Ports - GT750

[tz375]

I took the degrees pictured to be radial dimensions of the ports and not timing numbers.
A diagram like this for a 750B would be great.....anyone have one!
Even an untidy version would be fine!

They were supposed to be timing numbers - at least that's what the guy that drew it said he had calculated.

Exhaust on an MAB is quoted at 40.0mm below TDC compared to 42.5 on J/k/l motors. Timing is very conservative on the GT series and pipes on their own make a small difference, but work much better with revised porting. The easiest way to 80HP is a spacer plate, mild port clean up and a set of JEMCO pipes.

The only way top know exactly what your motor's port dimensions are is to pull the top end.

[sfront]

Take a look at how the upper and lower dimension lines extend to the radial drawings.
For all the ports.
Thats why the radial drawings are positioned like they are.
Definitely not timing drawings. Besides.....there's no math!!
I haven't seen a set of pipes for the GT750 that I have to have.
I have a set of Sugayas on the RGV and I love them.
It be nice to have a set of numbers for pipes that are known to work
rather than a bunch of theoretical dimensions from the software program.
I could also back into the port timing by trying to match resultant pipe dims.
I don't really want to build multiple sets just to see which one works the best.
I'm not racing this bike and if yours is faster than mine, I'm okay with that!

[Suzukidave]

If it helps any , here is a link to a degree wheel that can be printed out http://suzukidave.myphotos.cc/suzukidav ... heel_a.jpg" onclick="window.open(this.href);return false; and a mirror image one to attach to a piece of plexi glass http://suzukidave.myphotos.cc/suzukidav ... heel_a.jpg" onclick="window.open(this.href);return false;

[tz375]

Take a look at how the upper and lower dimension lines extend to the radial drawings. Thats why the radial drawings are positioned like they are.

Definitely not timing drawings. Besides.....there's no math!!

Yes, I can see that and I remember the guy that posted them proudly telling us he had the timing degrees on his diagram. I never bothered to try to work out what he had actually drawn. The subject at the time was port timing and that's what he posted for timing figures. We knew he had it wrong but that was as far as it went.

I work mine out allowing for piston pin offset and I like to double check with a 75mm range DTI and degree wheel. With different chamfers on all the ports it's actually remarkably hard to get an accurate reading.

Seriously, if you want a set of pipes that works well enough on the average GT, get a set from Jemco if you are in the US. If you design pipes using software, you at least need to model it to see what it does to the shape of the graph before committing to cutting steel.

[andersaGT]

I have measured both a J and a K barrel and to my surprise the K barrel (engine number 40258) has the modified timings. The measured distances are very close to the bulletin numbers 40 mm from the top of exhaust port to top of barrel and 106.5 mm from bottom of intake port to top of the barrel. The distance for the transfers is close to 52 mm and the deck height is 0.8-1 mm (guess the gasket will be compressed but that is probably negligible).

With a con rod length of 130 mm and a stroke of 64 mm we get with a deck height of 1 mm an exhaust and transfer duration of 169 degrees and 120 degrees, respectively. The numbers calculated with zero deck height gives 165 and 115 degrees…

My plan is to take the J barrel and raise the exhaust port 4 mm and use it with a 3 mm spacer. Minimizing the deck height close to zero would then give 33 mm and 49 mm from top of ports to top of barrel. Given in degrees that would give an exhaust duration of 191 degrees and a transfer duration of 129 degrees, with a nice blow down of 31 degrees. I think that should give a really nice engine to start with…

[tz375]

Good plan. Use the early barrel to retain enough intake timing, but remember that with a 3mm spacer, the intake floor will be 3mm closer to the top surface.

That allows enough metal to clean up the lumpy port floor and still leaves nice duration. It's unusual to get a K barrel with M dimensions unless a PO had modified it to be more like later motors thinking that would be an improvement.

Deck height is usually 0.7-0.9mm or so.

191 degrees for the exhaust is a lot of duration but with enough blowdown it could work.

Let us know how that works out for you.