IDI - effect of reducing compression ratio on hp & mpg

[Mark(The Miser)UK]

Mr. the Miser,

Who mentioned anything about the length of the test, or over what period?  I am not sure i see the point of your remark...

I have an idea for reducing Cr without changing headgaskets, that can be changed to suit.  

Also remember thicker headgaskets will destroy the quenching effect, not desirable.

My reference to length of test was only a suggestion. To run tests when you are way off maximum efficiency  for perlonged periods say a complete tankful is somewhat thumbtwiddlingly slow.  As you get closer to the goal; proof by perlonged testing may be desireable. Rather than re-engineer slabs of ally to alter compression ratio; slipping in gaskets temporarily is time efficient. Not intended to be permanant or for testing maximum powers.  Unless you always go to the same gas station filling tank needs plenty of iterations to average natural variations in squirting fuel in ...and even then it varies.

...I thought this thread was about changing things to get different results :shock:

[craiggroombridge]

when putting n/a injectors (135 bar) in a td without adjusting the timing to work with the injectors you effectily advance the injection timing that is why the topend had no power the injection was to early at max rpm with a td you could go to 163 (max limit) and you should get better power...
But at what expence... with a turbo the engine will use all the fuel you put to it,  Personly I will start with propane injection on my n/a to see if it helps me I'm still looking for a td to play with the possibility of nitrous ??
 :twisted:

Craig

[Mark(The Miser)UK]

Mr. the Miser,

Who mentioned anything about the length of the test, or over what period? I am not sure i see the point of your remark...

I have an idea for reducing Cr without changing headgaskets, that can be changed to suit.

Also remember thicker headgaskets will destroy the quenching effect, not desirable.


OOps sorry what was your method of changing compression ratio without changing gaskets? :?

What's the quenching effect?

[saurkraut]

I think i can vouch for not using thicker head gaskets to test for engine efficiency.

I put a TD motor together with new pistons, bearings, head, valves, and gaskets.  However, The block and crank were from different motors.  I was in too much of a hurry to measure piston protrusion, so I went with a three notch gasket.  I figured since its a summer only diesel, what could be the harm.

As a result, this thing has always had a less than pritty start.  i checked the compression, and all cylinders were in the mid 400 when the engine was below freezing temp.  It doesn't burn any oil, and will start without the glow system working, even down to 40F.  Its ugly, but it goes.  Even today @ 85F, I expect it to do some start monkey business when I go to lunch.

So we have:

Good stone cold compression
Burns no oil
Has seen nothing but synthetic since the first two pans of dyno oil
Starts about the same, with or with out a glow system.

The only thing I can figure is it has the wrong gasket thickness.

Although it may be doing me some good at high booste, at part throttle cruiseing, its probably not doing me any favors.

rather then pull the head and change the gasket, I'm going to build a 1.9 mechanical head, measure the piston protrusion, put in the proper thickness metal head gasket, and bolt on the 1.9 head.

This is not an invitation to solve my starting issues.  It starts just fine for me.  It is an observation on the posible outcome when testing engine efficiency with a thicker head gasket.  Its probably a less than optimal condition.

[Mark(The Miser)UK]

I think i can vouch for not using thicker head gaskets to test for engine efficiency.

I put a TD motor together with new pistons, bearings, head, valves, and gaskets.  However, The block and crank were from different motors.  I was in too much of a hurry to measure piston protrusion, so I went with a three notch gasket.  I figured since its a summer only diesel, what could be the harm.

As a result, this thing has always had a less than pritty start.  i checked the compression, and all cylinders were in the mid 400 when the engine was below freezing temp.  It doesn't burn any oil, and will start without the glow system working, even down to 40F.  Its ugly, but it goes.  Even today @ 85F, I expect it to do some start monkey business when I go to lunch.

So we have:

Good stone cold compression
Burns no oil
Has seen nothing but synthetic since the first two pans of dyno oil
Starts about the same, with or with out a glow system.

The only thing I can figure is it has the wrong gasket thickness.

Although it may be doing me some good at high booste, at part throttle cruiseing, its probably not doing me any favors.

rather then pull the head and change the gasket, I'm going to build a 1.9 mechanical head, measure the piston protrusion, put in the proper thickness metal head gasket, and bolt on the 1.9 head.

This is not an invitation to solve my starting issues.  It starts just fine for me.  It is an observation on the posible outcome when testing engine efficiency with a thicker head gasket.  Its probably a less than optimal condition.

[/size]

 Saurkraut, Having changed so much how can you be sure it's the gasket?

Are you saying 400psi is bad?

Many workshops just put in the 3 notch as a matter of course to save measuring as you did.

Difference between a thick and a thin gasket is 0.3mm...
So maximum possible affect with a wrong gasket is 0.3 x (76.5^2)PI/4 =1.3cc;
 but depending on engine may be only 0.65cc [or nothing :wink: ]

1588 cc TD has 23:1 pressure ratio. Stroke is 86.4mm
(76.5^2)PI/4  x stroke,  is swept vol... 397cc

calculation of combustion space
(397 +s)/s =23;  23s = (397 + s) ; s=397/22 = 18cc

Biggest compression drop is:
(397 + 1.3 + 18.)/(1.3 +18.) ... =21.6:1;

or 22.3:1; [0r no change] :roll:

A poorly seated re-ring could achieve this drop!

Add a second gasket; a (1 notch)...
1 notch gasket is 1.3mm thick...
1.3 x (76.5^2)PI/4 = 6cc

(397+6+18 )/24 =17.5:1

A  3 notch engine using 2 '1 notch' gaskets (6cc-1.3cc)
(397+ 4.7+18 )/22.7 = 18.5:1

Good for experimentation and then replaced with a taylor-made version metal or otherwise.

EDIT... Achieving compression ratios between 18.5:1 and 21.5:1  are not so easy without special gaskets ....
Unless an intact 'old one' has compressed slightly :?

EDIT... EDIT A real time engine's compression may be lower to start with so in practice those compressions are attainable. [Pays to have a tight spec. engine and a broken in specimen alongside]

[saurkraut]

Are we debating the same side of the compression issue?  I am really trying to say that running a thicker head gasket is not a good idea.  I screwed up and did it myself, and I'm trying to save others from making the same mistake.

For clarification, there were months between changes.  The entire process took over three years.  Each change was verified with a befor and after acceleration run over a known distance.  If it did not yeild an improvement in EGT, Boost, or speed, it was undone.

This motor was not slapped togather, and a bunch of changes done before the key was turned.  The motor went in bone stock.  The only difference between it, and a brand new motor was the 3 notch gasket, and 1st oversized pistons.  The pistons were even fitted to the block according to the honeing group number.

As far as I'm concerned, the engine performs to my satisfaction.  It goes like a scalded cat when I want it too, cruises on the highway just fine, and gives good fuel milage to boot.  I just wouldn't enter it into pritty cold start contest.  That is not its strong suite.

And, quite the contrary, I think mid 400 psi (i forget what it was exactly, 460 or so, with very little difference between cylinders) is just dandy compression.

If many work shops blow off piston protrusion measurements, it doesn't make it right.  It just shows there are alot of schlocky mechanics out there that are doing their customers a disservice.  Once agian, I don't think the German engineers were stupid, and made a mistake in specifying piston protrusion numbers that related to engine performance.

And yes, i can work a calculator too.  If an engine's piston protrusion calls for a 1 notch, and a three notch is installed, the compression change is very small.  It yields very small chainges.  Its not the way to go for lowerering compression.  Its a mistake I made, and I'm trying to communicate that this is not the way to go if you want to lower compression.

But what is changed significantly is the squish area between the head and piston.  When the squish area is correct, the head and piston act as if they are in thermal contact, and some cooling bennifit is enjoyed by the piston.  When the gap is increased, this bennifit disappears.

So my point is, going to a thicker head gasket to lower compression is not a good idea.  You don't achieve significant results, and unintended consiquences arise.

I'll try to find more information on squish band, and i'll post it here.

[Mark(The Miser)UK]

Saurkraut,
I'm not trying to accuse you of slapping the mods together; I'm just trying unravel the secrets of why some engines are more economic than others. Nothing against extra power either But I'm only really looking for smooth running compared with a farm vehicle and great economy. Perhaps I just want 'sufficient' power. What with all these speed cameras we have here in the UK. 1 bad week and the driving licence could disappear!

Are you still running the 3 notch gasket? If so then:

You actually don't know whether your 'play it safe' with a 3 notch was  in the 33% correct bracket?
I suppose all will be revealed in 100,000miles time when you change the gasket :wink:

With the lower power you felt you were getting did you try and add extra fuelling elsewhere to compensate?

That squish thingy sounds interesting. My thoughts are that a larger area  of compressed air above the piston with no fuel in it would help to cool the pistons seeing as the greatest heat is eminating from the prechamber fire.

So the compression wasn't affected much then mid 400's sounds excellent compared to some people 's 250 to 300!!

What's the size of the 1.9 head chamber? I'll see if I have the info somewhere!

[saurkraut]

I've worked backwards from a bunch of available information on the various VW diesel engines and get around 18.5:1 for a 1.9 head on a 1.6 block.  I don't have the stuff at my finger tips, but i'm faily confident in my mathematicle abilities to be satisfied.  If someone else want do do the research and comes up with 19:1, i will not be up set.  Its in the ball park, and it will do what I want.  There are poeple that have gone befor my in this endevour.  I am by no means the pioneer here.

My main goal for ruducing the CR is to reduce the load on the rods, bearings, head and whetever else, so I can run more boost and fuel.  Its a summer only diesel, and it still should start with a lower CR.  It might start even better with the piston clearence set properly.  If the fuel milage drops, I will not be concerned.  Its a diesel power go kart in my eyes.  A nifty toy.

Since I have blown off the piston protrusion measurement, I have no idea where the gasket situation is.

What I do know is this engine has always had a less than gorgious start, even with everything bone stock, and all the glow stuff PERFECT.  With everything essentially brand new, and set spot on, and nothing else weirded out, it points to the only variable I did not verify: piston protrusion, and hence the proper gasket.

Ive been all over the fuel thing.  Was even running 24+PSI with a soot cloud that blacked out the light and warmth from the sun.  Its a little more conservative now, but in no way pedestrian.

I hope to get to this head swap soon, and I'll gauren-gosh-darn-tee that I'll measure piston protrusion.  Its was appearently a big enough deal for the engine OEM to go through a few hoops to get it right, and thats good enough for me.

As a matter of fact, i'm gonig to determine what the squish is, and i might make it tighter if i can get away with it.

Here is some squish stuff that took me only a short time to find.  I don't think the detonation thing applies to us, but piston cooling, and turbulence are right up our alley:

What does having the squish clearance close do? It makes the compressed charge between the squish band and the top of the piston (at the top of the stroke) thin, thus allowing for quicker heat transfer between the piston and the squish band. This clearance change will produce MORE HP, MORE TORQUE, and the engine will not have to work as hard. You will be able to run a richer  setting, come off the turns faster, launch better, pull a bigger prop, run cooler and run faster! This is probably one of the 3 biggest failings of the Novice Boater!

One of the most important items in the design of a good combustion chamber is the squish band. I believe that a flat squish band produces much more power than an angled squish band. The flat squish band head has a flat area (squish band) around the perimeter of the head which comes in close proximity to the piston at top dead center. This squish band is designed to keep the layer of combustion mist very thin, in order to let heat travel quickly from a hot piston to a cooler combustion chamber (head). The thinner this layer (the closer the head clearance), the better this heat transfer is accomplished. If your head has its squish band to far away from the piston at TDC and the compression ratio is high, you will get pre-detonation (knock). You can tell if this is happening by looking at the squish band. If it looks like it has been lightly sand blasted, it is pre-detonating. Most people when they see this pre-detonation automatically raise their squish band piston clearance. That is the WRONG WAY!

Quench, or squish area is typically the flat area on the top of the piston that's almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to qualify as quench.
If you look at a combustion chamber, you will usually see these flat areas, and they will have the volume of the actual combustion chamber between them. When the piston is compressing the mixture, as the piston nears the head, the flat areas on the head and piston come together and force the mixture from those areas to "squish" into the chamber, where the spark plug and burning mixture reside, so you achieve a more complete burn.
The quench area also runs cooler than the rest of the chamber / piston. These lower temperatures are where the "quench" comes from.
When properly designed, the quench areas can have a tremendous effect on the quality of combustion, and allow higher compression ratios, and due to this they are considered "artificial octane" by scientific types.
Bottom line is "properly designed, quench is good

[Mark(The Miser)UK]

This squish effect seems to be directed at gassers. They have a larger readymixed combustion field that is prone to detonation at the more desireable higher compression ratios.  Does not seem relevant to diesels and a 20% gain on a gasser is still way below the efficiency of a diesel.
Improvements in maximum power at high speed can be found with 'smaller' inlet valves!

What's the compression ratio of a 1.9 diesel?
The purpose of experimentation is to vary the 'variable'; so how can you get other compression ratios if the 1.9 head doesn't give you an improvement? [Other than the improvement in maximum power for more fuel]

[saurkraut]

To say squish / quench doesn’t apply to diesels is making a very bold statement.  If we take a look at the tops of the pistons, and the bottom of the head, we see allot of flat areas that like it or not, are more than probably, acting as squish / quench areas.

Squish helps turbulence, which improves combustion.  We like to call the swirl chambers “precups” or “prechambers”.  VW calls them swirl chambers.  I wouldn’t be at all surprised if the swirl in theses chambers gets a big boost at the end of the compression stroke as the air is forced out of the areas were the piston and the head are very flat, and in very close proximity to each other.

Lets look at quench.  The main reason we put EGT probes in our exhaust manifolds is to keep from melting pistons (and a host of other things).  The top of the piston and the head are very close.  These areas are also very large.  I not ready to say that this close proximity is providing no quench either.  Increasing the gap between these surfaces may cause the piston to run hotter.

I am reluctant to say that VW didn’t design in quench and squish into this engine.

What I will say, with all the information I put forth, lowering compression with thicker head gaskets is a bad idea.

“The purpose of experimentation is to vary the 'variable'”

I’m not sure what your asking here.  I think I pointed out that I’m not throwing a bunch of stuff against a wall and see what sticks.

[Mark(The Miser)UK]

The close proximity areas in the vw diesel are only compressed air and not compressed mixture....
The air flow [volume/rate] through the entrance to the swirl chamber cannot be 'boosted' towards TDC because the piston travel is slowing down; hence the 'Dead' part of the term.

Can there actually be a detrimental effect because of the vaccuum affect of bringing two surfaces close and then pulling them apart quickly?

These forae are full of punters who have changed one thing and inadvertantly altered something else. [Not saying you have, but can you absolutely exclude it]

One factor that you may not have taken into account is the Tom, Dick and Harry theory.
Huh?... Basically take 3 identical engines/cars from the factory; and the figures for performance reliability and economy can be very different.
ie Tom is a great car, runs well, occasional problems and owner happy.
Dick is always in the garage, never had the economy of Tom and could give the vehicle a bad name and the owner won't be repeat buying.

Then there's Harry...
The owner loves this car and reliability is #1. Economy great and never lets the owner down. Engine does not wear. It's demise will leave the owner very sad 15 years down the line when it is shunted and written off by the insurance  co. ...

Whilst the above may sound a little abstract and non scientific it may be analysed by coincidence of machine tolerances that work together.

Having owned and driven 7 Quantum's; I can assure you that there is truth to this yarn. I also ran two identical Fiats from the 1980's which were far from identical.
My father bought 2 [UK] MINI pickups in 1959. One green and one blue. They were so different...

Throwing sticky stuff at a wall seems like a random event. Not the iterations that I'm proposing. I thought this thread was about finding out effects of compression ratio variation :?:

Sure you have what may or may not  be the wrong gasket. It may or may not be the reason that you are getting the symptoms you describe.
What you gonna do if it was the correct gasket? When you change heads please add to the data the actual piston protrusion measurement.

You aren't interested in the 'smaller' inlet valves creating higher speeds?  Not 'my' theory etc etc :wink:

[saurkraut]

"The close proximity areas in the vw diesel are only compressed air and not compressed mixture....
The air flow [volume/rate] through the entrance to the swirl chamber cannot be 'boosted' towards TDC because the piston travel is slowing down; hence the 'Dead' part of the term."

Were did this come from?  Do you have a link?

[Mark(The Miser)UK]

"The close proximity areas in the vw diesel are only compressed air and not compressed mixture....
The air flow [volume/rate] through the entrance to the swirl chamber cannot be 'boosted' towards TDC because the piston travel is slowing down; hence the 'Dead' part of the term."

Were did this come from?  Do you have a link?

What do you mean where did this come from?
This is general elementry physics.
In the IDI engine a piston travelling along the length of its bore pushes the air through a fixed orifice at a rate determined by 'PI r squared'  (the piston area) It is directly proportional to the rate of travel of the piston. The rate of travel of a piston is at a minimum at either end of its travel and a maximum in the middle of its travel. The minimum speed/velocity is zero [even though the crank is rotating constantly].
In addition you refer to the squish band allowing the piston to be cooled by the head. In the IDI the head is  being heated by the fire and compressed air in the swirl chamber whilst the piston is cooled by the incoming fresh air.

I again suggest that the squishy squashy stuff is related to DI gasser engines.; as your source suggests and my initial googling confirms. Please post links to diesel related squish /squash for IDI engine.

[jimfoo]

The piston periphery section 29 forms a squish area 31 of the combustion chamber 44 with the surface 48 of cylinder head 16. The squish area 31 allows more volume to be adjudicated to the lobes 20 and 22 and promotes rapid air motion into said volumes as the piston reaches TDC on its compression stroke for faster mixing and burning with the flames exiting out of the prechamber through the transfer passage (which have not yet been described).

[saurkraut]

"I'm not here to help... I'm here to Pro-Volke"

This is becoming more and more obvious.


"saurkraut wrote:
"The close proximity areas in the vw diesel are only compressed air and not compressed mixture....
The air flow [volume/rate] through the entrance to the swirl chamber cannot be 'boosted' towards TDC because the piston travel is slowing down; hence the 'Dead' part of the term."

Were did this come from? Do you have a link?"

I merely asked were this came from. It looks to me like its talking about the differences between compression ignition and spark ignition.  I wanted to see it in its full context.

"What do you mean where did this come from?
This is general elementry physics.
In the IDI engine a piston travelling along the length of its bore pushes the air through a fixed orifice at a rate determined by 'PI r squared' (the piston area) It is directly proportional to the rate of travel of the piston. The rate of travel of a piston is at a minimum at either end of its travel and a maximum in the middle of its travel. The minimum speed/velocity is zero [even though the crank is rotating constantly]. "

Geez, I apologise.  I didn't know that you were a Physicist.  How ever, the same is true for gas motors, the pistons slow down as they approach TDC.  I've degreed modified cams in turbocharged gas motors and set ignition and rotary valve timing in racing two stroke engines and I'm well aware of this phenomena.

“I again suggest that the squishy squashy stuff”

This is a response that engenders further cooperation.

I’ll see what I can find.  Like I said, I did a quick search (it wasn’t google). I’ll look some more.

The “squishy squashy stuff” has been around a long time.  I did not invent it.  It’s a basic design function that shows up in just about every gas motor on the planet.  The basic mechanical function of a diesel (pistons, connecting rods, crank, camshaft) is not so completely different from gas motors.  It may be there too, but not had a whole bunch of attention drawn to it.

I just think it very peculiar that VW specified different gasket thicknesses, when we can clearly see it does not affect the compression significantly.  Why did they do that? What is the important factor?

The reality of the situation is there have been a lot more gas motors that have been modified, and things studied more closely after modification