ESE's works engine tuner

[Haufen]

hello guys,
do anyone has a hint on where to find useful discussions on piston ported high-output engines in this forum?
the search button is a little bit of a nightmare for me...

i have some designs in the works and i'd love to know more about that

thx

Don't know about discussions, but Blair's recommendations worked well for me. In the end, the guideline for a piston ported high output engine would look something like this:

• effective area at the bore 1.05-1.15 (from memory) times the carburetor area
• inlet as straight (in all directions) and as short as possible
• start with low inlet timing (e.g. 80/80 on a high performance engine)

-> get the piston with the longest and widest skirt on the inlet side you can get, which will allow you to have a bridged inlet port which makes things (increasing area whilst keeping timing down and increasing unobstructed (by the piston) opening timing) much easier.
-> having a boost port is overrated and piston ported engines are much less noisy than reed engines.
-> if inlet length and inlet timing are off (e.g. too long in both cases), this can mess up your carburation and increase off powerband blowback in a huge manner, also killing all the power before the pipe comes on. (80/80 and carb directly at the cylinder was a quite safe starting point for me)
-> expect to have to run very small main jet sizes compared to reed engines, as there will be no blowback when everything is in tune.

[Frits Overmars]

...having a boost port is overrated and piston ported engines are much less noisy than reed engines.
-> if inlet length and inlet timing are off (e.g. too long in both cases), this can mess up your carburation and increase off powerband blowback in a huge manner, also killing all the power before the pipe comes on. (80/80 and carb directly at the cylinder was a quite safe starting point for me)
-> expect to have to run very small main jet sizes compared to reed engines, as there will be no blowback when everything is in tune.

Haufen, I agree with most of the above, but why would an engine that does not suffer from blow-back, require a smaller main jet?

[wobbly]

Very generalized idea's I dont agree with - a TZ250 that has a 20mm phenolic spacer between the 34mm carb and the cylinder will not run AT ALL without the spacer, impossible to tune.
A TZ 350G - 6 port with the 38mm long body carb, makes around + 2 - 4 Hp everywhere if you cut the bellmouth extension off and add a 20mm carb spacer, comes on earlier, revs on harder, and easier to tune.
Means you can cut 1.5mm off the piston inlet timing edge, that would normally overkill the mid power.

If you then cut the small cylinder extension off on an angle such that there is no step at the top, to lift the carb angle, and port the inlet like a G model TZ250, where the port roof at the bore is just above the ring, with
a tongue hanging down to support the ring gap, this adds another couple of Hp up top.

An engine like this won dozens of pre 82 Junior races with a 70 year old pedalling it.

[Vannik]

-> having a boost port is overrated and piston ported engines are much less noisy than reed engines.

What noise are you talking about? Inlet, mechanical or exhaust?

-> if inlet length and inlet timing are off (e.g. too long in both cases), this can mess up your carburation and increase off powerband blowback in a huge manner, also killing all the power before the pipe comes on. (80/80 and carb directly at the cylinder was a quite safe starting point for me)

This comment combined with Wobbly's on the inlet length: Inlet tuning on a piston port inlet is critical and it is very sensitive, it is a good idea to experiment with the length by using a spacer between the cylinder and the carburetor, not after the carb mouth!

The inlet should end at the end of the bell mouth, the extra piece that Wobbly cut off screws up the pulse shape as it is partially reflected by the bell mouth and partially reflected by the end of the extension. This smears the wave shape and lowers its amplitude.

If you have to run an airbox, either have the bell mouth end in the box or use a big connecting tube between the bell mouth and the box, at least twice the outer diameter of the bell mouth.

[Haufen]

To frame the post above, here's what I think I understood Gradella is planning and how I derived my recommendations:

I assume he is building a piston ported engine from scatch. Without further details, the recommendations turn out pretty general, of course. But what I think he was after and what my intention was, was to provide him with a robust baseline that works right from the start and from where he can then start optimising. I imagine having to fix a system you have no experience with, first, can be quite frustrating.

I once converted a cylinder reed engine to piston ported. For this, I used the dyno calibrated EngMod model of the reed engine (large vforce reed) and converted it to piston ported in the sim (Gradella, if you don't already use EngMod, this would be a good time to start. And if you already use it, you could post the file or send it to me and / or some of the members if you would like feedback). This is basically where the recommendations above were derived and which worked perfectly in reality.

To get the engine to rev on the dyno, however, I first had to deal with some real world problems, the sim did not tell me (at least not directly). The jetting.

Haufen, I agree with most of the above, but why would an engine that does not suffer from blow-back, require a smaller main jet?

I started out with the carburetion from the reed engine, which was a 155 main on a 38mm carb. Only to find that the engine did not rev past 8k or so. I was just about to check and change the ignition before I decided to try an even smaller main jet, first. And step by step I moved to smaller and smaller jets and found the new ideal jet size to be 132, on the same 38mm carb that is.
I did not analyse the reason for the small main jet to the very detail, but the reed engine had some visible blowback, whereas on the piston ported there was nothing at all (with this inlet length). Also, EngMod gave very good BSFC numbers for the piston ported engine, below 400 g/kWh if i remember correctly. What I was most interested in at that time was: "is this a safe jet size to run?" and both observations (end EGT numbers) confirmed me that it was.

Any reasons and explanations as to where the root cause for the small main jet is from the members here are welcome. What I wanted to tell Gradella with this was: once you've got it running, don't be afraid to lean it down, especially if you happen to have experience jetting a comparable engine with a reed valve.

Very generalized idea's I dont agree with - a TZ250 that has a 20mm phenolic spacer between the 34mm carb and the cylinder will not run AT ALL without the spacer, impossible to tune.
A TZ 350G - 6 port with the 38mm long body carb, makes around + 2 - 4 Hp everywhere if you cut the bellmouth extension off and add a 20mm carb spacer, comes on earlier, revs on harder, and easier to tune.
Means you can cut 1.5mm off the piston inlet timing edge, that would normally overkill the mid power.

If you then cut the small cylinder extension off on an angle such that there is no step at the top, to lift the carb angle, and port the inlet like a G model TZ250, where the port roof at the bore is just above the ring, with
a tongue hanging down to support the ring gap, this adds another couple of Hp up top.

An engine like this won dozens of pre 82 Junior races with a 70 year old pedalling it.

Do you happen to have pictures of the modifications and / or the engines? It's hard for me to put the words into a bigger context without knowing the engines or inlet shapes etc. On this TZ the carbs seem to be placed pretty close to the bore, which looks like a good starting point to me. https://motorrad-fuchs.com/en/single...,TZ_250_C.html . With "short and straight" I wanted to provide a baseline in the recommendations, it's easier to add a couple of mm in most cases. If you need 2x 90deg bends and the minimum length is 15cm before the carb can even start then this is not a partucilarly great prerequisite to start with.

In one of your other examples you made the port bigger and tilted the carb also reducing effective intake length all at the same time if I understood correctly. Were the changes made one after another (not always possible, of course)? Maybe the size and the length were positive and the tilting negative, power wise?

What would be your recommendations for a piston port intake if starting from a blank sheet of paper?

What noise are you talking about? Inlet, mechanical or exhaust?

Compared to the reed setup, it felt like inlet noise was completely gone and the engine was a lot quieter. I even had people come over and ask me what I had done to make the (race only) engine so quiet on several occasions (often followed by kneeling down, nose and eye right up to the carb bellmouth, opening the throttle and then saying things like "I can see your rod, amazing").

[Frits Overmars]

expect to have to run very small main jet sizes compared to reed engines, as there will be no blowback when everything is in tune......
the reed engine had some visible blowback, whereas on the piston ported there was nothing at all

If blow-back occurs, mixture that has already picked up fuel during its first passing through the carb, gets a second helping on its way back out, and a third helping when it is finally sucked in. By then it carries about three times the amount of fuel it was suppose to receive. So engines displaying blow-back get more fuel from a given main jet than engines with no blow-back. That is why your remark made me wonder.
I also wonder why the reed engine displayed visible blow-back. Tired reeds maybe?
One reason that piston port induction can live with small main jets is the stronger suction pulse through the carb, compared with a reed inlet system.

Compared to the reed setup, it felt like inlet noise was completely gone and the engine was a lot quieter. I even had people kneeling down, opening the throttle and then saying things like "I can see your rod, amazing").

Yup, I noticed that too. Piston induction and rotary disk engines produce a more pleasant intake sound than reed engines.
"I can see your rod" brings back memories of looking into the rotary inlet of an engine on full song on a friend's dyno, seeing a red-hot con rod and casually asking about the fuel-oil ratio

[wobbly]

All I was saying is that recommending the " shortest intake possible" is absolutely not realistic to actually run in many cases - that TZ250 picture is of an early model with limited port duration, the later engines had alot more
timing, and needed the factory added spacer to get the intake length matched to the port - remove that and it becomes impossible to tune correctly.

The TZ350 had the 5mm cylinder casting extension machined at an angle, such that nothing was cut off the bottom edge, so maybe 2.5mm of intake length was lost, but the duct was alot straighter - adding power.
Then a 20mm spacer was added, after the bell extension was removed - adding power.
Then the intake timing was increased, as the mid power with the longer tuned length was dramatically increased and the combination of the two worked perfectly - adding power.

All of this conjecture simply goes away, if you sim the thing in EngMod to find the perfect combination of timing duration, and inlet tuned length, as I did to see what was possible with the old 1980 - 350 engine.
The short carb and longer tuned length was super obviously superior - confirmed again when shorter/bigger Lectrons were added with a longer spacer again - adding power.
The Lectron's were tipped up to almost dead straight in relation to the duct axis - not possible with the Mikini carbs as under brakes the fuel level at the front of the bowl rises
and fuel spews thru the pilot jet on closed throttle.

The 15* bend to the left on a CR125 inlet is a 2 HP loss in 40 HP for a SKUSA engine over a dead straight RS125 manifold.

EDIT - Frits, the reason reeds can have " blowback " or more accurately " standoff " is due to the wave bouncing up and down the inlet tract off atmosphere is alternately returned as a + or - wave, depending upon if the reeds are open or closed.

[F5 Dave]

I remember being able to see the rod on my MB reed engine.

It had just thrown me to the ground. Roughly.(for the python fans).
Aftermarket rod was indeed visible, or some of it as it cleft the cases in twain.a bit.

I'd been running an RD piston and Mike of this parish exclaimed that it was likely the Yamaha parts trying to get out.

[TZ350]

"I can see your rod" brings back memories of looking into the rotary inlet of an engine on full song on a friend's dyno, seeing a red-hot con rod and casually asking about the fuel-oil ratio

We use 20:1 oil ratio. This rod has not failed but I noticed our cranks getting really hot on the dyno. They were hot when we were pulling 12,000rpm now we are pulling near 15,000 rpm they get really really hot.

How do the 20+ RWHP Dutch guys get their cranks to survive at 15-18,000 rpm?

I suspect that crankshaft flywheel heating is the major source of power fade we observe after a generous number of dyno pulls.

[wobbly]

TeeZee, what oil are you using.

[Frits Overmars]

We use 20:1 oil ratio. This rod has not failed but I noticed our cranks getting really hot on the dyno. They were hot when we were pulling 12,000rpm now we are pulling near 15,000 rpm they get really really hot.
How do the 20+ RWHP Dutch guys get their cranks to survive at 15-18,000 rpm?
I suspect that crankshaft flywheel heating is the major source of power fade we observe after a generous number of dyno pulls.

My casually asking about the fuel-oil ratio was a tongue-in-cheek way of saying that there was no oil at all in that fuel. I advise 1:20, same as you.
The 20+ RWHP Dutch guys rev between 15.000 and 16.000 rpm. The front runners have about 24 RWHP. I do not know of anyone revving to 18.000 rpm, let alone survive at those revs.
You're right about the crankshaft heating. That is also one of the reasons for me not to use direct fuel injection yet.
Dry air is not very effective at cooling the crankshaft.