PTS Flowbench Forum

I may be way off, but i thing the vac in the crank case is a lot like a brake vacuum booster.

The piston, like you foot on the brake pedal, take less effort when you are pushing again a vac on the other side of it.

John your theory, sound good, but the blow by is probably what mess with it.

Kinda acts like a backwards 2 stroke. Instead of fuel and air being forced on top of the piston past the piston/s rings as it goes down. Only it's traveling back into the crank case.

Beginning to believe that you could loose power in a stock eng, that has the pcv valve removed, in favor one of them breathable fill cap!!!!! Or, has a leak in system, like a valve cover bolt.

jsa wrote:Here's some number,

350cid/12/12/12=0.203125 ft³

Rotate that at 6000rpm

0.203125 * 6000 = 1218.75cfm

Someone want to throw an assembled short block (any short block) minus two pistons on a flow bench.

Put one open bore over the bench inlet. Draw air from the other open bore through the assembled crankcase and out into the bench.

Report back with cfm @ pressure drop and engine spec.

With flow and pressure drop data, some ballpark figure could be extrapolated for the running engine power loss.

John, yes I think ring seal is also part of the advantage of sump vacuum.

Boy,
That is a tough thing to do. I have no way to put an engine on my flow bench let alone turn it 6000 rpm. I cannot prove or disprove that. I guess your theory is safe right now. Let me ask you, have you ever done that on your flow bench? What were the results?

John

Here is a good discussion at Larry Meaux's forum about vacuum pumps that may give us some insight into why they are used. http://maxracesoftware.com/bulletinboar ... f=25&t=523
They are discussing the need for a larger vacuum pump on a methanol engine compared to a gasoline engine. Now why would that be needed if they are the same cubic inch engine and the "bottoms" of the pistons are pumping the same amount of cfm in the crankcase? This goes back to my argument it has to do with ring seal. This discussion was mentioning the blow by in the crankcase being more with the methanol engine than the gasoline engine. We know that blow by builds pressure in the crank case which pushes up on the bottom sides of the rings causing them to lose their seal earlier in the power cycle. The longer you can keep the rings sealed the more power you will make. This is why you need vacuum in the crank case to help keep the rings sealed longer for more power on the power cycle.

This is another reason why it is better to space the top compression ring and the second scraper ring further apart if possible. This is another reason you also see the accumulator groove between those two rings on most quality pistons now. This groove prolongs the build up of pressure on the under side of the top ring. Another help is to run tighter gap on the top ring and a larger gap on the second scraper ring to help bleed off pressure from the under side of the top ring.

In the past we were successful in running a two ring piston for a while. We ran a .031 Dykes on the top groove and then just an oil ring. Super light set up with titanium rods. Lots of vacuum pump to keep the Dykes sucked down as long as possible.

Gordon, You are really thinking this one out. That is good! It would help suck down on the bottom of the pistons coming down. What about the pistons going up? Would it not suck down on them equally? This I would think would negate the advantage on the ones coming down. Gordon would that be the case?

John

jfholm wrote:Here is a good discussion at Larry Meaux's forum about vacuum pumps that may give us some insight into why they are used. http://maxracesoftware.com/bulletinboar ... f=25&t=523
They are discussing the need for a larger vacuum pump on a methanol engine compared to a gasoline engine. Now why would that be needed if they are the same cubic inch engine and the "bottoms" of the pistons are pumping the same amount of cfm in the crankcase? This goes back to my argument it has to do with ring seal. This discussion was mentioning the blow by in the crankcase being more with the methanol engine than the gasoline engine. We know that blow by builds pressure in the crank case which pushes up on the bottom sides of the rings causing them to lose their seal earlier in the power cycle. The longer you can keep the rings sealed the more power you will make. This is why you need vacuum in the crank case to help keep the rings sealed longer for more power on the power cycle.

This is another reason why it is better to space the top compression ring and the second scraper ring further apart if possible. This is another reason you also see the accumulator groove between those two rings on most quality pistons now. This groove prolongs the build up of pressure on the under side of the top ring. Another help is to run tighter gap on the top ring and a larger gap on the second scraper ring to help bleed off pressure from the under side of the top ring.

In the past we were successful in running a two ring piston for a while. We ran a .031 Dykes on the top groove and then just an oil ring. Super light set up with titanium rods. Lots of vacuum pump to keep the Dykes sucked down as long as possible.

Gordon, You are really thinking this one out. That is good! It would help suck down on the bottom of the pistons coming down. What about the pistons going up? Would it not suck down on them equally? This I would think would negate the advantage on the ones coming down. Gordon would that be the case?

John

Hello John how are you? I'm back, you are perfectly correct. Here in Brazil the engines that have alcohol are common today manufactures flex engines. His statement is correct since a higher pressure chamber of this phenomenon that happens is loss of efficiency of rings, here for engines alcohol steel rings are used in the first channel and has a smaller gap between tips. Besides the answer (mind) of an engine on alcohol faster and generating more pressure in the crankcase.
For 2.0L engine manufacturers added vents to the engine size greater
Recalling that alcohol engines reached nearly 12:1 compression ratio and track come to 16:1 with nitro methane (crazy) but it works.

Gordon, You are really thinking this one out. That is good! It would help suck down on the bottom of the pistons coming down. What about the pistons going up? Would it not suck down on them equally? This I would think would negate the advantage on the ones coming down. Gordon would that be the case?

John

Hmmmm, OK time to real talk out my a$$

Well, If your on the moon, You weigh vary little compared to what you weigh in that same moon get up, on earth.

Yes low tension rings is the key to releasing more power, but i thing the rotating assembly is happy'r in a vac state as well.

As far as the vac pulling on the piston going up.....a.....well.......... hmmm, there's no compression on the rings to help push them at the cylinder wall to seal..... so develops a leak......and relieves the vac.

As far as the factory PCV ventilation, I have re thought that one........It's only has vac at idle, so it don't apply to the Facts/Theory from above.

Does methanol racing fuel wash away some of the oil at the ring and cylinder wall mating surface?

Would this allow blow-by of compression past the ring seal into the crankcase?

The vacuum pump helps in two ways:
1) remove excess pressure from the crankcase.
2) remove methanol vapors from crankcase.

Ray

jfholm wrote:[I have no way to put an engine on my flow bench let alone turn it 6000 rpm.

John

No No No John,

I meant use the flow bench to measure the flow @ a pressure drop on a static engine.

Idea being to mimic air travelling down one bore through an assembled crankcase and back up another bore. Like what happens when one piston descends and another ascends.

Hope that makes a bit more sense

Cheers
John

350cid/12/12/12=0.203125 ft³

jsa,
what does the the 3 "12" represent?

Gordon,

12 inches to the foot, convert cubic inches to cubic feet.

Cheers

John