Heres a couple of links;
http://www.compcams.com/Technical/FAQ/LSAproperties.asp
http://www.popularhotrodding.com/en...nginemasters/articles/hardcore/0606em_camshaft_specs/lobe_seperation_angle.html
http://www.chevyhiperformance.com/t...s_valvetrain/0905chp_camshaft_lobe_separation_angle_performance_test/index.html
I dont think that rod ratio has anything to do with LSA.
Guy

 

Do you guys think that there is an ideal lobe seppearation angle for a motor and if so, would it be based on the rod ratio?

You may enjoy reading some of the wisdom of Ron Iskenderian, son of the famous "Camfather", Ed Iskenderian. Read through these Tech Tips and enjoy....
http://www.iskycams.com/techtips.php#2005
I would also urge you to read all the Smokey Yunick that you can find.
http://www.amazon.com/s/ref=nb_sb_s..._i_0_7?url=search-alias=stripbooks&field-keywords=smokey+yunick&sprefix=smokey+

 

Rod length can have an effect but not at the level you'll be working with. Just plot the data, most of it can be approximated fairly accurately.

 

Well I ask this after running a chain calculation of cams thru this dyno sim and using the stock 283 I have here as the mule. Trying to learn about this has been a wonderful adventure for me, I really appreciate having a place to ask this stuff to people that think about these things too. If you ask somebody at a party if they think rod ratio has an effect on ideal lobe sepparation angle of a cam they just kinda look at you funny. I tried it couple times last weekend. They start talking to you about this and that, yadah yadda and then there's a break in conversation and they look at you like the smiles and the nods are no longer adequate responses, and so you say "So, do you think a longer rod ratio kinda wants a cam with a tighter lobe separation angle than a short rod combo?" Then they realize that I have been trying to figure out how to make that into an equation the whole time I was smiling and nodding at them. I hope I do get closer to figuring it out someday.

Here is the point, I am trying to see the trends in the chain calcs of the stock 283 on the sim. It seems to do better with tighter lobe separation angle but less duration. So, short duration but tight cam angle. Kinda backwards as to what comes on a modern engine, that being a wide lsa and a long duration. I just wondered if you guys thought it was just the 1.9:1 rod ratio of the 283 that caused that. If rod ratio plays a role in cam selection, then it could be thought that there might also be a theoretical ideal lsa for your inherent geometry, whatever that may be.

My theory is that long rod motors do better with tight lobe angle and short rod motors do better with wider lsa. I read that wider lsa gives better gas mileage, and that led me to thinking about all this to try and understand it. Wider lsa and long duration seems like it would be almost the same thing as narrow lsa and short duration, but it is interesting how the two do not test the same on the same engine on the dyno sim. Thanks

 

are you after power or mileage? Two very different goals.


Also, rod ratio will not have much affect considering your intake and exhaust are not tuned, nor are you running good heads, carb, etc. All those things have a FAR more significant impact than rod ratio.

 

are you after power or mileage? Two very different goals.


Also, rod ratio will not have much affect considering your intake and exhaust are not tuned, nor are you running good heads, carb, etc. All those things have a FAR more significant impact than rod ratio.

Thanks for the reply,

power or mileage? I believe they can go hand in hand don't you?
If you consider that the manifold and heads installed stock have characteristics and flow numbers that can be used to aproximate as an in tune condition, sure it isn't cross ram but it still a column of air moving and should behave in a predictable manner that would allow me to attempt to optimize what I have to work with here. The smaller runners of the stock manifold and small valves can work in my favor to create velocity at lower rpm and get a small amount of tune pressure despite my small displacement. I do want better heads and manifold someday though, don't get me wrong here. Just trying to use what I have for now, and if it costs 200 bones (for the custom cam)to find out if a hair brained scheme can really work or not then I consider it a bargain.

so power and mileage go hand in hand? I say yes because a tuned engine is an efficient engine and and an efficient engine gets good gas mileage.

Unless you are drag racing.

I am not trying to change rod ratio of my engine here at this time, this post is really about the lsa and how it affects an engine. I was just speculating that maybe the rod ratio played a key role in the cam design style whether it be tight lobe but short duration or long duration but wide lsa, and how the rod ratio should then go into consideration when selecting a cam. So 1.45 rod ratio people with all that great intake velocity and vacuum can get more power from a wide lobe cam with long duration and 283 or 327 guys over on the other end need a tighter lobe angle and real short duration to help make strong low end power. Lemme know if I am on the right track here

 

Well I ask this after running a chain calculation of cams thru this dyno sim and using the stock 283 I have here as the mule. Trying to learn about this has been a wonderful adventure for me, I really appreciate having a place to ask this stuff to people that think about these things too. If you ask somebody at a party if they think of a cam they just kinda look at you funny. I tried it couple times last weekend. They start talking to you about this and that, yadah yadda and then there's a break in conversation and they look at you like the smiles and the nods are no longer adequate responses, and so you say "So, do you think a longer rod ratio kinda wants a cam with a tighter lobe separation angle than a short rod combo?" Then they realize that I have been trying to figure out how to make that into an equation the whole time I was smiling and nodding at them. I hope I do get closer to figuring it out someday.

Here is the point, I am trying to see the trends in the chain calcs of the stock 283 on the sim. It seems to do better with tighter lobe separation angle but less duration. So, short duration but tight cam angle. Kinda backwards as to what comes on a modern engine, that being a wide lsa and a long duration. I just wondered if you guys thought it was just the 1.9:1 rod ratio of the 283 that caused that. If rod ratio plays a role in cam selection, then it could be thought that there might also be a theoretical ideal lsa for your inherent geometry, whatever that may be.

My theory is that long rod motors do better with tight lobe angle and short rod motors do better with wider lsa. I read that wider lsa gives better gas mileage, and that led me to thinking about all this to try and understand it. Wider lsa and long duration seems like it would be almost the same thing as narrow lsa and short duration, but it is interesting how the two do not test the same on the same engine on the dyno sim. Thanks

If you ask people at a party if they think rod ratio has an effect on ideal lobe sepparation angle even I wold look at you funny!

I think stroke have a far more effekt on what LCA an engine will have then rod ratio.I havent eader consider the rod ratios effekt on LCA before, but maby it has som effekt...

 

If you ask people at a party if they think rod ratio has an effect on ideal lobe separation angle even I wold look at you funny!

I think stroke can have far more effect on what LCA an engine will have than rod ratio. I haven't ever considered the rod ratios effect on LCA before, but maybe it has some effect...

a motor with more stroke would most likely also have a shorter rod ratio tho, rod ratio affects how fast the piston leaves tdc so a shorter rod makes a stronger intake pulse signal and so can benefit from a cam with more duration and is less diluted by wider lsa right? This is just what I am noticing so far

 

Thanks for the reply,

power or mileage? I believe they can go hand in hand don't you?

Not in a NA sbc

If you consider that the manifold and heads installed stock have characteristics and flow numbers that can be used to aproximate as an in tune condition, sure it isn't cross ram but it still a column of air moving and should behave in a predictable manner that would allow me to attempt to optimize what I have to work with here. The smaller runners of the stock manifold and small valves can work in my favor to create velocity at lower rpm and get a small amount of tune pressure despite my small displacement. I do want better heads and manifold someday though, don't get me wrong here. Just trying to use what I have for now, and if it costs 200 bones (for the custom cam)to find out if a hair brained scheme can really work or not then I consider it a bargain.

I don't know why you would think those components are tuned for anything, they're not, they were cast to serve a basic purpose- deliver air and fuel to the cylinder, it does that, but not in a precise manner.

so power and mileage go hand in hand? I say yes because a tuned engine is an efficient engine and and an efficient engine gets good gas mileage.

It can if you have a lot of time and money to go into development- a good example would be the new LS series, they have decent power and decent fuel consumption- it took 40 years of development for GM to do it.

Unless you are drag racing.

I've seen a stock air cooled VW bug drag race- it ran a 23.1 in the 1/4... Power is power regardless of where its made

I am not trying to change rod ratio of my engine here at this time, this post is really about the lsa and how it affects an engine. I was just speculating that maybe the rod ratio played a key role in the cam design style whether it be tight lobe but short duration or long duration but wide lsa, and how the rod ratio should then go into consideration when selecting a cam. So 1.45 rod ratio people with all that great intake velocity and vacuum can get more power from a wide lobe cam with long duration and 283 or 327 guys over on the other end need a tighter lobe angle and real short duration to help make strong low end power. Lemme know if I am on the right track here

For this engine a relatively mild LSA like 112º will be a good start- that has more to do with cylinder heads, exhaust, intake, carb, stroke, and other things than rod ratio- you really REALLY overestimate the impact of rod length.

What this really boils down to is wanting a custom cam for a stock 283- not really much need for it IMO, but if you want one just call up your favorite cam company and ask for one- the good ones will probably tell you to just buy a shelf stock cam as it is cheaper and will serve its purpose quite well.

 

a motor with more stroke would most likely also have a shorter rod ratio tho, rod ratio affects how fast the piston leaves tdc so a shorter rod makes a stronger intake pulse signal and so can benefit from a cam with more duration and is less diluted by wider lsa right?

no

This is just what I am noticing so far

What do you think affects piston travel more .500" stroke or .500" rod length?

 

What do you think affects piston travel more .500" stroke or .500" rod length?

I would rather have the half inch more stroke!

 

What do you think affects piston travel more .500" stroke or .500" rod length?

I am not sure I understand your question there. The position of the piston in the bore of any motor at a given crank angle depends on the rod ratio. This boils down to me wanting a custom cam for a stock motor is true, but I think it will help and will only cost an extra hundred bucks to find out. If it does no better than a stock cam then I will have learned alot for cheap.


The thing is what cam tho? The position of the cam in relationship to the piston is affected by the lobe angle, so I want to optimize the peak lift to be where the piston is creating the most air velocity is all right? The position the piston is in while moving in the bore when it is accelerating at the highest rate should be where the cam is wide open then right? On a long rod motor the piston accelerates from bdc faster than a short rod motor so it needs different exhaust valve timing because it has improved exhaust velocity, on a short rod motor it is when the piston is leaving tdc it accelerates away from tdc faster so a cam that starts opening the intake valve earlier than what was ideal on the long r/r would be better. So a cam that works really good on a 350 might not be best for a 283. It seems like most of these off the shelf cams are made with the 350 383 and 400 in mind given there popularity.

 

You ar rigt that a cam that work well in a 350" our larger engine dont neceserly are the best cam for a 283".I dont think it has much to do whit rod ratio tho.Maby a littel, but the cubic and stroke have alot more to do whit it.
If you build a 350" with 5.7" rod and one with the exakt same specs but with 6" rod i dont think they will ned mutch diffrent cams...

 

I am not sure I understand your question there. The position of the piston in the bore of any motor at a given crank angle depends on the rod ratio.
No it doesn't, plot the piston position with 3 different strokes, and 3 differet rod lengths and see what it looks like, 9 excel charts that I think will revolutionize your thinking on rod ratios

This boils down to me wanting a custom cam for a stock motor is true, but I think it will help and will only cost an extra hundred bucks to find out. If it does no better than a stock cam then I will have learned alot for cheap.


The thing is what cam tho? The position of the cam in relationship to the piston is affected by the lobe angle, so I want to optimize the peak lift to be where the piston is creating the most air velocity is all right? The position the piston is in while moving in the bore when it is accelerating at the highest rate should be where the cam is wide open then right? On a long rod motor the piston accelerates from bdc faster than a short rod motor so it needs different exhaust valve timing because it has improved exhaust velocity, on a short rod motor it is when the piston is leaving tdc it accelerates away from tdc faster so a cam that starts opening the intake valve earlier than what was ideal on the long r/r would be better. So a cam that works really good on a 350 might not be best for a 283. It seems like most of these off the shelf cams are made with the 350 383 and 400 in mind given there popularity.

see bold text.

 

You ar rigt that a cam that work well in a 350" our larger engine dont neceserly are the best cam for a 283".I dont think it has much to do whit rod ratio tho.Maby a littel, but the cubic and stroke have alot more to do whit it.
If you build a 350" with 5.7" rod and one with the exakt same specs but with 6" rod i dont think they will ned mutch diffrent cams...

A 283 with stock rods has much higher rod ratio than a 350 even with 6 inch rods though. Not just a little bit but alot different. If a 350 could be made that had the same rod ratio as the 283 it would need 6.7 inch rods. I argue then that this much difference has to be taken into consideration in cam selection. It is a known fact that narrow lsa angles help create strong intake pulses. That helps the long rod motor to make more velocity at low rpm because the long rod motor doesn't make intake velocity as easily as the short rod.

 

A 283 with stock rods has much higher rod ratio than a 350 even with 6 inch rods though. Not just a little bit but alot different. If a 350 could be made that had the same rod ratio as the 283 it would need 6.7 inch rods. I argue then that this much difference has to be taken into consideration in cam selection. It is a known fact that narrow lsa angles help create strong intake pulses. That helps the long rod motor to make more velocity at low rpm because the long rod motor doesn't make intake velocity as easily as the short rod.

I think you over think rod ratio, and I understand why peopel look at you funny att partys As I sad, allot of things in a motor afect what cam the engine wants way more then the rod ratio!If you have a stock 283", why dont just use the stock cam?I wold beleve GM have tested more camprofiles than you ever will...

 

It is a known fact that narrow lsa angles help create strong intake pulses.

That's not a fact.

There are a lot of partial truths that you assume to be the full story... You really need to research more, it'll save you time and money in the long run.

 

see bold text

The position of the piston in the bore of any motor at a given crank angle depends on the rod ratio.
No it doesn't, plot the piston position with 3 different strokes, and 3 differet rod lengths and see what it looks like, 9 excel charts that I think will revolutionize your thinking on rod ratios

.

yes the rod ratio is what determines the position of the piston. A short rod motor at 45 degrees past tdc and a long rod motor at 45 atdc will have the piston in a different place in the bore. The short rod motor will have the piston down in the bore a little bit more at the same crank angle on the way down. Might only be a little bit but I think it is enough to affect the tune and warrant buying a custom cam. And get the 4/7 swap for the heck of it. I wonder if the 4/7 swap makes it scavenge better on dual exhaust setups where there is no crossover equalizer pipe installed.

 

The stroke has more impact than the rod.

I've tried to lead the horse to water but it appears it wants to die of thirst.

 

The stroke has more impact than the rod.

I've tried to lead the horse to water but it appears it wants to die of thirst.

The stroke has more impact than the rod? What does that even mean? Impact on what?

This post is about how rod ratio has a role in cam selection. And the minimum length rod possible would do better with an entirely different camshaft, different lsa different lift different duration and everything than a motor with rediculously long rods. You can't argue with that really.

All I am trying to do is get you guys to help me figure out if there is an ideal lsa for a given rod ratio. I am thinking about it, could use all the help I can get. The maximum lsa is about 130 degrees, the minimun rod ratio is about 1.4:1. They could go well together all up the line then like that then. So an infinetly long rod motor would be better with a 80 degree lsa maybe.

 

You are oversimplifying many things in your "blinders on" approach of only looking at rod ratio as the sole criteria for the selection of any engine part used around them. The ideal lobe seperation for any goal, be it mileage, power, or whatever, is far more impacted by stroke length and cubic inch volume, port efficiency and air velocity, than it is rod ratio. Rod ratio consideration as impacting the selection of cam LSA would be about 99th on the list of things to place any value on in the selection process, especially with an un=optimized intake and exhaust tract....

Your tombstone is going to read "Pathologically Obsessed with Rod Ratio Theory"

 

The stroke has a greater effect on piston speed.

With a stock exhaust manifold you won't want a narrow lsa. A shelf cam would work well on your STOCK engine.

 

Words of wisdom from Ron Iskenderian.....

"Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype."

"In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!"

 

Hey I think you guys are missing the my point a little bit but that is ok. I do not want to change the rod length in my motor at all. I want to change the camshaft. The rod ratio I have is what determines the cam lsa that will work the best for me and my hardware. I think that a tighter lobe angle works better than a wider one in my motor because of the rod ratio effect on the breathing and the position of the piston relative to the crank. Same cam might be a total dog in a 350. If I had 4.8 inch rods in my motor, I would have the same rod ratio as a 350 chevy. I would also need a different camshaft strategy to optimize breathing. It would change the requirements. The off the shelf chevy 350 cam might just work fine in my 283 if I had 4.8 inch rods. That does not mean I need to have custom 4.8 inch rods made. That is just an example to try to help illustrate the concept. I simply want to try to determine if there is an ideal lsa, and if it depends on the rod ratio. The cam I am going to order is quite a bit different than the off the shelf rv cam.

 

The stroke has a greater effect on piston speed.

With a stock exhaust manifold you won't want a narrow lsa. A shelf cam would work well on your STOCK engine.

I am using 36 inch long headers 1.5 inches in diameter. My stroke is not going to change. I like the 283, I am just trying to use it to learn a little bit about cams.

The dyno sim tells alot about the factors involved, I make the best cam for the 283 using my heads and intake and headers, then change the stroke to 3.48 inches keeping the same heads cam exhaust etc. With 5.7 rods, the torque curve starts and ends a good 1500 rpm below the rpm of the peak torque of motor with 3 inch stroke. If I change the rod length to 6.8 inches so as to have the same rod ratio as the 283 then the rpm at which the power comes on is the same on both motors.