Pressure in the timing chest

I know the general consensus is that breathing (with valve) directly off the crank is a better bet than off the timing chest alone, but does anyone have any data regarding just how much pressure is passed into the timing chest? I'm curious because I've picked up a rear timing chest cover plate with a reed valve installed, and am debating whether or not to install it, which would effectively give me two valved breathers (71 with breather above primary, mikes XS valve mounted directly to case).

Thoughts?

acadian said:

I know the general consensus is that breathing (with valve) directly off the crank is a better bet than off the timing chest alone, but does anyone have any data regarding just how much pressure is passed into the timing chest? I'm curious because I've picked up a rear timing chest cover plate with a reed valve installed, and am debating whether or not to install it, which would effectively give me two valved breathers (71 with breather above primary, mikes XS valve mounted directly to case).

Thoughts?

Click to expand...

Well, any pressure buildup is going to be passed to the timing case.

Just not fast enough to make a reed breather create much of a vacuum at road speed. Jim

comnoz said:

acadian said:

...just how much pressure is passed into the timing chest? I'm curious because I've picked up a rear timing chest cover plate with a reed valve installed, and am debating whether or not to install it, which would effectively give me two valved breathers (71 with breather above primary, mikes XS valve mounted directly to case).

Thoughts?

Click to expand...

Well, any pressure buildup is going to be passed to the timing case.

Just not fast enough to make a reed breather create much of a vacuum at road speed. Jim

Click to expand...

How about venting the timing case into the crankcase?

Nater_Potater said:

comnoz said:

acadian said:

...just how much pressure is passed into the timing chest? I'm curious because I've picked up a rear timing chest cover plate with a reed valve installed, and am debating whether or not to install it, which would effectively give me two valved breathers (71 with breather above primary, mikes XS valve mounted directly to case).

Thoughts?

Click to expand...

Well, any pressure buildup is going to be passed to the timing case.

Just not fast enough to make a reed breather create much of a vacuum at road speed. Jim

Click to expand...

How about venting the timing case into the crankcase?

Click to expand...

Any pressure in the timing chest is going to come from the crankcase. It has to travel through relatively small holes though so it kills the pulse from the pistons that make a reed valve effective as a pump.

Without the pulses the reed valve is just a pressure relief valve, but that is still better than an open breather, particularly at lower RPMs.

The merits or relative effectiveness of a reed valve mounted on the backside of a timing chest can be argued up and down, but real-world experience on several dozen of them, over several years of significant road use clearly indicates they are indeed effective at positively venting the crankcase and eliminating typical niggly leaks.

In other words, HAVING one is better than NOT having one.

Having two breathers on one bike is not a recommendation I have ever made.

I run both a mike's XS reed breather on the timing chest back plate, AND the stock timed breather on the end of the camshaft on my '70 commando. I have no problems with them working together so far. I drilled a 3/8" hole between the timing case and crankcase above the oil pump and between the webbing of the crankcase casting to allow air from the crankcase to move towards the timing chest more easily, and I also drilled a 1/4" hole down under the oil pump so oil would drain back to the crankcase sump from the timing chest.

I'd like to hear what the criticism of running both breathers together would be,...

o0norton0o said:

I run both a mike's XS reed breather on the timing chest back plate, AND the stock timed breather on the end of the camshaft on my '70 commando. I have no problems with them working together so far. I drilled a 3/8" hole between the timing case and crankcase above the oil pump and between the webbing of the crankcase casting to allow air from the crankcase to move towards the timing chest more easily, and I also drilled a 1/4" hole down under the oil pump so oil would drain back to the crankcase sump from the timing chest.

I'd like to hear what the criticism of running both breathers together would be,...

Click to expand...

Not sure, to be honest, I've just purchased Jim Comstock's sump/breather plug and he advised me to block off any other breather outlets, which I will do. Perhaps Jim is the best person to chime in on this, but I think it may have something to do with the efficiency of a valve breather being diminished when a second outlet is present.

o0norton0o said:

I run both a mike's XS reed breather on the timing chest back plate, AND the stock timed breather on the end of the camshaft on my '70 commando. I have no problems with them working together so far. I drilled a 3/8" hole between the timing case and crankcase above the oil pump and between the webbing of the crankcase casting to allow air from the crankcase to move towards the timing chest more easily, and I also drilled a 1/4" hole down under the oil pump so oil would drain back to the crankcase sump from the timing chest.

I'd like to hear what the criticism of running both breathers together would be,...

Click to expand...

I doubt that running two breathers would make much difference when breathing through the timing chest.

With either a timed breather or a reed breather you only need enough flow capacity to flow the leakage that gets past the piston rings because air can not be drawn back in on the upstroke. So one breather with a 5/16 hole is big enough unless you have ring problems.

With my breather directly connected to the crankcase, I plug any other breather so the reed receives as strong a pulse as possible. That keeps it vibrating in sync with the rpms up beyond road speed. Jim

It's so hard to make a comment on parallel twin engine breathing without covering the whole subject. All I wanted to say was that I have a '70 commando that uses BOTH a stock timed camshaft breather AND a timing chest modifications with no ill effects (so far) Grandpaul seemed to be saying that HE doesn't recommend this arrangement and I wondered why that is....?? GP, care to explain?

If I could have bolted up Jim's sump plug breather, I would have, but the '70 commando's lower frame member is in the way. Certainly breathing directly from the source of the pressure wave in the crankcase would be the most efficient way to do it.

All I am saying to the Original poster is that I have the breathing set up which he's asking about, and it's working ok for me. It's stopped most of my oil leakage... I don't think that it gives me the same harmonic vaccum effect that jim's talking about. It just stops the most annoying effect of a highly pressurized crankcase which is oil leakage.... Beyond that, I don't have a dyno to see if there's any increase in HP due to some vacuum effect, nor was I looking for that...

acadian said:

Not sure, to be honest, I've just purchased Jim Comstock's sump/breather plug and he advised me to block off any other breather outlets, which I will do. Perhaps Jim is the best person to chime in on this, but I think it may have something to do with the efficiency of a valve breather being diminished when a second outlet is present.

Click to expand...

Well,..... I don't know about the '71 frame, but the '70 frame (with the camshaft breather) has a crossmember right near the drain plug that makes fitting Jim's sump plug breather impossible for me... If that's not the case with your frame, then I would block the timed camshaft breather as jim suggests...

... You didn't say that in your first post and I didn't see your follow up post until now... check that crossmember location for your 71 frame...

o0norton0o said:

... I have a '70 commando that uses BOTH a stock timed camshaft breather AND a timing chest modifications with no ill effects (so far) Grandpaul seemed to be saying that HE doesn't recommend this arrangement and I wondered why that is....?? GP, care to explain?

Click to expand...

I'm not recommending using 2 breathers, and not criticising or faulting using 2 breathers.

I advocate for the use of my reed valve breather by itself. Any other arrangement is at the user's discretion.

I've never had anyone install one of mine with some other breather installed at the same time. IF a client did so, and experienced oil leaks, I would not accept any responsibility until the other breather was removed and properly blocked off as though it had never been installed, then tested again with ONLY my breather. If it leaked then, I would have to be assured that it was not due to physical mating face or other damages, before returning the money paid for my reed valve (I would pay original and return postage as well; assuming I receive my undamaged reed valve).

What I am saying is that my reed valve (nor anyone else's breather kit) can stop static oil leaks due to physical oil containment damages such as dinged mating faces, stripped threads, cracks, etc.