Topic: Alternative performance pistons & squish

[livefast_dieold]

Hi everyone,

so at the last race of the season the engine "cooked" the JE pistons I was using (DP62/r) and now I have to change them. The engine is running quite hot (the upper part of the conrods was purple, and the piston pins has typical overheating purple circles), I guess because of a quite high compression ratio (12,5:1).
While brainstorming with Yossef, it has also due to the fact that my current configuration has little or zero squish, so it requires a quite high spark advance (40°).

I'm looking to install Kawasaki KZ650 pistons, which are almost a perfect fit for the CB750 (apart some valve groove work to do). I've also bought a set of Yamaha XJ650 pistons, that are 63mm in diameter with a nice dome. The only main problem is that they use a 16mm pin :-(

Does anyone here have bored out the conrods to fit 16mm pins? Seem pretty feasible to me... especially since the kawasaki kz650 pistons, the yamaha xj650 pistons and the JE pistons I was using have the same weight.

Also, how much squish are you guys using and how have you achieved it? Looking at the piston and head configuration there's very limited squish to achieve within the 750cc displacement...

Ciao!
Riccardo

[turboguzzi]

since ive seen the problem one on one, i am adding, to get a minimum of squisk in these deep chambers, a pstion that angles up directly form the edge is needed.
What do guys racing these in 750 displacement do?
Mark form M3 used to have unifnished high dome pistons to custom fit for racing application, do these still exist?
might have a rod solution for you ric, i think that gpz750 turbo rods have the same crank diameter of 35mm and 16mm pins, gotta chack tough...

[bwaller]

Those unfinished domed pistons Mark sold were Wiseco's, perhaps they are still available and probably lighter that the other pistons mentioned. I believe they are 62mm.

Those big chambers...nearly everyone on here building 750's are using bigger bores so this increases compression some. Plus some squish as a result is developed with a bigger bore than chamber diameter. Because of the larger bore Pewe had his chambers machined wider and developed a squish band that worked with whatever pistons he used. That works well for a much larger bore, otherwise welding in the chamber is one solution especially, (in the case of the 750) on one side of the K chamber....obviously TG has done this.

I dislike huge domed pistons although it's the easiest solution. As far as I'm concerned though, spark lead must have to be increased with these tall pistons as well, may not change your heating problem Ric. Large domes are leftover from the early days with wide valve angles and deep chambers.

The 650 chamber is even worse as a full hemi shape. That one is driving me nuts, because untouched, the only answer is a huge dome!  The much smaller 550 chamber offers potential for incredible squish and ignition advance down to 28 degrees.

Good luck, I'll follow your search with interest.

[turboguzzi]

indeed brent, somethign like this but i think that the crown inital angle should be steeper to match the chamber closer

from the markings it looks wiseco, but i dont see the 9720L06200 code in their catalog

and yes, in bigbore situations squish is easier to create, but limit here is 63mm

[turboguzzi]

muzzy racing pisotns for the gpz750

if you look a the two edges left and right you can see that crown angle starts at almost 40-45 degs....

crown choped off so doesnt stick out into the chamber as much like the wiseco.

[cbr954]

Dynoman can make custom pistons to fit your head.  Take a mold of the head and they make pistons to fit or to what you want for piston shape.

[Captain]

Weld up the chamber because you need a piston similar to the “muzzy” crown shape. Forget what pistons from other engines and get custom made “billet” type which can be made using all the latest tech and out perform anything else you could do (low weight, low drag, stronger). When pursuing this tech you need at least two flat area’s for squish of .040” max. The bathtub chamber you end up with should blend with the cylinder wall inline with the angle / direction of the port inlet charge. This promotes swirl and combined with effective squish promotes faster burn (as lower ignition lead numbers will confirm). Exhaust port is central and so doesn’t require this directional blending.

 Captain

[scottly]

so it requires a quite high spark advance (40°).

Why do you believe 40* of advance was optimum which such a high compression? Do you have dyno graphs with different advance settings to support this?

[turboguzzi]

right scottly, i asked riccardo the same.... but still, good squish would help further
captain this is exactly what i did in my kawi750 and it worked well, i proposed this to ricardo, just a bit more complex/scary to do
nevertheless, i thought that with all the racing 750s out there, an already developed squish piston could exist,
 

[livefast_dieold]

Problem is that even the DP62 should have been the non-plus-ultra for racing sohc within the 750cc displacement, guaranteeing the right squish and all:



This is a KZ650 pistons:



Also 0.04" squish area is not that much, big bore pistons have way more squish area. If this is the right value then it shouldn't be needed to weld the chambers

[Captain]

.040” piston to head “clearance” ..... Not area.
 If the bore size does not allow for large squish areas on each side of the chamber (above the wrist pin) then weld up the chamber until you can. The area I’m referring to will match closely the flat areas of the “muzzy” piston I indicated about previously and you will need a piston of your own to match.

Captain

[bwaller]

Pistons like the DP are generic "so one size fits all" with extra clearance. Not saying you didn't, but in the end trying to massage these into providing tighter clearances can be more difficult than having pistons "purpose built" after possible chamber preparation.

I've had pistons made two different times, one company would make only four pieces, last time Wiseco had a 12 piece minimum order. If you get to this point, check around. Just as an example and I'm not pointing you in any direction....but last time the rep I dealt with was excellent. Getting an accurate net dome displacement, a "drawing" before production gives the builder every opportunity to calculate exact CR etc, and get the piston dome right.

Off the shelf pistons come with a suggested CR value, but there are several different build variables that can change this final # significantly. I have found tight tolerances and especially squish is more important than crazy high CR.

[turboguzzi]

exactly what i was going to say, an aftermarket company can not take the risk that a piston will kiss the head, so they take extra clearance, bye bye squish....
BTW, it's the same story with even modern 4 valve stuff. in production line with all the variables, they stay at no less than 1.2-1.5 mm from the head, good squish works at 1mm. alot of gain to be had by adjusting the clearance to that distance

[livefast_dieold]

So I should weld in the highlighted zone:



I'm a bit concerned by this operation, to be honest, seems to me that no one here has done it :-)
Does someone have some pictures, even from other bikes?

[PeWe]

The overheating here reminds me about a piston meltdown I had to witness. Suddenly a lot of smoke in my mirrors, very dense white-grey.
I had to replace all 4 pistons and all 4 rods that were blue, guides too. Very smilar.

I think it has been running hot for a while, pinging/detonation as the last drop. Hole in piston on the IN valve pocket, plus one with a melted bump, piston 2-3.

It would be fine if they can be repaired, weld them and machined to correct shape and weight. I remember that the tuning shop guy was happy about the low weight.

Edit: chambers looked like the photo below. K6 head with F2 inlet valves
Rods are corroded after  + 20 years in a barn. Issue occurred 1984

[turboguzzi]

it's a 65mm piston, but i think it explains well the idea

see how the crown rises right from the edge of the piston? it doesnt have the step like in the dynoman....

thats the area that should produce the squish

it's from bore-tech.com, maybe ask them if they have something similar in 62mm?

[livefast_dieold]

it's from bore-tech.com, maybe ask them if they have something similar in 62mm?

Looks nice! And nice price too! Already asked them :-)

[johno]

Heres a good pic of the 3 types of 62mm piston, stock , Dynoman, custom.
Kind of highlights the edge of the pistons as per your discussion

[johno]

With the custom straight sided piston this was a map of the piston top to head clearance, as you can see it varies greatly and as I didn't want to do welding on the head I had to relieve the piston edge to get enough clearance to miss the head ( which had a standard type profile except for extreme valve area enshrouding)

The dynoman was easy peasy to fit into head but the rings really gave me the poos and I was glad  to do more piston work on the custom pistons just to get my old fashioned ring thickness back and be 100% confident the rings were correctly installed , unlike those skinny barstards that drove me nuts installing. In the scheme of things I felt the thicker rings were a better option for me after the troubles I had with the skinny things. I was confident that no minuscule HP gains were worth the pain especially with the bore and hone techniques employed by the machinist.

Lots of compromises chasing the dream squish

[PeWe]

Better to attach the photo in the post

[bwaller]

With the custom straight sided piston this was a map of the piston top to head clearance, as you can see it varies greatly and as I didn't want to do welding on the head I had to relieve the piston edge to get enough clearance to miss the head ( which had a standard type profile except for extreme valve area enshrouding)

The dynoman was easy peasy to fit into head but the rings really gave me the poos and I was glad  to do more piston work on the custom pistons just to get my old fashioned ring thickness back and be 100% confident the rings were correctly installed , unlike those skinny barstards that drove me nuts installing. In the scheme of things I felt the thicker rings were a better option for me after the troubles I had with the skinny things. I was confident that no minuscule HP gains were worth the pain especially with the bore and hone techniques employed by the machinist.

Lots of compromises chasing the dream squish

[/b]

That's for certain johno. (good to hear from you again) Stuffing a deep chamber with a tall dome makes for trickier measuring. What material did you use?

I know I've read you prefer leaving chambers untouched, because expensive heads aren't as disposable as comparatively inexpensive pistons, (good logic!) but I don't remember where you needed to set ign advance. Simply for further discussion....even with tighter squish isn't any tall dome going to require more spark lead which may have been part of lfdo's heating problem in the first place?

Welding in a chamber is a hurdle no question, but squish is maintained and may well in the end allow for a partially flat top piston, WHICH ought to also allow for less advance?

[turboguzzi]

hey guys, photobucket doesnt work anymore, load your pics on the message.... bet these are interesting pistons, just cant see them

[Medyo Bastos]

Photos show up fine fine for me


Sent from my iPhone using Tapatalk

[bwaller]

They did show for me earlier. johno this Photobucket deal is an absolute PIA. I was in the same boat until i read this from member 333. Shazzam it worked in my case & all my pics seem to open again. Take a look.


http://forums.sohc4.net/index.php/topic,168102.0.html

[Tintop]

Tried the fix with Chrome just now and it looks like all the pics are back in my threads.