Looking for some DOHC horsepower

[Half-Caf]

Hey Guys, as my bike is currently stripped down and I'll be opening up my head end soon, I've been thinking about doing some hp upgrades. I've been able to find a lot of info about sohc's relating to big bores, turbos, cams, etc... -but not so much on the dohc side of things.

I don't have a lot of $$$, but I do have some connections with a machine shop, so any boring or machine work would be cheap, if not free.
I found this kit on cycle x, but it's a little high for my price range:



Satanic Mechanic has a lot of good info on bore kits, but it appears its all directed to sohc's. I have no idea how much, if any of that info would be relevant for a dohc.

I don't know much about cam swaps or what options are out there, but I'll try anything once  (honestly, I don't know much about boring cylinders, either)

I've also thought that if the upgrades cost more than a few hundred $$, I may just look into purchasing a 900 engine. I know a guy who will sell me this 900c (running and titled) for $350.



Many thoughts running around in my head  Any and all help to make sense of them is much appreciated!

[shinyribs]

I have been wanting to do something similar to my SOHC. I'm not sure if the HP gains of an 836 kit are worth it. You see where people buy and sell them,but you never hear what they are worth. Dyno charts would be nice too look at,but I cant really find any. If you do the math,break it down by hp per cc.,using the factory hp rating on my bike,which yours is probably higher than mine already,an 836 kit would be worth about 9 hp.

67hp divided by 736cc= 0.09103 hp per cc

additional 100cc's @ 0.09103 hp/cc= + 9.103 hp

Thats above a 10% hp gain which is not too shabby,but is it worth it.

What does it do to the torque curve?Who knows

I dont know if this hold true in the real world or not,but that is the math. There would be other things that an overbore would create. It would help improve cylinder head flow by unshrouding the valves,but a larger bore always give you inherent frictional losses (cylinder wall drag/increased heat)that cannot be avoided just due to increased surface area.If you buy heavy forged piston that would increase rotating mass,which costs you hp. So if you factor in the inherent frictional losses and the rotating weight you could pickup that 9hp could drop to about 7 hp or so. But who knows really.

And the cams listed are just funny. They are described as "race","hot street","heavy duty"...i dont want adjectives.I want hp numbers. Show me a dyno chart where your cam makes more power than the other guys! Too bad these engines dont get the attention they deserve.

[Re-run]

It is impossible to give hard numbers since engines can vary so much. You can even take the same bike and dyno it at different times of the day and get different readings. As you can see, you really can't put a hard number down.

Now, an 836, will add some to the bike, BUT the weakest area is in the head. For best results, port that sucker, then bore and cam it and you really see a difference.

[Re-run]

Look back in the search here and you will find more info.
You can take cb900 cams and drop them right in.

[shinyribs]

You are exactly right about a hard number Rerun.What I meant Id like to see is someone make a few dyno pulls with a stock bike and then again with the same bike after an 836 alone went in.Just to give somewhat of an idea of the gains to be expected with an 836 kit alone.

The CB900 cams sound promising! I hate to even say it...but I wonder what kind of gains you would see with that.Mainly how the torque curve of the engine would be affected.

My new tires finally came in and when I went to pick them up Eric and I started talking about SOHC engine mods today...well,he was talking I was mostly listening and asking questions. The bike he races has an 836 with the forged wiseco pistons,a cam and port work like you mentioned. He couldn't remember the details of the cam,but that bike dyno'd at 93 rwhp thru a set of keyholes,pods and open 4-1.

The way these bikes are still pulling like mad at redline it's hard to believe that the heads could be any sort of restriction at all,but porting them seems to be a big deal. I'll probably start there with my new motor.

[Piute]

Re-run wrote

Look back in the search here and you will find more info.
You can take cb900 cams and drop them right in.

  Now these 900 cams that " drop in " are refering to only D.O.H.C. or  S.O.H.C. also ? Little away from post but 836 kits for F2 is different and any head work is better then none "is it worth much on A F2 ? SORRY,this has been bothering me .

[TOOLS1]

It's hard to improve on the Honda ports. Don't try changing them unless, you have a flow bench. They can go backwards real quick. The best thing to do is to just remove the casting slag, and shave the head. They only have 9:1 compression. It is safe to go to 10.5:1 with premium pump gas. The compression makes a big difference on the SOHC engines. Compression is also a benefit with the 836cc kit.
TOOLS  

[shinyribs]

Good advice Tools.Thank you. One guy a no "ported" a really nice set of trick flow sbf heads. No technique at all,he was just going for BIG. He flowed them later and lost over 100 cfm!!! He was sick. Velocity is more important then port volume

[Piute]

   The F2 has a bigger combustion area already,many that put in the bigg bore lost compression cause used wrong one (wasn't for an F2) Will be trying to find complete kit with cam for F2,If winter get ruff.

[Half-Caf]

shinyribs wrote

The CB900 cams sound promising! I hate to even say it...but I wonder what kind of gains you would see with that.Mainly how the torque curve of the engine would be affected.

Re-run, what you're saying does make the 900 cam sound like an attractive solution. Just to clarify, are you saying that the 900 cam is a drop-in for the DOHC?

What I've read elsewhere definitely doesn't make it sound like that's the case:



The fit issues mentioned in that write-up may be referring to the SOHC. However, I understand the stroke will be lengthened (article refers to using either shorter rods or some sort of spacer to compensate)
Can you verify that these aren't issues for the DOHC. I guess it would also be helpful for others if anyone could verify that the 900 cam would only work with modifications for the SOHC.

[TOOLS1]

Cam's, and crank. Two entirely different things. The cam's will bolt right in. The crank will not bolt right in.
TOOLS

[shinyribs]

That article that Half-caf posted is talking about putting a 900 DOHC crank into a SOHC. Cool.

[Piute]

   Sorry
   If I've confused your post..
  I believe Rerun was referring to the cam not the crank,(I was)that info U posted,is considered stroking yo bike,Lots involved in doing that fo sho.I'd say strictly track also.

[Half-Caf]

Way to put this newbie in his place  Good thing you admins are here -who knows what kind of rumors we could be starting!

Re-run, I was able to dig up quite the entertaining thread where you discussed this topic years ago:
http://honda-cb750-s.456789.n3.nabble.com/4-into-1-td461995.html#a462009

[Piute]

Half-Caf wrote

Re-run, I was able to dig up quite the entertaining thread where you discussed this topic years ago:
http://honda-cb750-s.456789.n3.nabble.com/4-into-1-td461995.html#a462009

   Entertaining ,Believe your drop in will be easyer compared to

[shinyribs]

Ok. The cams are from a DOHC. They are obviously NOT ever gonna work in a SOHC.No explanation needed. But since the cranks for the SOHC and DOHC are reversed concerning the charging and ignition systems that would be a real pain for a SOHC guy to make easily do-able on the street. It is interesting that the mains bolt right in,though.I thought the DOHC was a whole new design.Hmph,neat!

[shinyribs]

Oh,and that link...fun lil read








                                                                                               sic 'em Boss!

[TOOLS1]

Half-Caf wrote

Way to put this newbie in his place  Good thing you admins are here -who knows what kind of rumors we could be starting!

Re-run, I was able to dig up quite the entertaining thread where you discussed this topic years ago:
http://honda-cb750-s.456789.n3.nabble.com/4-into-1-td461995.html#a462009

Man, that was like having a flash back to the days I hung around that chopper forum.
TOOLS

[Re-run]

yes I remember that one!
My younger and hotter days! The cb900 cams drop into the cb750 dohc. And they DO add some but it is more to enhance mods you may have already done. It will still liven up a dohc engine a little.

As for porting, a guy named Mike came on here a couple times, I think mostly in the for sale section.
 If a person can, contact him. It is not cheap to port but he does an A1 job that can't be beat. And it will make a difference, especially when combined with an 836 kit. His cb750 does over 100 hp last I knew.

Visit here, a lot of info about modding an sohc.
http://www.satanicmechanic.org/ Axl is dead though so it is just sitting here right not.

[Piute]

   I've read most of his write up today this caught my thoughts;
Exhaust systems
When talking about exhausts here, we are speaking about 4-1 or 4-2-1 systems. Four individual exhausts may provide better performance, but at the expense of ground clearance and weight.

A 4-2-1 exhaust (which connects cyl. 1&4 and 2&3 first - this is the only way to do it right!) provides more mid-range and overall power, while a 4-1 exhaust has more peak power, both at the expense of peak power or all-around power, respectively.

Common knowledge seems to have it that the best header length (exhaust valve to collector box) is 70cm (27.5"). The secondary pipe on 4-2-1 systems should be 35cm (13.8").


      Guess thoughs nice loud pipe's going into the works on mine.