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Old January 15th, 2006, 01:26   #1
dvst8r
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Talking 15:1 or How low can you go...?

So the recent compound turbo disscusion has me thinking, i know what your thinking: "oh no there must be a fire now" Anyway my thoughts had to do with pcp and cr. More specificly what is the best way to go about lowering the cr: Head work, tuliped valves, thicker head gasket, even more machining of the pistons over and above the "kerma mod" ect...? With that in mind what is the general consensus of those in the know cough... cough... tdimiester cough.. and our other engineering friends. What the lowest managable cr is? By managble I mean it must be able to start without needing ether, or some other starting aid and must be able to idle on its own, however I don't care about off boost performance or emissions, but those considerations are open for disscution anyway.

I would start with my guess, but to be honest I don't even really have a clue.
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Old January 15th, 2006, 03:43   #2
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The motor which i have now was initially lowered to 17.1 or very slightly lower and had a very hard time in starting tdi-rs had to prolong the glow plug cycle 3 time's as long , and may question other people who state there running the same cr and is why it was raised up to 17.3.1 and ran just fine.

Its my belief that this is where a better fuel would also come into play and i know people say just add 2 lbs of boost pressure instead of desplacement which would give you hotter air anyway because its off the turbos efficiency map/line and you can't start the engine because there not enough heat/pressure produce because of the lowering of the cr where does that leave the more boost theory.
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Old January 15th, 2006, 04:19   #3
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I'm no expert,
so i'm probably wrong!!

But here goes:

If you lower the cr by X% how much more boost will you need to get the same amount of power??

So if my thonking is right (which I doubt) if you lower the cr, you need more boost, needing a bigger turbo, leading to more turbo lag???

Any comments????
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Old January 15th, 2006, 06:15   #4
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There was (or still is) an old gasser racer's trick that for every point less in CR, you add 6 psi boost. The point is, the more volume in the chamber, the more air you can pack in, which leads to more fuel that can be added, which gives more power.
Here is an example of reducing CR in a diesel...
http://www.62-65-dieselpage.com/finale.htm
Earlier releases of this article included pictures which showed the GM 6.5TD combustion chamber not unlike our TDI.
If you read through the article, they are making crazy power with this engine.
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Old January 15th, 2006, 07:27   #5
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Speaking of lower compression...
just back from the machine shop yesterday...thanks to the gf's brother who was able to help me out...










I put the compression ratio right around where Kerma's is (17:1 or so) with the exception that this was done on a precision CNC lathe so everything is exact to the .001.I believe his were on the bridgeport. I got lucky finding that someone could do this for me for next to nothing. Guess it pays, her brother runs a machine job were they make precision worm gear drives, etc. for nuclear power plants, windmills, etc.
Basically the same mod as he had his machinist do though...
I think the cold start problems relative to lowering CR are greatly about the way you go about lowering the CR. Was TDI-RS's original motor running larger van pistons but then also had a shimmed gasket?
Kerma reports very little in the way of cold start problems in previous threads about piston modification, etc. which DOES NOT increase the amount of dead space in between the piston and the head when it comes to starting. However, shimming the head gaskets does. Reading the SAE papers on the TDI motor orginally, dead space was something that the VW engineers worked very hard on to "limit". Only ill effects reported from lowering CR via piston modification is that cold starts were a bit more smoky, but not hard starting...
Maybe Kerma will chime

My pistons after I CC them here before I send them out will be going to SwainTech to be coated on the dome with a TBC or possibly Goldcoat to keep the heat where it belongs as well to block heat dissapation through the piston. They will also receive PC-9 on the skirts. With the lip now removed, CR is effectively lowered, potential hot spots removed and once coated should yield more of a complete burn because of coating barriers applied. I only wish I could afford to have the exhaust ports done too!

By the way...anyone have the stock CC measurement for a ALH piston? I wanted to calculate my "after" CR to the tee with that measurement so I had it for sure. I had it here before I sent the pistons out about a month ago for her brother to do but I recorded the information on a computer that recently crashed I lost all info as well as alot of other research and documentation I had made thus far. I wanted to see the before machining, machined and then CC again after coating just to see how accurate the coating claims are for applying the TBC coatings.



Joe

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Old January 15th, 2006, 08:27   #6
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Diesel-Des had the same coating done on the pistons and others...he couldn't do any before and after tests because a few days later the motor blow , i would think the way in which tdi-rs lowered this engine's cr not only by the pistons but also via the shortened rods contributed to the cr being some what lower than what you and others have taken it to .
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Old January 15th, 2006, 09:27   #7
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Car is smoky on the first cold startup of the day, but after that smokeless. Starts on the first turn of the key just like normal.

Lower PCP should allow more room for timing as well. Too bad IP limits timing.

There is an amazing amount of slop between the individual pistons' combustion chambers. Machining them like this could also provide a benefit in that regard.

IMO shimming the head or shaving the piston crown or shorter stroke is the wrong way to approach lowering the CR. No one can argue with Simon's results, but I bet that rig would perform even better if the work was done inside the piston cavity instead of outside.

6 psi per point of CR sounds like a lot. Gassers trend to run about half the CR, wonder if that has anything to do with it? 3 psi sounds more reasonable, just a shot in the dark.
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Old January 15th, 2006, 09:29   #8
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You may want to consider taking some emery cloth to round the sharp edge before having them coated.
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Old January 15th, 2006, 09:45   #9
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Each to there own here and i think tdi-rs has had 35+psi through this motor with no problems another sign its a proper lowering not a guesstimate .
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Old January 15th, 2006, 10:52   #10
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Kerma,

Yeap, was planning on cleaning the insides there before sending them out. I was anxious to take pics so didn't even do anything to them when I got them home last night.

Got something that'll help me CC them today, so I'm going to figure out what the CR is pretty damn close and report.

I had to go the route of machining the pistons...even in the past gasser motors I've done, the last being a Lysholm stg-4 'rado, the considerations were brought to machining before increasing dead space...but then, this topic has been hashed and rehashed. Your right though... the results speak for there own...

Joe
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Old January 16th, 2006, 00:35   #11
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Take my opinion FWIW, but I agree with Kerma above that shortening the rods as a means to reduce CR (or to achieve ANYTHING for that matter) is not the best way. ACHTUNG! I said, "Not the best way"; I didn't say it won't work, so don't flame me to taking my words out of context.

Shortening the rods without any other changes is no different to adding a thicker head gasket, and that destroys your squish zone and does all sorts of no-so-obvious badness as well. For example, with shorter con-rods, the pistons are permanently lower down the cylinder bore, meaning fuel is sprayed higher up the piston bowl ever closer to the thermally-critical bowl-lip area, and where fuel is now injected into a non-optimized area of air motion.

Not only that, shortening the con-rods while maintaining the stroke worsens an already disadvantageous L/R ratio of the TDI. The TDI engine already has a very low L/R ratio when benchmarked with other engines of similar design, 144/95.5=1.508. I would be pleased to debate with anyone the merits and demerits about low or high L/R ratios but I won't get into it now. It suffices to make my point that further reducing the L/R ratio from the current already very low value in stock form is plainly the WRONG direction.

Having said this, I go back to my first statement that this doesn't mean that the engine "won't work" or "will blow up." I'm just saying it's not the best way.
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Old January 16th, 2006, 01:41   #12
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Quote:
Originally Posted by dvst8r
More specificly what is the best way to go about lowering the cr: Head work, tuliped valves, thicker head gasket, even more machining of the pistons over and above the "kerma mod" ect...?
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.

The best is to enlarge the bowl volume, IF AND ONLY IF it is done with understanding and consideration of the fluid mechanics of the air flow and fuel injection spray. This means I don't subscribe to indiscriminate machining of the piston bowl either. I believe -- actually I know -- that VW has done extensive benchmarking, design, analysis and test-bench work over decades of development by people far more qualified to do this than anyone on this board, to arrive at what it feels is an optimal design for the engine's performance goals. I guess what I'm trying to summarise is that I think VW's design shouldn't be mucked around too much

Of course we are hot-rodders and have different goals in mind; that goes without saying. The problem is we look at individual components and individual problems (e.g. reducing CR) when we need to see the engine as an intimately interrelated, interacting system (e.g. increasing power output requires increased fuelling --> requiring increased air flow and boost --> requiring proper turbo matching --> requiring management of thermal and mechanical stresses (PCP --> CR reduction, etc.,etc.) and a whole cascade of other factors to be considered; what I've listed above is just the tip of the iceberg.)

I think the best compromise to reduce CR short of the availability of custom, properly-engineered low-compression pistons for the TDI is to VERY VERY slightly enlarge the bowl overall WHILE keeping the overall geometry and all the internal bowl proportions as much as possible, which also includes the ratio of the piston to bowl opening diameters. The rest can be achieved by increasing the depth (but NEVER enlarging the radial cross-section) of the NON-SQUISH areas of the piston crown, that is, the figure-8 shape for the valve reliefs. I believe there is sufficient thickness for a fairly substantial volume increase here (in the order of 1 mm or so more depth) with minimal consequences, but keep very importantly in mind the bowl lip will now be thinner by the same amount!

Quote:
What the lowest managable cr is?
I think this can only be answered through the extent of the machining that is done in a judicious manner with the considerations I listed above. The latest Audi V8 TDI has a CR of 16.5:1, so I would use that as a starting point. Properly done, I think 15.5:1 can be done without too much negative effects on cold-start (glow plugs would have to come on at a much higher ambient temperature threshold, though), driveability and efficiency.
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Old January 16th, 2006, 03:17   #13
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I was at 16.7:1 once with a thick gasket. I would start and run, but was smokey and would smoke at idle and light load even when warm. But 16:7 with proper pistons would be workable im sure, maybe even 16:1.

How about custom forged pistons? Anyone get in contact with a manifacture and pice a tdi piston up. Just make a bowl with no lip. They could machine that? Thanks,des.
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Old January 16th, 2006, 04:05   #14
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I think PCPs for an extra-high boost TDI can be managed though a combination of maintaining a moderate static geometric compression ratio (in the order of 16.5:1 or so) and intake valve closure timing. I did some calculations and simulations of a cam that closes the intake valves about 7 degrees @ .050 later than OEM (I have to look at my numbers again to confirm), which would reduce the effective CR by about a point.

5 cams were made and are in the hands of their owners now; one I am aware is already installed, but I am still waiting for some dyno results. Irrespective of any HP gains by the cam itself, I calculated that PCP is reduced by over 100 PSI or nearly 7 bar for any given torque output scenario. This doesn't sound like much, but 7 bar might be the difference that breaks the camel's back so to speak, between a borderline safe zone to something breaking (say 200 bar vs 207 bar)

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Old January 16th, 2006, 09:29   #15
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Quote:
Originally Posted by TDIMeister
VW has done extensive benchmarking, design, analysis and test-bench work over decades of development ... to arrive at what it feels is an optimal design for the engine's performance goals. I guess what I'm trying to summarise is that I think VW's design shouldn't be mucked around too much

Of course we are hot-rodders and have different goals in mind; that goes without saying.
VW's "PERFORMANCE" goals for this 90 hp car can be summed up in three design criteria:

1. Emissions
2. Emissions
3. Emissions

The bowl shape is there to optimize EMISSONS, performance be damned.

POWERPLUS nozzles already spray at an angle and shape that differs from stock. Voila, all that careful bowl shape design work is for naught.

Yes, we are hot-rodding and we have different goals.
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