77MM Pistons?

smsazzy
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Joined: June 24th, 2006, 5:56 pm

Re: 77MM Pistons?

Post by smsazzy »

I don't know the specifics of it, but I have heard of people intentionally putting a small "bow" in the crank at rest so it will straighten once under load.

I don't know the specifics behind it, but there are a lot of people smarter than me tinkering with this stuff. I'm just an old country boy from Georgia, so you're gonna have to talk real slow like to me. :-)
Stephen Saslow
FV 09 NWR
hardingfv32-1
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Joined: December 1st, 2006, 8:01 pm

Re: 77MM Pistons?

Post by hardingfv32-1 »

Let us review the logic of using a bent crank. I'm going to assume the theory is that the crank will run straight when the "proper" load is applied. That meaning someone has figured out what the load is at some RPM, say redline, and applied that to the crank and gotten a deflection number at the center main. While this is all possible, does it sound like anything that any of the major FV engine builders is capable of?

So now we have the right amount of bend in the crank for a given RPM. What happens at idle when there is not enough force to straighten the crank? We could just add a little more bearing clearance to handle the out of round situation at idle.

Wouldn't it be easier to support the crank with a stronger oil film that is provided by tighter clearances?

Next question?

Brian
brian
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Joined: June 26th, 2006, 12:31 pm

Re: 77MM Pistons?

Post by brian »

I am using one of the "bent" cranks in my high rpm motor and it works really well. Less vibration and no strikes on the center main. I doubt that oil film, regardless of strength, would ever be able to prevent crank flex. While bent cranks are not common in the vee engines I have seen, it's amazing how little it takes to straighten one of ours. The old school guys use a hammer to do it. A couple of smacks and it's done. The spintron films I mentioned before clearly show the crank flexing and twisting everytime a rod stops and starts. Granted, with our low compression and mild cams I doubt it's as much of an issue as it is with more stressed engines.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
hardingfv32-1
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Re: 77MM Pistons?

Post by hardingfv32-1 »

Less vibration ... what instrument are you using to measure that with? The seat of your pants?

I'll try and get some numbers to illustrate how strong the oil film is.

Brian
sharplikestump
Posts: 183
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Re: 77MM Pistons?

Post by sharplikestump »

Thank you, SMSAZZY. I always wondered just HOW MUCH Mag the As41 case had. That is more than I would have guessed, but it explains why it was SO bright when I had some chips light off while machining one on the B'port! Get's pretty exciting.
sharplikestump
Posts: 183
Joined: January 12th, 2009, 2:28 pm

Re: 77MM Pistons?

Post by sharplikestump »

On the issue of bent cranks:
I have had the last 100 + bent. The theory is simple (I'm a simple guy). Our cranks are a button on a string. I am just trying to compensate for the weight of the button. I have no idea at what point the crank is running "straight", and I would like to bend them more than I do, but it is a compromise between that and dragging them at idle/low revs. I do see a difference in the wear patterns of the center main and #4 main journals. To me, the proof that the crank is flexing at revs is the fact that all of the wear on #4 is diametrically opposed to the "button" formed by rod journals #1 and #4.
After what I have seen from a couple of thousand engines, while experimenting with a range of clearances, oil wts. etc, I have a hard time believing that a layer of oil molecules is going to stop the flex, and from what I understand, the impressive hydraulic numbers have to do with the wedge preceeding the rotation of the ROD journal, and I am just not sure how pronounced this is on the mains, but like I said.....I'm a simple guy.
Mike P.
hardingfv32-1
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Re: 77MM Pistons?

Post by hardingfv32-1 »

1) "it is a compromise between that and dragging them at idle/low revs"

What is causing the drag at idle/low revs, physical contact between the bearing and journal?

2) "I have a hard time believing that a layer of oil molecules is going to stop the flex"

IF the oil film is strong enough to support the journal at speed, why can't it prevent the flex? What is the cause of the flexing, poor support from the bearings/oil film?

Brian
Bill_Bonow
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Re: 77MM Pistons?

Post by Bill_Bonow »

hardingfv32-1 wrote: "What is the cause of the flexing"
VW's original design criteria of a crankshaft that will turn 3800 rpm maximum :mrgreen:
Bill Bonow
" I love Formula Vees, they're delicious!"
hardingfv32-1
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Re: 77MM Pistons?

Post by hardingfv32-1 »

I assume we are talking about crank flexing at the center bearing. Is it not supported by the center main bearing? How much flexing are you talking about? More than the .0015 clearance found on one side of the journal. I assume .0015 on each side of the journal for a total of .003 per bearing.

Brian
jpetillo
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Re: 77MM Pistons?

Post by jpetillo »

In the motorcycle world, to relieve the effects of load on the bearings some believe that we want to spin the engine faster. In other words, if we spin too slowly then the oil has more time to be squished out. For engines that spin more slowly and have high torque at such speeds, larger bearing surfaces are used to avoid this. I would have guessed that the problem with the Vee engines is that we have perhaps 30-50% higher torque than a stock engine was designed for with regard to the bearing surface area, and there is enough time for the oil to squish out under these circumstances. But, perhaps there are resonances with the crank deflections at the higher RPMs we run at, that exacerbate the problem. Just thinking out loud. John
smsazzy
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Re: 77MM Pistons?

Post by smsazzy »

Actually, the way I understand it, we actually have slightly lower torque than a stock engine. With the offset cam keys, free flowing exhaust, etc. we have shifted the peak torque rang up in the RPM's. While it equates to higher HP overall, the peak torque is actually a little lower.
Stephen Saslow
FV 09 NWR
SR Racing
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Re: 77MM Pistons?

Post by SR Racing »

Never actually measured the torque on a stock 1192 engine. However Stevan is correct. The peak torque on a stock 1192 is probably higher or close to the peak torque on a FV engine. (70+ lbs) We make more HP only because the peak point is at a much higher RPM. (HP=T x rpm / 5252).

In regards to OP. It is really only an indicator of what we THINK is happening between the bearing and the journal. If we were sure that area was ALWAYS saturated, 5 lbs of oil pressure would be fine. We aren't though. So OP is a (at least at our level) best guess and experimental result of what seems to work. The a/c vw is a little problematic in that there are no real rod side tolerances.

As mentioned, OP is additional work that the engine has to perform. We would like OP to be as low as possible while keeping all bearings covered. In a a/c vw the bearings take their most abuse at 2 points. 1. At LOW RPM in the peak torque range below 4000 RPM. This is why the dube buggies and stump pullers use the HV oil pumps. They need max pressure at low RPM. We are seldom racing at below 4000 RPM, so point 2 is where we can experience oil issues. High RPM, where the crank may be flexing and probably more so when we are pumping frothy oil, due to windage, high G corners and downshifts under heavy braking. In the last case we are experiencing mucho reverse torque at high RPM and all the oil is forced forward into the front (rear?) of the case and being churned into a nice froth.

A down shift will put the crank into a S shape far greater than the small amount of load that our pistons can generate at high RPM under acceleration. (Maybe 50 lbs on a rod.)

Anyways, I doubt oil pressure per se' is a problem with an FV as long as we have the journals full of oil. So what is the required minimum pressure? Only experience tells us that.

We spend a lot of time on the balancer to make sure that the engine is zero balanced from end to end. (Pulley to clutch cover) You would be surprised at the amount of lateral load on a crank with a few grams out of balance at 6500+ rpm.
sharplikestump
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Joined: January 12th, 2009, 2:28 pm

Re: 77MM Pistons?

Post by sharplikestump »

Good Points All.
Lacking a Spintron, I just kinda go by what I see upon teardown.
On the subject of crank flex: I am pretty good at listening (having learned that I learn nothing when it is me doing the talking), and will listen to anyone who wants to explain to me why almost all of the wear I see on the center main is in the area of the journal between the two center rod journals, and ALL of the wear on the 4th. brg. journal is 180 degrees opposed to that.
You read what SR says about the effect of just a couple of grams of imbalance on the assembly. Is it possible that we really want to balance that assembly where the crank is running at speed? Just asking.
sharplikestump
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Re: 77MM Pistons?

Post by sharplikestump »

Belated thought:
At least for my mind, the reason for the cracks that appear on stock pulleys is not the rate of spin, but I believe due to a flexing crank doing what it can to knock that pulley out of running on one plane.
jpetillo
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Re: 77MM Pistons?

Post by jpetillo »

I agree completely with Steven & Jim about the stock engine probably having higher peak torque, but at a lower RPM range. But we don't run there. Where we run, the stock engine can't get to RPM-wise. In my previous post I was talking about the torque being higher at the RPMS we run at - that's a given. But I do agree that this is most likely not the cause.

I think Jim and Brian put their finger on it. The rotating assembly will have a natural frequency based on the stiffness of the steel/material and the locations of any unbalanced masses - you can never get it perfect. When the RPM of the shaft is near the natural frequency and harmonics of it, it will distort the shaft the most - probably more than a third gear downshift would.

Brian, I'm guessing that we run through an RPM where the crankshaft harmonic mode has the crank shaft distorted into a shape to give you just that wear pattern. If you can spin the crank that fast externally you will probably feel the resonance pretty strongly when it hits that RPM. About balancing it at that RPM, that's a good idea, although the RPM will keep shifting as you balance. A balanced assembly should be a balanced assembly, but there are internal variations that change with RPM, so you really don't know if you're truly balanced with a static scale, for example. Since you can't disassemble the crank into separate pieces, I'm not sure you can figure out where the internal imbalances are to figure out where to correct them. If we could enter the exact crank shape with rods & pistons into a CAD model, and assuming that the materials in each piece are of uniform density, then with some straightforward analysis it would give us an idea of the locations of the gross imbalances - I think. But we're not going to do that. Again just thinking out loud.

I think just trial and error would be easier - keep up what you're doing. John
hardingfv32-1
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Re: 77MM Pistons?

Post by hardingfv32-1 »

Flexing? Are we talking about the crank bending or twisting along it's axis?

I think the front pulleys crack do to twisting. There is a loading and then unloading of forces that causes a vibration in the pulley. Having a fan belt in use a stock engine might help the situation.

I still don't understand how the crank bends if it contained by the bearing clearances. Chances are that the twisting is causes the crank to bend. It is common to see the crank bent after a rod bearing failure.

Brian
sharplikestump
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Re: 77MM Pistons?

Post by sharplikestump »

hardingfv32-1 wrote:Flexing? Are we talking about the crank bending or twisting along it's axis? Bending

I think the front pulleys crack do to twisting. There is a loading and then unloading of forces that causes a vibration in the pulley. Having a fan belt in use a stock engine might help the situation. I've had engines with stock cranks with which I could destroy a pulley at will, while far more powerful engines with counterbalanced cranks never suffered the problem

I still don't understand how the crank bends if it contained by the bearing clearances. Chances are that the twisting is causes the crank to bend. It is common to see the crank bent after a rod bearing failure. That is heat distortion and a totally different thing.

Brian
I am talking about Bending. We don't have anywhere near the torque to twist the crank that much. Also, twisting would not generate the wear patterns that occur on every crank that is run at the revs we turn. You cannot deny that pattern. Furthermore, if cranks did not flex, explain to me why any one or any manufacture would bother counterbalancing a crank, especially since adding the extra material (rotational mass) makes for a slower crank. Then there are the cranks that saw extreme revs, as in when a shift out of 3rd. at top revs went into 2nd. instead of 4th. The estimate was 9,000 rpm, and coincidentally the t.i.r. on the center main was .009. along with being cracked. There is no convincing me that a film of oil or a mushy alloy case is any match for an non-counterbalanced crank at high revs. I have witnessed center case journals beat in so badly that the 1st. o/s line-boring did not clean up the diameter.
Mike P.
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