Cylinder distortion

[70mc]

What would the specs. be for out of round and taper of the cylinder bores of a small block Chevy. It is a regular production 400 engine out of a 1976 car. It has been bored .030 over and honed with a torque plate, and will be used for racing.

 

[strokersix]

Will two engines, one with large skirt clearance forged pistons and the other with tight skirt clearance cast pistons, show different leakdown rates if they are otherwise identical?

It seems to me that the loose forged pistons can move when cold to a position where the piston axis is no longer parallel to the cylinder axis causing poorer ring to cylinder contact with a higher leakdown result. This could be easily tested by inserting shims to force a loose forged piston square for a comparison. Or perhaps heat the piston to expand it to it's operating size. Anyone investigated this? If this theory is correct then the leakdown test may be of questionable value for a forged piston racing engine.

 

[ivymike]

In most cases you're only talking about +/-0.4deg of tilt. Are you sure that will really have any effect whatsoever on ring sealing? It may cause an increase in ring gap of about 6um on a 100mm bore (vs 400um cold gap size), but I'm not sure that the effects will be noticeable. There should be negligible influence on effective ring tension, so conformability should be unaffected.

 

[MaxRaceSoftware]

you can "rock" the crank back and forth slightly at TDC and more at BDC on a loose Piston-to-wall clearance engine
to read a better Leakdown percent ...can be as much as 8 or 10+ % differences.

what counts is the Blowby meter CFM

and what counts more or more often is "if" the Engine will make more useable HP torque gains in your desired RPM range
and has proper oil control

i've seen plenty engines show great on a static leakdown test..but have more blowby CFM or run slower down the DragStrip



Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

http://www.maxracesoftware.com/

 

[ivymike]

that's interesting. I don't think it's the ring-liner contact that is changing significantly, but something must be. Perhaps it's piston-liner clearance near the top ring gap that's changing? I could certainly see something like that having a big impact...

 

[strokersix]

Or maybe it's the ring stays square to the bore and the ring groove doesn't?

Perhaps the leakdown test doesn't have enough pressure to seat the ring in it's groove or on the cylinder wall?

 

[ivymike]

Or maybe it's the ring stays square to the bore and the ring groove doesn't?
I considered that possibility, but decided against it because of the relatively small clearances involved. 0.4deg of piston tilt translates to 691um of axial displacement from one side of the bore to the other, which is probably 14x the axial clearance that the rings have in the grooves. The rings will be forced to tilt with the piston, mostly, although they could remain slightly misaligned within the clearance in the groove. I would expect even a slight amount of applied pressure to be enough to align the ring with the groove flank.

Perhaps the leakdown test doesn't have enough pressure to seat the ring in it's groove or on the cylinder wall?

100psi gage should be more than enough to seal a ring against a groove flank if everything is cold (and therefore relatively flat).

Sealing against the cylinder wall, I'm guessing, would not be significantly affected by rocking the crank slightly back and forth, although it may be the case that the ring finds regions of thick/thin oil film as it is moved back and forth, which would influence the size of non-conforming areas between the rings and the liner thru which gas passes.

 

[patprimmer]

Rocking could also effect the position of the ring in the grove, ie sealed against the top or bottom surface. These might preform differently.

Also, I still think that cylinder wall movement under dynamic loads might be large enough to cause a significant effect on a race engine. If the forces are great enough to leave a permanent distortion in cast iron, they must cause much greater dynamic distortion.

I really wish I had some actual data instead of relying on intuition, and drawing conclusions from semi related observations that all have other variables.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

Rocking could also effect the position of the ring in the grove, ie sealed against the top or bottom surface. These might preform differently.
yeah, that's basically the sort of thing I was referring to when I said "misaligned within the clearance..." I still think that it will only take a small amount of pressure in the cylinder to push the top ring to the bottom of the groove very quickly. The other rings don't see such a pressure difference, so perhaps they linger in odd positions due to friction - that actually doesn't sound all that unlikely, now that you mention it.

Cylinder wall motion under firing loads is significant to the piston behavior (tilt and wear), and very likely has a big impact on ring gap dilation too.

Dynamic cylinder wall shape can be calculated relatively easily by a skilled engine structural analyst (via FEA). If you've got a hundred grand (USD) or so to spare, I can put you in touch with one who'll do it on a consulting basis...

The effects on blow-by and engine performance are a little bit harder to calculate, but not impossible. There are commercial software packages on the market for that purpose (although the ones I've seen both assume constant block geometry during the course of the simulation - you get to look at a single distorted shape, not a time varying one).

 

[patprimmer]

sorry, I don't have $100,000 budget, and I supose a 99% discount is out of the question.

I think using static block geometry would be so far off as to be not much better than intuitive guessing from other observations.

I wonder what effect the dialation and contraction of the ring has on effective ring tension on the bores of a running engine.

Anyway, that is where I expect the fill helps, in reducing dynamic barrel shape distortion of the bore, and I suspect at our power level, and bore temperatures, the distortion from cylinder pressures could be a lot greater than those due to uneven expansion from heat.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[70mc]

Would still like to hear what everyone thinks about the new leakdown numbers using bolts now. Is the difference top to bottom thought to be acceptable,(from my post yesterday). Again thanks to all.

 

[SMOKEY44211]

My vote is for "run it as is". It would be nice if the leak #'s were lower right out of the box but I think they will come down to acceptable levels after a few hrs. run in time. If you do decide to go this route give us a report after.--------Phil

 

[patprimmer]

Run it as it is. What have you got to lose, and we really don't know what it will do under actual running conditions. As MaxRaceSoftware says, it's blowby when running at full power, not static leak down that actually counts.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

I think using static block geometry would be so far off as to be not much better than intuitive guessing from other observations.

nah, you can still tune a ring pack pretty well with the existing software, and troubleshoot problems successfully. It's hard to get blow-by within 10% if you don't have correlation information a priori, but if you do have correlation info then you can pretty well match ring pack behavior through a variety of operating conditions, and predict things like flutter, collapse, etc.

 

[patprimmer]

ivymike

Now you have me wondering if race piston manufacturers might have the software to perform this analysis. I might ask my supplier.

I really expect that due to required budget, it might be restricted to F1 and OEM.

If I have anything more to say, I will start another thread and stop my hijacking of this thread.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

I know that at least 2 major piston ring suppliers to OEMs use this type of software. It might be too much for a smaller shop to buy, but they could have such analyses done periodically on a consulting basis by a number of different companies.