I am running the NMP Velocity stack that you are using. It depends on the model you are flying. I can run a 160DZ with the stack in my Astral XXc at just a shade under 10lbs and its excellent performance perfect for what I'm doing. I use the stack


However on my Adventure Biplane using a 160DZ and the Velocity stack it needs the extra power on the top end. So when running a 160 in this model I run the engine without the stack. However with a 170 in either model the stack is a really good thing for both sound and smooth precise throttle control response.

My guess is that you 160 could be a little tired and needs a new ring or you model is likely a bit on the heavier side.

The YS engines have a very good throttle and torque curve. However like with most glow engines the last little amount of throttle barrel travel doesn't always yield an increase in rpm but does change in noise level. So what we started trying is putting a restrictor in the carb to eliminate this "dead spot" in the throttle travel, and reduce the noise. I had seen the Japanese pilots doing this and felt it may improve the torque curve. A friend and I ordered some of the stacks from Japan and the cost of about $45 USD each. By the time we paid the tax and got the shipping with some other little goodies sold in the Japanese market.

I did a bunch of testing with different diameters on the Velocity stacks. I tested 8mm which is the size of the original restrictor in the YS 140DZ. I then tested every size up to 10.5mm by 0.5mm changes. (8.0, 8.5, 9.0, 9.5, 10.0, and 10.5mm) I found the best performance-noise and effectiveness to be at the 9mm size. By the way the bore on the carb is 12.25mm

The 8.5mm to 9.5mm all performed about the same. You lose about 200rpm from full throttle wide open rpm. The 10.5mm version only lost about 100rpm and you could not hear any difference in the engine sound wise or feel any performance difference between with or without the 10.5mm. The 10.5mm was exactly the same performance, noise, and everything as if it was not there. As we restricted below the 8.5 and down to 8mm the result was the engine started losing more rpm (300rpm for the 8.0mm) from the top but really didn't improve the low end torque situation much more than the 8.5-9.5mm

So this is how we decided to have NMP produce the version you have. We found in several different configurations that the "sweet spot was between 8.5-9mm. This gave the best noise reduction, and throttle curve with the least reduction in top end rpms. Therefor the NMP velocity stack is at 9mm.

Yes it takes some power off the top end. However the engine still has its low end torque. If your model and situation doesn't work with the stack then I suggest you pull it out. Its not going to work in all applications. However a good model with excess power to burn benefits from velocity stack in the improved throttle response and noise reduction.

By the way you engine is down about 200-300rpm from what I am getting on my 160DZs with that same prop. this could be elevation related, air temps, time for a new piston ring? and many other things going into the rpm numbers you are getting. By the way 7900rpm on that prop is right on the noise limit for F3A. That is where I would run the engine for a competition. I actually reduce the throttle travel of the engine to get 7900rpm on the 170's with a similar prop. Wide open with the velocity stack my 170's will pull about 8300-8500 on that prop. So I dial down the ATV to get 7900 and this is where I fly the monoplane. The Bipe gets a different diameter prop and a slightly different setup. It needs an extra few RPM due to its extra drag. But I still run the velocity stack on the 170 in this model and reduce the top end rpm with ATV. For the 160DZ in the bipe I just remove the Velocity stack and run the same curves adjust the ATV to get same RPM as the 170 with the stack.

Both engines perform about the same in this configuration. You really have to play with props, and noise. Also the model is a big variable to see the performance changes when using a device like this to tune the setup. I have found the Velocity does the same thing regardless of 170 vs 160. The 160 just has a little less to give up at the top than the 170. And the prop selection will be a big determination on the performance of your model. The best full power rpm doesn't always work the best for throttle response and the noise limits.

The good thing about the NMP stack is it can be removed in about 30 secs. Its easy to do a bunch of testing back to back.

Troy Newman
Team YS