Piston difference

[Guy_b_g]

You have to do both or they'll have radically different compression ratios, as well as the possibility of piston/valve/head interference.

When a crank is "stroked" you are moving the centerline of the rod bearing farther away from the centerline of the crank bearings. This creates a longer stroke, and more volume in the cylinder at the bottom of the stroke because it is now farther from the head, hence the increase in displacement.

Conversely at the top of the stroke the piston is also closer to the head, which in conjunction with the greater volume of compressed gasses creates higher compression. To lower compression and avoid piston/valve/head contact, the piston wrist pin location must be closer to the top of the piston, which lowers the piston crown when at the top of the stroke, and reduces compression, as well as fixing the interference problem.

The advantage of stroked motors is higher torque and HP than a stock engine, (you get a bit more power than you would by boring out the cylinders to an equal displacement).

The disadvantage is higher piston speeds as they now have to move a greater distance for each revolution of the crank, creating more inertia, and lowering the redline of the motor.

Hope that explains things...

[XSokieSPECIAL]

I thought that you had to change the crank, not the wrist pin location in the piston, to get a differant stroke length?

Yes, that is correct. But to make up the difference in deck height of the top of the piston in relation to deck height of the cylinder either the length of the connecting rod or the position of the wrist pin in the piston must be moved to compensate for stroke length difference.

To further confuse you, the longer the connecting rod and the closer the wrist pin is to the top of the piston, the longer the piston STAYS at TDC. Thus creating a FALSE compression increase becouse the piston stays at TDC longer during combustion. This = more horsepower.

I don't fully understand that second part but I know it works because I've seen it in action in race car engines.

I think that is why Harleys sound the way they do. That's just an educated guess.

[Guy_b_g]

either the length of the connecting rod or the position of the wrist pin in the piston must be moved to compensate for stroke length difference.

I didn't mention the connecting rod option...

Trying to keep it simple..

The reason the piston is at or near TDC longer is that with a bigger stroke, the circle that the rod bearing turns is larger, and takes longer to pass a given point (at the top and bottom of the stroke, but a shorter time at the midpoints of the stroke) as it is translated into reciprocating movement at the piston.

It's that greater acceleration between the midpoints and top and bottom of the stroke that lead to a lower RPM limit than a shorter stroke motor (all other things being equal..reciprocating weight etc..).

[XSokieSPECIAL]

I didn't mention the connecting rod option...

Trying to keep it simple..

The reason the piston is at or near TDC longer is that with a bigger stroke, the circle that the rod bearing turns is larger, and takes longer to pass a given point (at the top and bottom of the stroke, but a shorter time at the midpoints of the stroke) as it is translated into reciprocating movement at the piston.

It's that greater acceleration between the midpoints and top and bottom of the stroke that lead to a lower RPM limit than a shorter stroke motor (all other things being equal..reciprocating weight etc..).

Ok, yea. That makes sense and now that you mention it someone explained that to me before but I had forgotten.


Thanks for reminding me.

[Guy_b_g]

NP, YVW Greg

I hope it is clear to the uninitiated...

[oseaghdha]

Just to derail the thread even more

The "Long Rod" theory is that the rod to crank pin angle dwells longer at or about 90 degrees. This is where the most force is applied to the crankshaft on the power stroke.

I verified this by programming it into a computer and plotting out the rotation per millisecond for several rod lengths at a fixed stroke.

Since you can't get something for nothing, you give it back in increased side forces on the cylinder.

Eventually, you run into piston height problems and start to give up rings.

[XSokieSPECIAL]

NP, YVW Greg

WTF is YVP? Not on my acronym list.

[Guy_b_g]

yvw...you're very welcome..

[Guy_b_g]

Actually, shorter rods create more side thrust...

Connecting rods were an overlooked part of buildups not too long ago but with the recent long rod craze you may be asking if you should be running long rods???

First lets cover the basics. In the image you will see the black lines indicating the center of both the large and small ends of the rod. This is the length of the rod. If you have a 5.7" or 6" rod it is measured at these points as are all other rods.

Rod to stroke ratio is a simple ratio. If the rod is 5.7" and the stroke of the crank is 3.48" you have a 1.6379 rod to stroke ratio. In theory the higher this ratio the more power it makes. The reasoning behind this longer rod is more power comes from a couple of aspects. One; Is that if you go back to basics of a simple machine the pistons, crank and rods are basically a compound lever. The up and down movement of the piston translates into rotational motion of the crank.

In an engine a longer rod can make more rotational TQ from the same piston force because it's a longer lever!

Two; Due to the angles involved a longer rod is less angular than a shorter one and side wall loading is less. What this means is that the piston is pushed more up and down the bore than to the side of the block thus reducing friction and increasing HP.

[Guy_b_g]

here is the rest of the article: from http://www.chevymania.com/tech/rod.htm

As you can see in the below example that rasing the piston pin height changes the rod angle noticeably. You will also notice that the piston top doesn't change. Many people believe that longer rods make an engine a stroker but this is simply not so. The only thing that can change displacement in an engine is the bore or the stroke.

Eventually, you run into piston height problems and start to give up rings.

This is true, as you can see in the second diagram....rings and wrist pins don't mix well.

[Flatlander]

and to DERAIL this thread even more..........

This is why I own a OLDS 403....well actually a 409. "THE" motor to have!

Well only if they (OLDS) put better factory heads on it! And the issue with the Siamese cylinders! And the windowed webs! And poor oil return! And......I'll stop there.....Well I guess it could have been "THE" motor to have! Darn them OLDS people...they alllmmmoosssttt had it!


But the Eleven Hundred is PERFECT....right???.....That's why I also have the snowmobile with it in it!!! Just it's big brother to the XS.


Thanks for all your info.....AWESOME!!! I didn't think it was just the wrist pin location.???

[XSokieSPECIAL]

Actually, shorter rods create more side thrust...

here is the rest of the article: from http://www.chevymania.com/tech/rod.htm








This is true, as you can see in the second diagram....rings and wrist pins don't mix well.

Gotta love those Chevy SBs Can't go wrong.

I appreciate all of the technical bull but I'm just a dumb nut twister.

All I need to know is what parts I need to make it do what I want it to.

Heh-heh, interesting stuff though.

[oseaghdha]

Actually, shorter rods create more side thrust...

Yep, I got that one wrong.
I misremembered from stuff I was doing a really long time ago.......
I have CRS, it's not my fault.

[tarzan]

HIGHJACKERS,lol
This thread is starting to remind me of that game when you are a kid.One person says something and by the time it comes back around the whole story changed.

[oseaghdha]

HIGHJACKERS,lol
This thread is starting to remind me of that game when you are a kid.One person says something and by the time it comes back around the whole story changed.

One o' them Okies started it!

I was minding my own business, reading about pistons........