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This is likely a stupid question, but I am largely stupid, so please bear with me. I have heard that stroking the motor increases the compression ratio. I was always under the impression that the height of the piston remained the same but the piston wrist pin height is what was changed. For instance lets say we have a block that is "0" decked for a 4" stroke and has 10:1 compression ratio. If you stroke that motor to a 4.25" stroke and the compression goes up, wouldn't that mean that the piston would be hanging out of the block a little bit? Thank you for any insight.
From: At my Bar drinking and wrenching in Lafayette Colorado
Well, if you really want to get into rod lengths and other engine design parameters, there are a lot of things to discuss. The OP only wanted to know why comp ratio increases when stroke increases... but we can certainly discuss the other parameters:
What you’re talking about are some fundamental engine PARAMETERS. An engine parameter is anything that defines the basic engine configuration, such as bore, stroke, rod lengths, valve sizes, etc. These parameters affect how much power and torque an engine can produce. These parameters have been manipulated and changed by engine designers as long as the internal combustion engine has been in existence, and they continue to be refined and changed to this day. There are thousands of combinations possible, and every designer who ever put together a combination is certain that HIS (or HER) combination is the BEST. So you’ll get a LOT of argument on what’s really the best or optimum combination, but let’s look at some facts and theory, and compare a few engines against each other….
The engine parameters have to be evaluated in some way in order to determine which are “the best” parameters (since this is the essence of your post…). We will do this evaluation by using three “performance factors:”
Torque – Twisting force. The moment of a force applied at distance. For a street-driven vehicle such as a Vette, we consider this to be of prime importance.
Horsepower – “Motivating Power.” It is a unit of time rate of work (essentially torque times rpm with a correction multiplier). This is of prime importance in a competition vehicle.
Volumetric efficiency – How effective of an air pump the engine is. Volumetric Efficiency, or V/E, is defined as the ratio of actual air pumped by the engine to the actual cylinder swept volume.
Important Parameters
To produce more performance, Detroit has been famous for the implementation of the old racers’ saying, “when in doubt, bore it out.” There’s just no substitute for cubic inches on the street. Whenever the public wanted more performance, Detroit gave them a larger engine.
So, it would seem, that if you wanted twice the power, you could build an engine of twice the displacement. But this is not true. Why?
Volumetric Efficiency (V/E) is the common factor for comparing all Chevy V8 engines. It has been learned (not by me, but by Detroit engineers…) that the V/E curve has the same shape as the engine torque curve….. Think about that – it’s important. V/E is influenced by many things, such as cam timing & profile, valve-area-to-bore-area ratio, intake and exhaust port lengths & diameters, etc. But if we keep all other things the same (ports, cams, manifolds, etc.), the only real engine parameter which affects V/E is stroke. Bore sizes in our Vettes engines change only slightly, but stroke changes quite a bit.
For matters of our comparison, let’s say that head design & flow, manifold efficiency, and other top-end factors remain the same for any given engine, since you would pick and use the best components no matter what your swept cylinder volume might be.
If we plot a curve for volumetric efficiency keeping all factors constant (except stroke), we see that the only engine parameter that affects V/E is the mean piston speed (MPS): When MPS is very low (idle and low-end rpm), V/E is low. V/E improves as MPS increases up to a certain point, then it will drop off quickly. MPS is merely the distance that the piston travels per crankshaft revolution times the crankshaft rpm. In formula form: MPS = STROKE x RPM/6.
We find that for most street-type engines, peak V/E occurs at 2000 feet per minute (fpm) piston velocity. Surprise! This is also the MPS at which peak torque normally occurs. In other words, the more stroke, the lower rpm at which the peak torque occurs (and you always wondered why a 427 could melt the tires off the rims at 1200 rpm….). But this is not the whole story, because actual engine torque is also directly proportional to swept volume (your engine’s CID). In formula form, TORQUE = V/E x CID (with a correction multiplier). Therefore, even though a 427’s torque occurs at a lower rpm, it also has a significantly higher percentage of torque than a 327.
We also know that horsepower is essentially torque times rpm (with a constant multiplier). Peak horsepower for most V/E curves occurs at 2800 fpm MPS where V/E = 0.85 (typical V/E for a healthy normally-aspirated Chevy V8). 2800 fpm MPS is a much lower rpm on a 427/455 than it is for a 327/350. Since horsepower is computed from the formula HP = V/E x CID x rpm/5252, we see that more cubic inches produce more torque, but everything else being equal, resultant horsepower is nearly independent of displacement…! In fact, analysis will show that horsepower is completely independent of displacement as far as V/E is concerned.
So what does this mean….? (now that I’ve rambled on and on…). It means two things:
For the street, run the biggest engine you can. You want torque on the street, and the 427/454 has the largest torque output that’s in a street-usable rpm range.If you don't have a big block, stroke a small block.
For racing, horsepower is not dependent in cubic inches. There are other factors that affect horsepower other than V/E.
So if total cubic inch displacement is not a concern, then what ratio of bore-to-stroke (the parameters affecting displacement) is the best? Almost all successful racing engines are “over-square,” meaning that bore diameter is larger than the length of the stroke. Clearly, a short stroke relative to bore size is beneficial since it produces less piston drag. Additionally, we see that a large bore allows the use of larger valves with less cylinder shrouding. Compare a few Bore-to-Stroke ratios of popular engines:
Chevy 302 4.00 Bore 3.00 Stroke 1.33 B/S Ratio
Chevy 331 4.024 Bore 3.25 Stroke 1.24 B/S Ratio
Chevy 454 4.25 Bore 4.00 Stroke 1.06 B/S Ratio
Mopar 426 4.25 Bore 3.75 Stroke 1.13 B/S Ratio
Another parameter that enters this picture is the ratio of the connecting rod length to the stroke length (R/S) as you noted in your post above. This ratio determines the maximum piston inertial loading and the optimum crankshaft angle (the number of degrees after top dead center at which the crank throw and the connecting rod are at right angles). We can plot a curve showing that the greater the R/S, the less the maximum inertial piston loading, and the more reliable the engine will be (or the greater the redline will be). The greater the R/S, the further past TDC the optimum crank angle occurs and the higher rpm at which the same maximum torque will occur with resultant higher horsepower (since HP = torque x rpm). Thus we can manipulate bore, stroke and rod length to change the torque and horsepower parameters/potential of an engine.
It is correct, as some people have pointed out to me in the past, that thermal losses in the combustion process are greater when you expose more cylinder wall area due to a longer stroke, but these losses are minimal compared to the losses and gains produced by the parameters discussed above.
I have some great ideas for a new ‘Vette engine. If you have a couple of $M to invest, stop on by for a beer or six, and we’ll design the next generation performance V8!!
From: At my Bar drinking and wrenching in Lafayette Colorado
Assuming that the stroke is increased (a "stroked" engine can be one where the stroke is either increased or decreased...), the longer stroke means that the engine has more displaced volume. You're compressing that larger volume into the same sized cylinder head combustion chamber. More volume compressed into the same chamber means much more compression.
Bence, if you used the same piston from the 4" crank on to the 4.25 crank, the piston would be sticking out of the block. You need to change to piston made for 4.25 crank.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by bence13_33
This is likely a stupid question, but I am largely stupid, so please bear with me. I have heard that stroking the motor increases the compression ratio. I was always under the impression that the height of the piston remained the same but the piston wrist pin height is what was changed. For instance lets say we have a block that is "0" decked for a 4" stroke and has 10:1 compression ratio. If you stroke that motor to a 4.25" stroke and the compression goes up, wouldn't that mean that the piston would be hanging out of the block a little bit? Thank you for any insight.
With a longer stroke crank you increase the swept volume, but you don't necessarily raise the compression ratio.
If you start out with an engine that has the piston sufficiently far down the hole at TDC, then yes, increasing the stroke while keeping the same pistons will increase the CR, as you're pulling in more air volume, and compressing it into a smaller volume than before. (As you noted above, if the piston isn't down in the hole at TDC before the stroke change, it's going to be sticking out of the hole at TDC after the stroke change. Not a practical engine configuration.)
In real life, when you stroke an engine you need to replace the pistons with appropriately shaped domes and wrist pin placements. These two changes (dome shape and wrist pin placement) will allow you to get a compression ratio higher, lower, or the same as before the change in stroke.
From: At my Bar drinking and wrenching in Lafayette Colorado
All things being constant, and only changing the stroke, an increase in stroke has a massive change on compression ratio. Use any one of the good compression ratio calculators (such as this one: https://www.rbracing-rsr.com/compstaticcalc.html) and keep all parameters the same while varying the stroke. It'll give you a good idea of what happens...
For example, using the noted calculator, run a stock-like 350:
Bore = 4.00
Stroke = 3.48
Head gasket thickness = .040
Deck height = .010
Piston top volume = 5cc
Combustion chamber volume = 64cc
Resultant compression ratio: 10.03:1
If you keep all numbers the same, and simply stroke the engine to a 383 (3.75 stroke, with new pistons to keep the deck height and piston top volume the same), the new comp ratio due to the stroke alone will be 10.73:1.
When building a custom "stroker" engine, the engine builder must consider all parameters, such as piston dome (or dish), deck height, head gasket thickness, and cylinder head combustion chamber volume to tailor the compression ratio to make the engine functional for the end user. A street engine with iron heads running on common pump gas will not be happy with a comp ratio over 10.5:1, so the example noted above, stroked to 383, would not make the car owner very happy in most cases. If the engine is for a bracket race car running on race gas, the owner might be very happy. Good decisions must be made when stroking and building custom engines, and intended usage and application must be carefully considered.
If the pistons are not zero decked and you offset grind your crank in order to zero them you could increase the compression ratio and slightly increase the displacement...
Get some longer rods and then your pistons "dwell" at TDC for a longer time building more cylinder pressure plus the longer rod results in less side loading of the cylinder wall.
From: At my Bar drinking and wrenching in Lafayette Colorado
Well, if you really want to get into rod lengths and other engine design parameters, there are a lot of things to discuss. The OP only wanted to know why comp ratio increases when stroke increases... but we can certainly discuss the other parameters:
What you’re talking about are some fundamental engine PARAMETERS. An engine parameter is anything that defines the basic engine configuration, such as bore, stroke, rod lengths, valve sizes, etc. These parameters affect how much power and torque an engine can produce. These parameters have been manipulated and changed by engine designers as long as the internal combustion engine has been in existence, and they continue to be refined and changed to this day. There are thousands of combinations possible, and every designer who ever put together a combination is certain that HIS (or HER) combination is the BEST. So you’ll get a LOT of argument on what’s really the best or optimum combination, but let’s look at some facts and theory, and compare a few engines against each other….
The engine parameters have to be evaluated in some way in order to determine which are “the best” parameters (since this is the essence of your post…). We will do this evaluation by using three “performance factors:”
Torque – Twisting force. The moment of a force applied at distance. For a street-driven vehicle such as a Vette, we consider this to be of prime importance.
Horsepower – “Motivating Power.” It is a unit of time rate of work (essentially torque times rpm with a correction multiplier). This is of prime importance in a competition vehicle.
Volumetric efficiency – How effective of an air pump the engine is. Volumetric Efficiency, or V/E, is defined as the ratio of actual air pumped by the engine to the actual cylinder swept volume.
Important Parameters
To produce more performance, Detroit has been famous for the implementation of the old racers’ saying, “when in doubt, bore it out.” There’s just no substitute for cubic inches on the street. Whenever the public wanted more performance, Detroit gave them a larger engine.
So, it would seem, that if you wanted twice the power, you could build an engine of twice the displacement. But this is not true. Why?
Volumetric Efficiency (V/E) is the common factor for comparing all Chevy V8 engines. It has been learned (not by me, but by Detroit engineers…) that the V/E curve has the same shape as the engine torque curve….. Think about that – it’s important. V/E is influenced by many things, such as cam timing & profile, valve-area-to-bore-area ratio, intake and exhaust port lengths & diameters, etc. But if we keep all other things the same (ports, cams, manifolds, etc.), the only real engine parameter which affects V/E is stroke. Bore sizes in our Vettes engines change only slightly, but stroke changes quite a bit.
For matters of our comparison, let’s say that head design & flow, manifold efficiency, and other top-end factors remain the same for any given engine, since you would pick and use the best components no matter what your swept cylinder volume might be.
If we plot a curve for volumetric efficiency keeping all factors constant (except stroke), we see that the only engine parameter that affects V/E is the mean piston speed (MPS): When MPS is very low (idle and low-end rpm), V/E is low. V/E improves as MPS increases up to a certain point, then it will drop off quickly. MPS is merely the distance that the piston travels per crankshaft revolution times the crankshaft rpm. In formula form: MPS = STROKE x RPM/6.
We find that for most street-type engines, peak V/E occurs at 2000 feet per minute (fpm) piston velocity. Surprise! This is also the MPS at which peak torque normally occurs. In other words, the more stroke, the lower rpm at which the peak torque occurs (and you always wondered why a 427 could melt the tires off the rims at 1200 rpm….). But this is not the whole story, because actual engine torque is also directly proportional to swept volume (your engine’s CID). In formula form, TORQUE = V/E x CID (with a correction multiplier). Therefore, even though a 427’s torque occurs at a lower rpm, it also has a significantly higher percentage of torque than a 327.
We also know that horsepower is essentially torque times rpm (with a constant multiplier). Peak horsepower for most V/E curves occurs at 2800 fpm MPS where V/E = 0.85 (typical V/E for a healthy normally-aspirated Chevy V8). 2800 fpm MPS is a much lower rpm on a 427/455 than it is for a 327/350. Since horsepower is computed from the formula HP = V/E x CID x rpm/5252, we see that more cubic inches produce more torque, but everything else being equal, resultant horsepower is nearly independent of displacement…! In fact, analysis will show that horsepower is completely independent of displacement as far as V/E is concerned.
So what does this mean….? (now that I’ve rambled on and on…). It means two things:
For the street, run the biggest engine you can. You want torque on the street, and the 427/454 has the largest torque output that’s in a street-usable rpm range.If you don't have a big block, stroke a small block.
For racing, horsepower is not dependent in cubic inches. There are other factors that affect horsepower other than V/E.
So if total cubic inch displacement is not a concern, then what ratio of bore-to-stroke (the parameters affecting displacement) is the best? Almost all successful racing engines are “over-square,” meaning that bore diameter is larger than the length of the stroke. Clearly, a short stroke relative to bore size is beneficial since it produces less piston drag. Additionally, we see that a large bore allows the use of larger valves with less cylinder shrouding. Compare a few Bore-to-Stroke ratios of popular engines:
Chevy 302 4.00 Bore 3.00 Stroke 1.33 B/S Ratio
Chevy 331 4.024 Bore 3.25 Stroke 1.24 B/S Ratio
Chevy 454 4.25 Bore 4.00 Stroke 1.06 B/S Ratio
Mopar 426 4.25 Bore 3.75 Stroke 1.13 B/S Ratio
Another parameter that enters this picture is the ratio of the connecting rod length to the stroke length (R/S) as you noted in your post above. This ratio determines the maximum piston inertial loading and the optimum crankshaft angle (the number of degrees after top dead center at which the crank throw and the connecting rod are at right angles). We can plot a curve showing that the greater the R/S, the less the maximum inertial piston loading, and the more reliable the engine will be (or the greater the redline will be). The greater the R/S, the further past TDC the optimum crank angle occurs and the higher rpm at which the same maximum torque will occur with resultant higher horsepower (since HP = torque x rpm). Thus we can manipulate bore, stroke and rod length to change the torque and horsepower parameters/potential of an engine.
It is correct, as some people have pointed out to me in the past, that thermal losses in the combustion process are greater when you expose more cylinder wall area due to a longer stroke, but these losses are minimal compared to the losses and gains produced by the parameters discussed above.
I have some great ideas for a new ‘Vette engine. If you have a couple of $M to invest, stop on by for a beer or six, and we’ll design the next generation performance V8!!
If the same piston top design is used, and the piston is in the same relative position to the milled surface for the head at TDC, AND the same cylinder head is replaced on the block.....
Compression ratio will be increased as Lars described. More swept volume in each cylinder; same compressed gas volume in the combustion chamber. Must be higher compression.
Of course, when actually doing this "engine stroking", you MUST change some parts and you COULD change many others that could impact CR and lots of other engine parameters.
The Engine Masters contest is very interesting to watch as folks manipulate all these factors to produce an engine to meet the current year's rules/parameters/ And you often see some pretty unusual approaches by some very smart folks that work.
One year Jon Kasse designed a small bore/long stroke/short rod combo to perform on pump gas. This a contest based on power/cubes so just building the largest engine doesn't work since it will be adjusted based on cubes. His approach actually sleeved a 460 Ford block to a smaller bore, made the sleeves longer into the crank case because the long stroke was pulling pistons too low. The short rods (and resulting heavy as heck piston) worked to pull the piston away from TDC quicker rather than follow the typical plan of letting it dwell longer to build pressure. This was a pump gas contest...so Jon decided to run higher compression ratio...but to minimize detonation damage he sped the piston up as it left TDC to reduce cylinder pressure quicker. He calculated what air flow the heads/valves could flow with the small bore and decided it could be assisted with cam timing...plus using canted valve heads like a BBC helps airflow by opening valves away from the bores.
It all worked to score MUCH higher than the competition. Jon wins this contest regularly with his "out of the box" thinking.
Say you have a 5" stroke with 2" compressed volume. Increase stroke by 1"...… Stroke goes from 5" to 6" witch is a 20% increase but the compressed volume goes from 2" to 1" which is a 50% decrease. In simple Jim2527 speak your pulling in 20% more air but squeezing it into a space 50% smaller.
Thanks for the help. I was just having problems understanding how if the piston remained at 0 deck on both motors you would have more compression, the added air/fuel volume is the answer. Thank you for all of the help.
With a longer stroke crank you increase the swept volume, but you don't necessarily raise the compression ratio.
If you start out with an engine that has the piston sufficiently far down the hole at TDC, then yes, increasing the stroke while keeping the same pistons will increase the CR, as you're pulling in more air volume, and compressing it into a smaller volume than before. (As you noted above, if the piston isn't down in the hole at TDC before the stroke change, it's going to be sticking out of the hole at TDC after the stroke change. Not a practical engine configuration.)
In real life, when you stroke an engine you need to replace the pistons with appropriately shaped domes and wrist pin placements. These two changes (dome shape and wrist pin placement) will allow you to get a compression ratio higher, lower, or the same as before the change in stroke.
I understand the switching of pistons (due to wrist pin height). Is it possible to have a 4.00" stroke at "0" deck and stroke it to 4.25" with the piston still remaining at "0" deck. Basically all things being equal (same cc dome, same deck height) the compression would still go up because of the added air volume being compressed? It makes sense that you have more volume shoved into the same space. For some reason I thought that the dome, quench and cc of the combustion chambers were the only contributing factors.
Last edited by bence13_33; Sep 16, 2019 at 03:05 PM.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by bence13_33
I understand the switching of pistons (due to wrist pin height). Is it possible to have a 4.00" stroke at "0" deck and stroke it to 4.25" with the piston still remaining at "0" deck.Yes, with custom pistons, as I mentioned earlier. Basically all things being equal (same cc dome, same deck height) the compression would still go up because of the added air volume being compressed? Yes. It makes sense that you have more volume shoved into the same space. For some reason I thought that the dome, quench and cc of the combustion chambers were the only contributing factors.
Those, along with the stroke, are the factors. With a longer stroke, the wrist pin height or rod length must be changed, or you'll shove the piston into the head the first time you tried to rotate the assembly. And as I mentioned before, the final compression ratio can be whatever you want, as it's just a matter of choosing the right piston shape/configuration to match the desired CR calculations.
given two bores of same diameter BUT of two different heights AND given both columns are compressed into the same size combustion volumes ...
... the taller the column of air ... the higher the static CR.
you do the maths and you will reveal that to yourself ... aka "a proof"
This is likely a stupid question, but I am largely stupid, so please bear with me. I have heard that stroking the motor increases the compression ratio. I was always under the impression that the height of the piston remained the same but the piston wrist pin height is what was changed. For instance lets say we have a block that is "0" decked for a 4" stroke and has 10:1 compression ratio. If you stroke that motor to a 4.25" stroke and the compression goes up, wouldn't that mean that the piston would be hanging out of the block a little bit? Thank you for any insight.
Revisit the definition of what a static compression ratio IS and it's really obvious.
Compression ratio is exactly what it sounds like, it's the ratio of the volume of space that you have in the combination of the cylinder + combustion chamber when the piston is all the way down in it's travel (starting volume there) compared to when it's all the way at the top of it's travel (finishing volume with the piston all the way up).
-Obviously, if you just stroke the engine and the piston starts down further the starting volume of air before compression begins is much larger even if the volume with the piston at the top of it's travel is identical. (The ratio of the total volume at BDC to the ratio of the total volume at TDC.)
Real example:
BDC Volume SBC 350: If you have a 5.7 liter SBC 350 with a set of heads that have 64cc combustion chamber, 5cc valve reliefs, a flat top piston that's literally 0 decked and then to make the calculation more simple, no head gasket, and the top piston ring literally at the top of the piston with no piston-to-bore clearance so I don't have to calculate that volume: You have 5.735 (rounded) liters + 0.064 liters for the combustion chamber + 0.005 liters for the valve reliefs or 5.804 liters of total volume when the piston is at the bottom of its stroke.
BDC Volume SBC 383: If you took that same 350 SBC and left the bore alone at 4.000" and added a typical 3.75" stroker crank you'd have a 377 or 5.804 liters in the bore + 0.064 liters for the combustion chamber + 0.005 liters for the valve reliefs so 5.873 liters of total volume when the piston is at the bottom of its stroke.
TDC Volume SBC 350: 0.064 liters combustion chamber + 0.005 liters for valve reliefs = 0.069 liters TDC total volume at top of stroke
TDC Volume SBC 383: obviously identical as the SBC 350 == 0.069 liters TDC total volume at top of stroke
Even if you don't change the volume at TDC, increasing the volume at BDC increase the compression ratio.
Now -why when I actually do the math above do I get an insane compression ratio of like 84:1?!?! I have no idea.... I'm guessing the head gasket volumes are not insignificant...
How Does Stroking An Engine Increase Compression?
