Thread: #6

**bucketracer** · 24th August 2009, 07:39

Originally Posted by SS90

Ermmmm

and your point is exactly???

**SS90** · 24th August 2009, 07:55

By all means, please continue with your experiences with blow down times!

It's not really possible to give a "number for all situations" when it comes to blow down times, so as such I am interested in what other people use for a given situation.

I remember in the ESE thread you wrote something (using an RGV250 cylinder as an example) and referred to some "blow marks" around the transfers.....then something about scroll diodes.

I think you will actually find what has happened there is a result of the power valves effecting the blow down times, if the engine is consistantly operated "around town" the blow down time (which is most efficient in the upper RPM.....particularly on the later "3 Stage set ups") in this (lower) RPM is completely wrong, which causes this "blow mark" situation.

Higher up in the rev range, as the power valves raise the exhaust port height, the blow down time is more suitable (of course the is only one RPM level when it is at an "optimum", and as such it is always a compromise.)

I suggest that the blowdown time that Suzuki use for the RGV250 is SOMETHING like 35 deg (just guessing) which isn't correct for higher trapped compression ratio provided by the power valves at that RPM, but as the revs raise (......and the power valves "open".....and the trapped compression lowers) the blowdown time is more suited to this trapped compression ratio/higher RPM)

Power valve transfers as well as exhaust valve would be the answer.

The blow down time could alter with the RPM increase/decrease, a fully programmable one would be horny...........

**F5 Dave** · 24th August 2009, 09:52

Originally Posted by SS90

Thats interesting with those blowdown times (assuming it is for your RG Fiddy)........ clearly that will increase your peak ) at the expense of range), but I remember you mentioned that RG fiddys arent partial to high RPM's....... I have found around 40 deg is suitable for around 14000RPM, have you found the same?

RGs run fine at 12,000 peak with new parts & good oil & will last a few seasons with over rev of about 500rpm, or usually a season with spin out to 13000. So it depends what you call high revs.

**speedpro** · 25th August 2009, 20:34

I've had another read of Wobbly's notes and I'm a couple of degrees short of desired port timing, though that could be due to measurement methods. I have always used a thin piece of lockwire in the port to stop the piston and read directly off the degree wheel. Target engine speed was 11,000rpm and BMEP of 165psi. The correct blowdown is supposedly 9.8 which is not a figure I understand, yet. Looks like I need to make minor changes to the combustion chamber as well. The squish % needs a small alteration and the radius into the bowl. All little bits not quite right all adding up.

**bucketracer** · 25th August 2009, 23:25

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"Blowdown" the period between exhaust opening and transfers opening. Now I got this from a reliable source. The rule of thumb is that blowdown should be for around a 1/4 of the exhaust port area.

Obviously for wide bridged ports and ex ports with supplementary side ports the number of degrees of blow down required will be less than for narrower higher single exhaust ports.

So a handy rule of thumb is blowdown = 1/4 of exhaust port area.

now that's so easy.

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**SS90** · 26th August 2009, 04:15

Originally Posted by bucketracer

.

"Blowdown" the period between exhaust opening and transfers opening. Now I got this from a reliable source. The rule of thumb is that blowdown should be for around a 1/4 of the exhaust port area.

Obviously for wide bridged ports and ex ports with supplementary side ports the number of degrees of blow down required will be less than for narrower higher single exhaust ports.

So a handy rule of thumb is blowdown = 1/4 of exhaust port area.

now that's so easy.

.

I have to disagree with you there Bucketracer.........

1/4 of the exhaust port time area?

No way.

Sure, the number of degrees in a general sense that you will find most engine's use is NO HIGHER than 1/4 of the time of the exhaust time area, but to say that "THE BLOW DOWN TIME SUITABLE FOR ALL ENGINES IS EQUAL TO 1/4 OF THE EXHAUST PORT TIME AREA" is very very incorrect.

For example.

I took this from Bucketracers previous post.

Trans 123deg duration = trans open 118.5deg after top dead center (TDC).

Ex 187deg duration = ex opens 86.5deg after top dead center (TDC).

Therefore blowdown is 118.5deg - 86.5deg = 32 deg.

32 deg seems in the ball park of 28-36deg.

Blowdown time area depends on the exhaust port width/shape engine capacity and rpm.

so, using the (I say incorrect) 1/4 "rule", 1/4 of 187= 46.75 deg

46.75 degrees of blowdown time for an exhaust port time area of 187 Deg?

Hell no!

If anything, with exhaust duration like speedpro has, LESS blowdown time is in order!

VERY VERY roughly, 45 degrees of blowdown time would be suitable for a smaller capacity engine (50cc) with an exhaust port time area of, say, "more than 200 deg"

Simply (as some of use mentioned already) there is really no hard and fast rule for blowdown times.

Factors to consider are

Exhaust port width
Capacity
Trapped compression ratio
Rev range
Exhaust design
Cylinder head design
crankcase volume
cylinder compression
Ignition mapping
expansion chamber design
available gear ratios
Rod length
Bore stroke ratio

Correct blowdown times for a particular engine only need to vary from 2 degrees to be innefective.

don't forget, most 125 GP bikes only run 30 deg of blowdown...........

You know f5dave, I see the reason for not "throwing around port timings"....it really is a can of worms!

**bucketracer** · 26th August 2009, 07:37

Originally Posted by SS90

I have to disagree with you there Bucketracer.........

1/4 of the exhaust port time area?

No way.

but to say that "THE BLOW DOWN TIME SUITABLE FOR ALL ENGINES IS EQUAL TO 1/4 OF THE EXHAUST PORT TIME AREA" is very very incorrect.

I didn't say that and for your info, a rule of thumb is just that a good place to start and as you should know not something to slavishly follow also exhaust port area is not time area either.

Originally Posted by SS90

for the times that speedpro currently has, I recon something closer to 25 deg would be in order

For an Exhaust opening 86.5deg ATDC

Try

180-86.5 = 93.5...........The exhaust port is opening for 93.5deg

1/4 of 93.5 is 23.4deg.

pretty close to 25 isint it?

So just like my source said, rule of thumb 1/4 of exhaust port area gets you into the ball park.

But I can see I wasn't very clear before about the degrees bit and how to work it out.

And its only a handy rule of thumb. But from someone who knows what they are on about.

I am more interested in your thoughts on how to refine the blowdown period required.

By all means, please continue with your experiences with blow down times!

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**SS90** · 26th August 2009, 08:04

Originally Posted by bucketracer

For your info, a rule of thumb is just that a good place to start but not something to slavishly follow and exhaust port area is not time area either.

Try

180-86.5 = 93.5...........The exhaust port is opening for 93.5deg

1/4 of 93.5 is 23.4deg.

See rule of thumb 1/4 of exhaust port area.

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Let's not continue down an all too familure path here Bucketracer.

I am quite aware I suggested around 25 Degrees but I don't use any 1/4 rule.

so now, you are saying... the 1/4 rule applies to THE EXHAUST PORT OPENING TIME?

That's different...........

It is a "ball park" figure, but as I say, there are some constants to take into account, and one or 2 degrees is really important (on some high spec engines)

I admit I did make a BIG assumption that you meant "amount of degrees the exhaust is open for" (DURATION) when you said " exhaust port area" (I have heard many make that statement, and simply assumed you meant the same)

I think Teezee wrote a few things a while back about time areas..... that's different again.

I shouldn't have assumed!

Exhaust port time area is neither Duration, NOR "opening time" (but you do need to know both these to calculate the port time areas)

This "rule of thumb" cannot relate to "exhaust port time area"

You need to calculate "exhaust port time areas" into sec-cm2/cm3

There is plenty of information available in books or on the web (gulp), but like I say, I am pretty certain Teezee covered it (a little)

What do you mean by exhaust port area? from the figure you are using, like I say, do you mean exhaust port OPENING time?

**bucketracer** · 26th August 2009, 08:14

Yes I wasn't very clear about how it was worked out.

and yes I am interested in how to dial in that last all important degree or two.

.

**SS90** · 26th August 2009, 08:32

The best way to find out what will suit an engine (unfortunatly) is "trial and error"

Using MOTA etc is great, but it really does take someone with years of experience to get it all right.

I really want to see how the original "Wobbly" engine goes...... my senses tell me (also reading the short durations Speedpro posted) that it will be a very torquey rideable bike with a wide long spread of power.

Because so many people don't realise that exactly that is what a two stroke is good at doing!

Like Teezee is doing, get hold of as many cylinders as you can, and measure everything.

I always look at "proven" cylinders I.E "ones that work well", opposed to, for example, on old TT2 Yamaha (which, sadly was a lemon from the factory, and required quite some work to be rideable), as it just confuses!

When it comes to getting a data base, try to look at two cylinders that are VERY similar (RS125 compared to CR125) both are similar in design, but the CR has power valves. Look at all the timings (incl blow down) and try and come up with a pattern as to why they are so much the same, but each slightly different.

One thing that helped me (other than my good looks and charm) was to look at a Kawaski "KIPS" system, I didn't want to use a power valve (complication) but I wanted to see what blowdown times where the best compromise compared to another water cooled 125cc engine with no power valves.

Using your method, you may well it works exactly the same as before, but another guy with the same exhaust etc just seems to have that little bit more every gear change, because he was able to get the whole engine working in unison....

Something to remember Bucketracer,

what "blowdown" allows is, time for the pressure in the cylinder to reduce down (blow-down) to BELOW the pressure in the crankcase (positive pressure will always try to flow to negative pressure to equalise) so the charge can flow out the transfer ports during the scavenging time.

How much time is needed to do this (at the intended RPM range) varies dramatically with different factors (exhaust crankcase volume etc)

The time available isn't much, so it really makes a difference to get it right for your engine.

It is another reason why the "top hand made" cylinders of GP teams are so damned expensive. They are VERY accurate!

**bucketracer** · 26th August 2009, 09:49

Originally Posted by SS90

What do you mean by exhaust port area? from the figure you are using, like I say, do you mean exhaust port OPENING time?

Handy rule of thumb, blow down in degrees = the number of degrees required to uncover a 1/4 of the exhaust port area.

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**SS90** · 26th August 2009, 10:06

Originally Posted by bucketracer

Handy rule of thumb, blow down in degrees = the number of degrees required to uncover a 1/4 of the exhaust port area.

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Mate, I think where your getting confused is "EXHAUST PORT AREA"

You need to find the correct terminolgy for what, in your example equates to 86.5 Deg. (degrees is a measure of time, but not area)

It's not "exhaust port area" it's an exponent of the total "exhaust duration" (which in this example is 187 degrees)

Like I said, exhaust port area is different!

**bucketracer** · 26th August 2009, 11:07

Originally Posted by SS90

You need to find the correct terminolgy (degrees is a measure of time, but not area)

Degrees are actually an angular measurement, in this case of crankshaft rotation.

The handy rule of thumb again. blow down in degrees = the number of degrees required to uncover a 1/4 of the exhaust port area.

That is the number of degrees of crankshaft rotation for the piston to uncover 1/4 of the exhaust port (area).

Because I was talking about degrees I used the term exhaust port area to differentiate from exhaust port height so the astute could quickly see that the shape and size of the exhaust port would influence the number of degrees of crankshaft rotation required for effective blowdown.

A tall narrow port would require more degrees of crank rotation than a wide bridged port with side windows for the piston to uncover a 1/4 of the total area of exhaust port window.

The handy rule was never trying to say anything about "Time Area" that is something else.

Is any body else finding this hard to understand?

**F5 Dave** · 26th August 2009, 13:17

Have to say I was quite surprised at my old RS how pedestrian the port timings were. They were more akin to what I would have expected in their CR. Of course I suspect that was largely the plan.

As with the MT that superceeded it the RS was simply a CR with the powervalves removed, a large carb & a clever pipe that made the most spread available so it made an excellent customer race bike. A wide spread is very useful for high top speed on long tracks with any old gearing & some modicum of reliability when the customer didn't keep to the maintenance schedule stipulated. Good for early 30hps.

Oddly the P.V. 95s didn't immediately capitalise on their change in architecture, but again they offered enough improvement having lost the archaic cantilever suspension & gone to usd that they didn't need to go crazy with the new engine. yet.

Sorry are we off the topic far enough yet?