Its all about the intake side

[Vintageman]

http://www.dragonfly75.com/motorbike/cr ... volume.pdf" onclick="window.open(this.href);return false;

Hello,

I find this old article very interesting and should be accurate for our 2 Stroke vintage Suzuki.

It was/is a difficult read in that it tries to cover multiple points (see conclusion). I am sure I can/need to read it several more times . There is a lot there.

What I like is it gives empirical data for changes in parameters of choice. You don’t see many book s like this (Some in two stroke tuning by Graham Bell for example. Anyone else?)

The intake side is very important: Delivery Charge! Experiments vary length from Carb to cylinder and even impact or air box, port timing, crank volume, and pipe. Crazy amount of stuff (almost lacks focus… too many drugs late 60s I suppose)

One point I get to achieve the higher RPMS a short restrictive intake track is needed ( bottom end torque is lost), This to me explains some changes you see in early versus late T500s and even T250 versus GT250. I think even the early GT750 has a better intake air box even though porting milder

Enjoy

Intersted to see what you think

[ja-moo]

Technical papers can be a bit confusing without the rest of the engineering background. Luckily, even on stock engines, the factory has done a lot of the work for you. And in the present time, there are a lot of computer programs out that take all this information and translates into a (mostly) user friendly system where you just plug in your inputs.

The biggest thing to remember, is an engine is just an air pump, the more efficient the pump, the more power output. Also, you will be moving the power around the rpm range, so more power up high, the less you have down low. The better designs will minimize the lower rpm losses, but you still have to look at the rideability issue.

If you figure out what your hp output level you want, the guys here can probably give you what needs to be done to attain it.

[tz375]

I love those old SAE papers from back in the days before computers.

Their findings were valid, but predated any of the work done by Jennings or Prof. Gordon Blair at QUB. The other thing to keep in mind is that the research for that paper was on a small capacity simple single cylinder motor running at low revs. It was also before more was understood about the importance of the exhaust pipe which has more effect than almost anything else.

Back then they were in a HP chase that led them up some blind alleys, but they did get some great results. For example they had discovered that raising primary compression raised power and they just kept stuffing crankcases until power was really high but with a razor thin powerband.

Now people use pipe wave theory to design ports and pipes that work much better with lower primary compression and make more power over a wider range. A good pipe will effectively suck the gas out of the crankcases and doesn't need much compression to do that.

There's a lot to learn though, and old papers make great reading and provide insight into the work that was being undertaken back in the day.

On the early GT750s what made them so torquey wasn't the intake system per se, but the shorter intake duration. If you want a great GT750 on the street always use an early cylinder and port the exhaust and leave the intakes. Or use a spacer and choose the intake timing you want.

[Vintageman]

I know the exhaust is tuned most and mainly via pipe can pull and push back: what a great thing and one reason I like 2 strokes. That is fact,

But an intake is needed too. That’one thing this paper is about.
I have never really seen data like that for the intake on piston ported. It matters big time.

I just finished boring carbs on a GT185 with chambers. They are now about 21.5 mm stock was about 19.5mm I followed this through the flange and into intake (only short distance needed 1 inch or so). I have those Jemco kit race chambers.
To my surprise, I picked up another honest 1500 RPM (maybe more, backed off when I saw +10000 RPM!).

I think this doc shows you can’t get upper RPM if your intake path won’t let you, regardless what you do or what you do elsewhere just won't works as good as it could. The path needs to be short and no restriction to achieve high RPMs

The loss of low end torque on piston ported must be lack of momentum versus when you have a long intake path for blow back is suppressed by momentum of mass flow. That is my thinking. Not sure if the same issue with reeds or rotary.

What I mean about the early GT 750 is I hear the very first one even though it has less intake and exhaust timing versus versu slater GT750 it still had 70 HP and at 7000 RPM (Higher peak than later 6500). If true, one and maybe only reason may have been intake design.

Same deal you see with say 70/71 T350 versus 72 T350, T250 versus GT250 first gen, early versus late t500 (not that this is the only thing) , but they have messed with intake side air box, length and even how far the slides go up.

cheers

[ja-moo]

You can stay "stuck" on short intake lengths if you like. But that is incorrect thinking. There are multiple reversion waves in an 2 stroke intake track. And thus can be tuned at different lengths.

The reasoning for a torque loss at lower rpms with increased intake timing, is due to longer and more powerful reversion in the intake system. With the later closing of the intake port by the piston. The "intake" charge becomes extremely rich, as the mixture actually flow past the needle jet 3 times picking up fuel each time. This greatly decreases power with the rich mixture. This also raises where the powerband kicks in as it takes a higher rpm to reduce the effects of the added reversion in the intake tract.

On the very high HP Kawi 750's, old school think was to machine the intake port short so bigger carbs would be raised to fit on the motor, and straighten the port. (as the original intake port angled down to level the carbs.) More HP was found when adding a wedge, lengthening the intake port and straightening the intake that way.

And I'm not sure to the strength of the 185 motor, but with a 49mm stroke, a very easy 4000ft/min stress factor is just under 12,500 rpm. !0,000 is really low on such a short stroke motor.

the carbs seem rather small, but that might be necessary with a piston port. On my "forever project" Kawi 250 triple roadracer, I am looking at 30 to 32 mm carbs.

[tz375]

I just finished boring carbs on a GT185 with chambers. They are now about 21.5 mm stock was about 19.5mm I followed this through the flange and into intake (only short distance needed 1 inch or so). I have those Jemco kit race chambers.
To my surprise, I picked up another honest 1500 RPM (maybe more, backed off when I saw +10000 RPM!).

What I mean about the early GT 750 is I hear the very first one even though it has less intake and exhaust timing versus versu slater GT750 it still had 70 HP and at 7000 RPM (Higher peak than later 6500). If true, one and maybe only reason may have been intake design.

Not exactly.

Your slightly bigger carbs allowed the bike to breathe better, for sure. The issue there is the size of the carb relative to revs and power (amount of air consumed) For low revs and small power it only needs to flow a small amount of air. Raise the revs and the carb needs to be bigger. Of course that only applies up to a certain point and above that you lose everything.

On a 750 for example, stock 32mm is enough carb. Start to tune it and at a certain point a 34mm is beneficial. But big carbs lose power at lower revs, so it's important to chose a carb that matches the performance. 34mm on a 750 is better than say 36 or larger up to about 120HP or so. Above that on a drag race motor that never sees low revs, huge becomes good.

On the 750, early bikes supposedly made 67hp and later ones supposedly made 70. Later ones peaked at higher revs. In reality, early 750s tested out at around 48hp power at slightly lower revs than later ones which made around 51 RWHP. Later bikes had longer intake timing, longer exhaust timing and slightly higher compression. And they changed the exhaust system too.

We modeled all the port dimensions of both and it was clear that the change in exhaust added more top end with little loss of low end. The change in intake timing however, lost a lot more bottom end for almost no top end gain. The biggest issue with longer duration intakes is that the port is open as the piston descends and at low revs that pushes a lot of the gas back out the port to create the reversion that Ja-Moo referred to. The longer the intake timing, the worse that reversion is.

At peak revs, things are different. That's where the ram effect comes into play but only at high revs and only where the exhaust is pulling in mixture. So on a mild motor, short timing is good and longer timing just costs tractability. On a high HP motor, intake width brings in more top end without losing too much bottom end.

40 or 50 years ago that was all just theory and deduction, but today the body of knowledge is much larger and we have access to great software tools such as Bimotion, MOTA and EngMod2T all of which were based on the work of Dr. Gordon Blair and his team at QUB.

[Vintageman]

Ja Moo and Tz375.

Good stuff.

When I have time I need to read thses 2 replies agin to get full understaning

First what I want to say about software simulation tools, as an engineer, software runs algorithms or models that need to have been developed and proven by theoretical data. Once you know the principles, you appreciate the speed and flexibility software tools gives, but without core theory, limits of models, you can get in trouble (where I am) I am trying to understand the concepts to better enjoy this two stroke hobby.

If I may set a focus, don’t change the intake timing just intake length which this paper talks about as one test. When they did just change length shorter made the Higher RPM/ Power

Ja-Moo,

Reversion waves, very good point. Shorter lengths, air box end through cab into piston skirt will determine the wave reflections period, the shorter, more off them and less loss each time.
I think blow back too has an effect once momentum is established (read somewhere), You may be correct that is at higher RPM.

Is this not what a Boost Bottle helps fix? It pulls from the once mixed air gas stored in bottle instead of multiple passes through carbs

Here is what I don’t understand (have heard a few times now) and find interesting,

How can (or can) you take advantage of this multiple oscillation? By tuning the intake?
What is the concept here?

For whatever reason, here is why I still think intake matters too and don't here to much about. Before I changed the GT185 Carb size, I took a set of pistons and tapered the exhaust side to advance timing. I also raised compression using a shaved head and copper gasket. Now my data is lost on specifics so what I saw is of less value, but change made no differences top end that was significant and was much worse lower half of RPM.

Tz you pointed out before that one would have to model all ports to understand which is weakest and in this case it may be the intake or simply carb was too small.

Do tell how to tune intake theory

Thanks

[tz375]

Absolutely correct that without a strong understanding of all the underlying principles it is hard to work out what might be important and what is not.

The more I read and research the more it I realize how little I really know. We are all learning as we go on.

One thing I have been able to do is to model an engine and then tweak one variable over a range to see the effect of that change in isolation. Of course I should probably have changed other parameters as well, but it's enough to indicate the probable direction in which to explore changes.

In the case of the GT185, I would hypothesize that the intake was too small in bore and restricted it from revving and making more power. Presumably the designer wanted that smaller carb to enhance the bottom end for town riding by people with limited experience.

A 125cc single two stroke can usually use 36-38mm of carb size when it's a fully developed race motor, so 16mm on a small twin is just a little bit smaller and is indicative of the design intent. Plus ports and pipes were not that well understood back then.

A quick rule of thumb is that the carb area should be slightly larger than the intake port area to promote a nozzle effect that causes gas velocity to rise to promote an efficient discharge. Too much and it becomes a problem and too little raises gas velocity too high in the carb. You want a mean gas velocity of around 0.55 Mach or less at peak revs. IIRC

[ja-moo]

Is this not what a Boost Bottle helps fix? It pulls from the once mixed air gas stored in bottle instead of multiple passes through carbs

Here is what I don’t understand (have heard a few times now) and find interesting,

How can (or can) you take advantage of this multiple oscillation? By tuning the intake?
What is the concept here?



Thanks

A boost bottle works with a reed valve motor as the section in front of the reeds is sealed off from the carbs.

The pressure will take the path of least resistance, so a boost bottle really doesn't work with a piston port.

And you would have to dyno tune your specific engine with different length intake systems to put the power where you want it.

[pearljam724]

I think another thing that is so often over looked in reference to the early model 750. They were also geared significantly lower. Within a few years, customers back then were crying more top speed. For the most part, Suzuki simply geared the later models higher. This also happens quite often today in reference to any make or model. Depending on feedback, safety issues. Or a simple claim to have supposedly improved torque on a recent model. I also think manufacturers do this on purpose to supposedly increase a performance aspect within a 2-4 year model run. To use at their disposal, aside of just changing the paint color only. Within the same model run.