Crazy Idea

[GearHead]

I started kicking a few ideas around at work. From what i can find the compression ratio on the 78 gs400 is 8.5:1. Soo what if you had some custom pistions made to raise it to say 9.5 or 10 ? Then throw in a little port work and gaskit matching on the heads. Some k&n pods and a highflow exh? Nuts huh. What do ya'll think?

[tkent02]

I started kicking a few ideas around at work. From what i can find the compression ratio on the 78 gs400 is 8.5:1. Soo what if you had some custom pistions made to raise it to say 9.5 or 10 ? Then throw in a little port work and gaskit matching on the heads. Some k&n pods and a highflow exh? Nuts huh. What do ya'll think?

Go for it. If you did it all up right, tuned it perfectly, it would probably go about like a good 550.

[SqDancerLynn1]

Sounds good, but I would surface the head & block. Maybe about
.050 tho total cut off & degree the cams. Then you could use stock pistons. You would need to check piston / valve clearances & piston deck height before hand

[Plooker]

Most anyone who can do good port work for you could reshape the combustion chambers at the same time to increase compression. Of course, without a good cam you're just doing half-measures. I suggest nitrous. I came across a nutter from England (who by the way is a big fan of GSs) who modifies industrial air control valves to work with nitrous up to 70hp or so. His web page has instructions as well as a whole lot of good reading, some cool pictures and some dangerous inspiration. Picture a 100hp GS400 for a hundred bucks worth of parts and a couple weekends work. Wouldn't that give the local squibs something to talk about. You can find his page here: http://www.nitrous.info/motorcycle-n...ion-system.htm
You can tell from the pic at the top that he's one of us.

[82Shafty]

See if you can shoe horn a 750 in the frame: something out of a wrecked gsxr LOL.

We shall call it, GS-rod w/ a 15000 redline.

[duaneage]

The money spent might be better invested in a larger bike. Many of these smaller bikes were intended for either beginner riders or for economy. With 400 CCs you don't have much displacement to play with,

[Big T]

Back in the 90s, there was a local racer who took a GS 500 twin and used a GSXR 750(?) head with the 2 outer combustion chambers cut off. That thing would really go, but I think other parts of the motor suffered.

[rapidray]

In the mid 90s we road raced a GS500 twin out of my shop in Hawaii. We put a pair of 39 mm Keihin FCRs on it with a V&H 565 piston kit & I put 2 mm bigger Titanium intake valves & 1 mm bigger stainless exhaust valves in it. I had Web make cams for it & we modified a stock head pipe with a V&H back half from a GSXR. This little bike was a TERROR at the track, out running FZR 400s! To give you an idea how quick it was, it ran 12.60s at the dragstrip! I spent about 10 hours porting that head but it sure was WORTH it! The bike is still being ridden on the street now in Maui. GREAT little bikes! Plus, you can put GSXR suspension on them & have KILLER brakes! Ray.

[trippivot]

mill the head (0.090" example) to gain compression advantage then get 1-2 mm larger intake valves. to fill the cylinder better and finally degree the cams to put emphasis on where in the rev band you want all this new found power.
.
the magic is in the head and you will have reliability as well as more power. this approach does not create more heat like adding displacement does. it will be nearly the same money but have a better outcome in the long run.

[asetech]

you could zero deck the jug also

[GearHead]

I think milling the heads and doing a good port and polish might be the way to go. That way i dont have to open up the bottom end. The nitrous has me thinking to... What is the recomended minimum valve clearance? Im also wondering if the cam profiles from say the 425 or a 450 would would be better, more lift and or duration ? Shoot, how about a bit of boost ?
Im not realy trying to make earth moving power, just do something a bit different. A bit of a sleeper to supprise the squid or harley guy at the next stoplight.
Thanks for the help.
GearHead