IDI - effect of reducing compression ratio on hp & mpg

[Audi80]

what does the thicker gasket give us? Or can we use 2 togeather  or is there something in Cu sheet?

Thicker gasket will lose quench-effect. It is important in gasser world, how about in diesel? I think even more important?

[rwest1]

Not sure adjusting HG thickness alone is optimum solution, or if changing cup dimension to keep ratio of cup to compressed cyl. volume app. the same would be best. Seems I remember a thread where one of the hot rodders had figured out how to use some other cup (think it was GM) in the VW head! Here is a Cu shim gasket supplier that (according to list) seems to have pattern for VW 1.6's (diesel?). If not, I'm sure they could use an old HG for pattern. If shimming the head alone will give satisfactory results then the different thickness's available should allow someone to built just about any CR engine they desire. Regards

http://www.headgasket.com/gaskets.html

[tylernt]

Definitely don't want to mess with the head gasket. If you use a thicker head gasket, a bunch of oxygen never makes it into swirl chamber. Need to have (as near as possible) 100% of the available oxygen in the swirl chamber when injection occurs for a good burn.

[jimfoo]

There was the thread about modifying the central groove that runs down the piston to get better high rpm performance. It would also lower the CR.

[Mark(The Miser)UK]

Definitely don't want to mess with the head gasket. If you use a thicker head gasket, a bunch of oxygen never makes it into swirl chamber. Need to have (as near as possible) 100% of the available oxygen in the swirl chamber when injection occurs for a good burn.

[/size]

Why do you need all or nearly all the oxygen/air  in the swirl chamber? With a turbo there is always a xs anyway

[tylernt]

I just remember from other threads on this board that thicker head gaskets are not a good idea.

[Mark(The Miser)UK]

I just remember from other threads on this board that thicker head gaskets are not a good idea.

[/size]

Are you including the 3 hole /notch standard gaskets or specially commissioned ones?

I wonder how much longer an engine will last if the compression is lowered. Less strain on bearings etc?

[xud9te]

Regarding - why does increasing Cr (or rV) not increase overall fuel economy?

Remeber there are two efficiencies here, which are COMPLTELY different, thermal efficiency (formula that uses Compression ratio) and mechanical efficiency.

For a better THERMAL efficiency, high CR is always better.  But it is just a theoretical number.  Mechanical efficiencies are based not on compression ratio directly, but more about material properties and engine speeds and loads and VE.

As an engine goes faster, mechanical efficiency drops, as does VE (cylinder filling).  Lower compresssion can increase mechanical efficiency due to less forces (friction > heat) onto contact surfaces.  

Think of it as two lines on a graph, compression ratio x axis, efficiency y axis. as comprssion increases, mech eff goes down.  So top left to bottom right.  Thermal eff goes up, so botom left to top right.  We have a big X.  Very simplified, i know but it is just an illustration.

Middle of the X is the best point for efficiency.  Seems to be 17:1 according to many sources on a diesel, but would expect 18:1 would be as low as you can go on these engines...

What was the numbers for the increase in compression volume too get 18:1 again?  I think I have an idea...

[Black Smokin' Diesel]

I just remember from other threads on this board that thicker head gaskets are not a good idea.

[/size]

Are you including the 3 hole /notch standard gaskets or specially commissioned ones?

I think he means stacking gaskets. It's common procedure with gassers but the cr and pressures within the cylinders are much less.

[burn_your_money]

Something to consider with the "well worn" engines. They are also burning oil which does produce power.

I'm not sure how much the swirl chambers can be modified. There was alot of engineering put into them to maximize air/fuel mixing etc.

[lyeinyoureye]

Unless it's really really bad I don't think it can add more than a mpg or so. Even at 5L per 2500 miles, that'd still only add a couple mpgs...

[Benjamin]

what do you guys think of this, the valvecut out in the piston is now deeper, he also have custom cams, its calculated and its reduced to CR16,9

this is a stock picture:



this is the modified


Greetz, Benjamin

[Mark(The Miser)UK]

Looks interesting Benjamin; but are there any performance/mileage figures to go with it?

[rwest1]

Funny pictures, all my old VW diesels have different oil supply and return hole placements. What do we have here???? Regards

[RabbitJockey]

he didn't keep the little stem thing that usually connects the 2 valve indent things... which aren't really valve indents, since they def don't line up with the valves, atleast not on a vw.  what is that motor for anyway?  is it a mercedes or something?