Topic: Pipes, Air cleaner and cam for a stock 78 CB750K

[maxheadflow]

I really didn't get much response over in the other section. Hopefully this performance section can get me some answers. Maybe even discuss a little cam theory.

I've been through a few places on this site and found a bunch of info on CB750s.  One thing I notice on the 78 CB750K is that the cam pretty much has 0 overlap likely due to smog.  Intake 0/40  Exhaust 40/0 This gives a nice linear torque curve but really limits peak torque and HP even when the intake and exhaust are opened up.  Can anyone suggest mild cam that can pep this bike up? While I've only ridden the bike for about 40 miles, it was pretty gutless compared to what I remember.  A good running K1 with pipes, modified breadbox AC with KnN and dyno tune would do 12.5 in the 1/4 back in the day.   While I really don't care for drag racing, I really want something with a little more pep for the mountains.

Looks like Webcam 341 or 341A or Cycle X CX1 would be the best bet.

[PeWe]

Stock pistons, stock head ports, and stock springs limit the cam alternatives.
A hot cam will give more overlap  and lower the dynamic compression making the bike's power very boring from low. Maybe some on top revs. A good cam for street has higher lift and relatively small overlap. Pistons with deeper pockets and higher CR is a must.
A good cam need better flowing CB750 head.

F2 stock cam if you can find one, web cam 41 can work. Megacycle make one too but cost a lot when they only make hardwelded cams.

I doubt 41a will work with stock parts.
Cams with higher lift need race springs, not only for the revs, avoid coil bind since race springs are thinner to be able to compress more as it does with higher lift

The head need to be in good condition, if in need of new valves guides, especially exhaust side a shop that can do that can also improve the ports.
Still limited by pistons and low comoression.

My stock K2 with fresh restored engine using F0-75/K7-78 cam
Has 150-160PSI when I tested the compression with cheap compression tester.

My modified K6 with JE 71.25 mm pistons and Mike's billet block (1005cc) has 190-195PSI with an RC295 cam that has rather high lift and relatively low overlap. This is a very good street engine!

It had a hotter cam before RC315 profile with lower lift on In and much more overlap ending up with low compression as my stock K2. Really dull engine all the way from low to high.
That cam must have CR 12.5:1 pistons minimum.

Same engine had similar compression 190-195 PSI with 836cc and slightly more modest cam ( similar numbers as 41a).
Good response all the way,  81whp.
This compression work fine on Std gasoline (Shell V-Power)

[maxheadflow]

Thanks for the reply. I understand the issue with running stock heads and pistons / compression ratio.  The selection tends to put you at the lower end of the scale.

As stated IMO the addition of overlap to a smogged motor can really help bring out the performance of the motor providing there is a decent amount of flow to start. Stock honda heads are nowhere as bad as say a Harley head. If bad, I would think some simple pocket porting would help.

When you say lower dynamic compression, I assume you mean "lower corrected compression".  It's a value based on intake close that sets the cranking compression.  The good thing is that if you look at say the Web cams 341, it's intake closes at 37.5 degrees compared to 40 for the stock smogged 78 CB750K. Cylinder builds a little more pressure which should help with low end.  The amount of overlap provides a means for intake and exhaust tract tuning.  Intake and exhaust tuning tend to have a specified RPM range but careful design can extend it.

Timing
                  In                    Ex
Stock 78     0/40                40/0
341             10.5/37.5        37.5/10.5



I've always considered dynamic compression as a compression ratio based of whatever the current volumetric efficiency is.  It is RPM dependent (dynamic).

Nice thing about doing big bores in a hemi motor is that they tend to create a squish area which helps with turbulence in the chamber.  Old Sir Harry Ricardo was an expert at this.  I think more recent 2 valve per cylinder designs move this direction. In doing so, swirl and tumble are less of an issue.

195 ccp sounds like a good build with squish.

You bring up an interesting point about head flow.  What do stock heads flow compared to the ported ones?

[scottly]

This is dated, but still relevant:
http://draftcycleworks.blogspot.com/2010/09/porting-honda-head.html
My 836 motor with 195+ cranking compression, a mystery re-ground cam, Weber carbs, and a stock un-ported K1 head makes about 70-72 HP at the rear wheel. With a ported head, it should be close to 80, maybe more. A stock 750 makes about 55 RWHP.

[PeWe]

Dynamic compression is the actual compression you get with running engine.
Compression ratio is a number calculated.
Different cams make difference.

The cam open up and let compression out.
Check the website Satanic Mechanic that has cam numbers of several cams and describe cam numbers in general.

It work both ways. High compression pistons need a hotter cam to not get too high compression and cam need pistons to not get too low compression.
The compression can't be too high, engine will ping. Racer engines runs on higher octane race fuel and can use higher dynamic compression without pinging blowing holes in pistons.

I guess 200-210 PSI by cheap compression tester will work on std fuel (195 no problems). Remove all plugs, connect the tester in one plug hole and run the starter. Full throttle a must.
This is a good way to verify that all cylinders are OK, same numbers on all 4 is good.

I have noticed the difference when I increased overlap just a little with my DP315 cam that worked best with 0.3 mm valve lash with my pistons. I tested 0.1/0.15mm as many other cams use. Compression with tester decreased almost 20 PSI and bike lost power.

Anyway, with stock pistons you are limited camwise.  Lack of compression (affects overlap) and piston-valve clearance (affects total lift).
Too high lift and the stock valve springs can bottom out by its coils hitting each other.

Or you can use the parts YOU want and check the result.

Also read the cheap 836cc pistons thread were many use the low priced Cruzinimage pistons, some with 41 cam. Same limitation camwise as stock pistons.

Low compression, really boring engine (if it has no turbo).

I have an extra cylinder to my K2 that has new CB750F pistons (392) to give it extra compression. Cruzinimage +0.5mm. If I need to change in the future.
The guy in the shop nearby will soon retire so this bore job has be done now.
A reground cam can be something increasing lift a little by grinding base circle.

Do the entire package with 836, restore + port head, springs and a good cam.

Edit: A slightly hotter cam with more overlap will give more at higher rpm but loose the low rpm torque if not increasing the compression.

A boring engine can be caused by a bad head in need of restore.

[scottly]

Do the entire package with 836, restore + port head, springs and a good cam.

And back down the rabbit hole we go..

[scottly]

 A good running K1 with pipes, modified breadbox AC with KnN and dyno tune would do 12.5 in the 1/4 back in the day.

Really??

[maxheadflow]

This is dated, but still relevant:
http://draftcycleworks.blogspot.com/2010/09/porting-honda-head.html
My 836 motor with 195+ cranking compression, a mystery re-ground cam, Weber carbs, and a stock un-ported K1 head makes about 70-72 HP at the rear wheel. With a ported head, it should be close to 80, maybe more. A stock 750 makes about 55 RWHP.

I've seen the Branch Article.  Too bad they didn't give any dyno results..   Unfortunately they don't talk about the exhaust.

I do find the HP claims here for stock cb750 a little high but I suspect that it's more of a dyno issue causing the different numbers then the bike. Most current dynos are inertial and not brake HP type dynos.  For instance, you say a Stock K1 puts out 55 hp.   The place I worked in 1972, a stock CB750 would pull about 34 hp out of a brake type dyno. The bike that turned the 12.5 made 44hp on that Dyno.  These numbers are all raw HP as there wasn't any correction on the dyno. There was an attempt to keep the room stable by pumping a lot of preconditioned air into the room.    We did a number of bikes there as the service manager successfully used the "put it on the Dyno instead of test ride" to convince the owner that it would be better.  It only worked for a little while.    When servicing a bike you still need a test ride.   Side note: The service manager had a first year XL250 that made 34 hp at the rear wheel.  The guy welded the dome on a stock piston, used a Yoshimura cam, 1 3/16 Amal GP carb, handmade megaphone exhaust pipe.   He was an old BSA Goldstar racer and had lightened and polished everything inside the motor. 

BTW I seem to remember guys with pre Ks get 12.5s at the same track.  I was able to get a 13.0 on a H1 with chambers with the carbs jetted too rich. It would start to break up right at the redline.   My Suzuki X6, road racer would do 15 seconds flat.  HD sevicar could do it in 22 seconds..    Track was close to sea level, air was pretty much always fresh and temps between 70-80 (Hawaii Raceway Park in the evenings).

[maxheadflow]

Dynamic compression is the actual compression you get with running engine.
Compression ratio is a number calculated.
Different cams make difference.

The cam open up and let compression out.
Check the website Satanic Mechanic that has cam numbers of several cams and describe cam numbers in general.

It work both ways. High compression pistons need a hotter cam to not get too high compression and cam need pistons to not get too low compression.
The compression can't be too high, engine will ping. Racer engines runs on higher octane race fuel and can use higher dynamic compression without pinging blowing holes in pistons.

I guess 200-210 PSI by cheap compression tester will work on std fuel (195 no problems). Remove all plugs, connect the tester in one plug hole and run the starter. Full throttle a must.
This is a good way to verify that all cylinders are OK, same numbers on all 4 is good.

I have noticed the difference when I increased overlap just a little with my DP315 cam that worked best with 0.3 mm valve lash with my pistons. I tested 0.1/0.15mm as many other cams use. Compression with tester decreased almost 20 PSI and bike lost power.

Anyway, with stock pistons you are limited camwise.  Lack of compression (affects overlap) and piston-valve clearance (affects total lift).
Too high lift and the stock valve springs can bottom out by its coils hitting each other.

Or you can use the parts YOU want and check the result.

Also read the cheap 836cc pistons thread were many use the low priced Cruzinimage pistons, some with 41 cam. Same limitation camwise as stock pistons.

Low compression, really boring engine (if it has no turbo).

I have an extra cylinder to my K2 that has new CB750F pistons (392) to give it extra compression. Cruzinimage +0.5mm. If I need to change in the future.
The guy in the shop nearby will soon retire so this bore job has be done now.
A reground cam can be something increasing lift a little by grinding base circle.

Do the entire package with 836, restore + port head, springs and a good cam.

Edit: A slightly hotter cam with more overlap will give more at higher rpm but loose the low rpm torque if not increasing the compression.

A boring engine can be caused by a bad head in need of restore.

You and I are saying the same thing about Dynamic compression.

I can understand where you are coming from. And 836 sounds like fun, but for now a simple street cam is probably all I want.  Adding compression is definitely the way to more power.  Intake close is the most important part to set when bumping the compression. It sets cranking pressures which determine what cylinder pressures you'll be running at lower RPMs. Getting them into the 195 range is good for performance, fer sure.    Overlap can kill low end by dirtying up the new charge but it will kill off some of the ping.

I take it that you adjust your timing curves when tuning?

Pretty much all the other bikes I own are not stock (except for my XR400).

This bike, here, made about 144 hp last time it was on the Dyno.

[scottly]

I do find the HP claims here for stock cb750 a little high but I suspect that it's more of a dyno issue causing the different numbers then the bike. Most current dynos are inertial and not brake HP type dynos.  For instance, you say a Stock K1 puts out 55 hp.   The place I worked in 1972, a stock CB750 would pull about 34 hp out of a brake type dyno. The bike that turned the 12.5 made 44hp on that Dyno. 

Your dyno numbers are bogus. In order to do the quarter in 12.5 seconds with 44 HP, the total combined weight of the bike with fuel and rider would have to be 435 pounds.

[maxheadflow]

I do find the HP claims here for stock cb750 a little high but I suspect that it's more of a dyno issue causing the different numbers then the bike. Most current dynos are inertial and not brake HP type dynos.  For instance, you say a Stock K1 puts out 55 hp.   The place I worked in 1972, a stock CB750 would pull about 34 hp out of a brake type dyno. The bike that turned the 12.5 made 44hp on that Dyno. 

Your dyno numbers are bogus. In order to do the quarter in 12.5 seconds with 44 HP, the total combined weight of the bike with fuel and rider would have to be 435 pounds.

They very well could be..   In 1971, microprocessors were just starting. Personal computers really didn't happen until the end of the 70s. That means the dyno I used was probably a hard wired calculator.  I don't know much about how it was designed but the brake transducer might not have been very accurate.  It was a funny machine. It only put out a HP number.  You basically opened the throttle and loaded the brake to get your number.   The numbers were consistent between runs tho. 

BTW. IMO 1/4 trap speed is a better indicator of HP tho the ET calc requires everything be perfect. 

[maxheadflow]

Has anyone checked actual compression ratio to see how close it is to advertized number?

[scottly]

Another clue was the 250 that made the same 34 hp as a 750. What sort of dyno was this? In my high school auto shop, we had a Go-Power water brake engine dyno, at about the same time. I don't think there were any fancy electronics? I do recall that the school had received a donated 427 Chevy motor, with stickers reading "do not run". It was torn down, inspected, re-assembled, and put on the dyno, where it pumped out about 200 hp.  

[maxheadflow]

Another clue was the 250 that made the same 34 hp as a 750. What sort of dyno was this? In my high school auto shop, we had a Go-Power water brake engine dyno, at about the same time. I don't think there were any fancy electronics? I do recall that the school had received a donated 427 Chevy motor, with stickers reading "do not run". It was torn down, inspected, re-assembled, and put on the dyno, where it pumped out about 200 hp.

Actually I do believe the XL did crank out the same HP as a stock CB750.  As the numbers were repeatable.  I rode the XL a couple times, thing felt like a strong running 450cc Maico.  The last 10 HP was (on that dyno) was made by setting the intake and exhaust length. No air cleaner, the exhaust pipe was a simple cone and pipe. Length of the pipe and cone were adjusted an run.  FWIW that was the best it ever did. Try to put an air cleaner on the carb and the power would drop. They put the motor in a Rickman frame and had to move the float bowl on the carb. It never ran tight after that except on the dyno.   The bowl was too far away from the main jet.

Calibration was kind of hokey as you measured pressure on the torque arm with a scale.  Come to think of it there wasn't any check for parasitic drag. It was a rear wheel dyno tho.  Don't remember the name on the dyno.

[maxheadflow]

Stock pistons, stock head ports, and stock springs limit the cam alternatives.
A hot cam will give more overlap  and lower the dynamic compression making the bike's power very boring from low. Maybe some on top revs. A good cam for street has higher lift and relatively small overlap. Pistons with deeper pockets and higher CR is a must.
A good cam need better flowing CB750 head.

F2 stock cam if you can find one, web cam 41 can work. Megacycle make one too but cost a lot when they only make hardwelded cams.

I doubt 41a will work with stock parts.
Cams with higher lift need race springs, not only for the revs, avoid coil bind since race springs are thinner to be able to compress more as it does with higher lift

The head need to be in good condition, if in need of new valves guides, especially exhaust side a shop that can do that can also improve the ports.
Still limited by pistons and low comoression.

My stock K2 with fresh restored engine using F0-75/K7-78 cam
Has 150-160PSI when I tested the compression with cheap compression tester.

My modified K6 with JE 71.25 mm pistons and Mike's billet block (1005cc) has 190-195PSI with an RC295 cam that has rather high lift and relatively low overlap. This is a very good street engine!

It had a hotter cam before RC315 profile with lower lift on In and much more overlap ending up with low compression as my stock K2. Really dull engine all the way from low to high.
That cam must have CR 12.5:1 pistons minimum.

Same engine had similar compression 190-195 PSI with 836cc and slightly more modest cam ( similar numbers as 41a).
Good response all the way,  81whp.
This compression work fine on Std gasoline (Shell V-Power)

I've been playing with a simulator and on thing for sure. A cam by itself won't do as much as I'd thought, probably because the cylinder displacement / engine size is less than what I'm used to.   The best way to get the performance up on stock would be to bump the CR to about 10.5 and work on the heads.   If you're pulling the heads, might as well do the bore kit.  Best I can see without pulling the heads is to open up the air cleaner and exhaust some.

[PeWe]

I miss a thread about most hp on the wheel for less money

[MRieck]

Another clue was the 250 that made the same 34 hp as a 750. What sort of dyno was this? In my high school auto shop, we had a Go-Power water brake engine dyno, at about the same time. I don't think there were any fancy electronics? I do recall that the school had received a donated 427 Chevy motor, with stickers reading "do not run". It was torn down, inspected, re-assembled, and put on the dyno, where it pumped out about 200 hp.

Actually I do believe the XL did crank out the same HP as a stock CB750.  As the numbers were repeatable.  I rode the XL a couple times, thing felt like a strong running 450cc Maico.  The last 10 HP was (on that dyno) was made by setting the intake and exhaust length. No air cleaner, the exhaust pipe was a simple cone and pipe. Length of the pipe and cone were adjusted an run.  FWIW that was the best it ever did. Try to put an air cleaner on the carb and the power would drop. They put the motor in a Rickman frame and had to move the float bowl on the carb. It never ran tight after that except on the dyno.   The bowl was too far away from the main jet.

Calibration was kind of hokey as you measured pressure on the torque arm with a scale.  Come to think of it there wasn't any check for parasitic drag. It was a rear wheel dyno tho.  Don't remember the name on the dyno.

a Schenck unit?

[maxheadflow]

The only things I remember about it are that it had sort of console / box on the right side that had the HP display on the far end, Inside the box was the water brake and torque arm right at the drum.   The only control was a cabled small box that had an up and down button to control the brake.   You could cheat the HP output by starting at the higher end of the RPM scale for the motor and pressing the increase brake button.  Number would shoot up.. Best way to get a reading load the brake and bring rpms up slowly. When HP peaked you hold steady state for a few seconds.   Don't ever remember even seeing a name but I did go through the manual.

Lead mechanic tried to run service manager's Goldstar on the machine and promptly smoked the clutch..