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Old January 16th, 2006, 10:28   #16
GoFaster
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Agreed, for most hot rodders that drive the vehicles on a daily basis the objective is to have an engine that makes power, doesn't blow apart, and still has decent driveability. The emissions aren't a factor as long as it passes local inspection. Machining the lip off the bowl might sacrifice emissions, but it does eliminate the highest-temperature spot on the piston and reduces surface area exposed to the heat, so from the performance and not blow the engine apart point of view, I still think it's the way to go ...

Reducing the surface area might even help with cold starting a little.
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Old January 16th, 2006, 13:28   #17
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TdiMeister..........since your good with the figures by how much would the ignition be retarded by lowering the cr by 1 or 2 points and by lowering the cr wouldn't the fuel that's use have to have a shorter injection combustion period because of this fact , and something that adding say 10psi of boost could never be able to address.
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Old January 16th, 2006, 13:39   #18
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Ignition delay is lengthened with reduced temperature at SOI, no doubt about that. How much can't be quantified easily because it depends on several factors including combustion chamber design and fuel. But it's nothing that cannot be solved through revised timing maps and higher cetane fuel. Having combustion delayed in a magnitude of a few microseconds can't be a bad thing anyway if the objective is to further reduce PCP. I mean, this is what we want, right? It would be nice to have the efficiency afforded by running high compression ratios with no need for regard for PCPs, but that is wishful thinking. Something has to give.
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Old January 16th, 2006, 13:42   #19
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Quote:
Originally Posted by TDIMeister
I consider thicker head gaskets and any other means that raises the squish clearance or reduces the squish area to be the WORST thing you can do to reduce CR, but is the most common practise because it's a) easy; b) cheap; c) undo-able; d) doesn't take much brains.


I think of squish in a much more simple way...

Squish = SACRED territory not to be messed with...

Machining the pistons was much easier with the build that I am doing and this is a spring/summer/fall car so it doesn't see winter. Also, emissions are an issue with me as it is going into my MK1 Rabbit. I like the do it right, do it once philosophy and as it has been outlined so many times before in the lowering compression threads, etc. increasing dead space via shimming works, but isn't the most "efficient" way to go about doing it.

On another note...no sooner got the pistons home that they are now on their way to SwainTech for their TBC and PC-9 coatings... luckily, they will be there tomorrow and there is a week turnaround so I'll be anxiously awaiting their return.

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Old January 16th, 2006, 14:26   #20
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Rabbit we'll see how much she smokes when you run her but if you think its going to be minimal with no lip uumm and Meister ignition delay is lengthened with reduced temperature" we know our chamber design...nothing changed there then and using a better fuel i will be doing .

And if on the same small scale 1 mill here and 1/2 a mill there gives taken from or added to an engine by lowering the cr 1 or 2 point is not minute and can equally be measured and has a bigger impact on running and power than one might think , i've also seen papers on the subject reporting as much which i have posted here in the fuels & lubricants section .

Take ether for instance if the right amount is used it will shorten the ignition delay but if too much is added it has a cooling effect which lowers efficiency and power and these are not big differences in quantities , and the same can be same of the lowering of the cr by 1 or 2 points = bigger changes in pressure and heat then one might think.
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Old January 16th, 2006, 15:20   #21
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Sounds good....can't wait for April then...

Kerma's kinda already dropped proof of the smoking though with the modified pistons and stock headgasket... Only on cold start...then nothing... yet a very fast and smooth running car. Efficient.
Mine is the same modification with the exception that I am having mine coated now and I'm going with a mechanical setup. I have airflow improvements with the plenum style manifold I'm cooking up, but fully expect there to be some smoke...
Just couldn't stand to get rid of that James Bond smokescreen feature! It was so much fun just a few years ago with the modified IDI TD going to WF03 leading a pack of wolves and smoking in a S-VR's screen Good times...

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Old January 16th, 2006, 16:00   #22
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BTW fuel economy isn't affected by what I did to the pistons.

Only downside I can see is the cold start smoke.
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Old January 16th, 2006, 16:43   #23
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So what if i said by lowering your cr by the 1 point or so it made the fuel burn as if it were down by 5/10 points cetane and that's why you have smoke first start up , and this in turn would lead to more smoke wot.

So if you've started with a 45 cetane fuel it could be burning as little as a 35 cetane'd numbered fuel.?

Want some super cetane now ?..............................i'm guessing so
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Old January 16th, 2006, 16:51   #24
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Yum... higher cetane would be nice.... 40 is the limit around here...suppose with the stanadyne additive I run in my current dailys (79 Diesel bunny) fuel it might bump it up 3-4 points...possibly. Plus, its winter...even worse quality fuel... not that the low sulfur fuel is great anyways. Replace pump seals!

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Old January 16th, 2006, 23:33   #25
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Well I suppose I should jump back in as I did start the thread . From some further reading on the subject. GM went to 16.8:1 in there new duramax LBZ coded engine this year some of the reason was for better emissions the other was to handle the added output of the motor, now the important part, due to the lower cr they installed a 1000watt grid heater for idle and low load conditions. Hmmm so does this mean that 16.5 to one is around as low as you can go before idle and low load problems start to occur?

Thanks for everyones imput I am learing more then i am forgetting

Also as I am a very visually orintated learner, could some one draw a sketch of where TDIMeister is proposing we remove the material from the piston. I just can't picture it in my head, even a ruff paint sketch will do.
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Quote:
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again, not a topic for this forum...take it to "people who give a ****" forum.....the only numbers hear we care about are hp/ftlbs...
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Old January 19th, 2006, 12:22   #26
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Credit for the original pictures go to tdi rs (Simon Cooper) who modified his TDI pistons way back in 2002. Now you know who did it first.

The full thread is here: http://forums.tdiclub.com/showthread.php?t=16470


The valve relief pockets highlighted in green are OK to deepen a bit; as you can see from the next picture, I believe 1mm still leaves sufficient meat around the critical bowl lip area. Do NOT, however, enlarge the diameter of the reliefs. This cuts into squish area, which is basically the entire area highlighted in translucent red (minus the bowl of course). You have no good reason to do it unless you are enlarging the valves and need to clear them. And that also means, do NOT touch any of the red area (that's why I made it red). If you think squish is not important, consider why VW would go to the trouble of having THREE gasket thicknesses only 0.08 mm (.0031") difference between each to maintain the proper squish clearance.



Plan view showing the machining TDI RS had made. I would do recommend pretty much exactly as he has done. IMO, I don't think the bowl lip should be completely eliminated, resulting in the straight up "W" appearance of the bowl when looked in section like #5 in the figure below. I think there should be some lip left, as TDI RS has done.




In this excellent picture of the TDI piston in section (thanks for sacrificing a piston in the name of science ), I've highlighted in dashed lines roughly how the original lip would have looked, but it is greatly exaggerated... what I'm trying to show here is that if you MUST muck around with machining the bowl to get more volume, roughly follow the proportions of the existing bowl, although the width (breadth) can be widened relative to increasing the depth by approx. a 2:1 ratio particularly around the 8 o'clock position. Try to avoid increasing the depth of the bowl; make most of the volume increase through an increase in the width, particularly around the 8 o'clock quadrant, because that is where the fuel spray is aimed at. The key is preventing impingement of the fuel spray -- under momentum due to higher velocity (injection pressure from upgraded pump) and higher mass (larger droplet size due to larger nozzle holes).

I've only sketched one-half of the section because it's exceptionally hard to get it right using the rudimentary drawing tools in PowerPoint, which I used to fool around with these pics. The centre hump should be left alone, with only machining clean-up. It does little in the combustion process, and significantly cutting into it will result in a low-swirl velocity in that area that will hamper mixing. and combustion.



View of TDI RS's pistons in their home. Looks great! The only think I would add to that is to do a thermal barrier coating to the pistons. and (at least exhaust) valves.



Another view.
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Old January 19th, 2006, 13:04   #27
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By the way, after machining the pistons, make sure you break ALL sharp edges with as generous radii as practical. Nothing burns pistons and causes stress risers than sharp edges. The only exception is, whatever you haven't otherwise touched, LEAVE THEM ALONE!
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Old January 19th, 2006, 13:57   #28
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I think the piston that are in there now are different are they not Simon , i'm thinking these were the first ones that were used.
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Old January 19th, 2006, 15:09   #29
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Well I never figured out what is the relationship between Simon and "fred dibnah," the latter of whom originally posted those pics. I always thought there were either business partners or relatives or online alter egos... that fred used to speak for Simon as much as you do now <shrug>
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Old January 19th, 2006, 15:30   #30
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Or we could be one for all and all for ???
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