904 questions

[wolfie]

I want to hear from the guys running 904 kits on this one.

What's been your experience with em? Are you happy with the results? Any regrets?

More specifically, is there anyone here who has converted an 865 to a 904? Was it worth it? What kind of gains? etc.

I'm wondering if the incease in compression ratio alone would make it worth it. Obvioulsy there isn't a lot to be gained displacement-wise.

Educate me please. I'm probably going to concentrate on suspension upgrades this year. Then, possibly cutting up the air box this year if / when I get the time, but I'm thinking I might go the route of a 904 kit in the long run.

 

[sweatmachine]

My bike was 865cc and has been bored out to 904cc with a Wiseco kit. The difference wasn't exactly night and day, but it was an improvment and was definitely worth it. I didn't do any before/after dyno runs but at a guess I think the 904cc kit added 8-10hp and a good amount of torque.

While I had the head off for the 904cc kit I also had 2mm larger intake valves and 1mm larger exhaust valves installed along with a mild port/polish job.

Later I installed Bonneville Performance's 42mm HSR Mikuni carb kit. This kit REALLY felt like it made a difference, even more so than the 904cc kit did, but it might have been the missing link to let the engine breathe properly.

Anyway, I'm happy with the motor (but I wish I had gone bigger ) and here is a dyno print out:

stock bikes typically make about 50-55hp on the dyno, and mine is making about 75hp, and has a really good flat torque curve, I'm making approx 50% more hp than a stock bike. I'm hoping to get some cams in the near future to raise the hp on the top end a bit.

Would I recommend the 904? Hell yeah!

 

[mikenva]

you know i hear this asked alot lately.I think if you look at the dyno test for 790 and 865 motors you will find out both run real close to the same.People say its only39 cc well thats true but think of this there are alot of small bikes with 50cc motors so that is still a gain.The other thing you need to look at is with a bp 904 kit you have a brand new bore instead of useing a old bore that you should have checked and maybe worked on.As far as power goes if both are done right the 904 will maybe only gain a little.If your going through all the work and money for gaskets kits and so on why not go with the bigger of the 2.To make ether work good you need good head work carbs and pipes.The better 904 motors make 80 hp + when they have everything right.

 

[Fast Freddy]

The hi comp pistons will help the most over the bore size. A fresh cylinder will always be thr better way if the budget is there. Heads and cams will be better and larger carb. thanks, and merry christmas freddy How much power are you wanting.770-572-9098

 

[Fast Freddy]

Merry Christmas To All Off You Guys And Gals . Fast Freddy

 

[Kirkus51]

If you really want to go all the way, stroke the bike too. Of course that would mean splitting the cases and now you're into actual Bongo bucks.:hurt:

I always ask myself if I would really enjoy that much more punch when I still think the fun factor is waaay up there for me with a stock motor.:headbang2:

But a 904 would be tasty with a cam and those 42 Mikunis. OOh La La.:cheer:

By the way, I really like the suspension bits I put on my bike. That's like night and day and I'm glad I put my money there 1st.:wave:

 

[localmotion411]

If you really want to go all the way, stroke the bike too. Of course that would mean splitting the cases and now you're into actual Bongo bucks.:hurt:

I always ask myself if I would really enjoy that much more punch when I still think the fun factor is waaay up there for me with a stock motor.:headbang2:

But a 904 would be tasty with a cam and those 42 Mikunis. OOh La La.:cheer:

By the way, I really like the suspension bits I put on my bike. That's like night and day and I'm glad I put my money there 1st.:wave:

I'm debating on doing engine mods in the future, but I'm wondering if a good set of lightweight wheels would serve me better and make the bike more nimble. It would benefit the bike in my opinion to lose weight as well as rotational mass and unsprung weight, much more so than a small increase in horsepower.

 

[Thruxtonboy]

I also changed my motor. One of the first things installed was the 904 kit, this was very important because of the higher compression. Without increasing the compression you want get the results.
After all the changes we made I have a motor with 115 Bhp instead of the 70 Bhp wich were stock. Im talking from a Europe perspective, our Thruxtons here have 70 Bhp stock on the crank. That's about 60 messured on the backwheel. We have now 98 Bhp on the backwheel (with te 904 kit, no stroking) That's an 63% impovement in power and I can realy tell you that this make a lot of difference, my bike is very, very quick indeed!

 

[daleCarlsbad]

904-cc Big Bore Cylinder kit $1100
Stage-1 head $800
Stage-1 Cams $800
Keihin FRC-39 Carbs $1200
Billet Intake Manifolds $100
Predator Exhaust Pipes $550
Airbox Eliminator Kit $300
K&N Air Filters $130
Hi-Po CDI Ignition $250
Nology Coils $75
Nology Ignition wires $75
NGK Iridium sparkplugs $35
Barnett Clutch Springs (green) $40
19-tooth front sprocket $25


Just to give you a rough idea of the cost involved. Is it worth the money....well what I like is that you can now pull that 19-tooth front and cruise at 80-mph @ 4K rpm.

There is so much usable torque that I rarely leave the 3k - 4.5k range.

 

[mikenva]

Keihin FRC-39 Carbs $1200
Billet Intake Manifolds $100
Airbox Eliminator Kit $300
K&N Air Filters $130
wow dale where did you buy all that stuff that is kinda high priced compared to some .

 

[mikenva]

I'm debating on doing engine mods in the future, but I'm wondering if a good set of lightweight wheels would serve me better and make the bike more nimble. It would benefit the bike in my opinion to lose weight as well as rotational mass and unsprung weight, much more so than a small increase in horsepower.

I dont care what wheels you buy they wont help your bike as much as a stroker kit will ,its not just a slight increase in hp its 25 to 50 %.

 

[Bonafide]

I dont care what wheels you buy they wont help your bike as much as a stroker kit will ,its not just a slight increase in hp its 25 to 50 %.

So you're saying a 904 stroker kit will make a Bonnie more flickable, will decrease braking distances and will make a positive change of it's handling characteristics by reducing unsprung weight?

pssst ... he changed topics from apples, to oranges.

 

[wolfie]

Keep the opinions coming folks! This is what I'm after. There wasn't a whole lot of discussion about the big bore kits to be found in the archives.

My next question is about the carbs. When one has done the 904 or other kit, is it necessary to install larger carbs, or will it run right with the stock carbs?

Same question for port and polish, cams etc.

Granted it makes sense to do the heads while you have them off in the first place.

Also, will a big bore kit necessitate heavier clutch springs?

 

[mikenva]

It will run with stock carbs and head just no where near as good

 

[mikenva]

dude he quoted Kirkus51 thats what my reply was aimed at ,guess it didnt come out right. I wasnt saying lighter wheels wont help ,just not as much as the hp gain.

 

[Bonafide]

dude he quoted Kirkus51 thats what my reply was aimed at ,guess it didnt come out right. I wasnt saying lighter wheels wont help ,just not as much as the hp gain.

You trying to say an upgraded suspension wont affect the totality of the bike as much as a 904 kit? That's really going to depend on the person riding it dont ya think? Personally, better handling means more to me than the limited increase in hp you'll see from the money spent on just the 904. I know the bike would be more fun to me with the suspension done first if I had to choose one option over the other.

 

[mikenva]

If you really want to go all the way, stroke the bike too. Of course that would mean splitting the cases and now you're into actual Bongo bucks.:hurt:

I always ask myself if I would really enjoy that much more punch when I still think the fun factor is waaay up there for me with a stock motor.:headbang2:

But a 904 would be tasty with a cam and those 42 Mikunis. OOh La La.:cheer:

By the way, I really like the suspension bits I put on my bike. That's like night and day and I'm glad I put my money there 1st.:wave:

here is what was said that localmotion was replying to hes talking putting a stroker in thats alot more power then just a 904. I agree it all depends on what kind of rideing one likes best as to what he or she should spend there money on.As far as how fast the bike is what you pick up by looseing the extra wheel weight cant add up to the hp and ft/lbs you get out of the stroker motor.

 

[Bonafide]

jeez .... i knew exactly the topic he was replying to. I didnt need you to go back and quote it. It's you, not me, who's not understanding the difference of the topics. The 904ci kit vs suspension changes are TWO different topics. Got it now? I, just like Kirkus, was saying, ... recommendations are first suspension upgrades, then go fast stuff. The costs associated with the increases of minimal gains in HP on a Bonnie are not worth it to me.

 

[GuyM]

On 904 kits...

Why is the TP USA kit $1,195 and the New Triumph Bonneville kit only $499.99 ???

I don't understand...

 

[mikenva]

your right im wrong sorry i was mistaken i just thought we where talking motors on this post sorry.