GT550 ports brass sticky up thingy????

[Vintageman]

Pistons may be more rare or $ than cylinders if you exclude boring if needed

Too bad cruzinimage_co on eBay does not have gt550 pistons. I just asked.

Others should ask too maybe he'll make them. I think many GT550s were sold and cyls have a lot of meat for a 1.5mm or 2.0mm over

I looked on eBay at cyls and those intake windows are very very height challenged.

I am am sure the exhaust timing is mellow as well,

I don't like raising compression above 7.0 on domed heads and if you add chambers Maybe others have had luck above this?

I have not pulled out my 73 GT550 yet, but juts realized I could use a timing wheel to figure out port timing pretty easy

good luck

[andyp666]

@tz375 Im going to build a set of spannies I have the software and tig skills etc. I was going to buy a load of bolt on stuff but since going thru some of these forums and seeing some of the stuff guys have done with their barn find wrecks, I am so inspired I thought sod it Im going to the brim of me wellies and beyond.... I have me eyes on mikuni tm's as far as carbs go but this will be a long build Im in no hurry.
I have a 1200 motodd laverda coming on aswell its all about triples with me and I wouldnt say no to a buckshee h2 if anyones biffing one out....
lolz

[tz375]

Motodd Laverda would be nice to have. Did yours come with Tracy Neave (sp?) sitting on it?

Port timings on the 550 are not wild by any manner. We ran a few simple sims a while ago and we lost almost 10hp at 5000 to get more from 6500 up, and we did get more - a lot more but that was for a race project. For the street a little more finesse is required. Port areas are small but that's a width issue more than a height issue.

Stock intake and transfer port timing combined with decent pipes and taller exhaust and wider on all ports could add 10hp at 6500 and hold that all the way to 8000 where a stock motor falls off. That same configuration with a little extra intake duration loses below 6000 and gains a little above that. It's all about the balance between all the ports and between high and low.

Lots of numbers to play with and pipes to design to work with your ports. Which s/w do you use? We have tested MOTA, BIMOTION and Wizard Pro and bimotion seems to create the best starting point and is the easiest to tweak to try different cones.

[Vintageman]

Here is the response fror more pistons


Thank you for your question. regarding GT550 pistons. These were sold out.I get these pistons at end of this month.
Thank you again,
mitsuo


- cruzinimage_co

If someone has the timing for intake, exhaust, blow down, and transfer please share.
We should have a spot to keep all port dimensions and timing

The only thing about going wider on ports is reliability of course. It depends if you are making a racer or high mileage street bike.
To me the GT550 intake looks wide just short. Exhaust is decent street width too, but I forget now how far down from the top it is.

I have had good luck now with a little intake timing changes on T500 and T350 alone, but I only look for a little more power and a tad more top end as not to destroy crank.
Importantly, those two bikes did not have the bottom end fall out with a little intake timing change alone.

Now you take the GT250A versus early ones, Suz raised the exhaust port 2.5mm (http://www.oldjapanesebikes.com/mraxl_G ... etins.html" onclick="window.open(this.href);return false;" , now the bottom end truly dropped away.
I recently made an exhaust only timing change on a GT185 and same thing, bottom end wiped out. That's why modern 2 strokes have dynamic exhaust valves I suppose

Maybe I don’t know which end is which?

Or it is Tz375 and others have said... Must consider all ports together.

[tz375]

Not all engines are created equal. Some are more restricted in one port than another. According to the not 100% accurate data I have, the 550 has fairly mild inlet timing and can stand "some" change before it makes too much of a difference at the bottom end. By comparison, a 750 already has too much intake time for a street motor and the MAB models have way too much and benefit from less inlet duration.

The question is how much is too much and that takes a bunch of time and the right software to work out.

If someone has accurately measured 550 ports that would be nice to see. We also need to know the deck height to get corrected port times. That typically is around -0.5mm. It would also be nice to see iron 550 barrels compared to later chrome plated barrels.