Bonneville America Forums Forums Technical Forums Performance Mods 904 big bore or thruxton motor?

hello,

maybe pat or someone who's done the 904 can help me out with my questions here.

i'm hoping someone can give me some feedback. my motor recently took a crap on me at 50,000 miles. i haven't taken it apart yet but i think it might be a bent valve or something came loose in the head. it just started making a bunch of clackity clack and that was it. fortunately i have someone who "rented" me a temporary motor while i take care of my problem. so i'm running that for now. i've been wanting to do the 904 big bore for awhile along with porting the head, and possibly the b/c stage I street cam, AND fcr or cr's.

my question is about the difference in bonneville, thruxton, and america motors. my understanding is the 270 firing angle makes for a smoother acceleration while the 360 provides more torque and power. if i'm after torque and power then is there any advantage to having an america motor?

next question is about compression? i don't really know why more compression is better but i know it is, if someone can explain. and i see that the thruxton comes 10:2:1 while america and bonneville is 9:2:1. what kind of compression does the 904 provide?

which leads me to my next question. i often go on long rides of 1,000+ miles and i would say i ride my bike pretty hard (80-95mph.) if the 904 adds more compression than originally intended for the motor, is something more likely to break under the stress? especially with more miles?

i've been wanting to upgrade the performance of my bike for some time from the setup i've been running (epco pipes, no snorkel, thrux needles) and i'm finally approaching the time to do it. but i was checking out the specs of the thruxton and i see they're almost at what the 904 does. there's a couple thruxton motors for sale now so it got me wondering. i plan on keeping this bike forever and don't want to mess with performance upgrades in the future. may i also add i would be doing this all myself and have never taken an engine apart before. but i do feel that i am pretty mechanically inclined.

what do you think? what should i do?
unbiased opinion please

Differences in motor:
Depending on the year of your bike the motor can be a 790 or an 865 cc. This won't matter much if you plan on going BB so just know that its one difference. For me the easiest way to know which is which is by color of the jugs. Grey are 790, black are 865.
Next difference in motors is the cranks. The America/Speedmaster/Scrambler is a 270 degree firing motor which is what gives them that "lump" idle that makes them more "cruiser".
Bonnies of all models and Thruxtons are 360 degree firing motors which makes them rev up faster (this is what I hear).
Both motors, aside from that crank, are mostly the same.
Keep in mind also that the 360 motors sit in a different frame and that means that things like the intake manifolds are different. The straight intake manifolds of the 360 motors won't work in the angled back backbone frames of the cruisers. You can reuse you intakes but that straight intake is part of what makes some of the power in the bonny motors vs the cruiser motors. Im sure its minimal but its part of it.
The bigger expense will be finding the right ignitor. The 270 ignitors will not work with the 360s due to firing degrees/timing.
From what I can tell motor mounts and such are the same. Side covers are different but are interchangeable (someone on there has done it).

If your motors out and the bottom end is still in good shape and you don't want to spend a ton of $$ the BB kit is the way to go. The BB with carbs, headwork and cams gets some great power gains. With tuning your knocking on the 100hp door. No one may answer but your knocking.

I see your in SoCal and are close enough to BC and TPUSA that it would be well worth going to both and tailing out your options. I don't know if BC has a shop where they do work but I know TPUSA does and Carlos is the guy to talk to to at TPUSA. I spoke to him a couple years ago and have a plan for my bike just not the money to follow through with it.

To answer your compression question: form Wikipedia

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency"

As you increase the compression of the engine you also increase the forces inside the engine (think about the dragster that blows up during a 1/4 mile run), there by making a higher compression motor less reliable or total engine life would be less...These numbers are not excessive or high (9 to 10:1)...if you start talking about 11 to 12 to 1 your are building maybe a 20,000 mile motor

Just got reminded by the post below...as your compression goes up you need to run a higher octane gas

When building a motor you want all the parts to work together so Talk to both TPUSA and B/C...I know that B/C has a "shop" but I don't know if they would on other bikes or just build test bikes....

One more think about the FCR carb...it is a very good carb but doesn't have a choke (fast idle)...you have to give the throttle a couple of turns and start the bike and keep the idle up it is warms up....for me this would not be a option because I don't want to idle the bike on my street at 6:00 AM when I go work...Right now I pull the choke start the bike and idle/low speed to the main street...keeps the neighbors happy.

You asked for unbiased opinions, but I'll chime in anyway.
I think you have it bassackwards with the 270º vs the 360º motors. The 270º is the preferred motor for the flat-track racers due to it's quicker response and torque ratings. I don't know all the high tech specifics of it but I have spoken with Bill Gately at Bonneville Performance and this seems to be the opinion of the serious racers that run our bikes.

All that said, I would stick with the 270º motor and toss in a big bore kit with upgraded valves as well. There are several kits out there now and the differences are what will make your mind up. There is the Wiseco that I use, has steel sleeves that samo say are not as long lasting as the others that bore and re-coat the cylinders with nickaseal (sp?). The issue I see there is that the cylinder walls are thinner and more prone to breaking under severe conditions. But I may just be speaking out of my arse on that part. I know mine has about 30k on the wiseco and going strong. But I have also known one to blow up, one to have to re-ring and now another that tells me he is using a little oil with many thousands of miles. I dunno, your choice, do the research and talk with actual users of the various kits, not just the sales guys. Real results are what you want.l

Also I believe the wiseco kit gives you a 10.5:1 compression rate as well, I run mid grade gas as regular here in the tundra has 10% ethenol and the regular can ping or chatter a bit if I get gas at one of the cheaper stations. 89 octane is no problem though and I think premium is a waste of money, dyno'd both with no difference.

I'd go with the 1050 triple upgrade

Dinqua, I have the 904 Wiseco pistons with the boreded nikasil (sp?) cylinders. No problems at all. Supposed to be 10.5:1, but I suspect it is slightly lower. Had some oil usage, but that got fixed. Turned out to be a bad valve guide stem seal. I always ran mid-grade even after the big bore work, but, changed to premium last year. It seemed to run smoother and my mileage is slightly better. Maybe just enough to offset the cost. No noticeable difference in power.

My bike also likes non-ethanol fuel the best, but I can't find it around here. When in NC, I was buying non-ethanol fuel and my mileage was consistently 10% better. If I could buy non-ethanol mid-grade, I try that.

I just checked the Triumph site and they list the Thruxton at 68 hp and the America at 61 hp...the difference is the compression ratio between the two motors

The hp ratings by Triumph are at the crank, not the rear wheel. On a dyno you can probably drop 10 Hp from Triumphs ratings. My 904 is getting almost 70 Hp at the rear wheel tested on multiple dyno readings. That's 15-20 Hp more than stock engines.

so far (for me at least) the 904 has worked out quite well, the 904 speedmaster has circumnavigated the great lakes, twice, as well as numerous trips 500 to 1000 mile weekends and a trip from Wi to the black hills. I tend to ride hard (like it was stolen) and put in long days. I have no complaints about the reliability of the 904.
If doing over, I would be a bit more hestitant to go with the 12:1 compression, not because of reliability but because of fuel availability, several times got into remote areas where only low octane could be had and the motor did not like that. Realisticaly 10.5:1 or 11:1 would be more viable for the street.

on the other side, I have a speedmaster with over 90hp at the rear wheel and it is just plain fun to ride.

What sort of mileage do you get ?

I get about 36 around town and 40 on the highway. Roughly about 5 mpg less than before I did all the mod work on the bike. Well worth the trade off.

I get a bit over 18.5 km per litre on the highway from my 865cc with pipes/air filter/re-jet/one shim on the needles.

About 45-46 mpg a U.S. gallon.If I did the calculations right.

Or the same 5-6 more that you had,before the B.B and mods..
That's very good mileage for the power you have.

38 to 40 mpg pretty consistant