GT380 exhaust boost ports

[Nemisis]

I have a few porting questions about my GT380. After doing some measuring on the cylinders it looks like there is plenty of room for some exhaust boost ports. Has any one tried this mod on these old cylinders? I plan on reshaping the tiny transfer ports to point more towards the intake side of the piston to help push the exhaust out better, my only concern with doing so is possibly making the exhaust side of the piston running too hot due to lack of fuel cooling. The other idea I thought about using was double base gaskets and milling the cylinder deck down and cutting the intake skirt about 1mm shorter. Lots of ideas just not quite sure which direction to take this old girl. Anyone want to give up just a little info on what will bring this little bike to life? And by the way Jemco chambers will be used.

[tz375]

Does it need more port time area or is something else the primary restriction? On most Suzuki triples the exhaust can be opened up enough but the transfers are the critical point.

In general, pointing transfers towards the rear tends to boost low end performance at the expense of top end power and raising intakes tilts the powerband the other way. Have you measured all the ports to determine what it needs. I have no data on the 380 to advise you but if a good T20/T250 makes say 30HP in hot street trim, a 380 should make 45 as it's basically a GT250 with an extra pot.

[Craig380]

The later GT250A/B models had boost ports either side of the main exhaust port, but they also had two extra transfer ports (4 transfers total). I don't have a port map but someone here may have

I have some info saved somewhere about a guy who milled a 'boost port' in the cylinder, using windows in the pistons, I'll try and find that.

From my own experience, just fitting chambers makes a huge difference to performance, the stock exhausts not only weigh a ton but are NOT designed for performance in any way. A decent set of chambers really liven the bike up, even with stock porting, carbs, filter & airbox.

EDITED to add this from Jay Mendoza, WAAAAYYYY back in 2000!

The 380 has true pure Schnuerle porting with no auxiliaries or boosters and the ports are quite small in height and width. I got a great idea from examining my GT250 which basically had the same porting but with one major difference: the exhaust port had 2 small ports on either side near the top to effectively widen it without the risk of losing ring support.

I know how to take a hint and drilled two 1/4 inch holes on either side of the 380's exhaust port, the tops of the holes being equal (in line, ed.) with the top of the exhaust port. Next, I used my Dremel and a carbide bit to drop the bottom of the new holes 3/4 of the way down towards the bottom of the (stock, ed.) exhaust port and slightly ovalled out the tops of the (new, ed.) holes.

Next, I drilled two 3/8" holes in the piston skirt on the intake side, just on either side of the intake port, but so as not to fall in the ring gap area.

Then using the Dremel I cut two troughs, or grooves if you will, in the cylinder walls that were even with the tops of the transfer(s) (ports, ed.) and the bottom of the grooves were even with the holes in the (piston, ed.) skirt at BDC. Basically I added 2 booster ports, angled right at the plug and widened the exhaust with webbed holes to support the ring(s).

Next, I had the head milled to set the squish at .040" and went up 2 jet sizes on the main(s) (jets, ed.), used Uni-Filter foam on the stock filter cages and repacked the silencers. Talk about a difference....It would do roll on wheelies in first...just snap the throttle open and enjoy the view of the sky ! The (stock, ed.) 380 would crap out at about 7K (rpm, ed.), but this one would pull hard all the way to redline...and you didn't need to take it that far to get all the thrills you wanted.

I got the booster idea, and drilling the holes in the skirt, from Aermacchis and RDs that I had seen back then. I never told anyone until now, as it was my big speed secret....and later I forgot about it all together. Now that I don't have the bike, I might as well tell everyone what I did.

jmendoza