valve speed vs built in lift

[Schtauffer]

I've asked this question here before, a couple of years ago. Didn't get the kind of answer I was looking for, just was told I was asking a question beyond my level. Ok, let me try again.

On a road-tripable street motor, why would it not be a good idea to have a cam ground with more lift built into it and run a small rocker with less spring pressure? I understand that less lobe lift with a more aggressive rocker and consequently higher spring pressures increases valve speed and picks up hp at the sacrifice of the cycle life of the parts.

My questions are: (A) how much usable power is there to gain by increasing valve speed; and (B) how much more durability is there to gain by increasing lobe lift while reducing rocker ratio and spring pressure to accomplish the same valve lift?

[joe_rocket45]

Re: valve speed vs built in lift

subscribing...

[urwurznitmahre]

Re: valve speed vs built in lift

I've asked this question here before, a couple of years ago. Didn't get the kind of answer I was looking for, just was told I was asking a question beyond my level. Ok, let me try again.

On a road-tripable street motor, why would it not be a good idea to have a cam ground with more lift built into it and run a small rocker with less spring pressure? I understand that less lobe lift with a more aggressive rocker and consequently higher spring pressures increases valve speed and picks up hp at the sacrifice of the cycle life of the parts.

My questions are: (A) how much usable power is there to gain by increasing valve speed; and (B) how much more durability is there to gain by increasing lobe lift while reducing rocker ratio and spring pressure to accomplish the same valve lift?

a few reasons,
1)the lobe ,must pass through the block cam bearing so it only can be so big..
2) lifter ramp speed.. height and width. the higher the lift the faster the lifter has to go in the same degrees, so you'd need stronger springs to have the lifter change dirrections and not jump off the nose of the cam lobe..
the stronger springs would tax the lifter rollers or oil film (depending on type, rolller or flat tap)
and twist the cam core.. that at best only screws with engine timing .at worse .breaks the cam in two

[TC]

Re: valve speed vs built in lift

On a road-tripable street motor, why would it not be a good idea to have a cam ground with more lift built into it and run a small rocker with less spring pressure? I understand that less lobe lift with a more aggressive rocker and consequently higher spring pressures increases valve speed and picks up hp at the sacrifice of the cycle life of the parts.

I think your a little confused here. More lift on the cam is going to need more spring pressure not less. As having less lift on the cam is going to need less spring pressure. Remember the spring pressure is there to keep the lifter riding on the cam at RPM.

Now realize it doesn't matter what the rocker ratio is, one motor can have 1.5 rockers, where the other can have 1.8 rockers. Fact is depending on how the cams for the two applications are ground you could have the same lift/duration specs at the valve. Only difference is the higher rocker ratio motor will need less spring pressure due to the lobe lift not being as big as the lobe lift is on the 1.5 rocker ratio motor. To give you an example the 1.5 motor would need .480 lift at the cam lobe to achieve .720 lift at the valve, whereas the 1.8 rocker motor would only need .400 lift at the cam lobe to achieve the same lift. Now you start getting into crazy rocker ratios like 2.1 and and all of a sudden you only need about .340 lift at the lobe to achieve 720 lift. And by reducing the cam lobe lift you will be able to spin the motor higher rpms and have a more stable valve train.

If I'm right this years Engine Master won with a motor that had crazy rocker ratios(2.1??) and a cam with very small lobes, spun the thing to the moon.

[TheSilverBuick]

Re: valve speed vs built in lift

And like Beagle I am probably talking out my rear, but to extend Beagle's point into TC's. Is that the point at which the fulcrum stresses increase and rocker arm failure issues? Or bad angles between the rocker arm and the valve stem?

[JeffMcKC]

Re: valve speed vs built in lift

The weight on the push rod side is not a huge deal, the spring pressure is to control valve motion, not lifter, if that was a big deal you could run a rev kit.

You would not know the difference because its not all that much power in the same RPM range. I would not go over 1.7 for the road it gets hard on stuff Rocker stuff, you could not kill a good set of springs and if you thought you would just run spring oilers.

There are plenty of more power things than this, you can get a lot of lift in a stock block .800 with out to much effort. but this will all take its toll and you wont feel it in the seat of your pants.

[revolutionary]

Re: valve speed vs built in lift

If you are trying to compare apples then I would have to say look at two version with the same lift and duration AT THE VALVE. Version one lets say has .375 lobe lift and a 1.8 rocker to get .675 gross lift. Version two has .450 lobe lift and a 1.5 rocker.

Version one - not very aggressive lobe so it is easy on the lifter bores but if the length of the arm is not taken into account with a longer ratio it could result in breakage at the trunion from a weak spot where the pushrod cup gets near it. If you are running a generic 240 pounds of seat pressure times a 1.8 ratio that would make 432 pounds on the lifter which might be a little high for street use. To get 400 pounds on the lifter would require 222psi on the seat

Version two - very aggressive lobe that is on the edge of what the typical .750 diameter wheel can handle in a small block core. Low rocker arm ration makes the arm stronger as it evens out the load on the arm from side to side and has less chance of breaking it. Running 240 pounds on the seat gives 360 pounds on the lifter which might not be enough to keep it from bouncing. To get 400 pounds on the lifter would require 266 on the seat.

In either case the seat pressure would need to be adjusted to account for whatever opening/closing rate is on the cam to control valve bounce but you can sort of see the main differences here. It is a compromise when picking these things out and you sort of have to go with experience as to what works. On a small block I wouldn't recommend bigger than a 1.6 without shaft rockers or more than 1.7 without extended length arms. The small block rockers we used for our 327 in the EMC last year were 1.9 ratio but the arms were almost the length of 18 degree stuff to get there safely.

[Schtauffer]

Re: valve speed vs built in lift

If you are trying to compare apples then I would have to say look at two version with the same lift and duration AT THE VALVE. Version one lets say has .375 lobe lift and a 1.8 rocker to get .675 gross lift. Version two has .450 lobe lift and a 1.5 rocker.

Version one - not very aggressive lobe so it is easy on the lifter bores but if the length of the arm is not taken into account with a longer ratio it could result in breakage at the trunion from a weak spot where the pushrod cup gets near it. If you are running a generic 240 pounds of seat pressure times a 1.8 ratio that would make 432 pounds on the lifter which might be a little high for street use. To get 400 pounds on the lifter would require 222psi on the seat

Version two - very aggressive lobe that is on the edge of what the typical .750 diameter wheel can handle in a small block core. Low rocker arm ration makes the arm stronger as it evens out the load on the arm from side to side and has less chance of breaking it. Running 240 pounds on the seat gives 360 pounds on the lifter which might not be enough to keep it from bouncing. To get 400 pounds on the lifter would require 266 on the seat.

In either case the seat pressure would need to be adjusted to account for whatever opening/closing rate is on the cam to control valve bounce but you can sort of see the main differences here. It is a compromise when picking these things out and you sort of have to go with experience as to what works. On a small block I wouldn't recommend bigger than a 1.6 without shaft rockers or more than 1.7 without extended length arms. The small block rockers we used for our 327 in the EMC last year were 1.9 ratio but the arms were almost the length of 18 degree stuff to get there safely.

Thanks Rev! That's the sort of response I'm looking for. The end valve lift being the same, what is the best compromise for a smaller street small block (say 400 cid or less)?

For example, my buddy has a 10.5:1 383 sbc; the engine builder installed 1.7 rockers and the valve lift winds up around .620". He's wiped out three cams; methinks the rocker ratio is part of the problem.

[TC]

Re: valve speed vs built in lift

If you are trying to compare apples then I would have to say look at two version with the same lift and duration AT THE VALVE. Version one lets say has .375 lobe lift and a 1.8 rocker to get .675 gross lift. Version two has .450 lobe lift and a 1.5 rocker.

Version one - not very aggressive lobe so it is easy on the lifter bores but if the length of the arm is not taken into account with a longer ratio it could result in breakage at the trunion from a weak spot where the pushrod cup gets near it. If you are running a generic 240 pounds of seat pressure times a 1.8 ratio that would make 432 pounds on the lifter which might be a little high for street use. To get 400 pounds on the lifter would require 222psi on the seat

Version two - very aggressive lobe that is on the edge of what the typical .750 diameter wheel can handle in a small block core. Low rocker arm ration makes the arm stronger as it evens out the load on the arm from side to side and has less chance of breaking it. Running 240 pounds on the seat gives 360 pounds on the lifter which might not be enough to keep it from bouncing. To get 400 pounds on the lifter would require 266 on the seat.

In either case the seat pressure would need to be adjusted to account for whatever opening/closing rate is on the cam to control valve bounce but you can sort of see the main differences here. It is a compromise when picking these things out and you sort of have to go with experience as to what works. On a small block I wouldn't recommend bigger than a 1.6 without shaft rockers or more than 1.7 without extended length arms. The small block rockers we used for our 327 in the EMC last year were 1.9 ratio but the arms were almost the length of 18 degree stuff to get there safely.

Thanks Rev! That's the sort of response I'm looking for. The end valve lift being the same, what is the best compromise for a smaller street small block (say 400 cid or less)?

For example, my buddy has a 10.5:1 383 sbc; the engine builder installed 1.7 rockers and the valve lift winds up around .620". He's wiped out three cams; methinks the rocker ratio is part of the problem.

If I'm understanding what Rev said, it's not so much the rocker as it is the matching the spring pressures so you have the right amount of pressures at the valve and at the lifter. Basically the springs have two functions, one is to prevent the valves from floating and other is to keep the lifter riding on the cam. Apparently changing rocker ratios can also cause the spring pressures to change from one rocker setup to the other.

So I'm thinking it maybe more of a spring pressure thing than the rockers why your buddy is eating cams. Basically if his spring pressures are setup for 1.5 rockers and he's running 1.7's, he most likely has to much pressure on the lifter and that is why it is wiping the cams out.

[Schtauffer]

Re: valve speed vs built in lift

The cam keeps getting wiped out for a lot more reasons than just the rocker ratio vs the spring pressure.

" ;) " :P :

[JeffMcKC]

Re: valve speed vs built in lift

If its a Flat tappet cam shaft, use a real light break in spring then switch but you will need to use a break in oil also


Thought I would add we have had very good luck on Flat tappet camshafts with Claimer cams out of Stock cars a good Lobe for holding up and making power too.

Also make sure depending on the rocker used, its not hitting the end of the slot for the studs

[urwurznitmahre]

Re: valve speed vs built in lift

If its a Flat tappet cam shaft, use a real light break in spring then switch but you will need to use a break in oil also


Thought I would add we have had very good luck on Flat tappet camshafts with Claimer cams out of Stock cars a good Lobe for holding up and making power too.

Also make sure depending on the rocker used, its not hitting the end of the slot for the studs

ok hyjack
on head valvespring change..
I've done it ..
but with high spring pressure. whats a good tool to use.. as the one for normal work isn't made to compress a hy lift roller spring..
I bent mine taking the roller springs that came with the track one heads off..

[JeffMcKC]

Re: valve speed vs built in lift

http://www.lsmproducts.com/product2.htm only way to fly

I dont know what you use a Hydraulic roller does not have much pressure

[urwurznitmahre]

Re: valve speed vs built in lift

http://www.lsmproducts.com/product2.htm only way to fly

I dont know what you use a Hydraulic roller does not have much pressure

solid roller,,

[JeffMcKC]

Re: valve speed vs built in lift

I mis understood your post, the LSM one Kicks Butt its what the Big Dogs Use, and Doc with the SeeRed car has also mine is less but when it gives up it is what I will replace it with not that bad on money.