purely theoretical... Can you have too much stroke?

journeyman

My only interest is in low end torque and possible second: efficiency (combustion, heat, and of course, mpg's)

I have always been told and also have experienced with my foot that longer strokes will give you better low end torque and "tend" to give you better fuel economy (I interpret this as higher efficiency, but have no data)

QUESTION:
When is more stroke too much? What are the effects? take an unrealistic extreme example: a tall deck 366 with a 3.935 bore.
If you could fit a 5.6" stroke crank in there you would end up with 545 inches, and a bore/stroke ratio of 0.7

Obviously that example is unrealistic, but really, when is more of a good thing (stroke) too much?

journeyman · Posted: Mon Sep 14, 2009 4:35 pm Post subject:

Also... I am assuming limits of rod angles/lengths, etc.
In other words, with proper geometry of the crank/rod/pin/piston.

squeeezer · Posted: Mon Sep 14, 2009 8:48 pm Post subject:

i think its piston speed you are asking about????

efficiency i would look to a bore that takes advantage of the heads
as well as compression ratio and quench

i would NOT assume stroke will help mileage in anyway unless you are towing

journeyman · Posted: Tue Sep 15, 2009 12:21 am Post subject:

Yes, mostly towing.

the engine will rarely if ever see 4500 RPM.
Most all running will be below 3500 RPM

One of the things I am puzzled by is the bore/stroke ratio.
What does that tell us?

10sec.et · Posted: Tue Sep 15, 2009 12:45 am Post subject:

after building a small block Olds with a big block Olds crank, im going to say that if you can fit it in the block, its not too much Wink

journeyman · Posted: Tue Sep 15, 2009 1:07 am Post subject:

But what are the issues with having too long of a stroke? Is it just angles and clearances? Or are there combustion issues (combustion efficiency, etc.)?

clay · Posted: Tue Sep 15, 2009 2:14 pm Post subject:

I would think that at some point, parasitic losses will start to factor in. Longer stroke of course equals faster piston speed which should change camshaft specs required but would also equal more friction. Also I would think the faster piston speed would put even more emphasis on better head flow. Look at the two Prostock classes, NHRA and IHRA - 500 ci. vs. something a little over 800 ci. Granted the larger engine is faster, but not faster by an amount that the displacement increase would lead you to believe. Clay

Big Dave · Posted: Tue Sep 15, 2009 7:22 pm Post subject:

journeyman · Posted: Wed Sep 16, 2009 12:51 am Post subject:

PROCV8 · Posted: Wed Sep 16, 2009 2:04 am Post subject:

A friend of mine used to spin their old 434BBC to 9200.

Someone correct me if i'm wrong, but, a 632BBC with 4.750" stroke & 10.200" deck height pulls the piston skirts out the bottoms of the bores; so reliability for a road scenario goes out the window.
This is why my new motor is only 615ci, to avoid the piston wear issues.

Big Dave · Posted: Wed Sep 16, 2009 2:30 pm Post subject:

journeyman · Posted: Thu Sep 17, 2009 3:40 am Post subject:

11.625"? Holy cow that's big.

I looked at just HOW big that was:

That is just wild.

I still don't have adequate understanding of the effect of [increased] stroke on combustion efficiency, but am seeing that a standard "tall block" (10.2") looks like the most practical starting point if more than 4.25 stroke.

Also in reading I found that logically, shorter rods give faster motion to the piston at TDC and BDC and therefore, better filling on the intake stroke. Of course shorter rods mean higher side pressures on the pistons.[/img]

jonny_b · Posted: Thu Sep 17, 2009 4:30 pm Post subject:

You want torque, you want efficiency. BUY A DIESEL!

But here is my thinking:
You can have as much stroke as you want. As you increase stroke you need to decrease speed. Look at the ol' steam ocean liners, HUGE steam eingines, with FEET of stroke, only turnned ~40 RPM, Made HUGE torque. So really you are only limited by the package your putting it into, and $$

af2 · Posted: Thu Sep 17, 2009 5:54 pm Post subject:

journeyman · Posted: Thu Oct 08, 2009 6:14 am Post subject:

In my quest for understanding in the question of stroke and its effect on efficiency (at low speed), I found myself in a quagmire of information, opinions, disinformation and what seemed to be superstition.
I am not much further along in understanding this today than I was a few weeks ago.

What I did find:
• Many people agree that 1000 – 1200 FPM piston speed is the most efficient range. Why? Most by how much? How was this measured? All questions that I could not find an answer.

• Also, it seems that the oil ring contributes significantly to the total friction of the engine. Some even claim that the oil ring is responsible for 10% of the total friction in the engine.

• Rod length, crank angle, and piston acceleration is a whole science in itself.

• I also found many, many megabytes of completely useless, baseless information.

In all this, I found my way back to Endyn. I had seen this years ago, but now it all seems pretty exciting and makes a lot of sense. His premise: complete combustion of a smaller amount of fuel/air is better (and far more efficient) than partial combustion of a large amount of fuel/air. Would be VERY cool to see this applied to the BBC.