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Hi All,

Been lurking on this list for almost a year even got some great answers to my questions in the past. I'm just about to start my cafe racer project, I wanted to start with Thrux/Bonnie with carbs but can someone tell me the pros and cons of 790cc v's 865cc I'm told that bigger is better and I had planned to go out to 900+ cc and bigger valves. I haven't really looked at the cylinder head but if the 790 ports are smaller that may even be a good thing because you may be able to smooth out the short radius and get better flow. Mostly at this point I'm talking outa my butt cause I've not seen a Hinkley engine apart, Any advise would be a great deal of help so please chime in. I have a line on four bikes that have all damaged to some extent 2 are 865's two are 790 so thanks in advance,

Richard
 

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I can't speak for changing the valves - out of my league. But I had a 790 and now an 865, both bonnie blacks. The 865 pulls like a train in comparison, but that could be due to it having no airbox, pod filters, and the baffles removed. But the difference is noticeable.
 

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A lot of the gearheads here say the camshafts are better in a 790; in what way I don't know.
If you are going to do big motor work, I don't think it really matters - get the best bike you can for the dough.
 

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Having had both in my garage at the same time in the form of the 2001 790 and my 2007 865 I have to say I prefer the 865. You wouldn't think it feels much different but the extra torque is noticeable. The big thing was the starting. The 790 is a reluctant starter especially under 10C where the 865 is an eager starter.

Personally I prefer the 865. There is nothing wrong with the 790 what so ever just the 865 pulls harder and seems smoother.

BTW, both bikes have BCustoms ark with sleepers on the 01 and preds on the 07
 

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The hot set up is the 790 cam in the 865 engine, you have to also use the 790 igniter to get the full benefit from what i've read.
 

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A lot of the gearheads here say the camshafts are better in a 790...
The 790 a far better performer when applying big bore..carbs...etc. Its the cams...
The hot set up is the 790 cam in the 865 engine...
But it's only certain early years of the 790 that have those hot cams, aparrently. This was gone over on several detailed threads on this topic in the past. Reurgene46, do a search and you find more detail that'll be useful to make your call.
 

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But it's only certain early years of the 790 that have those hot cams, aparrently. This was gone over on several detailed threads on this topic in the past. Reurgene46, do a search and you find more detail that'll be useful to make your call.
nope on the 360 motors all of the 790 cams are better then the 865 the early 01s are better then the later 790s though.
 

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If you go with TPUSA 813 cams & igniter remap on an 865 you'll be very happy with it. It will lose nothing on the bottom & gain big-time up top. Way fun to ride.
 

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Maybe I should swap the cams out of that 01/01 790 Bonneville sitting in my garage..... and put them in m 865 :)
 

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Since we're comparing the cams, tell me if I have it right: the 790 cams have more overlap duration, overlap as that little window when intake and exhaust valve are open at the same time. So they can make more hp at higher rpm. But they take a hit on emissions so the 865 cam has less overlap because of stricter emissions regulations. Let me know if this is close. I realize there's probaby more to it. But that's usually the method with performance cams.
 

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From what I understand this had nothing at all to do with emissions and much more to do with getting the bike to develop torque down at the operating RPM's. Dyno results have shown that the 865 engine delivers as much as ten more HP and close the same in torque increase at mid range operations. It also develops more HP on top and gives more top end than the 790. I owned a t-100, 03 and an 04 Thruxton at the same time and the Thruxton was smoother, faster all the way around and had at least 6 to 8 mph more top end, it also got there much faster then my 790 motor. The 865
had much more snap in the mid range (real world riding here), I found the 790 motor was slow to get moving and it developed power in a very short burst near the red line. It had a horrable power delivery method. They had the cams wrong on those bikes, in my opinion.
 

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From what I understand this had nothing at all to do with emissions and much more to do with getting the bike to develop torque down at the operating RPM's. Dyno results have shown that the 865 engine delivers as much as ten more HP and close the same in torque increase at mid range operations. It also develops more HP on top and gives more top end than the 790. I owned a t-100, 03 and an 04 Thruxton at the same time and the Thruxton was smoother, faster all the way around and had at least 6 to 8 mph more top end, it also got there much faster then my 790 motor. The 865
had much more snap in the mid range (real world riding here), I found the 790 motor was slow to get moving and it developed power in a very short burst near the red line. It had a horrable power delivery method. They had the cams wrong on those bikes, in my opinion.
Nope thats not true both make about the same rear wheel hp .The 865 makes more torque because its bigger but it runs out of rpm quicker then the 790.It had everything to do with emissions they could no longer pass emissions with the better 790 cams so to keep the hp the same they had to use a bigger motor.The 790 came with a 7400 rpm rev limit it never ran out of power the 865 has a 8000 rev limit it stops making hp before the limit.The 865 is pretty much dead meat at 7000 rpms + The 790 with the rev limit set at 8000 or above will keep gaining hp if tuned right.With the right cams a 865 will pull up in the rpm range allso and wont loose the torque on low end.865 cams are just junk.
 

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If the cams in the 865 are junk, why then when I owned both a 790 bike and an 865 bike the 865 engine could walk all over the 790 in any gear at any speed? HELLO? I've owned both side by side and have ridden the bikes back to back many times, the 865 performs better, that was and still is my conclusion. Those 790 engines develop power at the very high end of the rpm scale and only in a short burst. I found I was always having to downshift (790) to get the bike to move out of it's own way. Not so with the 865, it has good torque and power snap at a wider range in the rpm range. I think they got it right with the 865 engine.

Dyno results i have looked at on both engines show that the 865 develops almost 10 more HP and torque in the mid range then the 790 engine, it also develops about 6 more HP at the higher rpm's.

In fact my 03 was rated at 61 HP from the factory and the 07 is rated at 67 or 69 something like that.
 

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I have them side by side in my garage as well as I have previously stated and I would take the 865 over the 790 anyday.

Maybe the 865 plus the 790 cams are just that much better yet???
 

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Over an eighth of a mile drag race, Mrs Nickwiz 790 was consistently 0.8 or 0.9 secs slower than my 865(both are running TORS and jetting with standard airbox etc) We have the timing tickets to prove it. That leads me to believe the 865 has better low down power and torque.
 

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Lots of folks on this board know a lot more about engine process than I do so please help me out where I go wrong here:

From what I've learned on this board, the 865 cams seem to be optimized in such a way that high rpm horse power is more limited than it would be with a different cam. I take Mike's word for it that a primary reason they did this is to meet ever stricter emissions requirements. But I think the same optimization that robs high rpm HP may also benefit lower rpm torque.

Based on my limited understanding of how cams work, at high rpm in order to keep making HP you have to have a little extra overlap between the exhaust and intake valves. At high rpm the valves are closing so quick that it's more and more difficult to evacuate the exhaust so the extra overlap duration allows them to keep making power. Up to a point having more overlap helps make lots of HP at high rpm.

However, at lower rpm that extra overlap duration kills back pressure somewhat and so decreases torque. Race engines usually make plenty of torque and tend to operate at high rpm, so they mostly focus on high rpm HP. The extra overlap has the side effect that some unburned fuel is able to go straight through the engine during the overlap--some intake gas mixture gets sucked right out the exhaust valve during that overlap period when they're both open. That's bad for emissions so it's hard keep those kind of cams in a bike that has to pass regulations recently.

In the last couple decades, V-Tec and other variable valve timing systems are designed to allow both less overlap at low rpm and more overlap at high rpm (by varying valve timing of course). But on the Triumph engines the one cam set's lobes determine valve timing across the whole rpm range. (Having four valves per cylinder may allow the cams to address this tradeoff--I'm interested in learning more about how the 865 and 790 engines use the dual exhaus and intake valves.)

When Mike says the 865 cams are crap, it might be for reaons I'm not aware of, but the reason I think I understand is that he prefers cams that can keep making HP up throughout their rpm range rather than petering beofre redline like the 865 cams. If you want and need big HP at high rpm (like racers, many performance riders and tuners) then any cam that limits that is a disappointment.

But would it also be useful to view the 865 cams as torque cams? Cams that sacrifice some high rpm HP in favor of lower rpm torque--probably by limiting overlap duration. (In other automotive areas extreme versions are called "RV cams" because heavy RV's need torque more than they need HP at high rpm.) If so, it would explain what Mike says happens where the 865 stops making hp in the upper rpms at some point below its redline--the 865 would need more overlap to throw air/gas mixture that fast through the engine.
 
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