Learn Me GT380s

[Craig380]

@Markush - interesting that you use X7 pistons, do these have the same distance from piston pin to the crown / top of the piston?

I thought this measurement was different, for some reason ...

[Vintageman]

I think that bike is stock accept pipe. It does not look like 3 into 3 maybe 3 into 1. Occasionally you see him look down. This vid is before Gt550 carbs

I would say my 74 Gt380 run just like that one. If I accell it will pull past 9000 per tach not over rev/pull. It does so quickly in most gears (can't say I have ever tried in 5 th or 6th) it makes me nervous (rev limiter would be nice). It wouldn't do that with stock pipes and until I had my Iridium all ne points and caps). I think one limit on these bikes may be ignition.

I like the ZenZhin pipes out of Japan. Sent an email to learn more. Here is one for the GT750 here

http://www.motoparts-online.com/apps/we ... ow/2951142" onclick="window.open(this.href);return false;

[Pete O'Dell]

@markrush, am suprised your using x7 pistons as they are actually 5mm shorter than 380 half a mil taller in comp height and the ring pegs are in a different position 13 degrees if I remember correctly

[markush]

Hi!
I bought the pistons from well known Reiner Schneider (gtreiner1 @aol. com) as "X7 aftermarket-pistons". They are shorter then Suzuki-GT380-Pistons, not sure about the 1-2 mm (this was a few years ago), but certainly not the above mentioned 5mm. If I calculate with the current dimensions of the piston (from the catalog below) I get 2.9 mm shorter piston shirt. Compression height is equal.
At that time I had scanned the piston-package:

x7-kolben-nr.jpg

The company is now called T.K.R.J. Catalog download: http://www.tkrj.co.jp/pdf/catalog.pdf.
I had not measured the knock pin angle of these pistons. But the piston ring ends do not drive over any port of the GT380-cylinder.

I forgot to mention: I use contactless ignition, too.

some pictures in these directories:
pistons, pics 18+19:
http://suzuki-gt380.de/gt380_dateien/motor/zusammenbau/
ignition:
http://suzuki-gt380.de/gt380_dateien/el ... ktlos_neu/
3 into 3:
http://suzuki-gt380.de/gt380_dateien/mo ... ff/3_in_3/

[Vintageman]

markush

I see the X7 specs in the catalog, but can't find GT380 in the catalog? I am very interested in seeing this data


The X7 ring locator pins I think is like 1976/77 GT250 A/B with extra ports. You can put this pin location in an old GT250 or Gt380 for as you saw they clear all port. Just can't go other way around.

[markush]

Hi!

I see the X7 specs in the catalog, but can't find GT380 in the catalog? I am very interested in seeing this data.

There are no GT380-data in this catalog.

The X7 ring locator pins I think is like 1976/77 GT250 A/B with extra ports.

so I remember, too. (unfortunatly made no written notices in the days about the TKR-X7-Pistons.)

You can put this pin location in an old GT250 or Gt380 for as you saw they clear all port. Just can't go other way around.

You're right. GT380 and GT250M/A/B/C use the same pistons. (same part numbers)
There are some GT250-Piston-Infos about it at my site: http://suzuki-gt250.de/motornavi/motor.php?b=3
possibly use google-translator, but it might be a bit strange
http://translate.google.de/translate?sl ... D3&act=url

Markus

[Vintageman]

Markush

based on your we site and tkrj. If I am correct the very crown of X7 is a tad taller by 0.8mm (31.5mm vs 32.3mm) but crown starts at same point so just very top/center. Skirt shorter by 3.7mm (64.3mm vs. 68mm).

That does seam like a good hop up! Some more compression and intake timing too

I once compared the Intake port on a T305 versus T250. The T250, was much lower so greater timing. I forget how much maybe 4-5 mm. So I shave 2mm off the T305 piston. Dang that made a big difference. You say you saw 500 rpm on your gt380. with X7s, I would say I saw 1000 rpm more top end. Supposedly this rotates/pivots the power band. I so no real loss on low band as I ride down around 3000 rpm OK. The early versus late T500 has about a 3mm difference in intake port floor. the early is lower. I put early on a later engine all else the same and again big power gain feeling.

I thought T250 had a radical intake timing (when compared to T350)'

I wonder how the GT380 port timing compares to T250 or gt250. Maybe it is not the same as I assumed? Anyone know or can measure. I have a 74 gt250 cylinder and can make a paper trace measurement if some has a gt380.

I would love to know wouldn't you!

[Craig380]

Here you go - these are the port measurements from a '76 GT380. They're not done with a micrometer or anything, so assume the accuracy is plus or minus 0.5mm

Bore & stroke = 54 x 54mm, all measurements are the distance measured from the top of the cylinder liner.

Inlet: bottom is 88mm, top is 65mm (port height 23mm)

Transfers: bottom is 57mm, top is 43mm (port height 14mm). Transfer width is 26mm (measured straight across, not "around" the cylinder wall). Just one transfer port each side.

Exhaust: top is 33mm, width is 30mm edge to edge (measured straight across, not "around" the cylinder wall). The mouth of the exhaust port where the header pipe mounts is 36mm internal diameter.

The intake port is bridged, and the bridge in the liner is pretty thick too, as is the casting in the inlet tunnel.

The very low exhaust port height shows how mild the porting is, it could easily be raised 2 - 3 mm and widened a little without too many problems.

[markush]

Hi!
I made a mistake in my calculation with the X7-piston-data from the TKRJ-Katalog.
A new try:

Piston_GT_X7.jpg

Then the difference of the piston skirts is 4.5mm and we come into the range of the mentioned 5mm.
But in my memory, the difference between my built-in X7-pistons and the original wasn't as large.
May be, my memory is wrong, or my measurement in that time.
There are also different part numbers in the current TKRJ-catalog and on my old TKR-piston-box.
But it seems to be unlikely they have changed the dimensions(?)

However, the engine behavior has changed.
With the original pistons, from 3000 rpm, in 6th gear, I could give full throttle and the machine accellerates slowly but evenly. With no other changes except the X7-Pistons, this is now possible earliest from about 3500-4000, but the upper revs are higher, by the same range.

The port design of GT 380 and GT250RamAir-cylinders is quite equal.
e.g. Outlet height GT250(K/L/M): 32mm, the other ports also differ by only about 1 mm; among the cylinders I have here. This could perhaps be manufacturing tolerance? These GT380-cylinders do not have the additional small exhaust ports like the GT250s.

[Vintageman]

Craig380,
That was quick. I’ll try to get to measure the 74 GT250 soon.

Markush,
Yes 4.5mm. Quite a bit of difference in intake timing. .

Maybe use this piston and raises the jugs 1.5 mm and shave head 1mm is a nice mod too

I made a mistake when I said I compared T305 to T250. It was actually a T20.
On my T305 with 2mm off skirt I did note if I was down real low at 3000 rpm, then I blipped the throttle full it would not take it. I would take ¾ just fine. It may have done this before too but I think it is different. It always did if you were lugging at 4000-5000 and did it. Actually a ridiculous mode of operation… if I ever have the desire to do that and hold it WOT I should bolt on some CV carbs.

With 2mm off 3500 to 5000 rpm, the T305 lugs uphill just fine and see no difference. After 6500 bike wakes up and will pull beyond 9000 rpm ( I stop it not the engine). Before 8000 rpm was it and fell off hard and the bike just seamed flat up top (aka smooth versus a hit of power). Same factory tach was used. I think this is common understanding given how the intake port shifts the pivot point of the power band on these type engines. I also think it was Suz goal with T305/T350 to make it smooth so a couple of mm is OK if what you want

I am doing a T350 now with effective 1.5 mmoff intake and 0.5 higher exhaust and transfers

[Pete O'Dell]

Ohh Gosh!! I actually know what I'm talking about, bles my soul..........

currently building my son a race bike based on a Gt250B to replace the TS250 based race bike

[tz375]

I just ran some numbers for fun and fitting an X7 short piston should add about 2 hp at the peak and lose almost as many at 3,000 -4000. The curves cross at around 5,000.

But it still peaks at very low speed 6500 - 6750 because of the small/low exhaust port. I didn't calculate the differences that the taller crown might make. There's a lot of assumptions in those calculations, but it's indicative of what to expect. The loss at the bottom end is noticeable because there isn't a lot of power to begin with.

So what we really need is to transfer Pete's GT250 race porting onto a 380 with a set of decent pipes..

[Suzukidave]

I guess this would be one of those perfect times to get a base line dyno run before mods and then again after to get a real world comparative figure

[Vintageman]

Dyno is never going to happen on my T305 or T500 intake mod I imagine. You are right it is good proof but not practical for a hobbyist like me.

I shared my experience without unbiased proof, but I do believe, excited about it, and wanted to share. Given these facts, If anyone wants to try please do as low risk as possible, For example get a spare set of pistons as I did In case you are unhappy.

Here is the port info 1974 gt250 right cylinder port measurement via paper trace. so all port width measured flattened out

all similar but, it looks like GT380 has a lower intake port floor. To be sure I checked mine a few times


All measurements from top of cylinder

Exhaust:
Top 1.280”/32.5mm
Bottom 2.173”/55.19mm
Width 1.574”/39.98mm widest


Intake:
Top 2.572”/65.33mm
Bottom 3.340”/84.84mm
Width 1.574”/39.98mm across both windows

Transfer:
Top 1.667”/ 42.34mm
Bottom 2.217”/56.3mm * Tried to exclude long factory chamfering
Width` 1.260”/32mm

[Craig380]

@Vintageman, interesting, as you say the port dimensions are pretty similar, apart from the 380 inlet floor is lower.

Also the 250 seems to have a much wider exhaust port, although the port heights are similar.