.21 Engine Timings

[ozbryan]

Ok guys,

Interested in your thoughts here...

What are your thoughts on the ideal engine timings?

I know there will be a lot of variables so lets base it on this..

Nova Rossi 7 port engine.

Outrigger Hydroplane

Mono

Outboard

All opinions welcome.

Regards

Bryan

 

[Marty Davis]

My opinion is that Blow Down Time is much more important than specific timing.

Remember that the lower the exhaust timing, the larger the engine is effectively. The trapped charge is larger the lower the exhaust timing.

You can use the pipe length to generate the RPM so I personally like low timing and shorter pipe. My favorite is around 182 and 122.

 

[dave roach]

I like 185-125 30 deg blow down

Dave

 

[ozbryan]

My opinion is that Blow Down Time is much more important than specific timing.

Remember that the lower the exhaust timing, the larger the engine is effectively. The trapped charge is larger the lower the exhaust timing.

You can use the pipe length to generate the RPM so I personally like low timing and shorter pipe. My favorite is around 182 and 122.

Interesting

How is blow down measured?

Do you have a technical paper on this?

Bryan

 

[Marty Davis]

Blow down is the difference between exhaust timing and intake timing. For example: if you have 182 degrees of exhaust you subtract the timing of the intakes (side ports because the boost is so hard to measure). 182 - 122 = 60 and you divide that by 2 to get the blow down of 30 degrees.

It is really easy as you have exhaust - 60 degrees

185/125

190/130

etc, etc

 

[mtruex]

I like 185-122 30 deg blow down

Dave

Dave, 185-122 would be 31.5 blowdown

 

[dave roach]

You take the ext no.subtract intake no. and divide it by 2

Dave Roach

 

[dave roach]

Martin

Sorry for the misprint.

185-125 30 deg

Dave Roach

 

[kesso2118]

My opinion is that Blow Down Time is much more important than specific timing.

Remember that the lower the exhaust timing, the larger the engine is effectively. The trapped charge is larger the lower the exhaust timing.

You can use the pipe length to generate the RPM so I personally like low timing and shorter pipe. My favorite is around 182 and 122.

Interesting

How is blow down measured?

Do you have a technical paper on this?

Bryan

In addition to what Marty has provided here is a link to some 2 Stoke Definitions

http://www.2-stroke-porting.com/Terminology.html

Enjoy......

 

[ozbryan]

Blow down is the difference between exhaust timing and intake timing. For example: if you have 182 degrees of exhaust you subtract the timing of the intakes (side ports because the boost is so hard to measure). 182 - 122 = 60 and you divide that by 2 to get the blow down of 30 degrees.

It is really easy as you have exhaust - 60 degrees

185/125

190/130

etc, etc

Thanks Marty

That's good info!

You said you feel this is more important than timings.. What is a good blow down number and what are the effects of a higher or lower blow down?

 

[Marty Davis]

Blow down is the difference between exhaust timing and intake timing. For example: if you have 182 degrees of exhaust you subtract the timing of the intakes (side ports because the boost is so hard to measure). 182 - 122 = 60 and you divide that by 2 to get the blow down of 30 degrees.

It is really easy as you have exhaust - 60 degrees

185/125

190/130

etc, etc

Thanks Marty

That's good info!

You said you feel this is more important than timings.. What is a good blow down number and what are the effects of a higher or lower blow down?

One of the guys who I really value information from (Dave Marles) says that he also believes that blow down is super important. If I go above 30 degrees, the engine loses mid range and low end severely. It only takes a degree or two to make a big difference. You can get more low end with VERY low transfer timing, but the engine will not RPM very well. 30 degrees is just a happy number and one that I use on all engine sizes. You can see the difference in blow down time more on .21 engines because there is not any excess HP as there is on the larger engines. Just easier to see, but the effect is the same on larger engines.

Blow down has a direct relationship with time area of the ports too, but that is an area that has been difficult to analyze. There has not been a good way to analyze time/angle/areas until now. I will release a new version of the Engine Analysis Software that has time area analysis built in. Hopefully soon....

 

[Tim_Duggan]

Remember that the lower the exhaust timing, the larger the engine is effectively. The trapped charge is larger the lower the exhaust timing.

You can use the pipe length to generate the RPM so I personally like low timing and shorter pipe. My favorite is around 182 and 122.

Lower timings are generally more forgiving when travelling to different locations and big changes in weather etc.

However;

Until you get to the point (usually in a rigger) where you physically can't get the pipe any shorter due to the header, and are still chasing more rpm - then it is raise the timings or find a physically shorter pipe time. This is a big drawback to the 180 degree header. I say this because the original post specified NovaRossi 7 port as the engine.

Tim

 

[ozbryan]

Unfortunatly Timings numbers mean little without port area calcs

and time / area study

Greg

More good info Greg.

Care to share more on the above forthe not so knowledgable (me)

 

[steve wood]

for the last year, nova has been raising the transfer ports on the 5 and 7 port liners(maybe others,don't know).this makes it a little difficult to get 30° of blowdown.i usually set the nova about 188° exhaust and transfers at 128°. they have been creeping up to 134° on the transfers,out of the box.you have to lower the liner to get the transfers down,then raise the exhaust to get what you need.be carefull of lowering the liner,and be aware of sub piston induction.if you leave the liner alone and want 30° of blowdown you will end up with 194° of exhaust timing.too much in my opinion for consistant launch,and coming off the corners.don't forget to look at the crank induction also,as the 5 and 7 port engines are not the same,and need work here also.all this needs to work in a combination with head bowl volume,head to piston clearance,water jacket volume,HEAD MATERIAL(brass),and the correct pipe and pipe length to make it really rpm.not to mention a good boat and prop,and plenty of NITRO