"Stroker" on not "Stroker"........

[Pisobiker]

Hi guys,

as usually happens when I hear of techincal stuff which doesn't convince me, I try to investigate the theory to find physical explainations together with formulas and numbers that can confirm or belie it.
This is the time for the "LONG STROKE OR SHORT STROKE" issue, but more precisely:

ON A PURE MAX HP MATTER, IS IT BETTER TO USE A SHORT STROKE-SMALLER DISPLACEMENT-HIGH REVVING ENGINE, OR A LONGER STROKE-BIG DISPLACEMENT-LOWER REVVING ONE?

Despite I've heard of pople tending very clearly for one of the two against the other, given all other parameters the same, I can tell you, and will try to demonstrate, results are at the "PHOTO FINISH" and not so expected.

I'll start putting down the fixed points:

1) Engines of the same type;
2) Same bore, same pistons and rings, same kind of conrods;
3) Same intake and exhaust layout;
4) Same head, combustion chambers and porting job;
5) Same turbocharger;
6) Same auxillary systems;
7) Same ignition map optimization %;
8) Same fuel optimization %;
9) Peak rpm setted to give exactly the same inertial loads over the conrods.

For point #9 I had to build an excell sheet which could plot, at steps of 15°, inertial loads over the conrods for the whole 360 krankshaft degrees (didn't need to do it in the 720° range, because max inertial load happens at TDC in the intake stroke, so I focussed my attention from there to next TDC), depending of:
- Reciprocating masses;
- RPM;
- Stroke;
- Connecting rod lenght;

[Pisobiker]

For max simplicity I've tryed to analyze our IL5 turbo engine in the 81mm bore layout and:

A) 86.4mm stroke (2225 cc);
B) 92.8mm stroke (2390 cc);
C) 95.5mm stroke (2460 cc);
D) 77.6mm stroke (1998 cc);

the latter being "an outsider" 2.0L engine such as the NM engine (don't know exactly stroke, but since bore still is 81mm, stroke should be about the one I've assumed).

At this point, giving the same mass for pistons and rings, I had to make some assumptions about conrod masses and conrod lenghts.
I've assumed:

- for A) 144mm conrod lenght, 160gr conrod reciprocating weight;
- for B) 152mm conrod lenght, 184gr conrod reciprocating weight;
- for C) 156mm conrod lenght, 195gr conrod reciprocating weight;
- for D) 149mm conrod lenght, 175gr conrod reciprocating weight;

For D, not knowing how tall the NM deck is and if it could be used in a high power turbo conversion, I've assumed a lenght and wight which could approximatly be right when using the 86.4 stroke block.

Here is an example graph coming out of the excell sheet, which in this case is comparing the 86.4mm stroke with the 77.6mm stroke respectively @ RPM which gives SAME INERTIAL LOAD ON CONRODS for both.

[Pisobiker]

Therefore , supposing my calculations are correct, for having exactly SAME MAX INERTIAL LOADS over conrods for all of the above examples, giving 7300 rpm peak to the 86.4mm stroke one, RPM limits are:

A) 7300;
B) 6910;
C) 6750;
D) 7740.

If max load (therefore rpm) I've considered is too low, it will be on all of the examples for the same percentage, so that results will still be in proportion.


NOW IT'S TIME TO FIGURE OUT POWER OUTPUT:


If we assume same VE at peak HP for all of the examples, and same boost up to peak rpm, then, given all other parameters the same, if the 86.4mm stroke develops 500 Nm @ 7300 rpm then:

A) 86.4mm stroke, 500 Nm @ 7300 rpm -> ~532 Hp;
B) 92.8mm stroke, 537 Nm @ 6910 rpm -> ~541 Hp;
C) 95.5mm stroke, 552 Nm @ 6750 rpm -> ~544 Hp;
D) 77.6mm stroke, 449 Nm @ 7740 rpm -> ~507 Hp.


Of course my assumptions over mass differences among conrod lenghts might not be 100% precise, but error should be small enaugh not to change results significantly.


MY CONCLUSIONS:

results so far seem to me enaugh to say that, with the only purpose of choosing the option which will allow the max output and strongest power range, the "stroker" is best.
At an only slightly higher output on top rpm, it will show a significantly stronger pull from low revs and lower boost treshold (due to bigger displacement), making it also a lot better in a "all around" cruising condition.

Therefore, the chioce of going to a shorter stroke engine, for track and racing purposese, in my opinion can only be function of other considerations such as, one of the most critical, might be related to crankshaft's strenght and vibrations, when searching for the ultimate power and torque monster.

(Anyone who has detailed infos on this subject please tell)

[Pisobiker]

Last considerations over CONROD LENGHTS.


Lets start saying that, as I have had the possibility to check from different trustable sources, as long as LENGHT TO STROKE ratios don't go as low as 1.5, in a forced induction engine longer conrods don't give performance improvements, while keeping conrod short enaugh will help on having lighter conrods (at same max resistance) therefore rising accordingly acceptable peak rpm.

Just an example to show differences in load (therefore in max rpm) and possible output for different conrod lenghts and weights (wich cause different inertial loads both beacause of a different kinematics and different weight) using the same system and hypotesis than before:

Let's analyze the 92.8mm stroke (I choose this one because I was asked for, but results can be applyed with the right proportion to all other examples):

1) 152 mm conrod lenght = 1.64 ratio, 184 gr reciprocating mass, 537 Nm@ 6910 rpm -> ~541 Hp;
2) 144 mm conrod lenght = 1.55 ratio, 160 gr reciprocationg mass, 537 Nm@ 6985 rpm -> ~547 Hp;
3) 160 mm conrod lenght = 1.72 ratio, 205 gr reciprocation mass, 537 Nm@ 6840 rpm -> ~536 Hp;
4) 160 mm conrod lenght = 1.72 ratio, 160 gr reciprocating mass, 537 Nm@ 7070 rpm -> ~554 Hp;

Last example above, to show how things change if everything but reciprocating mass is kept the same:
at 160 mm lenght, going from 205 to 160 gr reciprocating weight will allow rising revs of 230 rpm therefore increasing power 3.35% (in this example = 18 Hp).


Of course conrods of different makes and/or better or higher quality materials, might have different or smaller reciprocating mass variations, but, as I told before, global proportions should stay more or loess the same.


Any comment or add-on welcome.



Marco

[Sidspud]

Marco, you have way too much time on your hands!
You lost me at the subject line!

[Pisobiker]

Marco, you have way too much time on your hands!
You lost me at the subject line!

Yo're probably right. My problem probalby is that, when I have some little time, instead of doing fun activities, I waste my time with this kind of junk....




Marco

[Pisobiker]

would like to cllearyfy, anyway, that I'm not comletely nuts (just a little)...

since I was asked to give my opinion on different stroke and conrod lenght options, while investigating the subject, I thought it might have been of interest for others as well, while having the opportuity to read other opinions as well.

Main problem, which started my research was a statement I heard sime time ago, which was sying that for track use the ultimate top power wapon was a short stroke engine, such as one in the 75-78mm stroke range, only because the shorter stroke allowed for much higher rpm......





Marco

[S2 Ant]

Makes for an interesting read, but agreed, you def have too much time on your hands!

[Pisobiker]

Jeez, how many misstyping.....

.... by the way, how come i cannot edit my posts to fix something out?



Marco

would like to cllearyfy, anyway, that I'm not comletely nuts (just a little)...

since I was asked to give my opinion on different stroke and conrod lenght options, while investigating the subject, I thought it might have been of interest for others as well, while having the opportuity to read other opinions as well.

Main problem, which started my research was a statement I heard sime time ago, which was sying that for track use the ultimate top power wapon was a short stroke engine, such as one in the 75-78mm stroke range, only because the shorter stroke allowed for much higher rpm......





Marco

[rotoryboy]

You should have 'Edit & Reply To Post' at the bottom right hand corner of your post.

[Pisobiker]

You should have 'Edit & Reply To Post' at the bottom right hand corner of your post.

Well, might seem odd but it does not show anything on my computer. But if I go over the area with mouse's arrow, then I can read the link to the "edit"......

Well.... good enaugh to me. The important is I discovered it works...


Thanks

[mbcouple]

Thanks Marco. That was a very nice analysis. I have several questions. In your example c, with the 95.5 stroke, could you really fit a 156mm con rod in there without moving the pin height? Also, comparing your numbers from the first examnple, it looks like the 92.8mm stroke with the lightened 160mm rods overpowers the 95.5. Would it be your opinion then to lengthen the rods, or increase the stroke? Lastly, by increasing the lenght of the rod and retaing a more mild stroke, ie the 92.8 vs the 95.5, would the block benefit from a longevity standpoint? The shorter the rod, the more wear on the cylinder, correct? So it looks to me from your data that the 92.8 is the winner? I hate to even ask, but is it possible to convert your spreadsheet to show the torque figures? As we all know, the audi 20v is all about torque. Maybe we are making the decision with the wrong data.
Thanks
Hans

[caterpillar]

I think this was nice,it will work for me.i have 94,5 oettinger crmo crank,and 158mm conrods,and 82,5 bore......what are you mean about 160 gr reciprocating mass ??? the conrods weight are some more,ca 560gr??????

[caterpillar]

do NOT edit pisobikers posts,i,ve learn somthing in this forum,hehe!

[CHEN]

Good read