.21 Engine Exhaust Timing

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Marty Davis

Well-Known Member
Joined
Sep 25, 2004
Messages
2,445
.The key is to get your pipes shorter and turn

more RPM fellas. 7" or less on the 21 and under 9" on the 45. RPM

is the key here for speed guys, and you will never get your pipes

short with high exhaust timing numbers. So back your 180+ timings

down to 175 to 178 degrees and shorten your pipes. Your motors

will then start to rev if you know what I mean?

Franco:

Interesting thoughts about exhaust timing. Would you expand on your thinking on this....

I also know that Rod Geraghty follows a similar belief about exhaust timing and short pipes.

Marty Davis
 
[

.The key is to get your pipes shorter and turn

more RPM fellas. 7" or less on the 21 and under 9" on the 45. RPM

is the key here for speed guys, and you will never get your pipes

short with high exhaust timing numbers. So back your 180+ timings

down to 175 to 178 degrees and shorten your pipes. Your motors

will then start to rev if you know what I mean?

Franco:

Interesting thoughts about exhaust timing. Would you expand on your thinking on this....

I also know that Rod Geraghty follows a similar belief about exhaust timing and short pipes.

Marty Davis

i didn't think you could get a nova or a cmb that low without having sub piston induction. also can't get most pipes that short on them also, without getting into the divergant cone
 
i didn't think you could get a nova or a cmb that low without having sub piston induction. also can't get most pipes that short on them also, without getting into the divergant cone

Steve:

You can actually turn a flat on the start of the divergent Cone and get it shorter that way. I used to do that with the thick Irwin Pipes.

Marty Davis
 
Marty,

That is my thinking on the 21 & 45 motors,it works well for me.

The pipes are really the key here, not just the numbers. Look

back a few more posts Marty, and you will see that it was my

thoughts not Frank's. Frank likes pretty high timed motors

from what he tells us.

Good Luck Testing,

Mark Sholund
 
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Marty,
That is my thinking on the 21 & 45 motors,it works well for me.

The pipes are really the key here, not just the numbers.

Good Luck Testing,

Mark Sholund
Mark:

Any reasoning, or just that it works for you?

Marty Davis
 
Marty

On the Mac and CMB 21 to get that low of ext.timing175- 178 you have to take .016 to .018 of the bottom of the liner and it will sub port.Been there done that.you have to make a piston .020 longer.

Dave Roach
 
Marty,

I am not an engine guy,but my theory on motors is really different

than a lot of others. I just know that lower exhaust timed motors work

well for me. I know with some of my other motors with higher numbers,

I have to run the pipes a 1/2" longer or more to get the RPM & Speed

that I like. Also,it seems that the higher exhaust numbers won't let you

spin the RPM,launch the propellers,or have great throttle response. I have

no problem getting the pipes short and launching the propellers still.

Keep Testing,

Mark Sholund
 
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MartyOn the Mac and CMB 21 to get that low of ext.timing175- 178 you have to take .016 to .018 of the bottom of the liner and it will sub port.Been there done that.you have to make a piston .020 longer.

Dave Roach
Dave:

On the recent CMB Engines that I have looked at and measured, I have seen some at 179. There is a good bit of ability to lower the liner on them also.

I am interested to see what people think about this.

Andy and I have talked over the years and each of us have visited moderate and VERY HIGH compression ratios and it seems that we are on the opposite end of that when we talk. BUT we both have visited both areas. It is the same thing with high and low exhaust timing.

Does anyone think it is interesting that the RC Car racers use timing that is VERY low with short pipes?

Has anyone considered the port time/areas of both approaches?

I realize that a GREAT reason to use one system over another is "it works for me" but I am really interested in the "WHY's of the approaches".

I can remember many years ago when Joe Kramer was running OS 21's and I was also. He ran the engine with low timing and I with high timing. He might have actually been turning more rpm than I was - not for sure, but it sure seemed that way.

I will be able to test both approaches EASILY on the dyno as soon as we get some projects completed. THEN I WILL KNOW FOR SURE.

Marty Davis
 
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Marty

I have a greenie and a Mac both with the same numbers right to the tee and the greenie is a lot stonger than the mac.I don't no if the opening in the case is different or what.Don't get me wrong the mac and cmb or both great engines.

Dave Roach
 
Adding to the fray here. Perhaps instead of looking at just the exhaust timing, whether high or low, what about it's relationship to the intake timing. The blowdown timing (when both intake and exhaust ports are open) would have a huge effect on the pipes ability or lack thereof, to do it's work. Too high and you can have a peaky somewhat torqueless motor, too low and you can have an easily driveable motor that won't have very high peak power. Dave Marles for one, stresses the importance of this when looking to modify a motors timing. After all, the entire system has to work together. Just a thought.
 
On the NR based engines you can use a car piston/sleeve. The car timing is around 170, xfer about 115. I just had to shim the sleeve in my EB Mods car motor up ~0.015 to get it to 176*/122*. It ran pretty good, but I wasn't able to get the pipe I was trying short enough. I'll work on the pipe to get it to slide over the header and try again.

As for the 45s, it might require a custom piston from Mr. Christy.
 
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MartyI have a greenie and a Mac both with the same numbers right to the tee and the greenie is a lot stonger than the mac.I don't no if the opening in the case is different or what.Don't get me wrong the mac and cmb or both great engines.

Dave Roach
I would be curious about the Crankcase volume of each. Why don't you measure it with the piston at the bottom on both engines and let us know what you find.

Marty Davis
 
Adding to the fray here. Perhaps instead of looking at just the exhaust timing, whether high or low, what about it's relationship to the intake timing. The blowdown timing (when both intake and exhaust ports are open) would have a huge effect on the pipes ability or lack thereof, to do it's work. Too high and you can have a peaky somewhat torqueless motor, too low and you can have an easily driveable motor that won't have very high peak power. Dave Marles for one, stresses the importance of this when looking to modify a motors timing. After all, the entire system has to work together. Just a thought.
Actually, it is much more complicated than that. BUT, you are correct in the analysis of "BLow Down TIme" being important for a pipe to operate efficiently.

It gets into time/angle/area of each of the ports AND the intake tract including the carb. For an engine to operate efficiently, there should be a balance in each component. If one is restrictive, it will limit the output of the engine.

The statement that "After all, the entire system has to work together." is correct, but involves the time/angle/areas of each component of the porting system of the engine.

Marty Davis
 
Last edited by a moderator:
Adding to the fray here. Perhaps instead of looking at just the exhaust timing, whether high or low, what about it's relationship to the intake timing. The blowdown timing (when both intake and exhaust ports are open) would have a huge effect on the pipes ability or lack thereof, to do it's work. Too high and you can have a peaky somewhat torqueless motor, too low and you can have an easily driveable motor that won't have very high peak power. Dave Marles for one, stresses the importance of this when looking to modify a motors timing. After all, the entire system has to work together. Just a thought.
Actually, it is much more complicated than that. BUT, you are correct in the analysis of "BLow Down TIme" being important for a pipe to operate efficiently.

It gets into time/angle/area of each of the ports AND the intake tract including the carb. For an engine to operate efficiently, there should be a balance in each component. If one is restrictive, it will limit the output of the engine.

The statement that "After all, the entire system has to work together." is correct, but involves the time/angle/areas of each component of the porting system of the engine.

Marty Davis
When you guys get it figured out .... i'm in the market for a new .21 engine !!!!
 
Marty- great thread, please keep us posted "when the shop gets caught up" and you run the dyno tests. I would be really interested in stopping by that day and going to school on these theories , I promise I will stay out of the way and TRY not to ask too many questions - LOL.

Andy
 
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