4 into 1

[CBShannon]

Just starting build up project on 82 750 supersport. Really just wondering where to start. Exhaust, Carbs,. Ignition, Etc. What will a 4 into 1 exhaust really do for me besides sound tough? I appreciate any input.

[Re-run]

Well a 4-1 does not always sound tough! A 4-1 usually drops your torque a small amount and puts it in topend. Whats best depends on what all you want and are doing to the bike. If it is just basic changes, then intake, exhaust, and ignition. A little more info would be best.

[CBShannon]

Well, I don't intend on racing it or anything of the sort but I would like to get better than stock performance. Right now it is completely stock and runs pretty good but I know it can run much better. I love these old bikes and I just want to really do this one right. I am willing to spend some money on her. Like I said I just don't know where to start. I appreciate the info.

[pickandpray]

-carb
-ignition
-cams
-exhaust

[CBShannon]

OK, so let's say we start with the carbs. I've had alot of buddies telling me get that air-box off and get some pods or velocity stacks on it, but since researching some on sites like this, I've read that these stock carbs need the air-box, and that only after-market carbs do well with the pods. I guess my question would be is the replacement and cost of new carbs in proportion with the gain of performance. Can anyone tell me what kind of gain I would get from that?

[Re-run]

Unless you do some engine work, no aftermarket carbs are not worth it. The average price is going to be $600 for good carbs. Unless I actually found that carbs were bottlenecking things, I would not bother with them other than jet changes.

Now lets set this at $1000. I know sounds like a lot. Find someone who is able to do some head work and get things opened up some. The will run 2-300. Now drop in a couple of hotter cams, Another 300ish. You should already have the CDI ignition so a new one may not be needed, so you might save there but check it carefully, A new ign system with coils will run about 250.

Now if you think this is bad, a decent exhaust will run you 300-400. Combine that with carbs at about 600 and you already spent 1000. I would do the engine and cams first.

[CBShannon]

OK, sounds like a good place to start to me. I do appreciate the information.

[OTTODOCTOR]

The stock 900 cams is a bolt in 3 hour job that will gain you around 10 horsepower over the 750 cams . I did mine for a little over a hundred bucks . Good used cams on ebay all the time , CHEAP .

[CBShannon]

OK, Thanks

[Re-run]

Given these bikes were not that high of hp, I would question the 10 hp increase. The bike was rated at about 65 hp which was technically less than the sohc bike but that does not take into account the losses in the drive train which would drop it to about 60 if not lower given that hp numbers back then were somewhat padded so once all said and done, probably closer to 55-60. Now 10 hp would be almost a 20% increase.
Now I am not saying it might not be possible but once you look at the numbers, it just does not seem possible with only a cam swap.

[OTTODOCTOR]

Ran the numbers on my computer program , ( performance trends engine analyzer ) which takes into account EVERY aspect of the engine . The only rating I used that was arbitrary was the cfm rating on the carbs . I used 420 c f m , and with the stock engine specs. the H.P. and torque ratings were within 2% of stock . -- The 72 advertised H.P. @ 9000 R.P.M. went up to 81.4 @ 10000 R.P.M. , which would be over the recommended limit , but mine sweeps to 10 grand easy . -- Mind you , I'm not recommending this on a regular basis , but once in a while doesn't hurt it . -- The power increase in the cruising range is what is REALLY impressive. -- I'm a big boy at 6'8" and 300 lbs. , and when cruising around at 35-40 I had to use 4th gear or the bike would start bucking lightly . Now I can use 5th gear and it's as smooth as glass . -- The seat-o-the-pants feel , which was , yeah,it's a 750 , is now OOOOOHHHH YEAH , now THAT'S a 750 .

[Re-run]

Ahh but that is a computer program and is no replacement for a dyno. The problem is that the program just cant take every variable into account. Such as friction in the engine. That can be changed by so many things that no simple computer program can account for. Nice toy to play with but again, it is no dyno. Like I said, an almost 20% increase is hard to see. I can certainly buy the seat of the pants difference as just a couple hp will make a difference.

Now I will not say to NOT do the swap as it obviously will still run, just not to get the hopes up that much.
Better to have low expectations and be pleasantly surprised than have high hopes and not see much.

[OTTODOCTOR]

Check out the program before you decide what it does and does not do . The program refers to your
"friction" as parasitic load . -- Your math is flawed as well , since the gain is actually 13.25 percent . -- I'm also trying to figure out why Honda would go to all the trouble to engineer a DOHC engine if the SOHC had more power ?????? -- Don't make no sense to this ol' boy.

[Re-run]

Well picture this then. Can the program take into account oil viscosities from all brands? No, because they constantly change them. Can it take into account friction changes from simple use? No, because no 2 engines will wear exactly the same, this is even more true in air cooled engines. Can it take into account valves that are too tight? No, it also cant account for the stiffness of valve springs. There is a whole world of things that software cant account for no matter how well it is written. not unless it is able to monitor the engine as it is running, in other words, a dyno. As for my math being off, I was throwing a ballpark figure, not an absolute. Which is why I posted it like this

"Given these bikes were not that high of hp, I would question the 10 hp increase. The bike was rated at about 65 hp which was technically less than the sohc bike but that does not take into account the losses in the drive train which would drop it to about 60 if not lower given that hp numbers back then were somewhat padded so once all said and done, probably closer to 55-60. Now 10 hp would be almost a 20% increase. "

I did it like this as I do not know the bikes base HP as it sits now at this point, stock. To claim an absolute without knowing this basic info is folly. So my math was not flawed, it was an estimate. just so you know, 10hp increase on top of 55 hp is about 18.2%

As for why honda would make the dohc, it is simple. What were all the other makes doing? Check it out, Suzuki had a dohc 750gs in 1976 "http://www.suzukicycles.org/GS-series/GS750.shtml"

In 1973, kawi released the z1 900 as a dohc. "http://www.cyclechaos.com/wiki/Kawasaki_Z1"

So in just a couple minutes I posted links for 2 of hondas biggest rivals going dohc 3 and 6 years before the cb750 did. If honda wanted to stay competitive in motorcycle tech, they HAD to go dohc.

Look I am not trying to call you a liar or anything, I just do not believe that a software program that is unable to monitor the engine while running before AND after the cam swap can accurately state what the hp increase will be. It just is not possible without that kind of interface.

[OTTODOCTOR]

In a perfect world , sheesh .............. Rerun , were you the captain of your high school debate team or something ?? -- All I did was offer a fellow forum member some VALID advice , just to receive criticism for / of it . -- It's a simple swap , and I think even you could do it  . -- I don't think it matters to you what the subject matter is , you know at least as much , and maybe more about it than the next guy does , and somewhere out there can find some computer link somewhere to quasi-back your claims . -- More power to you brother . -- As to the original posting author , I apologize for the actions of myself , as well as the debate team captain . -- Rerun and I seem to have a history . -- The cam swap IS a very worthwhile upgrade . -- You will NOT regret it .

[Re-run]

IF you read what I wrote, I never said to NOT do the swap. Not once. I only questioned the 10 hp increase, especially since it came from a software program and not a dyno. I totally gave you the seat of the pants part as I fully believe the 900 cams will bump power.
Again, I only questioned the absolute increase of 10 hp. I even went to the 750 dohc forum and found nothing there in the way of an absolute number for an increase. I DID find that the 900 cams can be used.

I am not sure why you are getting so upset when the only thing I am doing is being skeptical about the hp claims made by a piece of software.  I am SUPPORTING the cam swap. What gives here?   You just don't like when anyone questions you?

[OTTODOCTOR]

You sound like my wife , who I have told many times , it's not what you say , it's HOW you say it . -- It was not an absolute , chiseled in stone gain . It was ABOUT  10 h.p. -- Just accept it for what it was . -- Friendly conversation and erroneous information from an unreliable source . -- Nothing more . -- In defense of the computer program ,( Performance Trends , engine analyzer )  if you're interested , you should really check it out . It is a VERY extensive program that is used by engine builders to test how changes to the technical aspects of the engine affect performance without having to try the desired changes in the real world , which could get VERY expensive . No , it is not a dyno , nor will the results be absolute , but the cost differential between the two dictated that I buy the computer program , and I didn't think I could get the dyno to fit on top of my toolbox either .

[woodsrider250-2]

FWIW, Honda rated the DOHC 750 at 78hp, and the 900 at 95hp.  Thats crank hp, and factory numbers so you know what that means.  The 750 cams are in fact a joke.  Who knows how much more power the cam swap adds, but it's a good mod...Especially for what used cams can be had for over something like an andrews.  These motors are a joke for serious modding, the SOHC has definitely got them there, but the bike the DOHC is attached to is certainely a better ride.  Lots of BS EPA involvment at this time as well so who knows how that played into the engineering.  The other brands were definitely designed with 3rd party modding in mind.  The kaw is a DOHC 8v, which I always thought was interesting....It is a bad a$$ hemi head though for making power.

[Re-run]

Like you excepted my 20% statement? So I have to accept yours but you can nitpick mine? As for conversation, I started it friendly, it went south with your rebuttals. Even up till now, I have not been an asshole or confrontational, you just seem overly defensive about a piece of software I do not feel is worth the disk space it uses. A fun toy but you have no proof it is accurate for an air cooled engine. You already stated that motor builders use it but they are using it on an engine more advanced with tighter tolerances than these old air cooled jobs AND most likely recently rebuilt and probably not calibrated to take a possibly poorly adjusted carb and pump gas into account. I think I have stated enough variables to show how the program could be not giving an accurate display.
As for the 10 hp about you claim, if you used this program, it would have given you an exact number or number range, why didn't you post that? Why neuter the number? Just wondering. I mean if I got a dyno reading, I would put the number as the dyno stated it.
I don't know why you are so defensive and acting so offended here. I have not called you any name, unlike your insinuation or me being like your wife(but maybe she is the good half?).
I merely do not think the software is accurate in this situation.



Woods, where did you find that info? I ask as the numbers do not seem accurate to what I find.
http://motorbike-search-engine.co.uk/classic_bikes/honda-cb750.php
http://en.wikipedia.org/wiki/Honda_CB750

Even the later 750s did not have that much hp. I hesitate to go with manufacturer claims as those are often padded and not taken at the wheel.

[OTTODOCTOR]

You're right rerun . I surrender , and you were right about the wife too. She is definitely the better half . -- We are all entitled to our opinions , and  , as usual , yours and mine differ . -- Let's just leave it at that .