.21 Engine Exhaust Timing

[Marty Davis]

i am lost.. with all the crooked 2 x 4's and busted bricks
one person says high intakes works and exhaust.. then says low numbers are working?? which is it?? i now there is a thousand ways to do it. i now what works for me past and present..

I guess i am just trying to compair numbers from what others use vs mine..

but when i see people saying one works and then they say the other works i am lost.

oh well

Chris, This is a complicated subject.......success at the races. (plus a bit of luck).

Dave

Dave:

GREAT explanation of this topic!!!

In addition, the flow down period has a huge effect on the velocity of the intake charge. By getting the blow down period correct, the cylinder pressure is lowered and the crankcase pressure is much higher which will allow the charge velocity out of the intake ports to be much higher and create a much better charge loop into the combustion chamber.

I agree that close consideration of blow down time is VERY important.

Effort spent with compression ratio (head button configuration) and pipe selection will be the other important engine considerations which will yield the most performance gains.

Preston is CORRECT that setup is critical. The BEST engine in the world will not make a poorly setup boat great. As indicated by several who paid attention to setup, performance can be superb.

Dave, I am working on time/angle/area as an important addition to the Engine Analysis Software, but it is SUPER COMPLICATED. Hopefully, we will have that tool to use to balance the port and intake tract time areas.

For everyone trying to understand this complicated topic, you will use the intake port that opens first after exhaust opens to calculate blow down time. Reason: once either of the intake ports open, they are in effect all open.

Marty Davis

 

[nws]

Use to run K&B- the older OS Mac45- Piccos. They had the top of the ex. port

straight. The new engines have a arc. on the top ex. port.

Is the flat or the arc. the best on the top of ex. port?

Norris

 

[AndyBrown]

I'll bite....

what is it?? props.. hull drag.. areo? squish band??

chris

Just looking at the engine Chris. Yes squish band is an important item. I've seen seen small changes to the squish band effect a 3,000 rpm change without changing anything else.

 

[AndyBrown]

Finally, thanks dave.. I run numbers similar to you but have heard some people use boost and transters in the 140' to 150' and i cannot for the life of me figure out how the motor evens runs..


thanks again.

chris

I can't either.... LOL

But I do know that I ran 112 mph with a .45 engine that had 19 degrees of blowdown, The engine was very easy running and user friendly.

After five years that IMPBA record still stands, so I guess it must work!

 

[Jack ODonnell]

Use to run K&B- the older OS Mac45- Piccos. They had the top of the ex. portstraight. The new engines have a arc. on the top ex. port.

Is the flat or the arc. the best on the top of ex. port?

Norris

The arc in the top of the exhaust port helps to keep the piston from hooking the port. And the arc has a BIG influence on time/area . J.ODonnell

 

[Marty Davis]

Use to run K&B- the older OS Mac45- Piccos. They had the top of the ex. portstraight. The new engines have a arc. on the top ex. port.

Is the flat or the arc. the best on the top of ex. port?

Norris

The arc in the top of the exhaust port helps to keep the piston from hooking the port. And the arc has a BIG influence on time/area . J.ODonnell

WOW, look who showed up in this discussion.

Marty Davis

 

[Jack ODonnell]

Use to run K&B- the older OS Mac45- Piccos. They had the top of the ex. portstraight. The new engines have a arc. on the top ex. port.

Is the flat or the arc. the best on the top of ex. port?

Norris

The arc in the top of the exhaust port helps to keep the piston from hooking the port. And the arc has a BIG influence on time/area . J.ODonnell

WOW, look who showed up in this discussion.

Marty Davis

Hi Big guy;; You know I like this stuff !!!! Jack

 

[steve wood]

can the blowdown be calculated using the eap or do you have to use a degree wheel?

 

[DaveMarles]

Steve, deduct the transfer timing from the exhaust timing and divide by 2 to give the blowdown.

 

[AndyBrown]

can the blowdown be calculated using the eap or do you have to use a degree wheel?

I use a stop watch!

 

[DaveMarles]

[

For everyone trying to understand this complicated topic, you will use the intake port that opens first after exhaust opens to calculate blow down time. Reason: once either of the intake ports open, they are in effect all open.

Marty Davis

Marty I always calculate blowdown from exhaust to transfers not to the boost for the simple reason that this is a comparison of not just timing but time area and the boost port on the engines I use is quite small ( CMB's) and so a timing change on the boost relating to time area is comparatively insignificant compared to a timimg change on the transfers. The difference in boost timing to transfers on CMBs varies a lot and in practice makes little difference to blowdown requirement and in fact to the way the motor runs. Sometimes I think its not a planned change to boost timing by the manufacturer but something that occurs by accident.

Dave

 

[Marty Davis]

[

For everyone trying to understand this complicated topic, you will use the intake port that opens first after exhaust opens to calculate blow down time. Reason: once either of the intake ports open, they are in effect all open.

Marty Davis

Marty I always calculate blowdown from exhaust to transfers not to the boost for the simple reason that this is a comparison of not just timing but time area and the boost port on the engines I use is quite small ( CMB's) and so a timing change on the boost relating to time area is comparatively insignificant compared to a timimg change on the transfers. The difference in boost timing to transfers on CMBs varies a lot and in practice makes little difference to blowdown requirement and in fact to the way the motor runs. Sometimes I think its not a planned change to boost timing by the manufacturer but something that occurs by accident.

Dave

Dave:

I agree with that. Especially in machining the slanted Boost Port.

Have you ever tried the timing that the Car Engines use with EXTREMELY low timing on the exhaust? They are turning 40,000 rpm in race conditions. I realize that their loads are significantly less than ours, but there must be something that can be explored. Andy's statement regarding the LOW blow down would tend to make me curious. This topic started because someone said that they liked the low exhaust timing with a VERY short pipe.

Marty Davis

 

[DaveMarles]

Marty, Depends what you mean by extremely low exhaust timings. I have tried lots of high timings and low and high blowdowns. Everything is a balance, its possible that with a low blowdown you can haul a big pitch prop and possibly a very long induction timing and also the pipe can be designed to help motors with low blowdown but low blowdown is generally not the way to go. The more time that the motor has to get rid of the exhaust gas before the transfers open the better and the same with stuffing mixture back in to the cylinder before exhaust closing but you know this. Andy likes to throw these comments in to stir the pot.

 

[Chris Wood]

Marty and dave.

Do you think that the low timing on the car motors is due to the ability to idle.. and keep the motor cooler?? I was under the impression ( and i could be wrong ) that the lower timings would tend to make the motor generate less heat?? I know car guys fight heat..

maybe jack could chime in on this..

good discussion... always ready to try and improve on things.

chrsi

 

[Tom Foley]

Marty, Depends what you mean by extremely low exhaust timings. I have tried lots of high timings and low and high blowdowns. Everything is a balance, its possible that with a low blowdown you can haul a big pitch prop and possibly a very long induction timing and also the pipe can be designed to help motors with low blowdown but low blowdown is generally not the way to go. The more time that the motor has to get rid of the exhaust gas before the transfers open the better and the same with stuffing mixture back in to the cylinder before exhaust closing but you know this. Andy likes to throw these comments in to stir the pot.

Wouldn't an earlier transfer/boost opening time serve to " loop " out the gasses better with the new charge and then the pulse back at the right time (shorter pipe ) would serve to keep the new charge in the port till it closes ? This uses a lot more fuel but .....The way I understand it the time the pulse waves in the pipe have to travel has to be shortened to achieve higher rpm and power .

 

[dave roach]

Dave Marles

So what you are saying if you had 186 ext timing deduct 122 intake and divide it 2 would be 32 would that be 32 deg blow down.Or what would the 32 be.

Thanks

Dave Roach

 

[Marty Davis]

[

For everyone trying to understand this complicated topic, you will use the intake port that opens first after exhaust opens to calculate blow down time. Reason: once either of the intake ports open, they are in effect all open.

Marty Davis

Marty I always calculate blowdown from exhaust to transfers not to the boost for the simple reason that this is a comparison of not just timing but time area and the boost port on the engines I use is quite small ( CMB's) and so a timing change on the boost relating to time area is comparatively insignificant compared to a timimg change on the transfers. The difference in boost timing to transfers on CMBs varies a lot and in practice makes little difference to blowdown requirement and in fact to the way the motor runs. Sometimes I think its not a planned change to boost timing by the manufacturer but something that occurs by accident.

Dave

Dave:

I agree with that. Especially in machining the slanted Boost Port.

Have you ever tried the timing that the Car Engines use with EXTREMELY low timing on the exhaust? They are turning 40,000 rpm in race conditions. I realize that their loads are significantly less than ours, but there must be something that can be explored. Andy's statement regarding the LOW blow down would tend to make me curious. This topic started because someone said that they liked the low exhaust timing with a VERY short pipe.

Marty Davis

Dave:

For example, a Nova Rossi 8 port .15 has an exhaust timing of 161 and transfers of 114 for a blow down of 23.5. How can this engine perform at 40,000 rpm? Another a Traxas .15 with 156 degrees of exhaust and 117 degrees of transfers for a blow down of 19.5. Or a OS 12TZ with 155 degrees of exhaust and 119 degrees of transfer for 18 degrees of blow down. All engines are high reving engines used in buggys and cars.

I have no idea why these engines have such a configuration...

Marty Davis

 

[dave roach]

Marty

So on a 21 what blow down numbers would be good.

Dave Roach

 

[shoboat]

Dave,

We like between 19 & 22. Works Very Well!

Thanks,

Mark Sholund

 

[Marty Davis]

Dave MarlesSo what you are saying if you had 186 ext timing deduct 122 intake and divide it 2 would be 32 would that be 32 deg blow down.Or what would the 32 be.

Thanks

Dave Roach

Dave is saying that the optimum would be having the intakes at 126 rather than 122, to get to 30 degrees of blow down time, or using 128 degrees of intake and 188 of Exhaust which would also give you 30 degrees of blow down time. Study what he said about the intake duration also, as it affects the characteristics of the engines operation.

Personally, I like to have my intakes lower on the .21 that I run (MAC 21), the ports time area is more balanced. We increased the blow down time to almost 32 degrees. That reduced the cylinder pressure and increased the charge velocity out of the intakes which made the engine come alive. It might be even better by lowering the exhaust a few more degrees.

Boy, this topic sure would have been fun for my best friend, the late Tom Grannis. He LOVED talking about and analyzing things like this.

Dave Roach:

The blow down is highly dependent upon other things. As Dave Marles indicated, time areas of the ports is very important. It depends on the size (area) of the ports of the engine that you are running. Dave runs CMB, last time I heard, so his data would be different than mine. I might say that Andy designed the engine that I run and he liked the intakes up high. He obviously runs very well that way. Which system is best? I think mine is, but I bet that he would say the opposite. Since I don't run SAW (except at the Internats), I have no basis to show that mine is better. There is so much in the boat, setup, prop, etc that the engine is only part of the package.

Marty Davis