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6305 Views 33 Replies 12 Participants Last post by  RedBird
.....try again brooksie
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Thanks Born Again! Great help. Red-I was thinking along those lines on hp results. The bike gets a few hp just by going with a properly jetted set of pipes. The air box should add 4-5 more hp, putting the little beast in the mid-sixties. AFter that, displacement would be the only way to get real gain. I just don't think I'm ready to hog the poor little 790 out yet. She's been very reliable as is, just want to get around 70 hp, 120 top end.
"...120 top end...."
You sure don't need 70 Hp to get 120mph, IMHO, if that is what you are referring to. A NARKed and tuned 790 will run 120 mph (actual speed--not speedo indicated) easily with a 17 tooth counter sprocket.

I still believe the greater benefit from the BB kit is the higher compression pistons--not the extra cc's; however, we won't know until someone offers high compression pistons for the stock 790 bore or even the 865 stock bore.

imo your exactly right's the added compression that make's the 904 such a smooth torquey producer.

I recall reading that someone had installed aftermarket carbs and saying they felt the carbs added as much or more than the bb kit...after installing my carbs, I agree with that comment and would recommend anyone looking for performance enhancement to do so.
On 2006-11-03 11:06, FattRat wrote:
I had an '05 790 with the restrictor plate removed from the air box, K&N, bell mouth, Thruxton needles, and self modified D&D's. On the same Dyno I frequent for bikes of past, it Dyno'ed at 61+ SAE.

Then I sold the bike.
However before I sold the BonnieBlack, I returned it to it's stock configuration save for the ristrictor plate. I returned the stock needles, stock air filter, and stock pipes onto the bike.

My story:
I noticed little difference in the pleasure of riding my Bonnie when it was tamer and quieter with the stock/"restrictive" parts back in place. It was smoooother, the power more linear, and I can't help but think that those rubber mounts on the stock mufflers ELIMINATE a large bulk of (muffler) vibration.

What configuration will I settle on for my Red project bike?
I'm not removing the restrictor plate, nor do I have ANY desire to eliminate the air box. Gotta have the K&N & bell mouth ('cause I paid the big bucks for 'em.....), going with Thruxton needles, and plan to run my modified D&D's. And someday I may actually weld the factory muffler mounting system to the cans I end up with at the end of another year of riding.

My point?
Modify, rip up, tear into, Sawsall, tweek your Bonnie all you want; but do it because you enjoy doing that sorta thing. The gains in performance will be ever-so-slight, and your Bonnie twin will remain a Bonnie twin. You can not make a tame retro cruiser into a screaming big-twin monster simply by spending big bucks on cams, an air box elimination kit, and loud pipes. But if you like doing that sorta thing, there sure are a lot of parts-is-parts to choose from!
RATDUDE, as usual your gray beard all knowing Yoda common F***ing sense comes through. As a hobby modifying a motorcycle is he!!afun. The endless hours of tinkering, changing, polishing, adding chrome fru fru is some of the best fun a 15 year old man can have :-D .

The bottom line is the Bonnie/Thrux is never going to develop super bike HP and TORQUE. Enjoy what you have. My BMW R100 (past away Dad's) is a Torque Monster in compression to my other twins ( 1 litre does help). I enjoy motorcycles just like all of us on this forum. So did my Dad. He past on a few months ago at age or 94 and rode his BMW R100 with in a few days of passing :cool: :-D

GOD bless motorcycles and those who ride them.

Steve, Gears and all things Mechanical :lgh: :yayyy: :chug:
Red-you have given me great encouragment. Stop me before I mod again! I think I'll go with the air box kit as mentioned here and stop there. I really do not think I can make the Bonnie a Super bike. I have the 955i for that (or a GSXR in any size if needed) to handle the big speed stuff. I just love to tinker on the bike and get more out of it, and I am not really sold on the big bore kit. Thanks for the help again and ALL the great feedback!!
OK, so I have the TORS with a correct re-jet for them, other than that no performance mods, well i do have AI removal, but thats just for less pipe's bluing I think. I mentioned doing the airbox removal to my dealer when the bike was new, and he said that there would be a lot more intake noise (which I don't know if thats a positive or a negative) and that since there's a perfectly functional air filter and box on there now, why hack it up?. He also mentioned that with NYC being a city, with a little more soot, etc, in the air, and the generally more heavy duty kind of riding (stop and go traffic etc) that the stock air filter would do a better job of filtering the air entering the motor. So my question is, do these K&N filters allow better air flow to the motor at some expense of letting in foreign particles that the stock filter would otherwise have caught, and what kind of and how much extra noise is there with this mod. I like to cruise on my bike at pretty low rpm and really dont rev over 4k rpm's very often, so I think this mod isn't for me, but I am curious, I mean I was considering doing it at first.
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Gelamin-you bring up some great questions. I was against doing the air box removal for many of those reasons, but I have talked to so many people that have done it with nice results that I want to do it to enhance performance. Yes, the air intake in massively increased, which in turn allows quicker response from the engine and translates into increased horsepower somehow. If you commute with the bike, I would leave it alone. Others at this forum will have practical experience with this mod, but I am tricking the bike for casual riding and plain tomfoolery.
I'll try my best to answer some of your recent questions:

You have a Scrambler which is an 865cc 270 degree twin. The airbox mod for the other 270 degree twins (and I assume for the Scrambler) from is called the FREAK. I know little about 270 degree twin's response to airbox removal; however, I would suspect it would be similar to the 865cc 360 degree twins which to my understanding is not quite as beneficial as for the 790cc 360 degree twins.

For slow speed riding, you would be wasting your money and time doing this mod, IMHO. The hp increase on my 790 is mostly above 6000 rpm which is well above 80 mph in 5th. For the individual cone filters to let more air into the engine, they would have to do less filtering than the stock filter, IMO. The greatest restriction to the stock air intake setup is not the stock air filter, IMO--it's the restrictor plate in the airbox that restricts how much air can get through the airbox--again I'm not sure if the restrictor plate is removable for the 270 degree twins. The restrictor plate can be removed from the 360 degree twin airboxes by removing the right side of the airbox and simply pulling it out, and then resealing and replacing the right side of the airbox--there have been past threads on this with very good pics...Ridge's pics come to my mind first.

Intake noise: Ever hear a V8 auto engine with a 4-barrel carb without an airfilter? That's about what it sounds like--just not as loud--but it seems as loud because it's right between your legs. :-D Some do not like the sound--others like it...I'm one of the latter. The sound begins around 5K rpm when the throttle is wide open. The sound is often described as a "honk" sound. It is loud enough to mute my music on my Starcom 1 motorcycle communications system--which means the microphone inside my helmet is picking up a pretty loud volume of sound. :-D You would never hear this intake sound under "normal" (whatever that is) riding conditions, IMO. In other words, if you shift below 5K rpm and cruise under 90 mph, you will never hear the intake "honk". It "honks" when the throttle is wide open with the rpms above 5K. I cruise a lot at 5500 rpm and I never hear the intake because the throttle is far from wide open.


[ This message was edited by: RedBird on 2006-11-06 19:59 ]
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Despite some of the comments on this thread, airbox removal, K&N filters, rejetting and head work DO work, and worked very well on my bike. This work was completed by a highly respected tuner and represents an overall power and torque increase in excess of 30%. Stock Thruxton's on his dyno make 52 to 53 bhp before modifications.

Here are the dyno results from the tuner's alleged conservative rolling road. He indicated that other dynos give higher figures than his one. This guy prepares race tuned engines for club racers, so knows what he is doing. The figures shown below are measured at the rear wheel, not crank.

The most important point here is not the actual figures, as dynos all read different, but the improvements achieved with the modifications, all measured on the same dyno, carried out by the same operator.

1st dyno chart shows bike fitted with TORS (60.1 bhp), and afterwards with airbox removed, K&N's fitted and regetted to suite (65.1 bhp).

2nd chart shows airbox mod (65.1 bhp) and afterwards with head gas flowed and skimmed (now 10.4 to 1) now 70.6 bhp

3rd chart shows the torque figures for the 3 stages starting at 48.6 ft Lbs, ending with 61.3 ft Lbs
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I shouldn't be reading this thread...After I crashed and broke my thumb, I told myself its the straight and narrow from here on out, speed limit is good enough for me. Then I read this and start thinking "well, a little more torque couldn't hurt" and "It would be cool if teh bike was just a little faster" and "I wonder what a guy could get out of the bike with a airbox removal and the TT pipes I just put on..."

Also, I wonder if the changeover to all teh bikes now having teh 865 engine means no-one is going to bother making the high compression pistons etc for the 790s? There are a lot of 790s out there though..When I finally put on 90,000 or so and need to do a rebuild, I think I'll do the big bore, maybe a cam....At the current rate thats about 4 years away......
Okay, I'm convinced.

Though an air box removal & pods sound cool, I'm going to
come clean about being lazy..... even though the bike is
currently apart and air box easily accessable. 'Sides, I've spent too much money on this project as it is. So I'm keeping the air box in.

That said, may I ask for help removing the divider plate????

I've got the little 790 Black, and I've read differing stories about the removal. I've read that it requires breaking tabs, and I've read it can be done without any trauma to yer parts is parts.

Does somebody have a link to instructions for removing the divider/restrictor plate on the little 790?
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I don't have the link; however, you might try pm'ing Ridge--I believe he's the one who posted pics, etc of the restrictor plate. It is removed by removing the right side cover of the airbox. This should be much easier with the airbox off the bike than on the bike. The only problem I remember reading about was one needs to be sure the right side airbox cover is sealed good when replaced. The restrictor plate just slips out easily once the right side airbox cover is removed.

After sawing the box clean in half, you can get the plate out easily with a pry bar.
Hurry, someone take away his sawzall before he starts to work on removing the tank! :-D

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