Topic: Headwork and replacing valves.

[Soos]

Ok IF one were to replace only 1 set of valves(and seats) to a larger valve, Does upping the valve diameter of the intake or exhaust help up power more?


l8r

[mlinder]

Intake, generally speaking.

You have to rely on vacuum to get air and fuel in, so the bigger the valve, the better (speaking generally, of course... you can go too far.)
Exhaust is mostly pushed out by both the piston coming back up, and especially the pressure of fuel and air expanding as it's ignited.
There's a lot more getting the burnt AF out than there is pulling the  preburnt AF in, soa  smaller valave is allowable as there is more pressure pushing it out.

[w1sa]

A 5% increase in inlet valve diameter would equate to a 10% increase in potential intake volume and that could provide improved accelaration and higher achievable power/rpm at the top. Perhaps providing 5% or more power and usable rpm.

Generally speaking, I think once you exceed these volumetric levels you probably need to consider exhaust valve volume into the equation to ensure the engine can still breathe properly at the top end.

[turboguzzi]

go for bigger intakes (and let us know your suppliers for seats and valves... :-)  )

[esw07]

Definitely need to hear what real life power increase this gets after everything's installed. I may do something similar when I build my '76 550K up to a 750 with Paulages hybrid modification. Just one question where are you planning on getting valves and seats?

[Big Jay]

1 1/2 mm oversize intakes on the 750 does not require new seats.

[turboguzzi]

1 1/2 mm oversize intakes on the 750 does not require new seats.

OK, but do you get any benefit from just a 1.5mm larger valve without the bigger seat?

If yes, then why? 

[pddpimp]

1.5mm or .059" is a good sized upgrade on a valve.

What Jay is saying is that the outside diameter of the factory seat is large enough to be used without having to replace it with a larger seat. You can grind the factory seats for the bigger valve and then open/port the inner diameter of the seat up. Which in turn will aid in greater head flow.

[Soos]

Not too sure, still scavenging through a few heads to find good replacements.

The other option I had is no longer available to me as it was once.
I was thinking of stellite welding on the OD and radius and re-grinding roughly, re-heat treat it, and finish grinding of it.
My new workplace doesn't have any stellite rod, or a plasma spray system.




I will (like my conn rods out of a '05 GSXR1000) eventually come across a useable/modifyable set of valves.


Good knowing the intake is the side to do.
 


I wonder what size valves the KX650 or the KX750 has......
IF the valves are a indication from the piston valve pockets, they MIGHT do.
IF the stem is right, and IF I can re-locate(or not need to, yeah right... hahaha....) the keeper grooves.

Possibly something with a thicker stem might be able to be ground down on the OD and length to be used.....
Damn cb650 and it's nearly NON-EXISTENT aftermarket hop up parts......
But I love all 3 I got!
At least I refuse to give up, and have SO FAR been successful with upgrade mods/finds.




l8r

[bucky katt]

do you do grinding of the seats for larger valves Big Jay? if so, how much? i'll be doing the port work myself

[Big Jay]

We don't grind seats, we machine them on a Serdi machine. We would bore the seat throats out, then machine the seat out to the new diameter, using a full radius under the 45. Then blend the seats to the port.
Full valve job with oversize intakes would run $299

[w1sa]

Does anyone know off-hand what the remaining wall thickness of the  valve 'throat' would be after increasing the inside diameter by 1.5mm.
I'm just wondering whether the standard valve seat/throat thickness would be equally sufficient on all SOHC motors to withstand narrower walls and higher punishment, while 'sitting' in a lesser port shoulder.

[MRieck]

 You can fit a 34mm intake on the stock seat....probably a 34.5 if you could find one. You bore the seat ID out to 90% of the valve OD. I have done this to my head. It thins the seat but I've had no reliability issues.

[turboguzzi]

You can fit a 34mm intake on the stock seat....probably a 34.5 if you could find one. You bore the seat ID out to 90% of the valve OD. I have done this to my head. It thins the seat but I've had no reliability issues.

OK, so what you end up here is with a seat throat to which you cant really apply a good tri-angle job, right? the 45 degs bevel sitting right on the seat ID edge?

Are we all so sure that a smaller seat ID with a proper multi angle job wouldnt flow better? Just asking?....

TG

[MRieck]

You can fit a 34mm intake on the stock seat....probably a 34.5 if you could find one. You bore the seat ID out to 90% of the valve OD. I have done this to my head. It thins the seat but I've had no reliability issues.

OK, so what you end up here is with a seat throat to which you cant really apply a good tri-angle job, right? the 45 degs bevel sitting right on the seat ID edge?

Are we all so sure that a smaller seat ID with a proper multi angle job wouldnt flow better? Just asking?....

TG

There is enough room for a 60 degree cut (or light radius). You bring the 45 degree cut to the edge of the valve face

[754]

At some point , you have to trust the people that are doing & using this mod, that it will work and be reliable..

 Or, if you are just that paranoid.. stay clear of it..


 Come on guys, its a over built CB 750 head, not a Busa... you can get one every day for 150 or less..
 The CB 750 is quite overbuilt, more than newer engines.. Look at the bores.. 5mm oversize.. no problem.. good enough to tour on, you betcha..

[bucky katt]

i'll be sending a head off as soon as i have the money.

[Soos]

We don't grind seats, we machine them on a Serdi machine. We would bore the seat throats out, then machine the seat out to the new diameter, using a full radius under the 45. Then blend the seats to the port.
Full valve job with oversize intakes would run $299

What would be the damage for a cb650 head with that treatment?


l8r

[Soos]

Big Jay do you know if there is a OS (say 2-3mm) intake valve that will work in a cb650?
I can measure anything you need to know(I am a machinist), or even ship you a free intake valve for inspection if interested.




l8r

[turboguzzi]

Regarding valves, I believe that Kiblewhite will do one offs if what you need is not available in their catalog,

754, assuming your comment was addressed to me, then its not about being paranoid or not trusting people like Jay or Mrieck,

the issue here is that I am self tuning, have very limited time, money or spare parts (actually have no spares at all, have done three race seasons with a single motor) so I weight very carefully any improvement in terms of benefit vs. cost and also logisitcs.  Can only send my one and only cyl head to be worked on during the winter break in racing and I have to be sure that when I'll assemble the engine everything will fit.

That's why it's very tempting for me to stay with stock parts unless someone can put hard HP numbers on the benefit of a 1.5mm larger valve and seat throat.

I recently spent 400$ on a set of used CR29's based on the fact (?) that they should give 4-5 hp over my 27mm VM's. Will those 300$ worth of work on the seats + 200$ or whatever for oversize valves yeld a similar gain?    

Wish I had the dosh to try tons of stuff like that but i have to play it safe. So far it payed of, I started with 55hp three years ago and was up to 61hp last year, didnt have yet the chance to check what my 500/4 is doing now with the CR29's but since I shaved off two seconds per lap in last race, I'd say they were a worthy investment. I am third going into the last round in the championship against serious riders that have their bikes built for them by pro tuners, so I'd say that my cautions approach has paid off. If your hot rod motor breaks while cruising boulevards or in the canyons near home it's one thing. If your tuned motor breaks a valve seat in a race that you drove 600 miles to, payed inscription to, hotels, gas, etc. it can really hurt.

So then, can anyone quantify how much power you'd get from +1.5mm oversize valves?

TG      

[voxonda]

+1

Way to go TG, always do your own thing.

[turboguzzi]

rob

was reading yesterday Kevin Cameron's interview to Rob Muzzy "louder than money" from a 1983 issue of cycle mag.

Muzzy said there something like: You will get 85% of the improvement from a given job quite quickly, it's getting the other 15% that takes a lot of time. Dont waste time on that 15%, move on to other important stuff where you can make big gains.

That's from memory but i guess the intention is clear. I am trying to decide what are the "other important stuff"  for next season and been considering bigger valves and seats to solve the lack of breathing high up the rev range.

Somehow I get the feeling that I could gain a whole lot more by spending my time on a tuned airbox and velocity stacks or investing in a C/R box'. Truth is I dont know so all I am asking is what kind of improvement have people seen form the +1.5 mm bigger valves.  Even just flow bench figures would be interesting.

TG

[voxonda]

TG,

Think one of the flaws in the head design of 'our' period Honda's is the tendency to run out of breath higher on the rev scale. Think that only bigger valves will not do the trick without extensive porting. There is enough room for improvement, but got to keep in mind that it still is a 8-valve engine and will never match up to a 16-valve for that matter. A couple of years ago spoke to a (french) guy who had his 750 extensively tuned in the search for top power. They destroyed a head by using 35 mm intake valves and matched(?) the port but all it did was slow the mixturestream, in the lower rev regions so it was useless. That was what he told me, more or less.

Cheers, rob

[Big Jay]

Big Jay do you know if there is a OS (say 2-3mm) intake valve that will work in a cb650?
I can measure anything you need to know(I am a machinist), or even ship you a free intake valve for inspection if interested.




l8r

We would custom machine them from blanks. No problem.

Jay

[Bill/BentON Racing]

TG,way to go,Bring home that 3rd place,you can do it! I think the 1.5 mm OS valves has been researched more than your airbox mods.2 seconds a lap ...something worked!!Congrats,Also your gearbox has potential and what about ignition? Is your advance curves etc adjustable,wondered if any gains from Dyna 2000? You Go Man,we are proud of you and your results!SOHC's rule esp. built 500'550s!! ,Bill