 Re: Big Bore Versus Cams
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There is no greater thrill than being shot at with no consequence. -Churchill
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Re: Big Bore Versus Cams
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Joined: Jan 2005
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Bar Shake
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Bar Shake
Joined: Jan 2005
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Quote:
Cowboy, I don't think you understand what hp and torque are. They are directly related, and one is not made "near the top of RPM band" or "at lower RPM". You cannot increase torque without increasing hp.
Yes, the 865 cams make more POWER at lower RPMs, but increasing the bore and compression will net more POWER at all RPMs, not just a specific range.
This link has been posted before:
Horsepower, torque, etc.
Interesting reading for those who are interested.
Boring as heII for those who aren't.
Here's a bottom line for those who don't want to read through the whole thing:
horsepower = torque at 5252 rpm.
Always.
This is known as a constant.
At 5252 rpm, your torque and horsepower will always be the same.
That is; if you have 100 hp at 5252 rpm, you will also have 100 foot pounds of torque at 5252 rpm. And vice versa.
Of course, where your maximum hp and maximum torque come in are a different matter entirely.
Contra todo mal, mezcal; contra todo bien, también
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Re: Big Bore Versus Cams
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Joined: Aug 2006
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another point, the stock carbs suck. I'm going to get some mikunis as soon as funds allow, and I may also look into the thunderbike cams for my bonnie...
most people that get the HSR mikuni kit from Bonneville Perfomance report more gains w/ the carbs than the big bore, even on a stock 790 you can gain rear wheel dyno-verified 10hp.
Last edited by sweatmachine; 07/28/2007 7:45 PM.
There is no greater thrill than being shot at with no consequence. -Churchill
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Re: Big Bore Versus Cams
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Joined: May 2007
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Worn Saddle
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Worn Saddle
Joined: May 2007
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Quote:
you also need to understand that triumph is a small company who is still not in the main stream aftermarket. performance mods are expensive on other brands nevermind smaller ones like triumph, so bang for the buck performance and triumph do not go together very well. especially if you have to pay for the work to be done
Frank
I never expect to "save" money on restorations, speed shop work, or, especially so, fitting my ride to suit me. I bought the Triumph knowing full well I'd have to pay a bit more and be more creative in finding usable components to suit the quest in order to be in this elite group. What I enjoy here is a civil debate about how everyone is making their ride suit their needs and the vast perspectives that such a diverse yet single minded group brings to the table.
Triumph, it's an obsession, now, isn't it? 
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Big Bore Versus Cams
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Joined: May 2007
Posts: 65
Member
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Member
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Quote:
the stock carbs suck. I'm going to get some mikunis as soon as funds allow
I agreee if you are building a 'race macnine'.
I've tried to crakc the throttle a few times from stop and it bogged. I'm guessing because of the vacuum assist? One ting I have found, if I roll on the throttle, she really comes alive over 4K and after 5K,
So if you want a 1/4 miler, change the carbs. If not, roll on the throttle!!
(Not busting on your efforts Jeff! I really like hearing about what you are doing!)
03 Red/Black Speedmaster
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Re: Big Bore Versus Cams
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You dont need to be building a race machine to swap out the stock cvk's for mikuni's. While they do not provide the "drivability" that the cvk'
s do...( they are flat slide carbs )...they far outperform the cvk's. They just perform differently, you need to be smooth when applying throttle.
I gained 8 rwhp from installing 42mm mikuni's....worth it imo.
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Re: Big Bore Versus Cams
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Joined: Jul 2006
Posts: 1,337
Learned Hand
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Learned Hand
Joined: Jul 2006
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sweatmachine: Man, I'm sure your bike runs great. But you cannot get away from the fact that your HP starts falling off above 6000 RPM, where a 790 motor keeps pulling.
BTW- I understand what HP and Torque are. I've built hot rods for 20 years- petrol and diesel. Do I know it all? No. Not saying I do. But I do understand how to build power and what does what in an internal combustion engine, spank you very much.
This thread started about making HP. The 865 was designed to make more torque than the 790. The 790 beats the 865 in HP. The two "wheelie points" you describe are in the sweet spot of your torque band.
Let me put it this way, (as if it is really this important at this point in the discussion)- a 904 kit in the 865 will NOT make as much HP as a 904 kit in a 790 motor. It will, however, make more torque. If you want the max HP numbers, you'll need to upgrade the cams.
The original post was about the best bang for the buck for HP- not overall power. If you ask the question what makes the most power overall power increases for the buck, I'm certain the 904 would win.
Maybe I'm out of context or being too literal here. When we were building power in diesels (I just sold mine last month), some guys were building them just to blast dynos. They weren't interested in driveability or much of anything else- just big HP. I took this thread to be along those lines.
If I'm all wet, well I've been wrong before and am happy to admit it.
But here is the real deal, and oldroadie has touched it. If you are indeed making 79+ RWP (and you very well may be) how much torque are you making?
2002 Bandit 1200/ GSXR cams/ 1277 BB Kit/ Holeshot header and can/ 38mm flatslides/ a good head/lotsa hp/lotsa tq- lots of rear tires...
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Re: Big Bore Versus Cams
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Joined: Jan 2005
Posts: 3,753
Loquacious
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Loquacious
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since carbs were brought up, i changed my stock 790 (for now) to the 42 miks. at first i was actually nervous i made a mistake, but when i get the jetting better and figured out how to ride with the new carbs i am very happy. the bike starts better, idles better, and just feels better all around. but they are not for everyone
Frank
(Former)05 BA tbike pipes, ai removed, Freak, mikuni hsr 42's, 904, ported/polished head, 1mm oversized valves
NOW-2010 silver and black tbird
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Re: Big Bore Versus Cams
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Joined: May 2007
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Worn Saddle
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Worn Saddle
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It only makes sense to up the venturi size once the engine is bigger, a bigger drinking straw for a bigger gulp. A sniffer and a dyno would be really handy trying to get that A/F ratio close to 13 across the rpm range, but I'll bet checking the plug tips and the seat or your pants wil get you really close.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Big Bore Versus Cams
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Keep in mind as well regarding cams that the profiles are pretty different between the 270* and the 360* motors. As for the 360* motors it's the difference in the amount of overlap that defines the higher hp for the 790 motor.
In my 790 I advanced the exhaust cam 1 tooth to achieve a "inbetween" overlap figure. I see no reason that the 865 engine couldnt benefit from a inbetween overlap figure as well by retarding the exhaust cam 1 tooth.
I'm suprised noone has tried this.........
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Re: Big Bore Versus Cams
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Joined: May 2007
Posts: 6,432 Likes: 1
Worn Saddle
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Worn Saddle
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Actually, I've been pondering stealing the 4 degree timing advance from sa750.com and seeing if a similar mod could be done to the 790...
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Big Bore Versus Cams
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trim, I've been thinking about retarding the exhaust cam 1 tooth, which way would I need to turn it?
I think it still would not have the duration of the 790 cams though, so it would not be as good or make as much hp, but like you said it might be a nice in-between.
Last edited by sweatmachine; 07/29/2007 7:58 PM.
There is no greater thrill than being shot at with no consequence. -Churchill
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Re: Big Bore Versus Cams
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Jorgen would be the best person to ask this question....I honestly cannot recall which way I rotated mine when having the exhaust cam advanced.
I'm wiped out....and going to bed
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Re: Big Bore Versus Cams
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Joined: Jan 2005
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Bar Shake
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Bar Shake
Joined: Jan 2005
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Quote:
I've been thinking about retarding the exhaust cam 1 tooth, which way would I need to turn it?
I think it still would not have the duration of the 790 cams though, so it would not be as good or make as much hp, but like you said it might be a nice in-between.
Changing it a tooth cannot affect the duration, that's determined by the grind of the lobes.
It will just change where in the rotation the valves open and close. If they're opening later, they will also close later.
It would be interesting to see the dyno results on the 790 as well as the 865 after changing cam timing.
A computer model would be interesting too (if anybody has that capability).
Specialty performance camshafts are ground to open and close the valves faster (steeper ramps) and hold them open for specific (usually longer amounts of time. Of course the timing is figured into that also.
Of course the faster opening and closing is harder on all the components, resulting in a shorter service life.
Changing cam timing by the seat of your pants is a really good way to burn valves.
Contra todo mal, mezcal; contra todo bien, también
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Re: Big Bore Versus Cams
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All you have to do is look at the spec's in the service manual. Clearly with 39* of overlap that the 360* 790cc motor has, advancing the exhaust cam 1 tooth cause's no issues.
This mod changes the overlap from 39* to 24*...still more than the 360* 865 motor, giving you a "inbetween" figure.
As for burning valves and such...it is a non issue with the 360* motor. I know this cause I've done the mod, it works.
As for the 270* motors....I have no experience with this, but judging from the overlap I wouldnt advise this mod.
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