Want a 541cc motor w/ 78mm pistons?

[werase643]

yes you will probably have to rejet...big deal

compression should go up...probably...


just remember....when you modify it....stuff will break

[fat_sac]

Wouldn't there be a need to increase the amount of air and fuel to compensate for the new increased size chamber?  Or would the size of the new chamber consequently pull more air and gas in?

If you mill that heads a good bit, then the chamber won't be that much bigger. On that thought, won't you NEED to mill the heads to keep the compression the same?

[dgyver]

The compression will increase with these pistons without milling the head. These have a high dome that really fills the head cavity. I tried running a milled cylinder and milled head and the piston was hitting the spark plug. Even wiith that setup and high lift cams, there was still plenty of valve/piston clearance.

Yep, a rejet is needed. But not like most would think. Larger carbs will help as well as flatslide carbs. They are better than CV carbs.

[budget speed demon]

hold on here, so what your saying is that for under $400 I can make the gs's 487cc motor into a 541cc motor with nothing more than milling out the cylenders and bolting on the new pistons? come on, theres got to be more to it than that... sounds far to easy

[dgyver]

Yes it is that simple.

It may cost less (or maybe more) depending on what the machine shop charges to bore the cylinders, there is a lot of meat to remove. Good thing the GS only has 2 cylinders. I felt like I may have paid too much but then I spent several hours talking to Ed about motor building. It was well worth the cost for the info I gained.

[budget speed demon]

did you notice a big power difference when you put the bigger pistons in? alot more power? less power? the same?

[dgyver]

did you notice a big power difference when you put the bigger pistons in? alot more power? less power? the same?

More low end torque. Bigger displacement and greater compression will increase power.

[Chris_B]

so what kind of a CR are we talking about?

[dgyver]

I have never calculated it, but I guess it would be over 10:1 and probably close to 11:1. Stock is 9:1. You need to run premium gas.

[dbarile]

Is any one actually running one of these on the street?

Sounds like a straight forward mod/rebuild.

Bike, money, parts, machine shop, inclination.

Or am I missing something?

[dgyver]

I do not know of anyone running anything larger than 75's on the street.

A $400 mod is not something that many people are willing to do on their entry level bike if they are planning on upgrading. As for racers and track junkies, that cost is nothing in the quest of more power.

As for your second question, it was answered 5 posts back.

[dbarile]

Second question?? That's a statement, I saw the original post. It was kinda my own personal inventory. Inclination and money I have, parts and a good machine shop I'd have to look for. However, I must admit I confuse even myself.

I agree your not likely to see someone spend $400.00 on an entry level street bike when they are considering upgrading.  But then you don't expect them to put a turbo on it either.

I guess if you go into it knowing you aren't going to recoupe your initial investment you'd be OK. Like owning a boat or plane.

Nothing to stop you from doing the engine mod, running on the street and then going to the track if you wish I guess.

I appreciate all the info you've set out for us.  It's one of the things that makes this message board worth visiting.  

(I just had to use those emoticons)

Thanks

[dgyver]

Ok fair enough.

Yeah, doubtful that you would recoup all of the cost but it would definitly help with resale. I would look into if I was having to do a rebuild, it is not that much more than going with factory oversize pistons.

When I sold my bike with the 555, he drove from MN to pick it up. I bought the bike as a cheap track toy and ended up spending a lot of time and a fair amount of money on it, which I did recover in the sale.

[Chris_B]

I need to rebuild mine  it only has 90 pounds.... Was it 10-11:1 with or without milling the head to? Also, the stock pistons are 74mm, right? are the stock liners thick enough to be honned 4mm?

[dgyver]

My guess of 10-11:1 is with a stock head. Most sportbikes are around 12:1.

74mm is the stock piston size. The stock liners can be bored up to 5mm over for 79mm pistons. Do not bother looking for any 79's... they have not been made since '92. I found 2 of them and built one motor, acquired another and looking for just one more......

[5thAve]

You can get a reuseable copper head gasket from Cometic. They are about $30. I took a stock head gasket and enlarged the holes with a drum sander, no problems.

Does anyone have info on which gasket to buy from Cometic?  I see they sell one for a GS850.  Or are you talking about taking a 4-cyl gasket and cutting it down?
Thanks.

[lopee]

Could something like these be used?

http://www.usa2strokers.com/forum/forum_posts.asp?TID=28757&PN=1

[Merritt]

The compression will increase with these pistons without milling the head. These have a high dome that really fills the head cavity. I tried running a milled cylinder and milled head and the piston was hitting the spark plug. Even wiith that setup and high lift cams, there was still plenty of valve/piston clearance.

Yep, a rejet is needed. But not like most would think. Larger carbs will help as well as flatslide carbs. They are better than CV carbs.

Hate to 'revive' an old thread....

So what did you do about the new pistons hitting the spark plugs?  Could the top of the pistons be milled down or is that something that you just don't do?  Also, did you end up getting new carbs or staying with stock (and if new ones, what'd ya get)?

[The Buddha]

You do not machine the top of a dome piston. The top is like an arch, and machining that is like removing 1 brick from the arch.
You set it to not hit or space the plug up or well ... deck it less, or extra base gasket or whetever.
Cool.
Buddha.

[5thAve]

 
Warning!

Wiseco seems to have changed the profile of the dome on their piston number 4304M07800.


It doesn't fit under a GS500 head any  more. I'm investigating to confirm the changes in the design, and I'm looking for solutions...

Meanwhile, I don't recommend anyone order this piston unless you can confirm it is stamped with '4304PS' on the top, like the ones that Dgyver has.