More Porting Stuff

[Vintageman]

Check out the site if you have not

http://edj.net/2stroke/jennings/" onclick="window.open(this.href);return false;

Go to bottom and see the Articles. They are all interesting but
see "some development" specifically

I like figure 4 and Figure 8 which I believe are both piston ported bikes like our beloved Suzukis.

It makes sense to me why the later GT750 changed both Intake and Exhaust port timing to get more perfromance, but suffered bottom end versus early GT750.

It looks like you can get a lot out of a little changing intake port timing. Similar to exhaust.

Not too much is said here and one is generally directed to exhuast

Maybe exhaust is popular for it can coordinate with Pipe too.

First Page under intake talks about "owing to the restriction of the layout for the whole vehicle ... the bullets 1 and 2 Makes you think of what happened to early versus late T500 and Kawasaki s3 versus KH400 fro examples)

Enjoy

[oldjapanesebikes]

It makes sense to me why the later GT750 changed both Intake and Exhaust port timing to get more perfromance, but suffered bottom end versus early GT750.

They also changed the pipes eh ? Have you read Suzuki bulletin GT-7 ?

[tz375]

Jenning was certainly on to something and fortunately there is now a large pool of knowledge that goes way past where Jennings left off.

The way to look at an engine is as a complete system. Changing one aspect only improves things dramatically if that was what was out of balance. For example and engine with all the ports designed for high rpm saddled with one port that's out of step, will make little difference until that one thing is addressed.

On an engine like a GT750, the motor is in a low state of tune, but not horribly out of balance. It has enough intake area to support a much higher output, so increasing the port timing doesn't help much but it does change the way that the motor makes power. The exhaust on the other hand is a major obstruction and relatively small changes can have a significant effect across a wider range. Changing inlet timing on a 750 rocks the power curve around a 5500-6000 pivot point. More duration makes a small improvement above 6k and a big hole below 5500. And people make the ports taller and even worse than stock because they don't know that.

Look at the 500 and things are not necessarily the same. On that motor some other port may be the hold up. And just because a change to say exhaust port height makes a big difference on one motor doesn't mean it will have any beneficial effect on some other motor.

If you want to understand a motor, start by analyzing the ports and for a particular output level, work out how much time-area is needed and that will indicate which ports are significantly below target and which are close.

The old school way was to get a whole pile of cylinders and modify them and test them and keep lots of notes and try to work out what works and why, and work towards a good solution. Software may seem expensive, but it's cheaper than buying sets of barrels and a die grinder.

[Vintageman]

Tz375;

Even worst today for an experimenter: parts aren’t readily available.

Have you ever run the T500 through software to see “which ports are significantly below a target and which are close”

How does the T500 ports/timing compare to the GT750 is like the early one or later one or is it unique?

When you talk about GT750 isn’t the early and late very different?

I don’t have the GT750 just interesting stuff

[tz375]

GT750 changed in 1975. Inlet port floor dropped from 101mm down to 104mm down and the exhaust roof changed from 42.5mm down to 40mm IIRC. The exhaust system changed to delete the cross tubes and it used to be generally accepted that the exhausts were the reason for the huge drop in low end pulling power in later bikes.

Haven't sat down with these data to compare port widths or opening durations from the 500 to 750. It's on the list of things to look at soon.

[oldjapanesebikes]

GT750 changed in 1975. Inlet port floor dropped from 101mm down to 104mm down and the exhaust roof changed from 42.5mm down to 40mm IIRC. The exhaust system changed to delete the cross tubes and it used to be generally accepted that the exhausts were the reason for the huge drop in low end pulling power in later bikes.

And they changed the inside diameter of the first baffle in the pipes - 29 to 20 mm (page 2 of the service bulletin).

[Vintageman]

Have the stock port dimensions ever been published for GT750 (new or old)?

Either officially by Suz or someone here has traced out and stated.

[tz375]

I'm sure that someone has posted stock port maps at some point or another. I posted one a while ago, but I don't remember which variety it was.