Topic: Building a QUICK Honda SOHC for Street.

[754]

IF you were to build a  QUICK MOTOR TODAY, WHAT would you use?

 I ask becouse knowing how quick they used to be there must surely be room for improvement. I mean the 70,s were not the zenith of engine development, I am sure there is improvments in a few areas..

Porting,I hope in 30 years, there has been a bit of increase in flow. I see smaler stems, which should help.

Cam designs

Exhaust

Induction, I am sure over 30 yrs carbs may have improved or injection could be fitted, ditto for ignition

Tires, pretty sure they got better
Chassis and clutches.. got be a gain there somewhere

Bottom line, it should be easier now to go quick than it ever was..

[Gordon]

IMO, weight savings is one of the best things you can do to improve the performance of any bike.  Lighter wheels minus tubes, 4-1 exhaust, fiberglass fenders/tank/seat pan, are just a few things that come to mind.  I know, I know, it's not as exciting as things like boring out the cylinders or fitting an FI system, but it's a great supplement to increasing the actual power output.

[voxonda]

Agree with Gordon. Seen bikes filled with power that could not 'turn' around corners. Feel that first you should improve the total package and then improve on the power. Think that, even only 67bhp, can give a lot of fun once's you can use it all.

[destroyer]

Well put vox.   Nobody is really going to use 150hp or anything in a reasonable setting.  If you can learn to utilize the power properly and learn to corner right you will rarely need over 60hp to have fun.  In my opinion, fun is cornering and skill, not drag racing.

[CBJoe]

IMO, weight savings is one of the best things you can do to improve the performance of any bike.  Lighter wheels minus tubes, 4-1 exhaust, fiberglass fenders/tank/seat pan, are just a few things that come to mind.  I know, I know, it's not as exciting as things like boring out the cylinders or fitting an FI system, but it's a great supplement to increasing the actual power output.

Speaking of lighter things..... Atkins diet and a horse jockey for a rider are sure to gain a bit of quickness  

Sorry ... I know this doesn't help, but when trying to decide if I should loose some weight or go with lighter wheels, it made me snicker 

[Bodi]

The physics of internal combustion engines haven't changed and aren't about to. The technology has definitely evolved, and you can go to the local dealer and buy a vehicle with more horsepower per cc than a 70's formula one car had.
Current sportbikes are better in every measuirable way than our antiques. There's no point in trying to turn a 70's SOHC4 into an "CBR750".
You can increase power substantially from stock... but the 2007 CBR600 gets about 118HP. That's close to 0.2HP/cc; a 750 at that output would have almost 150HP.
The CBR does it using lots of tricks: water cooling, oversquare bore/stroke, 4 titanium valves on rockerless dual cams, short and straight intake path, EFI with large injector bodies and multihole injectors, minimized friction and turbulence losses in the crankcase, excellent exhaust headers and muffler, etc... HP is torque times RPM and the CBR gets to 15000RPM which would transform the SOHC cam gear into bits of scrap.
The air cooled CB750 has two steel valves in an undersquare design engine with carbs feeding a long convoluted intake path, a heavy crank with large plain bearings that fans a lot of air around, and an exhaust system designed mostly for looks.
It's best not to compare the frames. You may as well compare a model T to a Ferrari F2008. They both do roughly the same thing but that's where similarities end.
So, what can you do? Start throwing money at it and after about $10,000 you'll have a big bore kit, HD studs, ported head with oversize valves, lightened knife-edged crank, Carillo rods, undercut and polished transmission, bigger CR or maybe MIK flatslide carbs, and a sweet exhaust system. You should get around 100HP. Fuel injection is possible... but there's nothing ready made so you'll be designing and building it yourself for little gain over carburetion - the bottleneck is really the intake path and single valve. You still have that model T frame and skinny tube tires to deal with, as well.
The good part: The SOHC4 engine is a beautiful piece of machinery - the CBR engine is hidden under a lot of plastic crap. You can work on a SOHC4 - have you EVER seen someone in their driveway doing any work beyond washing it on a "new" sportbike? You can go fast enough to scare yourself stupid, especially on a curvy back road. You can break any speed limit I know of. And there's the unmeasurable part: you'll have more class than a hundred CBRs.

[GammaFlat]

Bodi Bodi Bodi,

You simultaneously made me feel really bad and really good.  Is that good?  Now I don't know whether to $4!+ or go blind. 

John

[jason teamshralp]

... there's the unmeasurable part: you'll have more class than a hundred CBRs.

That is the same way i feel. These bikes get my attention more then any new street bike.

I also agree with the weight saving that Gordon mentioned above.

[Terry in Australia]

Well written Bodi, I thought it was all gonna be "gloom and doom" for awhile there, but you recovered well when you spoke about classy classics versus modern, forgettable, race replica's.

Like most CB750 owners I want to be able to go faster, and the good thing is that there are plenty of good (slightly used and NOS) performance parts available, if you are prepared to be patient, or new stuff if you've got lots of money and want to do it now.

I've been collecting performance parts for several years, and recently built my 836cc engine with the following:

12.5:1 Arias pistons (used, but within tolerance)
RC modified OEM rods
RC Cylinder studs
Tsubaki HD cam chain
M3 cam chain tensioner
New OEM primary chains and tensioner
Mike Rieck stage 3 head
Megacycle 125/75 cam with M3 adjustable cam sprocket
Keihin CR 29mm carbs
Dyna S ignition and Dyna 3 Ohm coils + NGK Iridium plugs
4 into 1 race pipe
"Telcool" oil cooler.

All up, with a fair amount of "horse-trading", that engine cost me no more than $2K and I swear I've seen 120 MPH @ 11000+ RPM in third gear, using standard 18/48 gearing. That's plenty fast for me, and my K1 handles it well, with the addition of good rear shocks, newly rebuilt forks with a "Telefix" fork brace, tapered roller steering head bearings, bronze swingarm bearings, new Bridgestone BT45 tires front and rear, twin disc conversion with F2 MC and braided lines and RK 530 X Ring Heavy duty chain, with new sprockets.

With the help of Mike Kramer and Big Jay I recently received a lightened and balanced crank from APE, so that's added another 400 bucks (with shipping to Oz) to the above costs, and once installed, that engine will be going into a much more "sorted" frame I have leaning against the wall in my cluttered garage, and hopefully I'll be able to try to take it somewhere closer to it's full potential as another CR replica.

Nothing to rival a modern sportsbike of course, but total exhilaration to ride, it stands out from the crowd, and like Bodi says, you can fix it in your driveway! Cheers, Terry.

[heffay]

You can work on a SOHC4 - have you EVER seen someone in their driveway doing any work beyond washing it on a "new" sportbike?

 
i KNOW you weren't talking about me... cuz i was in the garage.   

[kghost]

He said Modern heffay 

[eldar]

Looks modern to me but then one piece of plastic looks the same as another!

[heffay]

He said Modern heffay 

u r rite about that!  its one of the last of the carbies... cr flat slides    uber finicky

add a muzzy, 4- 1.5 inch breathing holes in the box, jets turned river spillways  , and that was about all i could do to get it to "kindof" idle.   

don't fret guys... cb carbs are cake     i'm still saving the money to buy the tool to be able to adjust the pilot screws on the 7r... even after modifying the motionpro tool, it won't reach.    ... morgan carbtune 90 degree screwdriver, i'm sorry i ever doubted you.

[turboguzzi]

From my understanding, the biggest limiting factor in the 750 is the very deep combustion chamber and the high included angle between valves. It doesnt really let you create big squish areas that are key to good, quick combustion without detonation. Ducati has continued 2 valve development till these days for their aircooled twins and the combustion chamber looks totally different, I think their down to less than 60 degs between stems and the chamber is a bath tub style. Just to let you understand how efficient these heads are, they are now up to 10,5:1 CR stock with 98mm bores.  People who race these things go up to 12-12,5....

Todd henning got CB450/500 Twins up to 60 hp which would mean 120hp from a 1000 but had to re-angle the valves to get the modern chamber shape he wanted.

my 0.02

TG

[754]

Nothing to sop anyone from changing valve angles on a CB head.. or build one out of billet or cast one.

There has been DOHC heads made for CB 750, and I have seen raised port heads...

[heffay]

sorry for my earlier threadjacks   

back to the topic... i'm a firm believer in the cafe style and how it relates to performance.

usually loud pipes breath better (754 obviously knows the difference between unbaffled and less restrictive pipes).
lose everything that doesn't matter... if it did... it wouldn't be so ugly. 
drop the bars, get down on the bike and cut the wind without having to situate yourself on the bike differently than it is set up.
polish your bike frequently... every granule of weight counts and clean bikes look faster.   

[754]

Lowering my bike  about 6 inches and stretching the swingarm 6 sure made a difference..

 BTW when I said quick, I meant accelleration, as opposed to handling, or high speed.  However any improvement to any of it are worthy of discussion.  In my own case I am interested mainly in 1/8 or 1/4 mile racing & holeshots..

 The funny thing is the look on the sportbike riders face if you do pass them or get  the holeshot on them.. its priceles.. they just assume they are going to walk all over you..

[heffay]

Lowering my bike  about 6 inches and stretching the swingarm 6 sure made a difference..

 BTW when I said quick, I meant accelleration, as opposed to handling, or high speed.  However any improvement to any of it are worthy of discussion.  In my own case I am interested mainly in 1/8 or 1/4 mile racing & holeshots..

 The funny thing is the look on the sportbike riders face if you do pass them or get  the holeshot on them.. its priceles.. they just assume they are going to walk all over you..

ok then... race gas.

[Sweep]

My bike is so fast getting into the higher rpm ranges right now that I can barely hold on yet accelerates nicely off the line.  I set out to build a streetable, fast bike and I'm happy with it as such.  This is the description and state of tune/conditions.  NOTE: I need better brakes, the stockers aren't cutting it.

The motor and trans were rebuilt completely using:
-APE balanced/lightened crank.
-Carillo Rods
-APE head port and valve job.
-Web 41a cam.
-New stock primary and cam tensioners
-Wiseco 836cc forged pistons.
-CR29 carbs. (125 main jets on 2nd clip position from bottom for 65 degrees F.)
-Dyna 200 ignition/Dyna green coils. (Switch position 4, 32 degrees base timing)
-Kerker 4 into 1 exhaust.
-Wheels rebuilt, powder coated/polished.
-Forks rebuilt/polished.
-Progressive rear springs.
-Brakes rebuilt/blasted.
-Swingarm needle bearing
-Tapered head bearings.

[eurban]

-Dyna S ignition. (Switch position 4, 32 degrees base timing)

Did you mean Dyna 2000 or has the Dyna S now evolved to control advance?

[MRieck]

-Dyna S ignition. (Switch position 4, 32 degrees base timing)

Did you mean Dyna 2000 or has the Dyna S now evolved to control advance?

Yeah...that's the 2000

[Sweep]

-Dyna S ignition. (Switch position 4, 32 degrees base timing)

Did you mean Dyna 2000 or has the Dyna S now evolved to control advance?

Sorry, Dyna 2000 now with 3ohm dyna green coils.

[scondon]

  As far as pure acceleration goes the CR carbs and the right cam make a huge difference and good porting/valves really seem to keep it pumping at high rpm's. I've been curious about 1/4 and 1/8 mile timing and just how much is lost getting the bike off the line. Once it's rolling it seems there's no end to the power but seems there's a bit of time to be gained on the initial start. Haven't set a bike up strictly for drag, just curious.

   As far as cornering and handling goes I've been sold for a long time on the notion that these bikes will always be a bit squirrely when pushed hard through rough roads. New bearings throughout, fork brace, steering damper,rear sets,few welds on the frame, the right tires, and some thorough thought put into the suspension and I am no longer believing that these bikes can't take a modern bike. For sure the skill of the rider is the deciding factor. I ride regularly with a group of modern sport bikes(Suzuki,Honda,Ducati,BMW) and have moved from the rear to chasing the top 5 riders without testing the limits of traction and suspension.

    A big nod to whoever mentioned that the bike should be built for handling/braking before much thought is put into getting the most from the motor. Straight line acceleration is a blast but eventually there is a turn or a slow moving object

[Gordon]

Let's see...  A rider with a stock CB750 carrying around an extra 50 pounds in his beer gut and man-boobs could either stop eating so many burgers and stop drinking so much beer and drop that weight, or add an extra 3hp to his machine. 



Increasing hp is starting to sound a lot more attractive!

[scondon]

Let's see...  A rider with a stock CB750 carrying around an extra 50 pounds in his beer gut and man-boobs could either stop eating so many burgers and stop drinking so much beer and drop that weight, or add an extra 3hp to his machine. 



Increasing hp is starting to sound a lot more attractive!

   Well said and pass the fries