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Blow-by curve

General engine tech -- Drag Racing to Circle Track

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R.Brown
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Blow-by curve

Post by R.Brown » Sat Jan 03, 2015 12:18 pm

For those of you who actually measure blow-by on the dyno, what would be your opinion if you saw a curve like this... (picture attached) The purple trace in the picture, start of pull around 2CFM, end of pull just over 11CFM.

I have measured blowby on the engine and chassis dynos for many years, and in the last year I have seen two very odd (to me anyways) blowby curves. Strangely enough, both of these engines were Dodge B/RB engines (wedge).

One was a World Block 500+ CID, and the latest is a stock 400 block (451 CID). The curves are almost identical in shape, just shifted up in the RPM band on the 451.

Info from the engine builder on the 451: Icon pistons, 4.375 bore (sonic test results put the cyl wall thickness .220-.250), 3.750 stroke, 14:1 compression, 6.76 rod length. Block is filled to 2" of the deck, then plate honed on CV616 after a couple of weeks of cure time.

1/16, 1/16, 3/16 ring pack, .018 gap Moly top ring, .020 gap napier second, low tension oil.

Engine is built for methanol, but all pulls so far have been on gas.

Making pulls with the vacuum pump in place, it would only muster about 3" of PAN VAC, start of pull around 3.4, down to 2.x (on gas), whereas this same pump on his other (very similar) 451 pulls twice the vacuum ON ALKY. My experience has been that a vacuum pump will pull more on gas, so if we were to switch to ALKY on this engine, I would anticipate it would reduce to the pan vac even more.

We disconnected the pump and put the blow by sensor in place and saw this curve.

The performance of the engine is decent, 560 TQ, 720 HP... plugs are pretty dry. We have about 10 pulls on it so far.

It is still on the dyno, need to regroup think about this for a bit, maybe one of you smart guys can throw me a bone here... I've got a couple of people telling me that this is normal for a stock block, but I am not convinced.

Thanks in advance.
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Re: Blow-by curve

Post by Adger Smith » Sat Jan 03, 2015 12:45 pm

You have a PM
Good luck
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Re: Blow-by curve

Post by R.Brown » Sat Jan 03, 2015 3:51 pm

Thanks for that Adger.

To be clear, a typical blow by curve I see is flat, trending slightly upward with RPM, and in the 2 - 4 range (on gasoline).

rb

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Re: Blow-by curve

Post by lada ok » Sat Jan 03, 2015 4:25 pm

Is it something to do with the hardness of the iron casting Vs hone Vs ring type ? all being Dodge blocks ( actually they aren't are they ?? ) But they are wedge type heads, Then as below

Being an alky compression on gas .... piston rock with high combustion pressures ( knock ) Perhaps you can't hear or see it

0 .002 '" / inch of bore top ring, .. 1 1/2 for 2nd

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Re: Blow-by curve

Post by racear2865 » Sat Jan 03, 2015 5:47 pm

I use blow by meter on my dyno also. This is not uncommon on a OEM 400 SBC. I also saw it twice this summer on two 2300 4cyl FORD. On the Ford 4 cyl, it was much worse than what you saw. Later got to tear apart both 2300's and #3 cylinder split in both engines. What was odd that the owner said he had both blocks sonic tested and #3 cylinder was not the thinnest. I sonic tested both blocks and found 2 middle cylinders were at best .100 thick. Compression was well above 12 to 1 and on methanol.
reed

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Re: Blow-by curve

Post by Adger Smith » Sat Jan 03, 2015 11:18 pm

If I'm reading that right you have more about peak Tq. That is where the highest cylinder pressure is. It might respond to a higher quality wrist pin that flexes less. I've seen it on big bore NOS engines. If the piston is a Swiss Cheese design it could be letting the rings do strange things when it flexes. A good pin will keep flex out of big bore pistons. Just another thought if what I PMed doesn't help.
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Re: Blow-by curve

Post by PackardV8 » Sun Jan 04, 2015 10:18 am

Just asking:

1. What is the piston-to-wall clearance?

2. Cranking compression?

3. Have you pulled a vacuum on the crankcase at rest to confirm seal? Sometimes the various seals and gaskets just aren't holding as well as we think.

4. Has anyone else seen big bore stuff developing crankcase pressure via under-piston pumping pressures? Since this has a vacuum pump, one would think it could cope with that, but back in the bad old days trying to road-race big blocks, when crankcase volume and venting was insufficient for the sustained RPMs, an engine with good ring seal could be blowing oil out the breather just from pumping pressures.
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Re: Blow-by curve

Post by R.Brown » Sun Jan 04, 2015 1:29 pm

lada ok wrote:Is it something to do with the hardness of the iron casting Vs hone Vs ring type ? all being Dodge blocks ( actually they aren't are they ?? ) But they are wedge type heads, Then as below

Being an alky compression on gas .... piston rock with high combustion pressures ( knock ) Perhaps you can't hear or see it

0 .002 '" / inch of bore top ring, .. 1 1/2 for 2nd
Thanks for the input lada,

When on gas, we are testing with Q16 fuel, and pretty conservative with ignition timing (31 degrees - Indy heads) so I don't think it is unhappy with combustion. Also, when I have seen un-stability with combustion or piston rock unseating of the rings, it has (from what I have seen) shown a sharp increase in blowby or sharp reduction in pan vac.

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Re: Blow-by curve

Post by R.Brown » Sun Jan 04, 2015 1:30 pm

racear2865 wrote:I use blow by meter on my dyno also. This is not uncommon on a OEM 400 SBC. I also saw it twice this summer on two 2300 4cyl FORD. On the Ford 4 cyl, it was much worse than what you saw. Later got to tear apart both 2300's and #3 cylinder split in both engines. What was odd that the owner said he had both blocks sonic tested and #3 cylinder was not the thinnest. I sonic tested both blocks and found 2 middle cylinders were at best .100 thick. Compression was well above 12 to 1 and on methanol.
reed
excellent information!! thank you!

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Re: Blow-by curve

Post by R.Brown » Sun Jan 04, 2015 1:34 pm

Adger Smith wrote:If I'm reading that right you have more about peak Tq. That is where the highest cylinder pressure is. It might respond to a higher quality wrist pin that flexes less. I've seen it on big bore NOS engines. If the piston is a Swiss Cheese design it could be letting the rings do strange things when it flexes. A good pin will keep flex out of big bore pistons. Just another thought if what I PMed doesn't help.
If I am on the same track here as what you are saying, the curve on the graph is the purple line and it trends higher with RPM, not following the torque.

Just gotta run to kids hockey, lol.

rb

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Re: Blow-by curve

Post by Warp Speed » Sun Jan 04, 2015 3:16 pm

Really appears to be loss of ring control/seal. If it was a seal type leak, it would just be low and wouldn't gain with piston speed and mass air flow through the engine. That graph is typical of rings not sealing.
Now finding why is another deal.
You can pull a depression on the crankcase to verify the gasket/seal deal, but as long as there are no big problems there, hook the vacuum pump back up and hook the blow by meter to the pump exit. This will show you overall volume (cylinder and seals) and some depression may help with ring seal, especially with the low tension oil set. See what, If any changes it makes to the graph. It could come around, but at this point it doesn't look good....................

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Re: Blow-by curve

Post by Adger Smith » Sun Jan 04, 2015 6:16 pm

Notice the sharper rise at or near peak Tq in the purple line. Then it rises a little more with the RPM increase. Then sharply at 7,000.
There are a couple of things going on. Increased blow by at Peak Tq cylinder pressure and then that pressure falls, but the RPM/Speed of the piston-ring package takes off. I think it is a ring sealing problem. 1. End gaps too tight to begin with and the filled block didn't cool them adequately to begin with. 2. Cylinder distortion between the top of the hard block and the top of the cylinder when it gets heat in it.
You might have an application that responds to hot honing. I'll buy you a cup of coffee if it doesn't have scoured cylinder walls from ring scuffing. Leak it with the pistons up and down in the bores with it hot. Scope it before you pull it apart. Good luck
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Re: Blow-by curve

Post by racear2865 » Mon Jan 05, 2015 9:57 am

Adger Smith wrote:Notice the sharper rise at or near peak Tq in the purple line. Then it rises a little more with the RPM increase. Then sharply at 7,000.
There are a couple of things going on. Increased blow by at Peak Tq cylinder pressure and then that pressure falls, but the RPM/Speed of the piston-ring package takes off. I think it is a ring sealing problem. 1. End gaps too tight to begin with and the filled block didn't cool them adequately to begin with. 2. Cylinder distortion between the top of the hard block and the top of the cylinder when it gets heat in it.
You might have an application that responds to hot honing. I'll buy you a cup of coffee if it doesn't have scoured cylinder walls from ring scuffing. Leak it with the pistons up and down in the bores with it hot. Scope it before you pull it apart. Good luck
Adger/Warp Speed
What I think is the walls on a 400SBC really begin to flex when you hit peak power. If the block doesnt split out the bottom first, it most of the time will split a cylinder. And you are correct, hot water honing helps better than just torque plating but most shops cant do that and it way to expensive for some one doing a OEM to start with. Or at lest that is what they tell me when they come here. Also they read forums where people say they get 600 hp with a OEM block. And they can but for how long is a wild arse guess. Got three 406 in here now, and I specifically suggested they not do it and I wrote all on their invoices not suggested. Got one guy doing a Oh-- cast crank kit with a 3.875 stroker and hyper piston at 12.5 comp for a mud truck. I told him I would do the machine work but will not assemble the engine and I will not do it on my dyno. crank kit was suppose to be balanced external. Put his "stuff" on balancer, was 42 grams off on rear and 31 off on front?????
And both of you have done more dyno pulls than me. I got my first dyno in 1975. As Warp said, have pulled some levers during that time
reed

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Re: Blow-by curve

Post by R.Brown » Tue Jan 06, 2015 1:45 pm

Bit of an update here, I appreciate the ideas so far. Monday morning warmed up the engine and leaked it between 6% and 10% (all air coming thru crankcase)... not great to some, but I am not much of a slave to the leak down number. Scoped it, and saw no indications of bore damage. Tried hot water and oil vs cold water/hot oil with no real change in the numbers, so we switched to the methanol system (carbureted still).

Hooked up the vac pump again, and the power was up to 790 / 620 TQ. Pan vac did not fall off as much as I was expecting on methanol, started around 2.8 and still managed to pull 1" HG at the end of the pull. Engine still makes peak power at 7200, pulled to 7500.

Warp Speed: I did hook up the blowby meter to the exhaust of the vac pump this time, but the numbers were huge then... plus 15 and up from the start of the pull from what I recall. But that is what I have seen in the past when connecting the meter like that. I have attributed that to the fact that if there is a gasket/air leak anywhere, there will be more air being pulled thru the system, so I have always treated it as an arbitrary number. Does anyone else see the same thing when hooked up like that?

Not sure what kind of rear seal was in this engine, so air could be coming in there, and in general I can't speak to how well the engine was sealed in a static state... Either way, at this point the engine is making great power, plugs & piston tops are dry, and the owner of the engine is happy with the power...

They may decide on pulling it down to see about the rings touching (I was told yesterday the top gap was actually .022-.024), revisiting the bore finish, but finding a smoking gun may be elusive. I have been thru some unintentional R&D where a different design piston in a known combo has helped a bunch with blowby numbers, so at some point, like with a bunch of things in the engine world, ignorance is bliss... without the blow by numbers, this engine would have a clean bill of health.

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Re: Blow-by curve

Post by MadBill » Tue Jan 06, 2015 10:38 pm

I can't quite make out the blow by CFM on your screen, but assuming it's ~ 4 CFM, another 9 CFM of air leakage is a ton. If you can track down and eliminate the source(s), the pan vac. should go up big time and significantly help power and sealing, even without anything too trick in the ring department.
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