Topic: cb750 low end power upgrade

[metric-wrench]

im looking for the best way to get some more low end power out of a fairly stock cb750?
i have already installed 4 into one and kerker exhaust. also K&N filters (properly jetted).
i have considered buying 900cc (66.7 bore) cylinders from a website that are surfaced, sleeved and glass beaded.
it seems porting and getting aftermarket carbs is more for racing and does nothing until you reach higher RPMs.  i strictly want more snap of the line and throttle response.  will going to all the trouble of a bigger bore get me the power im looking for?  what other things should i be looking at for the power i want?
thanks in advance!!!

[Rock Store]

The most effective way to gain low end torque,is to change the sprocket ratios' , in a positive way. @ the cost of some top end HP,Either install a larger countershaft sprocket or smaller rear sprocket.
http://www.motorcycleanchor.com/motorcycle/how_to/mc_sprocketmath.html

Dom

[Retro Rocket]

The most effective way to gain low end torque,is to change the sprocket ratios' , in a positive way. @ the cost of some top end HP,Either install a larger countershaft sprocket or smaller rear sprocket.
http://www.motorcycleanchor.com/motorcycle/how_to/mc_sprocketmath.html

Dom

It doesn't change the HP at all, just lowers top speed, HP remains the same. Also you have the sprocket changes back the front, Smaller front or bigger rear will increase low down performance at the expense of top speed...

[cougar]

Hey metric-wrench ; If you've got the coin, definately get the port job. Our engines are basically an air pump. 2 main things have a great effect on that. Getting air in and out. Porting first and a cam are the best ways to accomplish this with a n/a engine. Besides if you get a Real nice port job now then anything else you do to the motor will be more noticeable if your head can breath  . ...cougar...

[Stev-o]

Put an 836 kit in there! I have one in my K4, it is scary fast!!

[ekpent]

The 4 to 1 header and pods filters has probably actually hindered the LOW end power if that is what your looking for. Stock airbox etc. would actually be better for low end grunt.

[Doctor_D]

No matter what you do it's not going to be a tractor motor. (thank god)

You can build more torque in a usable lower-rpm range.  A mild aftermarket camshaft (Megacycle 125-00 or Webcam 41a) and basic porting will fatten up your torque curve where you'll most feel it - 3,000 to 6,000 rpms.  Better throttle response can be had by modifying the slide-cutaway on the stock carbs. (CycleX performs this service.)

Added displacement will also provide a benefit, but not as much as you'd get without doing headwork and changing the cam as well.  Together, the CycleX 915 kit with a mild cam and Mike Rieck's stage 2 head work would give you stump pulling potential.

[Coyote13]

Anybody know how much Mike charges for headwork?

[jneuf]

The 4 to 1 header and pods filters has probably actually hindered the LOW end power if that is what your looking for. Stock airbox etc. would actually be better for low end grunt.

+1

You have to be careful it it's low end torque you're looking for. There are a lot of mods that'll increase your HP at higher RPM's, but you'll often get it at the expense of losing it down low. That being said, the same can go both ways...increase your low end torque, and you might lose some power up top.

Without changing any parts, you can advance the timing 5 degrees, and it'll lower the RPM where you hit peak torque.

[grcamna2]

The 4 to 1 header and pods filters has probably actually hindered the LOW end power if that is what your looking for. Stock airbox etc. would actually be better for low end grunt.

+1

You have to be careful it it's low end torque you're looking for. There are a lot of mods that'll increase your HP at higher RPM's, but you'll often get it at the expense of losing it down low. That being said, the same can go both ways...increase your low end torque, and you might lose some power up top.

Without changing any parts, you can advance the timing 5 degrees, and it'll lower the RPM where you hit peak torque.

+1 

[lucky]

A performance cam is the fastest way to get more power for the money. The most bang for the buck.

Porting is not going to help very much. Matter of fact just polishing the ports may hurt. Honda  company has said the the roughness of the ports actually helps cause turbulence and better mixing of the fuel/ air .

If you have pods and did not change the idle jet or change the needle clip position, or shim a non adjustable needle, you can still get a  lot more power out of it.
As much as 9 HP.  You said it was properly jetted. How do you know? What was done?

Want some good info.
Get a dyno run on it. Then you will know exactly what is going on.
Well worth the money. The dyno info can tell if you have a lean mixture etc.,.

Can you tell us what you have for a muffler?

EXAMPLES....

From the internet videos: CB750
Keihin CR31's, 65 slow jet, 128 main jet, YY7 needles, clip pos 5, mixture screw 1/2 turn, 30mm velocity stacks. No screens. The rest: JMR "Stage 4" ported head, oversized 5mm-stem valves, titanium retainers, bronze guides Wiseco 836cc, 10.25:1 Megacycle 127-75 cam APE lightened crank Polished & shot-peened rods w/ F2 rod bolts APE cro-mo cylinder & case studs Back-cut transmission Dynoman HD clutch Dyna 2000, 3.0ohm coils, & 8mm wire, Denso iridium plugs Replica CR750 exhaust

85.74 HP  rear wheel.


Another EXAMPLE.
Specs:
1975 Honda CB 750 Block
Wiseco 836cc Pistons
10:25:1 compression
F grind race cam
Stage 3 ported and polished heads
Keihin CR 29mm carbs
K&N pod filters
Barnett clutch and springs
Dyna 2000 ignition
Yosh 4 into 1 pipe

85HP.  Rear wheel.

ANOTHER EXAMPLE.

HONDA 4 CB750 Big bore kit conversion to 1050cc by Robinson Precision Engines.
 reground cam, Knife edge crank, bananced, ported heads,bigger carbs, Designed hand made connecting rods.

95 HP rear wheel - 85 ft. pounds torque.

Comment by Lucky. Only 10HP for 214cc increase.

ANOTHER EXAMPLE...
CB750 F2 Dyno test. (best numbers)
Mild port, 63b cam, stock exhaust, stock carbs and air box.

66HP rear wheel. 44 ft.pounds torque.

[metric-wrench]

Holy man!  Thanks guys. Much useful info here.  It seems I have some decisions to make

[Retro Rocket]

Porting is not going to help very much. Matter of fact just polishing the ports may hurt.

1, there is more power to be found in the heads that anywhere else on these engines, to say porting isn't going to help is ill informed and incorrect. I suggest doing a search and having a read.

2, you never polish the intake ports, they have heaps of scope for porting but do not polish the intake, the exhaust side is completely different, polish as much as you can, the faster those gases get out the better...

[Doctor_D]

Porting is not going to help very much. Matter of fact just polishing the ports may hurt.

1, there is more power to be found in the heads that anywhere else on these engines, to say porting isn't going to help is ill informed and incorrect. I suggest doing a search and having a read.

2, you never polish the intake ports, they have heaps of scope for porting but do not polish the intake, the exhaust side is completely different, polish as much as you can, the faster those gases get out the better...

+1

While badly ported heads can ruin performance, good street port jobs maintain port velocity, for good throttle response and a broad powerband, all while increasing critical low-lift flow.  Proper porting improves exhaust scavenging, and cylinder charging across the desired operating range - making the engine more efficient and more powerful at the same time.

[Spanner 1]

I'm confused !.... OP wants more 'low end power' and then says he wants better acceleration... two different things. Looks like he just wants to get into the powerband faster... poor man's fix would be a smaller front sprocket. Any motor mods would change the powerband, move it up the rpm range usually and narrow it too.
Whatever floats yer boat..... thought about a sportbike instead of a 40 yr. old CB ?

[HondaMan]

Like Ekpent and Spanner said...

The chopper guys always wanted more "low end squirt". Here's how I did/do it for them:
Stock K4 typical engine: 52 HP on the ground at about 7000 RPM.

836cc kit (your choice of Wiseco 10.25:1 or lower-compression 9.6:1 pistons, only 2 HP difference between them if done right).
Ported head.
Stock carbs (there's a good reason for this).
Stock airbox, a little modified (see comment above).
17 tooth front sprocket on engine.
If your engine is post-K2, see about getting a 1-row final drive bearing and shaft, swap them out for the 2-row in the later engines.
Stock cams: if a K0-K6 cam, advance it about 3-4 degrees. If a cam from the post-1976 bikes, advance it 7-8 degrees. You can still find sprockets that have these holes predrilled for you: I just saw one on the internet last week.
Typical power output: mid-70s to low 80 HP range, depending on the arrangement of parts and a dozen other details.

The pipes you choose will make almost no difference: less than 3% total, best-to-worst, with Dunstall 4-2 pipes the best outside Yosh megaphones. (In my experience.) No 4-1 will improve power over HM300 pipes, nor even HM341 in most cases.
These changes will move the power peaks band down from 7000+ RPM to 6000 RPM, and the power will start around 3500 instead of 5000+ RPM. The difference is more dramatic than it sounds, and tends to produce a wheelie machine.

[ekpent]

Those mods sound like some nice affordable,reliable and useful, fun real world street bike upgrades for an excellent street cruiser. I have an old RC836 done up pretty similar and its got more low oomph and better power all round.

[jneuf]

Excellent info, thanks Hondaman!

[metric-wrench]

Thanks again to all who took the time to help me out

[TwoTired]

im looking for the best way to get some more low end power out of a fairly stock cb750?
i have already installed 4 into one and kerker exhaust. also K&N filters (properly jetted).

A 4 into one that is properly engineered increases high RPM power at the expense of low end.

i have considered buying 900cc (66.7 bore) cylinders from a website that are surfaced, sleeved and glass beaded.

Yep.  displacement, for increasing throttle twist response.

it seems porting and getting aftermarket carbs is more for racing and does nothing until you reach higher RPMs.

I agree.  Unless you are going to bigger valves, low end gains for porting are marginal.

i strictly want more snap of the line and throttle response.  will going to all the trouble of a bigger bore get me the power im looking for?  what other things should i be looking at for the power i want?

Here are some suggestions:

Increase the displacement.
Supercharging
increase the displacement.
Have the engine be revving higher in the power band with lower gearing.  (Torque multiplication.)
Increase the displacement.
Higher compression ratios.
Increase the displacement.
Exotic fuels.
Increase the displacement.
Turbo charging (if you can stand the turbo lag).
Increase the displacement.

Back in the 60's I had a 55 Chevy.  In it was a 327 CID engine.  But, it had the early 265 heads, with very small valves, and a fairly small throated 4 barrel carb.  Still, it was a hot rod.

The racers were revving the little V8 to 7000 RPM and Beyond, using big valves and big CFM carbs.  They made great power at the strip (when they didn't break), but were fairly awful on the street at lower RPMs especially with the Hi perf cams.  I couldn't afford those go fast parts, or I probably would have done the same thing.  However, all were impressed with my set up for street response.  (I didn't design it, it was just the parts I had. ) The cam was standard 237. But, the small valves and the carb kept the air velocity into the chambers high, and the carbs was always working well and providing good atomization.  Throttle lag was no existent.  The cam kept the vacuum up even at idle.  When I blew up the rear end (Wheel hop in the rain) I put 4.56 gears in the little car it and would lift the front wheels off about an inch by just nailing the pedal while rolling in first gear.  I don't think the car would go faster than 90MPH (speedometer was uncalibrated for the 4.56 gears).  And, it was pointless (rather gutless) to rev it beyond 6000 RPM.  But, it sure was quick around town and seldom lost at the stop light Grand Prix.

Cheers,

[Hinomaru]

Increase the displacement.

The black art of stroking is another option that adds displacement. Combined with a big bore kit this will be expensive, but will deliver the low end power or torque that you seek. Here’s an older thread that briefly discusses stroking the CB750.

http://forums.sohc4.net/index.php?topic=39469.0

It looks like APE:  www.aperaceparts.com/ still offers their crankshaft stroking services:

http://www.aperaceparts.com/crankwork.html

[stickman]

Metric-wrench,   Been there done that here. 
It sounds like you want a little more take off to not be too embarrassed at a red light by the R-6 or other modern day sport bike in the neighborhood.  Or maybe I am just remembering what I wanted  many moons ago. 

One life's lesson I have learned the hard way( expensive way) is that it does not matter what you have or what you have done to it, there is always going to be someone better, faster, stronger, quicker, younger and prettier than you or what you are riding waiting for you at the next redlight with a big smile.  Point is performance is only limited by the amount of money you want to spend or have to spend.

 The more one modifies a ride to increase performance, the more you are shortening its life and decreasing reliability and increasing maintenance.  In short, remember that in most cases we have a near 40 year old machine that is obviously very dependable but not capable of truly competing with modern stock technology and sport bikes.

My suggestion is  pick up an R-6(hell I don't even know if the R6 is still a modern day bike, maybe just my perception of one) or other brand of choice.  You will spend less money going this route in the long run. 

Also consider replacing your counter sprocket with one tooth smaller to satisfy that urge to feel more take off torque and speed.  Make sure you have a near new chain as it will be worked harder with the smaller sprocket.   Keep that take off sprocket because you could end up putting it back on after the experiment.  A simple, easy, inexpensive way to satisfy the need to tweek your ride.

Next time you are sitting at a red light and an R6 pulls up beside you in the next lane and you both do that sneak a look at the other guy thing, consider that the R6 rider very well could be thinking, "wow, what a really nice old school ride.  It looks and sounds great and he has kept it stock.  Wish I had been that smart".

Sorry if I have rambled.  Cabin fever here for sure.  Enjoy your ride.  Stickman

[Duke McDukiedook]

Drop in a V4 or a V twin engine, inline fours don't do low end torque.

[TwoTired]

Drop in a V4 or a V twin engine, inline fours don't do low end torque.

This.^

Throttle twist is a Harley thing.  It doesn't need much skill with a transmission or a tachometer.

Tough to make a silk purse out of a sow's ear.

Anyway PM scondon.  He has a 900 kit installed in his.  He likes the "throttle twist" action. 

Cheers,

[HondaMan]

But, it sure was quick around town and seldom lost at the stop light Grand Prix.

Don't you miss those days, TT? I sure do...