Learn Me GT380s

[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.

[Vintageman]

Craig380,

I think you said you measured straight across in the cylinder. I did a paper trace then laid the paper flat. So I was thinking that is why mine shows longer.

Is that true

For a paper trace I covered just the port I wanted so I could tape it in place on the cylinder wall so it won’t move. Them just use a soft pencil.

Update: Oops I did mess up the exhaust numbers as I looked again. repeated intake

I accidently found if you take tape (I used a black duck tape like stuff) and put it in the cylinder and rub over port and top of liner with your fingers it make a perfect sharp line in glue to measure from
Exhaust:

Top 1.280”/32.5mm 1.295”/32.893mm
Bottom 2.173”/55.19mm
Width 1.574”/39.98mm 1.426”/36.22mm 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

[markush]

Hi !
I took some values, too.
Some are equal as posted above, some are different.
All my cylinder ports were in stock condition.
Bore 54,5mm, all.
all measures in mm.
width = chord (not circumferential).
Distances from cylinder top surface.

GT380B
average values of 3 cylinders. (one set of Left+Center+Right)
Outlet
width : 33,7
top : 33,4
bottom : 56,2
(heigth : 22,8)
Inlet
width: 36,8 (over all)
(center bar width : 4,5)
top : 65,5
bottom : 84,4
(heigth : 18,9)
transfers (measure for one)
width : 27,5
top : 43,4
bottom : 56,4
(heigth : 13)


GT250L/M
Shown values are averages of 4 cylinders. (two sets of Left+Right)
The range off differences between cylinders was max. 1 mm.
Outlet
width : 33
top : 32,5
bottom : 54,5
(heigth : 22)
small additional Outlets (measure for one. Lying at the same heigth/position as transfer ports)
width : 6
heigth : 12
Inlet
width: 36 (over all)
(center bar width : 3)
top : 66,8
bottom : 84,7
(heigth : 17,9)
transfers (measure for one)
width : 26
top : 43,3
bottom : 55,3
(heigth : 12)

Shapes of outlet ports of 250/380 are a bit different.

Auslassschlitz_GT250_380_dr.jpg