How do you hot up a GT380?

[Craig380]

I've never seen an article anywhere about hotting up the 380 but it's the simplest of 2-stroke designs - just a 4-port cylinder (a bridged inlet, two transfer ports and a plain exhaust port).

There is a step on the floor of the exhaust port which can be smoothed out, and you could easily raise the roof of the exhaust port by a millimetre or so. Any more than that and you'll need to skim the cylinder heads by maybe 20 thou' / 0.5mm to compensate for the lost compression.

The heads have no squish bands and the spark plugs are not centrally-mounted, so you can't go too radical on skimming the heads, or you'll risk detonation.

After this level of modification, you'd be paying a pro to do the porting as you'd have to modify the transfer port windows etc, which most people don't have the equipment to do.

[Vintageman]

The heads have no squish bands and the spark plugs are not centrally-mounted, so you can't go too radical on skimming the heads, or you'll risk detonation

The GT380 head do have a squish area.

Here is some directions if you want to increase compression->
http://www.oldjapanesebikes.com/mraxl_G ... etins.html" onclick="window.open(this.href);return false;

I have measured the early versus late GT250A head (have both) and the difference is 0.010” That is there is some cylindrical section left on both before you get to the squish section.

This brings it from 7.2:1 down to 6.8. 1 so 0.010" is 0.4 points in compression change. Should be similar results for GT380

[jabcb]

Vintageman, do you know why Suzuki changed the GT250A compression during the model year?

[tz375]

As Ja-Moo so eloquently said, It's a starting point from which it is possible to improve. That on board shot from Spa mentioned that he used a 1.24mm spacer plate to raise all the ports. That takes care of transfers and exhaust timing. He took 1.4mm off the head so that equates to a net 0.16mm off the head to raise compression slightly.

I would have taken it off the cylinders and corrected the squish but that's just me.

For the street, I'd get some decent pipes, maybe larger carbs and raise compression when setting the squish.

The place to start with porting is to read everything you can get your hands on and then measure the ports width and height from TDC and work out what stock settings are and from there you can decide which way to go on the ports. You will probably widen the exhaust, fit a spacer plate and widen and clean up the intakes slightly. small changes can make big differences.

I haven't looked at a 380 barrel but they use GT250 pistons, so I would expect there to be some similarity in porting and some ideas there.

For a Cafe Racer look, check these:

http://www.youtube.com/watch?v=58qt_ZTa-oc" onclick="window.open(this.href);return false;

http://www.youtube.com/watch?v=Hi08KvfgJ3g" onclick="window.open(this.href);return false;

or http://www.youtube.com/watch?v=sqLkaxuHG3A" onclick="window.open(this.href);return false;

[Craig380]

The GT380 head do have a squish area.

What I meant was, it is not as pronounced as it is on, say, a Yamaha RD head.

I have a 380 head that was skimmed by 20 thou which leaves just a tiny trace of a lip around the combustion chamber. It did sharpen up the low / mid-range a little. However, one of the combustion chambers was scarred by debris from a previous problem, so I don't use it now.

The stock head gaskets are at least 1mm thick after torquing down, and the piston is level with the top of the liner at TDC so there is some clearance to work with, there.

[ja-moo]

I haven't delved into the Suzuki port timing "numbers", my guess would be they are very tame. I don't "like" lift kits (yes I know they are necessary on the 750s) as the true timing ratios are are around 4:1 from EX to transfer timing, not the 1:1 with a lift kit. The problem through the decades has been, it's easy to do intake and EX mods so their timing gets extended and the transfers are left mostly stock, as they are a beeach without the correct tools, causing a pipey motor. EX port widths are left small from the factory for good ring durability. There is probably decent power in the cylinders with the proper porting, pipes and maybe carbs.

The heads are the problem with the offset plugs, but money can fix that....lol.

[Vintageman]

Vintageman, do you know why Suzuki changed the GT250A compression during the model year?

The reduced compression since they were seeing melted pistons early on. They also increased Main jet size while reducing Needle Jet (I suppose mid was too rich with larger main) And the ran a cooler plugs. Trouble with small bike you are always WOT. The later GT250 needed WOT more so due to the raised exhaust port, larger carb and extra transfers

What I meant was, it is not as pronounced as it is on, say, a Yamaha RD head.

I think the little they have helps. So GT380 is like GT250. The higher compression GT250 has another 0.010" before you get to the squish that's it.
I find it surprising how much 0.010" gives in compression rise. Again I would not go too high.

Speak of a lot of squish. Kawasaki S2 heads and I think H1 are real freaky with squish versus combustion area. Poor MPG I think is trade off with too much

Another GT380 mod: there is a lot of meat on those cylinder walls.
Larger piston oversize could help. I have seen Wiseco P8 for 2mm over that’s it (surprising). Bore to max then widen EX and intake to take advantage of extra CCs

The early GT250 has those extra little holes either side of EX. But unlike some Rotax engines they are at the bottom of port not top so not sure how much they help down there

[sportston]

This is all very interesting and useful stuff. Thanks guys. How/where do you get a spacer made up?

[ja-moo]

Lets not forget, if the squish clearance is much over .040" There is no "real" squish effect.

[tz375]

For best results,

Step 1: Read

Step 2: Measure

Step 3: plan

Step 4: machine/purchase

Not the other way around.

[sportston]

For best results,

Step 1: Read

Step 2: Measure

Step 3: plan

Step 4: machine/purchase

Not the other way around.

Excellent advice! And always measure twice and cut once. I am at step 1 for the moment.

[Craig380]

Another GT380 mod: there is a lot of meat on those cylinder walls.
Larger piston oversize could help. I have seen Wiseco P8 for 2mm over that’s it (surprising). Bore to max then widen EX and intake to take advantage of extra CCs

A good point. As TripleRocky has pointed out before, in Italy the GT380s had a 55mm bore as standard, making them 384cc (apparently because of some insurance / tax bracket). One of Rocky's bikes was a 384cc on +1mm oversize, making it a full-fat 400cc, and apparently it does make fair difference, even with stock everything else. In the UK, +2mm piston kits are available from Mitaka ....

[tz375]

If someone has a GT38-0 with aftermarket pipes and can measure the pipes and ports, I'd be happy to see what the software says. Failing that if anyone has port dimensions I can work from there

[Craig380]

@ Richard - I couldn't find a flexible tape measure to check the circumference of my chambers, but was able to measure the port dimensions, so for what it's worth ....

Bore & stroke = 54 x 54mm, all measurements are the distance measure 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)

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

The exhaust port dimensions alone show how conservative the porting is ...

[ja-moo]

Gheesh, you can safely widen the EX 6.7mm...........