side note I also have a Bonanza A36 1981 IO-520 with the original engine and 1600 hrs. It must have the old style pistons because it's compression is 68 to 74 over 80 on all cylinders. It has sat around a lot and shows corrosion in all cylinders at the top but still has the compression and OK oil consumption.

My view of the Humidity is that when it sits and a valve is open you can get the air movement and moisture in those cylinders but the Valve is stainless and very low corrosion prone where the cylinder barrel is Nitride Steel which is very hi corrosion prone. The valve seat is the only other steel of interest and while it could corrode, somehow I haven't seen that much, so running it like you are now and taking another compression test may make a difference. Other wise you need to plug the exhaust pipe when you park it for extended periods.

I'll post this on the LML and others may have more input/corrections.

Regards,