- Jul 12, 2011
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on a stock short block you can see when the piston is at TDC it is flush with the deck. So wouldnt that mean when you put a bigger crank in and increase the stroke the piston would go past TDC and hit the head??
When stroking an engine
- Thread starter foxbodymike87
- Start date
Nope, and this is why: If you add .400 inch of stroke, all you have to do is move the piston pin down on the piston .200 to allow for the extra stroke, thus the piston stays at the same theoretical deck height. Or you use a shorter connecting rod, or a combination of different pin height and shorter connecting rod, which is why a stroker kit usually includes crank, rods and pistons so that it all ends up in the right places.
When stroking an engine use plenty of baseline and make sure the door is closed behind you ... sorry this title made me lol
- Jul 12, 2011
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but if you use a shorter connecting rod arnt you just contradicting the whole point..you mine as well have not done it at all..a stock engine at TDC is flush andis a complete stroke..but if you get a larger crank and short rods wouldnt it be the same stroke and the smaller crank and longer rods?
The rod does not have any effect on the stroke length. The stroke length is all in the crank. If you take a 5.0 that has a stock stroke of 3.0" and add .400" to the length of the stroke, if nothing else was done to the rods and pistons, then the piston would stick above the deck by .200" To "fix" that, you can either move the piston pin down on the piston by .200" or make the connecting rod shorter by .200" or a combination of shorter rod and relocated piston pin. Either way you still have a 3.40" stroker engine that now displaces 342 cid assuming a stock bore size.
The piston has a total stroke length of 3.40" from top to bottom with the 3.40" stroker crank regardless of piston pin height or rod length. The whole reason for the shorter rod and/or relocated piston pin is to keep the piston from sticking out of the top of the deck. A stock 3.0" stroke crank only moves the piston 3" from top to bottom and that is why it has the length of rod and piston pin height it does, so as to also keep the piston from sticking above deck height. Obviously compression and rod ratio are just of few of the items also taken into consideration when designing a particular engine for displacement and type of operating range that it will see.
The piston has a total stroke length of 3.40" from top to bottom with the 3.40" stroker crank regardless of piston pin height or rod length. The whole reason for the shorter rod and/or relocated piston pin is to keep the piston from sticking out of the top of the deck. A stock 3.0" stroke crank only moves the piston 3" from top to bottom and that is why it has the length of rod and piston pin height it does, so as to also keep the piston from sticking above deck height. Obviously compression and rod ratio are just of few of the items also taken into consideration when designing a particular engine for displacement and type of operating range that it will see.
txstang84
15 Year Member
You're correct about the rod length not having anything to do with the stroke, but a couple other things aren't quite right...when stroke is increased, rod length is typically increased (at least in a 302 based stroker) to clear the extra throw on the crankshaft unless the rods are already plenty long enough to compensate. E.g., a stroked 302 to 347 uses a 5.4" or in some cases a 5.315" rod to clear the extra length in the crank throw (counterweight) at BDC since the piston is further down in the bore. Rod ratio is another discussion altogether and used to be pretty debated...the whole piston side loading/thrust side of cylinder discussion.
- Jul 12, 2011
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C'mon! It's simple when you sit and think about it. You already know that leaving all things alone, and only adding stroke to the crank, the stock piston sticks out of the hole 1/2 the added stroke. (the other half is added to the bottom of the stroke in the crank case). Like TX said, added stroke typically includes increased rod length so that the bottom of the piston will clear the crank counterweights when the piston is at the bottom of the hole. To compensate for the added stroke and increased rod length, a stroker motor has pistons with the pin moved way up toward the ring lands. ( so much so as to actually get into the bottom oil ring land, requiring special piston pins or oil ring spacers) Capiche?
- Jul 12, 2011
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i get that..it just all the numbers and wich numbers make so many cubes and what not
use a calculator like this,....makes all this 
easy.
http://www.mindsciencemotorsports.com/graphics/tech.htm
edited by fastdriver. please do not bypass the language filter.
easy.http://www.mindsciencemotorsports.com/graphics/tech.htm
edited by fastdriver. please do not bypass the language filter.
txstang84
15 Year Member
The stroke increase is only part of the equation...it changes the whole dynamic via piston speed and position in the bore relative to crankshaft position. Cam timing also plays a part because of this. This is why many people will tell you to get a custom or purpose-ground cam for stroker based engines because most cams are not ground for those variables. What happens inside the engine is altered...that is not to say that a cam and induction setup for a 289/302/5.0 wouldn't work-far from it-but it does mean that fuel curve and timing events will probably be a little different to make best power or efficiency and cam timing events will need to be altered to make the most of the displacement increase because of those changed variables.
Madmike's calculator is very useful-here's another one you can play with to get all kinds of fun numbers relating to compression ratio...
http://www.torinocobra.com/Randys_tools.htm
...it gives you displacement as well
Madmike's calculator is very useful-here's another one you can play with to get all kinds of fun numbers relating to compression ratio...
http://www.torinocobra.com/Randys_tools.htm
...it gives you displacement as well
It's really not that difficult. In grade school, you learned how to calculate the volume of a cylinder, right? The displacement of an engine is nothing but finding the volume of a cylinder times the number of cylinders in the engine. The bore is the diameter of the cylinder, and stroke is the height of the cylinder.
pi x ((bore/2)^2) x stroke x (number of cylinders)
Your right, I dont know what the hell I was typing, my fingers got ahead of my brain.......derp.
