Bonneville America Forums Forums Technical Forums Performance Mods What is involved with a big bore?

I'm not a good enough mechanic to attempt this myself, nor do I have a garage to work in. So I would be taking it somewhere.

But before I start looking into that - I have only the vaguest concept of what all is involved. Or how much more is involved between say the 904 and the 988.

Not to steal Pat's thunder, but...

http://dinqua.com/Triumph/BigBore/bb1.html

If you're paying someone then all you really need to know is a 904 will cost you around $1500.00 give or take, and a 988 would be a couple thousand more.

A 904 kit is bolt on parts....a 988 kit is a stroker motor. Far more costly.

I have absolutely no idea how relevant this is but I notice you have the '05 SM (as I do) which has the 865cc engine.
I mention this because I have noticed that the 904cc BB upgrade is normally only done (at least on this board - unless I am wrong) on the 790cc engine.
Is this a valid consideration?

many have done it on 865's and been very happy with the results

Frank

on the rat site and most are standard bonnies. should have specified that sorry

Frank

It is a consideration - I've wondered what another 39 cc would really get. So I'm interested in what the extra work necessary for the 988 entails as well.

With the 904, what you will get is more torque, probably not much more HP.

The 904 is definitely more cost effective for the 790s

OK, I'm convinced. I plan to have my dealer do the BB 904 on my bike next month. I have a 790 and primarily want more low end torque. The dealer (D&D Cycles, Pensacola) offers a choice of pistons. Either the standard Weisco or their own designed slightly higher compression pistons. I assume the higher compression produces a little more hp, but low end torque is my main objective. What would be the pros/cons of the higher compression pistons?

What is the higher compression?

I think it is about 10.2:1 or 10.5:1. They showed me the two pistons. The higher compression one has a slightly dished out head.

The Wiseco is 10.5:1

Then it must be slightly higher than 10.5:1 - I'll find out. I'm still curious if there are disadvantages to going up from that to get more hp. As I said before, I'm not as much interested in the high rpm hp as wanting more low end torque. My book shows the stock set up as 9.2:1. The dealer told me you could still use mid-grade gas in both versions. Seems like if it's more than 10.5:1, you would need premium.

stay with 10.5 so you don't have to worry as much about pinging. plus it is very safe for a street motor ridin a lot

Frank

I would suspect that a bit higher compression would not hurt these bikes and would probably get you a little more torque.

After the big bore, I could not get rid of pinging until I switched to high test.

Used to run lo grade...