Bonneville America Forums Forums Technical Forums Performance Mods big bore pistons

That was the very reason I didn't want to go further than the 904. A properly set up 904 is a MAJOR difference from a 790 or 865 with all the normal mods. There is a huge torque difference (66 ft. lbs. @ 3400 on dyno) making it like you definitely put a larger engine in the bike.

If I ever feel like that's not enough, I'll go with a whole bigger bike. I've had my 904 for almost a year and a half and still can't believe the difference. It cost me a total of $1250. including parts, labor, dyno runs and tuning. I'll say it again, no better bang for the buck. The fact that it is every bit as reliable as stock should make it a no brainer.

if the 904 isn't enough, head work and carbs should get another 10-15 hp

Question regarding the Wiesco kits.
When you bore out the barrels to fit the sleeves, what is the interference fit?
Do you have to hone out the sleeves after they have been pressed in?

I got mine already pressed from bonnevilleperformance.com

You know, I never really thought of my 790 as underpowered. After doing airbox and carb mods and when I had the 17T sprocket, my Harley friends had trouble keeping up with me.

I changed to the 18T and liked it. But then felt, especially two up, that I wanted more oomph starting out. After the BB, I decided to stay with the 18T. I have plenty of oomph starting out and never look for a 6th gear. I also love the slightly lower pitch the BB makes with my SS pipes on the highway.

I never felt that the 790 was weak, either. It just doesn't have the torque that I wanted for 2 up cruising. Now I can accelerate just as fast with my girl on the back as I used to be able to solo with the stock motor.

I'm not knocking the 790, just noting that by comparison it "feels" weak. I put 25,000 miles on my bike before the BB. It always felt adequate. I have just become accustomed to the much better torque of the 904. I wouldn't be happy with a stock 790 again. If for some reason i found myself in posession of one, I would immediately do a BB on it. I highly recommend it!

I can certainly understand that. I probably wouldn't like my "old" bike near as much as I did now that I'm used to the BB version.

Thanks Mic

I finally got to ride the beast (12:1 in picture). It has be a slow torture as winter keeps dragging on, but we got a couple day warm up, so...

So far only about 5 miles on (rode to work this am) the roads are still slick with snow melt and salt/sand. It seems to pull harder than before and quicker. The balance seems a little off but not to bad, I'll have to get at the carbs tonight. If the sunshine holds today I have to skip out early and try for an extra few miles. I am really wanting to open it up.

This is in reply to the question of 100 hp .You can make that much with 988 with my ported head and cams on pump gas. 770-572-9098

Freddy, what is your recommendation for the ultimate 904 set up (to get the absolute most out of it)? Having asked the question... what would the numbers be after all the mods were made?

Triumph-Performance USA 11.5 904 kit my stage 3 heads will make it 10.8 . bored stock carbs and intakes, T-P USA mufflers ,pie man module. 85-90+ hp ,ALSO 39FCRS for the guys who can tune them,great carbs though. This is a shim over bucket kit with T-p USA 813 cams .this will really fly.

Is that for the 709 or the 865?

Does not matter as long as you have a 904 kit.

I thought that I should wrap up this post as I finally got the bike runing where I expected to to be when I started this BB project (arctic cold weather gone for a few months now). Spent a couple of hours on Saturday with the gas analyzer tweeking the carbs. After 3 jet changes the motor really running sweet, pulls hard from the line even with the 19T sprocket. Still have yet to get it onto a dyno (looking for a convienient shop in my area).
All of the wrenching and machine work I did myself. So here is what I done did:
D&D 904 kit - 12:1 JE pistons, nikasil barrels
stage 2 head work
matched intakes to manifolds to rubbers
Thunderbike 270-B cams
green clutch springs
K&N pod filters
Scepter pipes
jetted K160, thruxton needles, 2 shims, drilled slide 3mm
light weighting efforts (bike not me) approx 20 lbs.