1979 GS425 Building more power

[Guest]

Me and a friend are building a cafe racer out of a 1979 GS425 and are in the process of rebuilding the top end of the motor. The valves will be lapped, the cylinder honed, new rings etc to get us as close to factory spec as possible.

However any extra hp we can coax out of it would be a bonus, its already going to get pods as well as a less restrictive 2 into one exhaust (this is also for aesthetic purposes, and yes, I know pods can be a pain to jet right, but its what we're going to do), we are also going to hopefully port the head.

I'm curious if we can have the head shaved down any to increase compression? If so how much can we tale off? Are there any other easy ways to make more hp on these?

I was also contemplating a more powerful ignition system so we could run a wider spark gap, has this been done? any suggestions?

-thanks,
Dude99

[Badooka]

As in any hot rot endeavor, when you get into issues of power and HP gains- it's pretty much a custom choice. Also know that in doing major adaptations, once changed it can be difficult to go back without extra parts. I would do research on shaving those heads- and more compression doesn't always mean more power. I would go for the best ignition, wires, and plugs and of course make sure those valves are adjusted 100% correct. You can spend your time wrenching or riding, so enjoy it as your are in it- and nothing beats that first turnover after your all done! Wider spark gap can be good for power but too hot can be a bad thing- I went through about 4 different plug styles before I found one that gave me what I wanted. I think those pods are going to be your weakest link, it was a pain for me getting correct jets after drilling my exhaust so good luck with it!

[dude99]

Thanks for the info. A question regarding ignition. The stock system uses two separate coils, one per cylinder. Will the firing arrangement allow me to run a wasted spark system with one coil? Or do I need to run 2 coils….

[Big Rich]

The quickest and easiest way to make more power? Find a GS500 motor and make a custom exhaust. There's no replacement for displacement ......

I'd be willing to bet you could fit up some GS500 carbs / K&N filter to your current motor. To make better use of the power you already have, look into a 520 chain conversion. Of course, cutting weight around the entire bike helps too.

[Badooka]

The quickest and easiest way to make more power? Find a GS500 motor and make a custom exhaust. There's no replacement for displacement ......

I'd be willing to bet you could fit up some GS500 carbs / K&N filter to your current motor. To make better use of the power you already have, look into a 520 chain conversion. Of course, cutting weight around the entire bike helps too.

That's an old age adage that was blown to bits by the Buick grand National.
Check out the Ford Focus 4 cyclinder twin Turbo- they call it the giant killer- I didn't believe it until I saw it smashing mustang GT's like toys- I'm a SBC guy- but all bigger size can be overcome.

[Badooka]

Thanks for the info. A question regarding ignition. The stock system uses two separate coils, one per cylinder. Will the firing arrangement allow me to run a wasted spark system with one coil? Or do I need to run 2 coils….

I can't answer the wasted spark question but I know for a fact the new Dynatek DC1-3's are power houses- there is no comparison to the ACCEL- And I have been ACCEL from the beginning. When I roll that throttle out on my bike, I have to really hang on.

[barnbiketom]

The quickest and easiest way to make more power? Find a GS500 motor and make a custom exhaust. There's no replacement for displacement ......

apples to apples, and oranges to oranges,
I agree...THERE IS NO SUBSTITUTE FOR CUBIC DISPLACEMENT..

[Badooka]

apples to apples, and oranges to oranges,
I agree...THERE IS NO SUBSTITUTE FOR CUBIC DISPLACEMENT..

Yes there is.

[Agemax]

Thanks for the info. A question regarding ignition. The stock system uses two separate coils, one per cylinder. Will the firing arrangement allow me to run a wasted spark system with one coil? Or do I need to run 2 coils….

it is running wasted spark already. you need one coil for each cylinder as you have a 180 degree crank, so one coil will only fire at the correct time when the piston is in the uppermost position, the other cylinder will be nearer BDC.
Therefore you need one coil per cylinder

[Nessism]

You won't pick up power from the coils or ignition, but you could improve reliability. You also won't pick up much from a header and pods - maybe 5% if you get a good header and jet the carbs correctly. You may loose power with a junky header like a MAC so good luck finding something suitable. Also, don't use those cheap junky Emgo pods. Get real K&N's or use the airbox. A K&N inside the stock airbox (if they make one that is) would be preferable to Emgo pods, just leave the lid off the airbox.

Good luck

[dude99]

Lots of good info, thanks. So as far as exhaust is concerned we plan to make one ourselves. Is there a known best exhaust system for these bikes that I could copy the dimensions\header lengths from?

[furu14]

Use the stock headers w/balance pipe-just mount some less restriktive mufflers.

[mrbridgeport]

That is a very sought after bike for ahrma superbike lightweight. Big rich is right you can make it a 500 pretty quick. I would just get the bike clean and reliable and go have fun on the track for a season and make the big plans for after season. Pretty easy to miss race season don g engine mods at this stage. Then swap the suspension and do the top end. Just my two cents

[dude99]

Agreed, we received the same advice from several other people in the know so we are following it. The engine is currently being restored to stock (or close to it). We a re-ringing it and putting a new gasket set in it. We will be focusing on the ascetics and suspension this year, the engine build will come next winter.

We will likely be using the stock headers (or at least keeping the length\diameter the same, however we are probably going to go for a 2-1 setup.

[pete]

I can tell you with K&N pods and a custom fairly free breathing (not open) 2 into 1 pipe my 450 is good for a bit over 32hp at the rear wheel, not exactly earth shattering...

My engine is stock, the only thing I've done really is jet the carbs, chop 20mm off the slide springs, and take the vacuum port on the slides out to about 3mm which has made the throttle a lot more responsive.