.67 goldhead cmb 2 glow plugs information

[Davidkrienke]

After the T-porting era, we found that slotting and window porting the piston closest to the boost port worked best. It's possible to also window port the boost port in like fashion, but we couldn't see a noticeable change in on the pond performance. Maybe different on a decent dyno, who knows. This is a NR67 flow configuration that works really well. Didn't grind new timing figures into it, but a .010" lift (shim) under sleeve flange woke things up....had to take .010" off the deck on the sleeve to compensate, too. I guess a .010" cut on the button touch down surface would've maybe been a better way to go. Buttons are cheap, compared to new sleeve + labor time.

Eventually, we're going to apply the same thought to our CMB 67's and 80's.....we'll keep you posted, Frank.

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That's great info. I have also been thinking of porting the piston sides and letting the fuel get up the transfers as some of the engines I have seen have it some dont. So your making a hole for the rear transfer in the piston? do you have a pic of the piston and sleeve to see what yours looks like or is the piston in the engine still ?
I had a scooter racing business for 10 years or so importing and making engines for drag racing scooters and kids 4 wheeler racing. I used to have to sit 5 hours a day porting cylinders for customers to get sent out.. I just had to look at some pics thinking about the jb weld. I remember if you keep licking your finger and smoothing it until it sets up you could really make some smooth transitions.
I forget alot of what I learned and did .

I just always am afraid making more volume in my crankcases for flow on the sleeves I see some really thin down the ramps up to the port . Its just a small engine and keeping it tight for velocity I know helps at least on the larger engines I have done. i usually end up shaping things with jb weld to smooth and flow while not adding crank case volume.

 

[Davidkrienke]

Its funny how all the engines I have done higher or lower timing that golden 30 degree blowdown ratio always seems to work out best. From 2.5cc to 500cc they all react the same. I have just found if I set say the tranfers at 127 or so and the exh at 187 to 190 is the max I have been able to do and still keep a broad engine power band. when you go 192 and up it just seems so peaky.

 

[Wasted wages]

when you go 192 and up it just seems so peaky.

It does,, but it works better for riggers where you can unload the motor… I don’t go that high with exhaust timing in monos…

 

[Timbo Whalen]

That's great info. I have also been thinking of porting the piston sides and letting the fuel get up the transfers as some of the engines I have seen have it some dont. So your making a hole for the rear transfer in the piston? do you have a pic of the piston and sleeve to see what yours looks like or is the piston in the engine still ?
I had a scooter racing business for 10 years or so importing and making engines for drag racing scooters and kids 4 wheeler racing. I used to have to sit 5 hours a day porting cylinders for customers to get sent out.. I just had to look at some pics thinking about the jb weld. I remember if you keep licking your finger and smoothing it until it sets up you could really make some smooth transitions.
I forget alot of what I learned and did .

I just always am afraid making more volume in my crankcases for flow on the sleeves I see some really thin down the ramps up to the port . Its just a small engine and keeping it tight for velocity I know helps at least on the larger engines I have done. i usually end up shaping things with jb weld to smooth and flow while not adding crank case volume.

No, don't have photo handy...basically, as piston travels toward BDC, trapped fuel within the piston evacuates into transfer(or boost, depending) for a given dwell period (hence the finger slot in sleeve). The objective is to get as much fuel up into the chamber as possible, per cycle.

 

[Don Ferrette]

Its funny how all the engines I have done higher or lower timing that golden 30 degree blowdown ratio always seems to work out best. <snip>

Absolutely. A number of years ago I had an old CMB purple head CAM 45 I did with 195 exhaust and 135 boost/transfers that was sick fast in a 45 rigger and still throttled quite well. Engine case was also missing a fair amount of metal inside as well.....

 

[Timbo Whalen]

Nice work BTW… looks clean!
I really like the finger porting CMB did on the exhaust ports,, cleaning those up and the exhaust port in the case can work wonders in these motors..

You talking about the 'cat eye' 3 port exhaust?

 

[Timbo Whalen]

Absolutely. A number of years ago I had an old CMB purple head CAM 45 I did with 195 exhaust and 135 boost/transfers that was sick fast in a 45 rigger and still throttled quite well. Engine case was also missing a fair amount of metal inside as well.....

Yessir, at full tilt, case volume really isn't an issue....at that point, the ol' pump is working OT!....

 

[Davidkrienke]

That's great info. I have also been thinking of porting the piston sides and letting the fuel get up the transfers as some of the engines I have seen have it some dont. So your making a hole for the rear transfer in the piston? do you have a pic of the piston and sleeve to see what yours looks like or is the piston in the engine still ?

I just always am afraid making more volume in my crankcases for flow on the sleeves I see some really thin down the ramps up to the port . Its just a small engine and keeping it tight for velocity I know helps at least on the larger engines I have done. i usually end up shaping things with jb weld to smooth and flow while not adding crank case volume.

No, don't have photo handy...basically, as piston travels toward BDC, trapped fuel within the piston evacuates into transfer(or boost, depending) for a given dwell period (hence the finger slot in sleeve). The objective is to get as much fuel up into the chamber as possible, per cycle.

Yeah I was not thinking. all those scooter motors had dual ports in the piston skirt on that rear boost port and I used to widen and cut shorter the bottom of the sleeve and open those way up. I just get frazzled and cant think sometimes. At first when I asked I was thinking you were doing side transfer ports and then I realized you were talking rear. Again I am a super nervous mind and I get nervous talking to you guys because you all know so much and I always feel I will come off dumb.

 

[Timbo Whalen]

Yeah I was not thinking. all those scooter motors had dual ports in the piston skirt on that rear boost port and I used to widen and cut shorter the bottom of the sleeve and open those way up. I just get frazzled and cant think sometimes. At first when I asked I was thinking you were doing side transfer ports and then I realized you were talking rear. Again I am a super nervous mind and I get nervous talking to you guys because you all know so much and I always feel I will come off dumb.

Don't feel bad....I've the same problem - look right past the obvious sometimes.

 

[Timbo Whalen]

Absolutely. A number of years ago I had an old CMB purple head CAM 45 I did with 195 exhaust and 135 boost/transfers that was sick fast in a 45 rigger and still throttled quite well. Engine case was also missing a fair amount of metal inside as well.....

Guy we both know ran an RS EVO up to 205 in a lightweight modified JAE flat plane design, fairly extreme blow down number, but interference waves from other boats in heat race would make it difficult to drive....buzzed up, sounded like a hot rodded 21. Oh, he even lightened the crank flywheels!