Author Topic: Oppy 42 ci head: flow optimization / grinding issues  (Read 14978 times)

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Offline GermanFred

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Oppy 42 ci head: flow optimization / grinding issues
« on: November 30, 2016, 05:54:12 am »
Dear Gents,
I am all new here in HeyMow/ Mowbetter, writing from Germany. We also have some mower sports over here - even though a very little scene compared to the US one. Your forum is a great source of information, a heck lot better than everything we have here in Ol'G! Also, your guys' attitude is much better than the German fellow racers' one. You help each other out rather than showing off each others selfish, egocentric I-am-the-boss character.  Thanks for that!

Now, here's my question.
I am working on a 42 ci Briggs Opposed Twin at the mowment. Event though I read trough much of the oppy-related articles here, I am still puzzled with the cylinder head itself. I did a crude littel sketch:

 :confused:
I would very much appreciate any help from you, thanks a lot!
Cheers,
Fred
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we race on fields, no ovals

Offline Jeff McKelroy

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #1 on: November 30, 2016, 08:54:31 am »
Do nothing more than blend. remember the more you remove the lower the compression, which is already somewhere in the kerosene range, HAHAHA.

As you increase valve lift, even with a stock diameter valve, you will have to machine reliefs around the valves for clearance.

At higher RPM, more mixture will be following the piston downward than what's running across the head anyway.

Offline Jim Knutson

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #2 on: November 30, 2016, 09:29:14 am »
As Jeff said, just blend it in....the "green" area for sure. I personally have found  better gains by softening the "yellow" area on the exhaust side of the head.
If you quit when you're behind, you will never win.

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #3 on: December 02, 2016, 03:38:03 am »
Cheers, appreciate your replies.
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we race on fields, no ovals

Offline flatlanderfornow

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #4 on: December 02, 2016, 05:18:23 pm »
the newer style 42 head is cut out a little more on the exhaust side about like you have it marked
Todd Potter
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Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #5 on: December 05, 2016, 05:37:25 am »
the newer style 42 head is cut out a little more on the exhaust side about like you have it marked

You mean like this?



I was already wondering why every picture of 42 oppy cylinder heads I found on the web did not look like mine (far right one). There is the answer, I guess.



Do you think it is worthwhile grinding off the bit within the red circle? (not caring about losing compression by this)

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we race on fields, no ovals

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #6 on: December 05, 2016, 06:43:31 am »
Oppy mechanics manual says:



However, do you think, re-facing the valves and seats both to 30° is advantageous? Because of:




Moreover, the exhaust valve head's upper most part is rather thick compared to the intake valve head. I guess, this is due to better heat transfer enhanced by more material. On the other hand, I definetly believe this thick valve head reduces gas flow. See lower valve on this Picture; it really sticks out:


What do you think, should I "flatten" the valve head by grinding off some material?

The Oppy mechanics manual states minimum thickness of valve head(s) to 1/32" - this would be at least half of what it measures right now.
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Offline PJG56

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #7 on: December 05, 2016, 11:51:44 am »
You mean like this?



I was already wondering why every picture of 42 oppy cylinder heads I found on the web did not look like mine (far right one). There is the answer, I guess.



Do you think it is worthwhile grinding off the bit within the red circle? (not caring about losing compression by this)



No on the red circle,it does nothing for intake flow,these heads flow plenty on the exhaust side as is
Paul Guptill  (oppy) #238modX
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Offline flatlanderfornow

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #8 on: December 05, 2016, 06:37:49 pm »
You mean like this?



I was already wondering why every picture of 42 oppy cylinder heads I found on the web did not look like mine (far right one). There is the answer, I guess.



Do you think it is worthwhile grinding off the bit within the red circle? (not caring about losing compression by this)


yup.
What else are you doing to your engine?  Unless you're running a cam and other mods the heads are not likely to be the bottleneck.
I use the small chamber heads on my pullers and cut off the ridge around the combustion chamber to raise the compression-we're
not allowed to run cams so this helps them out at the lower rpm levels we run (4000 or so under load).
Todd Potter
603-922-5269

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #9 on: December 06, 2016, 03:39:32 am »
No on the red circle,it does nothing for intake flow,these heads flow plenty on the exhaust side as is

Alright, so do you think, I should grind sort of the same geometry into the intake side of the head?
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we race on fields, no ovals

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #10 on: December 06, 2016, 03:45:30 am »
yup.
What else are you doing to your engine?  Unless you're running a cam and other mods the heads are not likely to be the bottleneck.
I use the small chamber heads on my pullers and cut off the ridge around the combustion chamber to raise the compression-we're
not allowed to run cams so this helps them out at the lower rpm levels we run (4000 or so under load).

I only do porting, head works and tweaking/ tuning the carb. Ah, sorry, and 'oil mods' of course!

I do not plan to install a racing cam, billet rods or forged pistons.
These components shall stay stock to keep (a) costs down since this engine will only race in 'farmer's fun races' and not competing in a national championship, (b) it is very hard (impossible) to come by such high-performance components here in Germany which means ordering them from the USA -> leading to very high costs, (c) max. RPMs will be kept under 4000 revs; I am looking for more grunt from below (to better get out of turns on muddy soil) rather than high-flying horses , yeah, that's it I guess.

The stock flywheel should be changed to a selfmade steel or (heavy, i.e. thick) aluminium one with bolted-on platic fins - this is the ultimate plan. I have some fear spinning the stock flywheel near and constantly about 4000 RPMs...
By the way, does anyone have dimensions as a drawing of the stock flywheel? This would save me a lot of work measuring everthing myself ;-)
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we race on fields, no ovals

Offline flatlanderfornow

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #11 on: December 06, 2016, 06:15:28 pm »
advance tne cam 1 tooth and the timing 10-12 degrees
shave the ridge off the small chamber head and do a good valve job on it 3 or 4 angle on the intake wide contact on exhaust to dissipate heat better
make sure the valve lash is right  .004-.006" intakes .007-.009 exhaust
use the 4-bolt carb as it runs better stock than the 3-bolt much more responsive and atomizes better even though the hole is smaller
fancy porting and head work won't get you much with that pathetic stock cam but the above setup will make that engine very
torquy down low where you will be running
Todd Potter
603-922-5269

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #12 on: December 08, 2016, 05:39:02 am »
advance tne cam 1 tooth and the timing 10-12 degrees

Really??
I mean, isn't that double-advancing the timing by 22-24 degrees in summary?
See post:
http://www.heymow.com/index.php/topic,16604.msg171186.html#msg171186

If you advance the cam by 1 tooth -> Rooster: "Yes the cam moves approx 6 deg, but the crank moves twice as far as the cam, so yes it is 12 deg on the crank."

 :confused:
Fred
<<< visit www.wxpss.tk for german lawn mower racing information >>>
we race on fields, no ovals

Offline flatlanderfornow

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #13 on: December 08, 2016, 07:40:02 am »
Ignition timing
 1 tooth on the cam 10-12 degrees on the ignition
Todd Potter
603-922-5269

Offline GermanFred

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Re: Oppy 42 ci head: flow optimization / grinding issues
« Reply #14 on: December 08, 2016, 09:35:34 am »
Ignition timing
 1 tooth on the cam 10-12 degrees on the ignition

i.e. advance the cam only by 1 tooth which results in 10-12 degrees advanced timing of the ignition. Correct?
<<< visit www.wxpss.tk for german lawn mower racing information >>>
we race on fields, no ovals

 

anything