pwood

Hi guys.

Does anyone have "rule of thumb" or any other info on port timing and compression ratio settings.

I have messed around with compression on my old Rossi engines but never really known what i was doing. I now have a Nelson QM40 that needs to be setup.

Out the box it has a total of 8 thou shims under the liner, giving a port timing of 196 deg. And a 5 thou shim under the head giving a head clearance of 14 thou.

I read in an earlier post that 196 deg is good. What will ha-pen if it was more/less.

One person said they used 12 thou bump clearance and another said 17...

I could go and do a whole bunch of tests, but if some one has a table or something to give pointer it would be much appreciated

Paul

Ed Smith

I do not know quite what "Bump clearance is"

The factory settings for the Q40 motor are:

Deck height, 0.193" (Distance from the top of the liner to the top of the piston at TDC.) Some people set this at 0.195", this would raise the timing 0.002. I would doubt that the ports are machined that accurately anyway so whether it makes a difference would have to be determined by testing.

Head clearance, 0.016" (Distance from the underside of the squish band to the top of the piston at TDC.)

The deck height has to be set first. To do this do not force the piston in the liner to TDC. Measure from the top of the liner to the top of the piston at BDC. From this distance subtract 0.706" (The stroke) the result is the deck height. Shim the liner accordingly.

Ed S

pwood

Thanks Ed

Bump clearance might be a local term for head clearance.

I will check my motor against your "factory" settings.

What i was asking is are there any general factors to look for if you have a deck height too high or low etc. and the compression wrong.

What i am getting at is i was battling to get my motor going.....would not get on the pipe without getting lean and only turning 21000rpm.

The compression was to high (14 thou clearance) I tried 17 and then 20 thou. 17 was better. I then tried an extra 2 thou under the liner to raise the timing because it was the 50/50 if i tried to raise it or lower it.....

It now comes on the pipe easily, the needle is useable and is still a bit ritch at 22000 rpm.

Still a bit low on the rpm...

The motor just doesn't seem to want to go to 23-24000 like it whould.

If there are any guys who fly at high altitude, is there anything else i can look at that might help. The 5500' altitude might be taking too much power. I would like to cut the prop down but the rule don't allow.

I am useing 15% omega morgan fuel and the apc 7.4x8.0 prop

Ed Smith

Sorry I cannotoffer more help. I have no experience flying at higher altitudes. The average around here is about 250'.

Ed S

daven

My Feedback: (1)

Locally, we can turn that same prop in the 22,500 - 23,000 range and it goes pretty good with the same fuel you are using.

Do you have a smaller prop you can try to get it up on the pipe?

Something doesn't sound right. Why don't you contact Dave Schadel and get his advise.

bl10

We have, on occasion, raced at Reno NV. Alt 5500 ft and Mexico City which is even higher. For Q500 engines I ended up lowering the head to about 7 thou total or about 3 thou less than I generally run. We ran the same Q500 props (8.8 x 9) as at sea level and lived with about 700 less RPM. We tried smaller props to bring the RPM up but went slower. The Q40 engines seemed to run the best with the Sea Level setup, about 15 thou total head clearance. At the time we didn't have the APC Q40 props but were down at least 1000 rpm with the wood props. As with the Q500s we were better off living with the lower RPMs rather than going to a smaller prop. So long as the Q40 engine will come on the pipe it should be fine. The APC 7.4 x 8.0 prop is hard to pull much above 23000 in any case. My best engine will pull it to 23900 on a hot dry day in Los Angeles (about 1200 ft alt). That same engine on a cold damp morning will only pull it to about 22800. My advise to you would be to find a combination which needles well and practice practice practice. All of us worry far to much about how many ground RPM some else has or having the latest trick airfoil when what we should be worrying about is flying.


Barry Leavengood

DHG

My Feedback: (7)

Barry,

Amen to that. Practice first, worry about the other guys' setup later. Re the rpm difference due to altitude: Several years ago I won a Q40 race in Texas (though no Texan will ever admit that, and if it did happen it was only because they all had a hangover that day) and then immediately packed everything in the car, drove to Denver, and without even stopping at the house I went to the field and test-ran my engine with the same prop, same plug, same fuel, etc. The weather was as similar as you could hope for, i.e., drier than Texas but about the same temperature. I measured a difference of 500 rpm and have used that as my standard ever since.

The APC prop is big -- sometimes won't pipe up at this altitude but usually you can get away with using it if you set the engine rich and don't let it blaze away too long on the ground. A good substitute in wood, if you can find them, is the old Rev-Up 8-3/4 x 7 or 9 x 7 (Series 400, the narrow racing props they used to sell for Q500)cut down to a diameter of about 7-1/4". Or a Zinger 8 x 7 will work. Don't be afraid to cut the diameter down to 7" if necessary to get the rpm, but of course make sure to balance every one. And if you feel or hear a nasty buzzing sound or a lot of vibration, use it to stir paint -- don't fly it.

Tree-Hugger

pwood

Hi guys

Thanks for all the replies.

I found 9 thou under the liner and a 16 thou head clearance gave a nice setup. Easy to get on the pipe and a little rich. It gives 22500 rpm on the ground and in the air it howls along with a bit of a smoke trail and leanes a bit more in the turns. Sounds really good.

This is the fastest thing i have ever flown. I had an old Miss RJ fai with a Rossi 10 years ago. that was a pain.

It looks like QM40 is going to take off here in South Africa.

Thanks again