exhaust timing on a .21. How much is too much?

[RodneyPierce]

Guys, how much exhaust timing is too much on a .21? For, lets say a Sport 20 boat. I have been doing some reading and digging, and it seems that alot of guys, that i greatly trust seem to like the 183 range. But with further reading and digging, seems alot are running the 190 range, and some higher. How much is too much? When does it get picky on launch and hard to needle?

 

[RodneyPierce]

i know there are plenty here that know

 

[Jerry Wyss]

Well Its going to get real picky @ 190* I'll guarrantee that. I went 190* once with TT sleeve, took forever to get that thing on the pipe but when it did it was like a 30-06 goin' off, YeeHaa! On a cool morning its easier to get up & go, but as the temp goes up,nadda.

I would say 183* is about there with a direct drive but a geared set-up you could take to about 188* for max usable. Just my opinion based on past experiece.

 

[Marty Davis]

One thing to consider.....

The higher the exhaust timing, the smaller the engine becomes. Why, because the trapped charge (when the exhaust closes and starts compressing the charge) becomes smaller the higher you move the timing. It is a significant difference between 182 and 190 in the effective size of the engine. I personally run low timing for this reason. I can get Super RPM easily by using pipe length and combustion chamber design.

OK, a specific example. I built a .21 engine with 182.3 degrees of exhaust timing and though about raising the liner .015" to make the timing 187.3 degrees. When I measured the difference in trapped charge, I saw that the engine was 4% smaller (too big a loss). Decided to stay at 182.3 and it runs great with a ton of RPM.

I don't really think that exhaust timing has that big an impact on overall performance other than the loss in effective size in the engine. As long as you have 30 degrees of blow down time, the engine should be great. Blow down is MUCH MORE IMPORTANT than high exhaust in my opinion.

 

[dave roach]

Rodney

A good no.I use is 186 on the ext.

Dave Roach

 

[Andy Brown]

One thing to consider.....

The higher the exhaust timing, the smaller the engine becomes. Why, because the trapped charge (when the exhaust closes and starts compressing the charge) becomes smaller the higher you move the timing. It is a significant difference between 182 and 190 in the effective size of the engine. I personally run low timing for this reason. I can get Super RPM easily by using pipe length and combustion chamber design.

OK, a specific example. I built a .21 engine with 182.3 degrees of exhaust timing and though about raising the liner .015" to make the timing 187.3 degrees. When I measured the difference in trapped charge, I saw that the engine was 4% smaller (too big a loss). Decided to stay at 182.3 and it runs great with a ton of RPM.

I don't really think that exhaust timing has that big an impact on overall performance other than the loss in effective size in the engine. As long as you have 30 degrees of blow down time, the engine should be great. Blow down is MUCH MORE IMPORTANT than high exhaust in my opinion.

If we look at the engine in static terms the argument for making the engine smaller with increased timing holds true. However, the engine works in a dynamic realm, not static. With a properly proportioned exhaust system, a little more timing can actually increase the total trapped fuel mass.

The old terms of non-piped timing and piped timing still hold true today.

Currently, in control-line speed, the fastest non-piped .21 engines are using over 180 degrees of exhaust.

I still hold the IMPBA D hydro SAW record that was set with an engine only having 20* of blow down. It was not a purpose built SAW engine and the engine had lots of drivable torque for heat racing....something that goes away with increased blow down timing.

As for my opinion on what is good exhaust timing, it all has to do with other proportions in the engine and pipe system. Some .21's will go really fast with only 175*. Others won't get moving until you get them at 190*. For the typical Novarossi 180* to 185* is good.

 

[dwilfong]

This is a very interesting discussion. There seams to be two different thoughts here on timing. The idea of more timing with the right pipe giving more time to charge the cyl with pipe pressure.

Very interesting.

David

 

[Jack ODonnell]

One thing to consider.....

The higher the exhaust timing, the smaller the engine becomes. Why, because the trapped charge (when the exhaust closes and starts compressing the charge) becomes smaller the higher you move the timing. It is a significant difference between 182 and 190 in the effective size of the engine. I personally run low timing for this reason. I can get Super RPM easily by using pipe length and combustion chamber design.

OK, a specific example. I built a .21 engine with 182.3 degrees of exhaust timing and though about raising the liner .015" to make the timing 187.3 degrees. When I measured the difference in trapped charge, I saw that the engine was 4% smaller (too big a loss). Decided to stay at 182.3 and it runs great with a ton of RPM.

I don't really think that exhaust timing has that big an impact on overall performance other than the loss in effective size in the engine. As long as you have 30 degrees of blow down time, the engine should be great. Blow down is MUCH MORE IMPORTANT than high exhaust in my opinion.

If we look at the engine in static terms the argument for making the engine smaller with increased timing holds true. However, the engine works in a dynamic realm, not static. With a properly proportioned exhaust system, a little more timing can actually increase the total trapped fuel mass.

The old terms of non-piped timing and piped timing still hold true today.

Currently, in control-line speed, the fastest non-piped .21 engines are using over 180 degrees of exhaust.

I still hold the IMPBA D hydro SAW record that was set with an engine only having 20* of blow down. It was not a purpose built SAW engine and the engine had lots of drivable torque for heat racing....something that goes away with increased blow down timing.

As for my opinion on what is good exhaust timing, it all has to do with other proportions in the engine and pipe system. Some .21's will go really fast with only 175*. Others won't get moving until you get them at 190*. For the typical Novarossi 180* to 185* is good.

Andy is absolutely wright on the properly proportioned exhaust system. The question i have is when people say they have 175 to 190* of exhaust timing,are they talking total width of the exhaust port ?

 

[dwilfong]

One thing to consider.....

The higher the exhaust timing, the smaller the engine becomes. Why, because the trapped charge (when the exhaust closes and starts compressing the charge) becomes smaller the higher you move the timing. It is a significant difference between 182 and 190 in the effective size of the engine. I personally run low timing for this reason. I can get Super RPM easily by using pipe length and combustion chamber design.

OK, a specific example. I built a .21 engine with 182.3 degrees of exhaust timing and though about raising the liner .015" to make the timing 187.3 degrees. When I measured the difference in trapped charge, I saw that the engine was 4% smaller (too big a loss). Decided to stay at 182.3 and it runs great with a ton of RPM.

I don't really think that exhaust timing has that big an impact on overall performance other than the loss in effective size in the engine. As long as you have 30 degrees of blow down time, the engine should be great. Blow down is MUCH MORE IMPORTANT than high exhaust in my opinion.

If we look at the engine in static terms the argument for making the engine smaller with increased timing holds true. However, the engine works in a dynamic realm, not static. With a properly proportioned exhaust system, a little more timing can actually increase the total trapped fuel mass.

The old terms of non-piped timing and piped timing still hold true today.

Currently, in control-line speed, the fastest non-piped .21 engines are using over 180 degrees of exhaust.

I still hold the IMPBA D hydro SAW record that was set with an engine only having 20* of blow down. It was not a purpose built SAW engine and the engine had lots of drivable torque for heat racing....something that goes away with increased blow down timing.

As for my opinion on what is good exhaust timing, it all has to do with other proportions in the engine and pipe system. Some .21's will go really fast with only 175*. Others won't get moving until you get them at 190*. For the typical Novarossi 180* to 185* is good.

Andy is absolutely wright on the properly proportioned exhaust system. The question i have is when people say they have 175 to 190* of exhaust timing,are they talking total width of the exhaust port ?

I was thinking about that. Would a flat or very small arc in the port top made very wide with less timing and less blow down work better. Than a say a top hat made small with high timing and more blow down.

David

 

[Andy Brown]

One thing to consider.....

The higher the exhaust timing, the smaller the engine becomes. Why, because the trapped charge (when the exhaust closes and starts compressing the charge) becomes smaller the higher you move the timing. It is a significant difference between 182 and 190 in the effective size of the engine. I personally run low timing for this reason. I can get Super RPM easily by using pipe length and combustion chamber design.

OK, a specific example. I built a .21 engine with 182.3 degrees of exhaust timing and though about raising the liner .015" to make the timing 187.3 degrees. When I measured the difference in trapped charge, I saw that the engine was 4% smaller (too big a loss). Decided to stay at 182.3 and it runs great with a ton of RPM.

I don't really think that exhaust timing has that big an impact on overall performance other than the loss in effective size in the engine. As long as you have 30 degrees of blow down time, the engine should be great. Blow down is MUCH MORE IMPORTANT than high exhaust in my opinion.

If we look at the engine in static terms the argument for making the engine smaller with increased timing holds true. However, the engine works in a dynamic realm, not static. With a properly proportioned exhaust system, a little more timing can actually increase the total trapped fuel mass.

The old terms of non-piped timing and piped timing still hold true today.

Currently, in control-line speed, the fastest non-piped .21 engines are using over 180 degrees of exhaust.

I still hold the IMPBA D hydro SAW record that was set with an engine only having 20* of blow down. It was not a purpose built SAW engine and the engine had lots of drivable torque for heat racing....something that goes away with increased blow down timing.

As for my opinion on what is good exhaust timing, it all has to do with other proportions in the engine and pipe system. Some .21's will go really fast with only 175*. Others won't get moving until you get them at 190*. For the typical Novarossi 180* to 185* is good.

Andy is absolutely wright on the properly proportioned exhaust system. The question i have is when people say they have 175 to 190* of exhaust timing,are they talking total width of the exhaust port ?

I was thinking about that. Would a flat or very small arc in the port top made very wide with less timing and less blow down work better. Than a say a top hat made small with high timing and more blow down.

David

I have never really tried full width timing other than what may have come on stock engines with narrow exhaust ports. The reason being that when using a very wide port, which has benefits, there is chance of hooking the piston in the port. Therefore I use a small gradual arch for the purpose of gently squeezing the piston into the center of the bore.

I suspect, and have heard that the power would be greatest with the straight across top but would be very peaky in the power range. While an arch, top hat or some type of gradual opening would produce a little less power, but would give a wide range.

So to answer Jack's question, my exhaust timing has a gradual closing point.

 

[Chris Wood]

i am just curious to hear from andy and jack..

what are your thoughts in regards to timing when it comes to motor that uses a DOME piston vs a flat top.. i am under the impression the dome runs a higher compression ratio. also the exhaust open with that piston gives it a longer duration with the piston going up and down on the stroke. ( stays open longer and opens sooner )

I have heard good and bad with this set up.

your thoughts please.

thanks

chris

 

[Martin Hamilton]

On my .21 tether boat engines ive allways used T shaped or reverse top hat exhaust that is straight accross the full width so timing is the same accross the full width.Most specialist control line speed aero motors also use straight full width ports as well all be it at much higher timing than we use in tether boats. I find that 176 degrees exhaust & 123degrees transfer is the best for my motors & 26.5 degrees blow down. I find that the pipe bangs in much harder when it comes in with much more rpm. Allthough my timing my be considered relatively low in terms of other motors, In the same boat my motor is 10-12mph faster than a CMB RS7. Martin.

 

[Terry Keeley]

The CMB RS21 comes with 188 and seperate eyebrows at 184 right outta da box!

 

[Jerry Wyss]

The CMB RS21 comes with 188 and seperate eyebrows at 184 right outta da box!

yeah and thats why us average joe's can't make'm run fer squat. Sorry Terr, I had

to say it, cause its true.

 

[RodneyPierce]

I wont tell anyone what I made the timing number, and we will see how it runs on sunday.

 

[Martin Hamilton]

The CMB that i used was the Valvola RS7 which had 182 degrees exhaust.

 

[Terry Keeley]

The CMB RS21 comes with 188 and seperate eyebrows at 184 right outta da box!

yeah and thats why us average joe's can't make'm run fer squat. Sorry Terr, I had

to say it, cause its true.

Dunno about that, think you just gotta find out what it wants. I ended up making a smaller head for it, the stock at .19cc plus the dish was a little soft. Here's the rest of the numbers if interested:

https://www.intlwaters.com/gallery/albums/userpics/10068/2006_CMB_RS_21.xls

 

[RodneyPierce]

Terry, I cant view the file here at work, but what did your head end up at?

 

[Terry Keeley]

Terry, I cant view the file here at work, but what did your head end up at?

0.16cc plus the dish for .17 total.

 

[RodneyPierce]

Very nice damn I need a lathe