Topic: 750 Homebrew port job

[Brantley]

Well I figured I'd give my fingers a rest from the bleedin' sandpaper and do some photo taking and typing to get some feedback. Although I've smoothed out rough edges in intake ports before this is my first attempt at removing a little aluminum. I only took photos of #1, as it was the worst, but it's indicative of the others. Sorry I'm too lazy to take before photos. I plan on bead blasting when finished. Whatta y'all think? It's a K5 head, by the way.

[LesterPiglet]

Any bloody photos would do.   

[Brantley]

Thanks, Les. Workin on it, pal...

[Brantley]

Not sure why I'm getting "request entity too large" for photos that are well below parameters...

[Brantley]

More:

[LesterPiglet]

Not sure why I'm getting "request entity too large" for photos that are well below parameters...

Some groovy colours goin there. 

Are you doing this manually?

Try photobucket.com for pictures, it's free.

[Brantley]

No, Les, just tryin to light em up so you can see the details.

[lucky]

Honda wrote an article that was in a motorcycle magazine that said they left the ports rough, and that it created more turbulence and got a better mixture.

[LesterPiglet]

I meant aren't you using grinding tips on a drill....sandpaper?

A flow bench is required Lucky. I homebrewed one years ago with some tube, ink and water, plywood to make into a box and my home vacuum cleaner.

[cgswss]

Seldom can you do a port job by hand that will help your performance.  The thing you can do by hand is "matching the ports"  Basically taking any "step" out where the manifolds join the head.  I can't tell you how many hours we spent working on ports trying to get it right, and we had access to a dyno.

Some things to think about.  When you take out material, you are have more volume and therefore slow down the gas flow.  This means you move your "power" up the RPM range.  Or in other words, you reduce your performance at low RPMs.  We also found by testing that you don't want slickly smooth ports.  We found that the limit was around 80 grit.

Frankly you will gain more performance with a 3 angle valve job then 10 hours of port work.

I really hate to say it, but unless you spend most of your time up around 10,000 RPM you are wasting your time.

[LesterPiglet]

Woohoo, thanks for that nugget. Now wheres me grinder?   

[MCRider]

"I really hate to say it, but unless you spend most of your time up around 10,000 RPM you are wasting your time"

Its curious you would say this. its a well known fact and first hand experience to me, that a ported head on a CB750, which includes some enlargement, Increases power throughout the band sub redline at 8000.

There is a bump in the intakes as they turn down to the valve, which when removed, really helps performance. Just too much work for the factory to go after. Then also as you mentioned, matching the screwed in manifolds to the head casting is important, still looking at sub 8000 RPM.

I can't comment on a 3 angle v a 10 hr port job. Both would be best.

[MCRider]

There's a porting piece by Branch Flowmetrics, written back in the 70s, which has cutaway diagrams of the porting they did to the CB750, removing the hump on the intake, care not to undercut the seat, etc. Its in the FAQs or the Technical part of the home page or somewhere.

They went with a hemi dome, elminating the swirls, to good effect. Though i know thats controversial.

[cgswss]

We were a racing shop and did a lot of mods on a lot of bikes.  Now "removing the hump" would be a good mod, because you are removing an obstruction.  In effect you are keeping the port from getting smaller-not making it larger.  If you are getting a measurable performance increase from porting, I would guess that the hump-tecty is the reason, not enlarging the ports.  But doesn't matter if is a Hemi or a Honda, increasing port size (cross section) will slow the gases down and that will NOT increase low RPM performance.

The most important thing is that you didn't screw things up and your happy with the results.  But it sounds like you did a lot more homework before starting.

[killersoundz]

My neighbor also told me that from his experience working in a machine shop working on engines, he learned that it is probably better to leave a rough surface in there.

[lucky]

I meant aren't you using grinding tips on a drill....sandpaper?

A flow bench is required Lucky. I homebrewed one years ago with some tube, ink and water, plywood to make into a box and my home vacuum cleaner.

In 1968 Honda had a 3 million dollar DYNO.

[Retro Rocket]

Its curious you would say this. its a well known fact and first hand experience to me, that a ported head on a CB750, which includes some enlargement, Increases power throughout the band sub redline at 8000.

+1 Ron, the single biggest performance gain on these engines is in porting the head, The heads on the SOHC honda's are quite restrictive. We have a couple of head experts on this forum that will back that up in an instant....   A friend an I ported a 4.2 liter Aussie V8 in an old SLR Torana i once had, we removed a cup and a half of material from each head, i then had custom made extractors made and an Edelbrock manifold fitted that i had to port to match the heads, that asthmatic little engine turned into a fire breather and surprised a lot of people. Never say Never....

[Brantley]

Ok, sorry boys... I didn't specify I removed the material MC pointed out via grinder and rattail file. Sandpaper was used to smooth transitions and, shall we say, act to knock the casting snot out like the blue bulb thingy does a baby. I actually used 80 grit emory cloth to smooth, rather than polish, the cuts I made. I think I understand the theory behind this and didn't remove anymore material from one port (in one area) than wasn't there in any other... if that makes sense. In other words if one was shaped (as far as feel) like a grape, one a pear, one an apple, one a banana I was going for the hybrid feel of all four. In other other words I am trying to match these things. My line of thinking is you don't have to necessarily know exactly what you're doing or give a $h!t about it on a factory production line as long as you meet a quota. Anyone ever heard the phrase "good enough for government work" on a jobsite?  All that aside- can I get a comment on the work done?

And Les- sorry thought you were talking about photography... HA!
sgswss- I left out the part where a valve job is next... who gets abrasives near a valve seat without planning on a valve job?
And Lucky, there's no way a 3 million dollar DYNO was used on every machine that left a few different factories.

[MCRider]

Regardles of how much was spent on the Dyno, a factory must always compromise between the engineers, the bean counters and what they perceive the public wil buy. Cost is always a limiting factor in how much time a factory will spend getting a head right v right enough.

10 hours of proper work will yield huge results. A production machine would never get that time.

[Don R]

I am positive the engineers didn't get their way on many items. Athough they were revolutionary, a price point had to be met. Many things can be improved they just couldn't sell it then.
 Case in point, the disc brake lugs on the right fork leg. If the engineers didn't think two were better they wouldn't have the lugs cast in and the parts wouldn't be reversable. My opinion is, bean counters cancelled the twin disc. Later when other bikes had them, Honda did too.

[MCRider]

Good point on the discs, Don.

[cgswss]

10 hours of proper work will yield huge results. A production machine would never get that time.

if you rewrite that as "10 hours of proper work{by someone experienced in port modification} will yield huge results {in the upper RPM ranges}"  I would be happy to agree with you.

However if you write it "10 hours of work{by an inexperienced shade tree mechanic} will yield huge results{ in all RPM ranges.}"  I will disagree.  I have seen way too many heads ruined by someone with a die grinder and a set carbide burrs.

[Don R]

He did say proper.

[MCRider]

He did say proper.

Thanks, I choose my words to keep the posts short.

[Retro Rocket]

10 hours of proper work will yield huge results. A production machine would never get that time.

if you rewrite that as "10 hours of proper work{by someone experienced in port modification} will yield huge results {in the upper RPM ranges}"  I would be happy to agree with you.

However if you write it "10 hours of work{by an inexperienced shade tree mechanic} will yield huge results{ in all RPM ranges.}"  I will disagree.  I have seen way too many heads ruined by someone with a die grinder and a set carbide burrs.

I'm curious, how many 750/4 heads have you ported, or how many 750's have you ridden with ported heads.? In my experiences, improvements were noticeable throughout the entire rev range, not just up high in the rev range, carburetion is also a contributing factor as is the cam and even pipes....