Porting with flow check or DIY?

1971 cb450 total engine rebuild, 3rd overbore (70 to 71.5mm), would love to run velocity stacks, hone, external oil lines, and stock carbs.

One shop says they can't flow check but they will polish and port for 300$

Another shop says they CAN flow check and port but will cost 400-450$

Thinking out loud here: I'll be up to 464ish cc's from 444, and the stock cb450 head is identical to the cb500t down to the diameter of the valves so it can obviously support at least that much displacement.

Has anyone had this done? Is it worth putting the money down (I don't mind if the engine will perform better) or should I just do the best I can myself?

BTW the valves were already vacuum checked and are perfect so I don't think I'll even bother having them lapped.

Porting for what? if you are looking for HP or torque or both a cam may be a better choice. think of it like this:

0><O if you make that 0 or O bigger or slicker how do you get the middle to allow more air to pass through?

I don't know the answer for the 450 I'm just askin

Excellent question, porting and polishing is something I'd only do with the engine completely apart and I've heard several times that the intake ports on this engine could be improved.

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yeah, its way better if you tell us exactly what you want to do.

do you want more bottom end or more top end so you can rev it farther?

are you going to change the cams?

what pipe do you have etc?

porting is a science and it is easy to f up a head . . it is not just a matter of making the holes bigger because this will actually lower velocity which will reduce bottom end power.

one thing that will increase air flow at low lofts without killing velocity is simply back cutting the intake valve.

also, some of these bikes have less timing then they could be running and in these cases, increasing the timing will increase power and throttle response.

also...compression is your friend . . if your is 140 or less, increasing it to around 170 will increase power EVERYWHERE.
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That makes sense, I was planning on having a 2-1 exhaust made, and this will be a mostly highway bike so better revving at higher RPM is the goal

Finnigan said:

That makes sense, I was planning on having a 2-1 exhaust made, and this will be a mostly highway bike so better revving at higher RPM is the goal

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ok, more rpm will be achieved from a different cam . . a different cam will require the head to be ported to perform optimally.

unless you have a LOT of experience grinding, i would NOT try it yourself because you might find that, the grinding bit spinning at 10,000 rpm just might take off a bit more material than you planned.

if you post a close up photo of the ports, i cam probably tell you what might help, or someone here that does a lot of porting can also help.

SLIGHTLY bigger carbs should help after the mods but then your intake boot and port must be the sane size to really benefit from it which is easy enough to do.

you might get lucky and find an old bassani exhaust on ebay.
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There are all sorts of answers to that question. I prefer to have a good porting shop run flow tests before I anyone starts and if I go ahead and have it ported, I like to see the "after" results too.

No let's say the flow numbers are higher - or lower - or not much different. How does that make you feel and much more importantly, what does it tell you? And what do you do with that information? There is software that will predict power curves from gas flow and other data but that's a whole lot of work to interpret and besides, what do you do with that set of data?

If you are interested in the design aspects of your motor, that is all a good investment and if you just want a little more power, maybe not so much.

Let's tackle this from the other end. What has anyone else done with that motor to get a mild increase and how successful were they? What does the body of knowledge suggest and how accurate is that pool? We know that stock Honda motors of that era were more restricted in exhaust port flow than intake and improvements in flow are not always easy to get but are worthwhile.

Let's say you increase the flow through the ports, what then? You would probably be looking for a cam that better matches your port's flow characteristics and allows the motor to rev higher and then you will want lighter pistons with thinner rings to allow it to use those extra revs.

And then you will want different carbs and exhaust and a decent ignition system and for some of us that would be teh way to go and for others of us, not so much.

I realize that didn't actually answer your question because I don't know what your objective is with this bike. And that's where you may want to think about where you want the journey to take you. Do as much research as you can. Talk to guys that race those things in AHRMA or UCRA etc in the "almost stock" classes and see what they did and watch their bikes to see how well they perform.

The real trick to performance though is add lightness and make it handle and brake well.

Spend money on cams and higher compression before professional porting.

Why? I'm sure the experts here can tell you.

Thank you. This thread will be amusing.

teazer said:

I realize that didn't actually answer your question because I don't know

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teazer said:

The real trick to performance though is add lightness

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Priceless. Fucking priceless.

Someday I will actually waste my time tearing your b.s. apart.

DohcBikes said:

Priceless. Fucking priceless.

Someday I will actually waste my time tearing your b.s. apart.

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Interestingly, there are some folks around here that occasionally actually stumble onto getting things right. I don't think it is necessary to condemn them for it. We all know how much simpler it is for the inexperienced among us to race old Yamaha 2 strokes. Come on - Mercy is no hardship to the mighty! I heard some guys here can even get their bare bones wiring harnesses to run for 100 miles or so without calling a tow truck. Not like I know anything about three track vehicle dynamics - seems like there would be some similarity though- go figure.

jpmobius said:

Interestingly, there are some folks around here that occasionally actually stumble onto getting things right. I don't think it is necessary to condemn them for it. We all know how much simpler it is for the inexperienced among us to race old Yamaha 2 strokes. Come on - Mercy is no hardship to the mighty! I heard some guys here can even get their bare bones wiring harnesses to run for 100 miles or so without calling a tow truck. Not like I know anything about three track vehicle dynamics - seems like there would be some similarity though- go figure.

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Yep after nearly 2 months of urban riding and 500 miles in harsh conditions i had a wire rub through what was basically a test harness due to time constraints on a basically scratch built rat bike, and I am HUMBLE enough to share that mistake. Teazer just talks, and talks, and talks about things that he is obviously not educated or experienced enough to talk about. Most of the time he is regurgitating unrelated information.

Most people would not have made it through the first day of my trip. But thanks for reading mobius.

jpmobius said:

Interestingly, there are some folks around here that occasionally actually stumble onto getting things right. I don't think it is necessary to condemn them for it. We all know how much simpler it is for the inexperienced among us to race old Yamaha 2 strokes. Come on - Mercy is no hardship to the mighty! I heard some guys here can even get their bare bones wiring harnesses to run for 100 miles or so without calling a tow truck. Not like I know anything about three track vehicle dynamics - seems like there would be some similarity though- go figure.

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We need an option to like posts ! I give this 2 thumbs up

1fasgsxr said:

We need an option to like posts ! I give this 2 thumbs up

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Thats one thumb for jp and one for yourself!! Enjoy.

Does anyone here know anything about porting or are you just gonna let this guy hang?

Told ya this would be amusing.

Ha, yeah. Looking into cams

DohcBikes said:

Does anyone here know anything about porting or are you just gonna let this guy hang?

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i usually just send my stuff to a guy named mickey onelap, he's a master porter.

DohcBikes said:

Thats one thumb for jp and one for yourself!! Enjoy.

Does anyone here know anything about porting or are you just gonna let this guy hang?

Told ya this would be amusing.

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At least our thumbs are in the air and not our ass ... ;D

Just remove the rough casting marks and it will help you some..Dont go from a 26mm port to a 36 mm port and you wont hurt yourself...less is more unless you are building a Pro Stock drag bike..port match your intake boots if they dont match and remove the roghness of the original casting and leave it at that... dont change very much of the diameter of the intake runner, maybe a 1/4 of an inch down the inside..buy an extra head and practice on it..what can it hurt if you mess up the extra head

I looked last night, the intake boots match the intake and they are smooth in the sense that there are no big imperfections. I might try and polish best I can and call it good.

Thanks for the honest reply

Finnigan said:

I looked last night, the intake boots match the intake and they are smooth in the sense that there are no big imperfections. I might try and polish best I can and call it good.

Thanks for the honest reply

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its been determined that polishing can do more harm than good on the intake . . it has to do with laminar flow and atomization of the fuel.

if the casting is not flush with the valve seat, blending this area will help.

doing a 4 angle cut on the valve seat can help.

always back cut the intake valve if there is a lip on them

shortening the valve guide and narrowing the intake side of the intake guide so the air can flow around it more smoothly will help.

if there is a sharp turn on the short side of the runner this can be radiused just a tiny bit.

as i mentioned, photos might help.

1fasgsxr said:

At least our thumbs are in the air and not our ass ... ;D

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You don't know what fun is then!