Porting torque vs hp

[Stump Shot]

Porting a 2-stroke is very much like changing a cam... The porting layout in a 2-stroke basically does the same thing a cam does in a 4-stroke, only with more voodoo! LOL!

Too much voodoo for this simple country mechanic. My only interest is to be able to tell if I have a dud or a stud before I rebuild a power saw. When I need port work done I will leave it to one of the experts here. Sometimes you just need a professional to get the job done right. I do have a project coming up, when all the pieces finally come together, that is exactly what I'm going to do.

 

[jmssaws]

I and I'm sure others port mostly by eye,I make a line for height and the ring ends and that's about it, if I have enough skirt to get 70% I'll mark that but usually not any other time.

You learn what it should look like.

 

[Iron.and.bark]

Thanks to sawfun more mentioning application. Which is essentially the horse I have been flogging to get to the finishing line

Take cakes for example.

HP & rpm biased saw is the absolute champ when cutting sponge cake, even a 60" spong cake.

A torque biased saw is the champ when it's a 60" rock cake baked by a demented lady using cement instead of flour.

What my saws have to deal with is positively demented, and what would be pleasant is an acknowledged that not everything is sponge cake

Now as for porting/modding for torque. I learned from an eccentric old coot who was a high level bike race mechanic. Truth be told I have forgotten a lot of the terms. Anyone ever given a port a moustache? Also never seen it mentioned apart from when I did in the 2100 thread any mention of the exhaust port shape being designed to work with less blowdown.

As my limited and inferior unstanding of things go. For torque maximum.

Squish (compression), muffler, ignition advance, exhaust port widened, intake widened.

Transfers left stock (polished if possible), intake & exhaust not raised or lowered.

Basically it's a heck of a lot easier than the jobs I see you fella's do with consummate skill on here. It's something a ham fisted Neanderthal like myself can accomplish.

Though on the transfer side, have been thinking about how the old Villiers 2 strokes (very torque biased) were designed. Danarm used them on their saws, have one scored at the tip.

These have an overly large lower transfer and extremely tight uppers. This is also something exhibited in the 144,152 dolmars. Slow beasts with max rpm, but my 144 holds it's rpm very well with a 44" 8 tooth sprocket and .404 under sponge cake free conditions

 

[MustangMike]

I believe a 661 also revs quite nicely, and there are many other factors (other than just bore & stroke) that determine an engine's personality.

For example, just putting a larger carb on a car may give it more maximum HP, but may also ruin mid range torque, engine response, etc.

Port shape, size, coarseness, etc all play a roll, and in a car, a cam can turn Jeckyll into Hyde.

And while multi valve engines on bikes and cars have resulted in dramatic performance/emission improvements, I am in awe of what GM engineers did to achieve similar results with traditional design 2 valve motor by changing the ports size & shape (& length), altering the combustion chamber design, and getting a near perfect air/fuel mix to improve both power & emissions.

Several of the builders on this site do incredible work, and share very helpful & valuable information, but I don't think there is one of them that will tell you they are still not sometimes surprised by what does or does not sometimes work.

 

[mdavlee]

Aha! So there is more than just convenience of easy upgrade by putting a 7900 topend on a 6400 case? Just a stab in the dark here, but that kind of leads me to think that the 6400 came first and the 7900 is a factory "big bore"?

Solo and Dolmar designed them together. They made them all off the same case for convenience I believe and it was built with the 7900 in mind. Solo decided to one up dolmar with the 1mm of extra stroke and a different jug on it. Look at a 365/372/Xpw. I think the 65cc has the widest powerband. Could be the case to cc ratio is about perfect.

 

[jmssaws]

Solo and Dolmar designed them together. They made them all off the same case for convenience I believe and it was built with the 7900 in mind. Solo decided to one up dolmar with the 1mm of extra stroke and a different jug on it. Look at a 365/372/Xpw. I think the 65cc has the widest powerband. Could be the case to cc ratio is about perfect.

Like a 85cc 064,the 92cc cylinder works great on it but the 85cc one is damn close and unless you have a good 066 cylinder it can be hard to beat the smaller one.

 

[CR500]

Like a 85cc 064,the 92cc cylinder works great on it but the 85cc one is damn close and unless you have a good 066 cylinder it can be hard to beat the smaller one.

Just what I like to hear lol... good reinsurance I made by not hybriding the064

Also did not want go make it where the 661 becomes a back up. I really dig the 661 J.

Gotta check the plug and make sure she is not lean with the MaxFlow.... should be fine I imagine?

I swear this made a difference in her besides filtration.

Sent from my non internal combustion device.

 

[smokey7]

Nope. Everyone that has big bored a dirt bike will tell you it gives you more bottom end with little to nothing on top.

It will give u power up top if you gear for it. My yz 250 -310 bike was a violent bike jetted sharp properly adjusted and trimmed powervalves, and a gold series fatty with the powercore silencer. It pull stock gearing no problem with a violent powerband. Then came the big bore...... powerband not as violent but bike was much faster the power delivery was so much more well mannered that it would hook up day and hands down walk away from others built like my 250 geared stock. In the sand the 250 with a paddle had it over the 310 all day but gear that 310 and it was too close to call which was faster. I still liked the 250 better all around rippin with buddies cuz i loved thst hit it gave me all the time especially if i got hooked up.
If i really had to chose a bike tho for all around well mannered setup with lots of room to play with a 310 big bore set up well is a sick mfer. Back to porting tho.... i wish we could play with jets and springs like sled, bikes and skis. Its stupid the power you gain or leave behind when ulyou jet for *s-word.

 

[Terry Syd]

Good thread, thought I'd throw my 2 cents in. A bigger flywheel on an engine does not give it more torque, it gives it more INERTIA. The rotating parts store energy and that energy can be used, but once used, that's it.

Big bore kits generally give more power down low and not much on top end - unless you match the rest of the components to the new displacement. If the old components would only flow a certain amount, the bigger displacement makes that maximum flow occur at a lower rpm.

Ignition timing on saws is a fixed advance. Advance the timing and you can get more HP up on top end at the expense of some torque down low. Tuning the ignition timing for a cookie cutter can kill some of the grunt of the engine. - Horses for courses, what do you want the saw to do.

Proper carburetion is critical for a broad powerband. I like to keep pulling the revs down to see when the engine bogs, then try a bit more low speed circuit flow, retune and try again. When the saw stops bogging and the clutch starts slipping, I've reached my goal. - Top end HP doesn't change, the engine is still moving the same amount of mixture at max power, but the powerband is a lot broader.

Porting - I remember a test done on DT-250 cylinders where all cylinders had the same time/area for all the ports. The only things that changed were the timings, area and angles - there was a 15
% difference between the best and worst cylinder. That 15% was pretty much over the entire powerband - it demonstrated a good loop-scavenging flow in the cylinder.

 

[concretegrazer]

Solo and Dolmar designed them together. They made them all off the same case for convenience I believe and it was built with the 7900 in mind. Solo decided to one up dolmar with the 1mm of extra stroke and a different jug on it. Look at a 365/372/Xpw. I think the 65cc has the widest powerband. Could be the case to cc ratio is about perfect.

How does the 7300 stack up with them?

 

[mdavlee]

How does the 7300 stack up with them?

It's usually real hard to tell it and a 7900 apart without a stop watch when it's 20-24" bars

 

[Locust Cutter]

This all makes me want to find a '70 Kawi 50cc triple and go let it rip! Three cylinders of air cooled 2-stroke goodness!!!