Help me understand camshafts...
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
With a 4" crank and ls7 rods in a 3.78" bore, you get 359ci.
Throw a 4.8 crank and rods in a 4.125" bore block and you have 353ci.
The 6ci difference is negligible.
But the cylinders require entirely different flow characteristics to fill properly. So the valve events have to change.
Right?
The 3.3" stroke equates to .0183" per degree of crank rotation.
The 4" crank is .0222" per degree.
That's a ~17% difference in piston speed, and I find it very hard to believe that wouldn't have a profound impact on valve events.
Throw a 4.8 crank and rods in a 4.125" bore block and you have 353ci.
The 6ci difference is negligible.
But the cylinders require entirely different flow characteristics to fill properly. So the valve events have to change.
Right?
The 3.3" stroke equates to .0183" per degree of crank rotation.
The 4" crank is .0222" per degree.
That's a ~17% difference in piston speed, and I find it very hard to believe that wouldn't have a profound impact on valve events.
Last edited by DavidBoren; Aug 6, 2015 at 01:59 PM.
TECH Apprentice
Joined: Feb 2014
Posts: 328
Likes: 1
From: Baton Rouge La
But in the set dimensions of the typical LS motor, we cannot have a 4" stroke and a 6.7x" rod, so everything is different from a 3.6" crank. The rod ratio with the 4" crankshaft ends up in the 1.5:1 range. Displacement per degree change is different. Piston acceleration per degree will be different.
I should have stated that I was using LS motors as reference for this conversation. In theory, yes, if the rod ratio remains unchanged, then changes to stroke wouldn't change much.
But putting a cam designed for a 4" stroke and 1.5:1 rod ratio in an engine that has a 3.3" crank and a 1.9:1 rod ratio HAS to be different.. right?
I should have stated that I was using LS motors as reference for this conversation. In theory, yes, if the rod ratio remains unchanged, then changes to stroke wouldn't change much.
But putting a cam designed for a 4" stroke and 1.5:1 rod ratio in an engine that has a 3.3" crank and a 1.9:1 rod ratio HAS to be different.. right?
TECH Apprentice
Joined: Feb 2014
Posts: 328
Likes: 1
From: Baton Rouge La
With a 4" crank and ls7 rods in a 3.78" bore, you get 359ci.
Throw a 4.8 crank and rods in a 4.125" bore block and you have 353ci.
The 6ci difference is negligible.
But the cylinders require entirely different flow characteristics to fill properly. So the valve events have to change.
Right?
The 3.3" stroke equates to .0183" per degree of crank rotation.
The 4" crank is .0222" per degree.
That's a ~17% difference in piston speed, and I find it very hard to believe that wouldn't have a profound impact on valve events.
Throw a 4.8 crank and rods in a 4.125" bore block and you have 353ci.
The 6ci difference is negligible.
But the cylinders require entirely different flow characteristics to fill properly. So the valve events have to change.
Right?
The 3.3" stroke equates to .0183" per degree of crank rotation.
The 4" crank is .0222" per degree.
That's a ~17% difference in piston speed, and I find it very hard to believe that wouldn't have a profound impact on valve events.
https://ls1tech.com/forums/generatio...gth-part1.html
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
In that discussion, the two theoretical engines in question both had the same rod ratio, though. That doesn't necessarily apply to how the valve events would be off if you put a ls7 cam in a 4.8L engine.
In your example, you gave two separate distances traveled @ 20 ATDC. Your theoretical examples had the same rod ratio, so the swept volume was the same, even though distance traveled was different...
In an LSx motor, notably a stock deck LSx engine, the rod ratio is different. The swept volume is different. The piston speed and rate of acceleration are both different. Everything is different, even if you can get close to the same displacement... Like in my oversquare 353 vs undersquare 359 example.
So, my question is, if we were to take an LS7, and put a 4.8 crank and rods in it (custom pistons, or pretend that the LS7 pistons by chance work perfectly with the 4.8 rotating assembly, whatever)... But we left the LS7 cam in it...
How far "off" would the events be? How many degrees too late or too soon would the LS7 cam be for a 3.3" stroke, 1.9:1 rod ratio?
What effects would it have on performance?
Could you install the cam retarded or advanced to compensate for it?
In your example, you gave two separate distances traveled @ 20 ATDC. Your theoretical examples had the same rod ratio, so the swept volume was the same, even though distance traveled was different...
In an LSx motor, notably a stock deck LSx engine, the rod ratio is different. The swept volume is different. The piston speed and rate of acceleration are both different. Everything is different, even if you can get close to the same displacement... Like in my oversquare 353 vs undersquare 359 example.
So, my question is, if we were to take an LS7, and put a 4.8 crank and rods in it (custom pistons, or pretend that the LS7 pistons by chance work perfectly with the 4.8 rotating assembly, whatever)... But we left the LS7 cam in it...
How far "off" would the events be? How many degrees too late or too soon would the LS7 cam be for a 3.3" stroke, 1.9:1 rod ratio?
What effects would it have on performance?
Could you install the cam retarded or advanced to compensate for it?
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
More food for thought on this topic, or maybe just a better explanation of what I am interested in...
For this specific question(s), we will use the oversquare 353 (B 4.125xS 3.3) and undersquare 359 (B 3.78xS 4.0).
The 353 has a 1.9:1 rod ratio.
The 359 has a 1.5:1 rod ratio.
We will use 6000rpms for both.
6000rpms/60 = 100rps
4" X 2 (up and down travel per 360°) = 8ipr, or inches per revolution
8ipr X 100rps = 800ips, inches per second
3.3" X 2 = 6.6
6.6 X 100 = 660
In relation to valve events, increasing stroke is the same as increasing rpms. Valves need to open sooner, close later, in general.
So the ls7 cam would be opening valves "X" number of degrees too early and closing them "X" number of degrees too late, when used on a smaller stroke engine, or at least an engine with a higher rod ratio.
Opening the exhaust valve too early would be detrimental to torque production, right?
Opening the intake valve too early may lead to reversion of the intake, right?
Can someone please explain exactly what would be different in terms of valve events and performance, when going from a 1.5:1 rod ratio to a 1.9:1 rod ratio using the same cam...
For this specific question(s), we will use the oversquare 353 (B 4.125xS 3.3) and undersquare 359 (B 3.78xS 4.0).
The 353 has a 1.9:1 rod ratio.
The 359 has a 1.5:1 rod ratio.
We will use 6000rpms for both.
6000rpms/60 = 100rps
4" X 2 (up and down travel per 360°) = 8ipr, or inches per revolution
8ipr X 100rps = 800ips, inches per second
3.3" X 2 = 6.6
6.6 X 100 = 660
In relation to valve events, increasing stroke is the same as increasing rpms. Valves need to open sooner, close later, in general.
So the ls7 cam would be opening valves "X" number of degrees too early and closing them "X" number of degrees too late, when used on a smaller stroke engine, or at least an engine with a higher rod ratio.
Opening the exhaust valve too early would be detrimental to torque production, right?
Opening the intake valve too early may lead to reversion of the intake, right?
Can someone please explain exactly what would be different in terms of valve events and performance, when going from a 1.5:1 rod ratio to a 1.9:1 rod ratio using the same cam...
TECH Fanatic
Joined: Sep 2003
Posts: 1,058
Likes: 1
On mobile so limited typing. Any case do not overlook that ls7 cam was not optimized for power but power and emissions. Just maybe it's s good power cam in another but smaller cid motor. Not so sure it was the ultimate event cam for the ls7.
Longer crank throw = more tq for a given cid since tq derived from power stroke and circular motion of crank, same piston swept area so same crank degree for cam timing I would think.
Edit : disregard I gotta reread all this later. Def gonna create software now
Longer crank throw = more tq for a given cid since tq derived from power stroke and circular motion of crank, same piston swept area so same crank degree for cam timing I would think.
Edit : disregard I gotta reread all this later. Def gonna create software now
Last edited by 03EBZ06; Aug 6, 2015 at 04:28 PM.
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
Ok, found the actual valve events of the LS7 cam. Now I need to know how "wrong" these events would be for a 3.3" crank, or more importantly, a 1.9:1 rod ratio, or a piston traveling 18% slower... However you want to word it... How far "off" are these events for a 4.8L crank and rods?
Advertised:
IVO 8 BTDC
IVC 89 ABDC
DUR 277
EVO 80 BBDC
EVC 35 ATDC
DUR 295
@ .050:
IVO 18 ATDC
IVC 49 ABDC
DUR 211
EVO 52 BBDC
EVC 2 BTDC
DUR 230
Which events are too early and which ones are too late for the 4.8 rotating assembly?
How would this effect performance? Now, I'm not asking how well an ls7 cam performs in a 4.8L engine, that's not what I'm asking, at all.
If, for example, the EVO event of 80 BBDC, is 15° "too early" when used in conjunction with a 1.9:1 rod ratio rotating assembly, what effects would it have on performance? I don't know if it's actually 15° "off", I made that up just for the sake of this discussion.
I would like to know how far "off" each event is. If you "scaled" the valve events to work with the higher rod ratio, what would they look like?
I would imagine you could keep close to the same duration, both intake and exhaust, but move the events to however many degrees they are "off".
That's why I asked if you could install it retarded or advanced to compensate.
Am I making sense? The events would be different if you made the ls7 cam specifically for the 4.8 rotating assembly, right? Even if you wanted similar duration and lift and lobe separation, blah blah blah. The events would still shift to compensate for the slower piston speed, correct?
Advertised:
IVO 8 BTDC
IVC 89 ABDC
DUR 277
EVO 80 BBDC
EVC 35 ATDC
DUR 295
@ .050:
IVO 18 ATDC
IVC 49 ABDC
DUR 211
EVO 52 BBDC
EVC 2 BTDC
DUR 230
Which events are too early and which ones are too late for the 4.8 rotating assembly?
How would this effect performance? Now, I'm not asking how well an ls7 cam performs in a 4.8L engine, that's not what I'm asking, at all.
If, for example, the EVO event of 80 BBDC, is 15° "too early" when used in conjunction with a 1.9:1 rod ratio rotating assembly, what effects would it have on performance? I don't know if it's actually 15° "off", I made that up just for the sake of this discussion.
I would like to know how far "off" each event is. If you "scaled" the valve events to work with the higher rod ratio, what would they look like?
I would imagine you could keep close to the same duration, both intake and exhaust, but move the events to however many degrees they are "off".
That's why I asked if you could install it retarded or advanced to compensate.
Am I making sense? The events would be different if you made the ls7 cam specifically for the 4.8 rotating assembly, right? Even if you wanted similar duration and lift and lobe separation, blah blah blah. The events would still shift to compensate for the slower piston speed, correct?
Last edited by DavidBoren; Aug 6, 2015 at 05:14 PM.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
Five Reasons the Camaro Was the Most Pivotal Player in the Pony Car Wars 2.0
Brett Foote
10 Reasons the LS7 Is GM's Most Extreme Naturally Aspirated V8 Engine Ever
Verdad Gallardo Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
Maybe I worded this incorrectly. The cam isn't "off", it just radically adjusts the conditions the 4.8 has to operate under.
For example, in order for the 4.8 rotating assembly to match the piston speed of the LS7's 4" stroke, the 4.8 has to spin between 3000-8500 to match the 4" piston at 2500-7000rpms.
It's retarded. It would be anemic down low.
I guess I just answered my own question then. If you put a camshaft designed for a longer stroke into an engine with a shorter stroke, it is retarded the amount of difference in piston speed or the rpms the smaller crank has to spin to match the longer stroke piston speed.
Now, where or what do you adjust to account for this?
Is it done through lobe separation changes?
Shift the intake valve centerline?
If you were building a cam for a theoretical square port 4.8, and you wanted a 7k redline, and a duration @ .050 of 211/230 how would the valve events be different from the LS7 cam?
I'm thinking everyone is going to get caught up on how vastly different the displacement is between the two, and lose focus on the piston speed part of the question. But oh well. If it makes it any easier, we can pretend that the bore changes to allow displacement to remain the same.
With a set displacement, and equal head flow, and desired operating range of, say, 2500-7000rpms... what would change on the cam going from a 4" stroke to a 3.3" crank? All else being equal, the only thing changing is piston speed. What changes on the cam, and why?
For example, in order for the 4.8 rotating assembly to match the piston speed of the LS7's 4" stroke, the 4.8 has to spin between 3000-8500 to match the 4" piston at 2500-7000rpms.
It's retarded. It would be anemic down low.
I guess I just answered my own question then. If you put a camshaft designed for a longer stroke into an engine with a shorter stroke, it is retarded the amount of difference in piston speed or the rpms the smaller crank has to spin to match the longer stroke piston speed.
Now, where or what do you adjust to account for this?
Is it done through lobe separation changes?
Shift the intake valve centerline?
If you were building a cam for a theoretical square port 4.8, and you wanted a 7k redline, and a duration @ .050 of 211/230 how would the valve events be different from the LS7 cam?
I'm thinking everyone is going to get caught up on how vastly different the displacement is between the two, and lose focus on the piston speed part of the question. But oh well. If it makes it any easier, we can pretend that the bore changes to allow displacement to remain the same.
With a set displacement, and equal head flow, and desired operating range of, say, 2500-7000rpms... what would change on the cam going from a 4" stroke to a 3.3" crank? All else being equal, the only thing changing is piston speed. What changes on the cam, and why?
Piston speed is very important to me.
If I have two engines that are both 427", one uses a 4.125" bore and a 4.0" stroke, the other uses a 4.070" bore and a 4.100 stroke.
In my world, the longer stroke engine regardless of cubic inches being the same gets a different cam.
Changing piston speeds affects port velocity by a large margin which affects flow characteristics. It also affects harmonics in the intake and exhaust tracts as well.
If I have two engines that are both 427", one uses a 4.125" bore and a 4.0" stroke, the other uses a 4.070" bore and a 4.100 stroke.
In my world, the longer stroke engine regardless of cubic inches being the same gets a different cam.
Changing piston speeds affects port velocity by a large margin which affects flow characteristics. It also affects harmonics in the intake and exhaust tracts as well.
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
I'm sure my inability to clearly communicate my thoughts is detrimental to this discussion, but thank you for replying. I understand that a lot of you probably have much more important things to do rather than try teach rocket surgery to a Neanderthal. But I do very much appreciate everyone's help.
Martin, about piston speed and its relationship to cam profile... You said different strokes get different cams, regardless of displacement.
If you were to spec a cam for each of your 427's both set up for the same purpose, how would they differ?
You've probably already made cams for both, but I'm not sure if they are both made to operate in the same conditions. So, if both of your 427's were to be used for the same purpose, how would you spec the cams differently between the two?
Martin, about piston speed and its relationship to cam profile... You said different strokes get different cams, regardless of displacement.
If you were to spec a cam for each of your 427's both set up for the same purpose, how would they differ?
You've probably already made cams for both, but I'm not sure if they are both made to operate in the same conditions. So, if both of your 427's were to be used for the same purpose, how would you spec the cams differently between the two?
I'm sure my inability to clearly communicate my thoughts is detrimental to this discussion, but thank you for replying. I understand that a lot of you probably have much more important things to do rather than try teach rocket surgery to a Neanderthal. But I do very much appreciate everyone's help.
Martin, about piston speed and its relationship to cam profile... You said different strokes get different cams, regardless of displacement.
If you were to spec a cam for each of your 427's both set up for the same purpose, how would they differ?
You've probably already made cams for both, but I'm not sure if they are both made to operate in the same conditions. So, if both of your 427's were to be used for the same purpose, how would you spec the cams differently between the two?
Martin, about piston speed and its relationship to cam profile... You said different strokes get different cams, regardless of displacement.
If you were to spec a cam for each of your 427's both set up for the same purpose, how would they differ?
You've probably already made cams for both, but I'm not sure if they are both made to operate in the same conditions. So, if both of your 427's were to be used for the same purpose, how would you spec the cams differently between the two?
It would get a 1-2* earlier IVO event as added piston speed will increase the demand for airflow on the intake stroke. Opening the intake valve earlier helps get more intake lift at the beginning of the intake stroke. Since the port flows more air at higher lifts, and the added stroke is wanting more airflow, opening the intake valve sooner will help satisfy this increase in demand.
The EVC event I may well leave 100% alone. I may change my mind between now and tomorrow though. Haha!
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
So, I chewed on this overnight. Read Martin's answer a couple more times to really let the information set in.
I love that you have the two engines of the same displacement with different architecture. That locks in a really good example of what I needed to help me better imagine how these things would differ.
Anyways, I very much appreciate your taking the time to entertain my curiosity.
Opening the intake valve sooner, to achieve more valve lift earlier in the intake stroke makes so much better sense to me now. Shorter rod ratios associated with longer strokes accelerate the piston near top dead center faster than the piston with a higher rod ratio, so that demand for more air earlier in the intake stroke makes sense.
Even with displacement remaining the same, the longer stroke has to be treated as a larger motor, or one that is always revving higher than the other. You said you would open the intake valve ~2° sooner AND close it ~2° later... That increases the duration by ~4° total with only a .1" difference in stroke length.
I'm actually starting to be able to visualize this and put it all together.
Thanks again for the replies, everyone. I'm going to review and ponder what I have learned, but feel free to continue to add to this discussion. I will be back with more questions, I am sure.
I love that you have the two engines of the same displacement with different architecture. That locks in a really good example of what I needed to help me better imagine how these things would differ.
Anyways, I very much appreciate your taking the time to entertain my curiosity.
Opening the intake valve sooner, to achieve more valve lift earlier in the intake stroke makes so much better sense to me now. Shorter rod ratios associated with longer strokes accelerate the piston near top dead center faster than the piston with a higher rod ratio, so that demand for more air earlier in the intake stroke makes sense.
Even with displacement remaining the same, the longer stroke has to be treated as a larger motor, or one that is always revving higher than the other. You said you would open the intake valve ~2° sooner AND close it ~2° later... That increases the duration by ~4° total with only a .1" difference in stroke length.
I'm actually starting to be able to visualize this and put it all together.
Thanks again for the replies, everyone. I'm going to review and ponder what I have learned, but feel free to continue to add to this discussion. I will be back with more questions, I am sure.
So, I chewed on this overnight. Read Martin's answer a couple more times to really let the information set in.
I love that you have the two engines of the same displacement with different architecture. That locks in a really good example of what I needed to help me better imagine how these things would differ.
Anyways, I very much appreciate your taking the time to entertain my curiosity.
Opening the intake valve sooner, to achieve more valve lift earlier in the intake stroke makes so much better sense to me now. Shorter rod ratios associated with longer strokes accelerate the piston near top dead center faster than the piston with a higher rod ratio, so that demand for more air earlier in the intake stroke makes sense.
Even with displacement remaining the same, the longer stroke has to be treated as a larger motor, or one that is always revving higher than the other. You said you would open the intake valve ~2° sooner AND close it ~2° later... That increases the duration by ~4° total with only a .1" difference in stroke length.
I'm actually starting to be able to visualize this and put it all together.
Thanks again for the replies, everyone. I'm going to review and ponder what I have learned, but feel free to continue to add to this discussion. I will be back with more questions, I am sure.
I love that you have the two engines of the same displacement with different architecture. That locks in a really good example of what I needed to help me better imagine how these things would differ.
Anyways, I very much appreciate your taking the time to entertain my curiosity.
Opening the intake valve sooner, to achieve more valve lift earlier in the intake stroke makes so much better sense to me now. Shorter rod ratios associated with longer strokes accelerate the piston near top dead center faster than the piston with a higher rod ratio, so that demand for more air earlier in the intake stroke makes sense.
Even with displacement remaining the same, the longer stroke has to be treated as a larger motor, or one that is always revving higher than the other. You said you would open the intake valve ~2° sooner AND close it ~2° later... That increases the duration by ~4° total with only a .1" difference in stroke length.
I'm actually starting to be able to visualize this and put it all together.
Thanks again for the replies, everyone. I'm going to review and ponder what I have learned, but feel free to continue to add to this discussion. I will be back with more questions, I am sure.
Thread Starter
TECH Addict
Joined: Jul 2013
Posts: 2,189
Likes: 123
From: Portland, Oregon
Just for reference, in relation to piston speed:
3.3" @ 1k = 545fps, @ 6k = 3270fps
3.6" @ 1k = 610fps, @ 6k = 3660fps
4.0" @ 1k = 667fps, @ 6k = 4000fps
The 3.3" crank has to spin @ 1120rpms to match the piston speed of the 3.6" crank @ 1k. And it has to be revving to 6715rpms to match the 3.6" crank @ 6000rpms.
To match the piston speed of the 4" stroke @ 1k, the 3.3" crank has to be spinning @ 1223rpms. And to match it @ 6k, the 3.3" stroke must be @ 7339rpms.
Food for thought when thinking about using the cam from a longer stroke motor in an engine with a shorter stroke.
3.3" @ 1k = 545fps, @ 6k = 3270fps
3.6" @ 1k = 610fps, @ 6k = 3660fps
4.0" @ 1k = 667fps, @ 6k = 4000fps
The 3.3" crank has to spin @ 1120rpms to match the piston speed of the 3.6" crank @ 1k. And it has to be revving to 6715rpms to match the 3.6" crank @ 6000rpms.
To match the piston speed of the 4" stroke @ 1k, the 3.3" crank has to be spinning @ 1223rpms. And to match it @ 6k, the 3.3" stroke must be @ 7339rpms.
Food for thought when thinking about using the cam from a longer stroke motor in an engine with a shorter stroke.