Pushrod Deflection

[HG]

I have a question which is aplication specific but may also make for good discussion. I tried the search function but didn't find what I was looking for.

I a comp eliminator style engine with a relatively short stroke and no clearance issues in the bottom end. Does using a block with a raised cam bore still give an advantage with being able to use a shorter pushrod.

My theory is that using a shorter pushrod will reduce the amount of deflection that can take place. Is this a reasonable statement to make?

Now for the application specific question. If the above statement is true is it worth the extra cost in the block and the jesel belt drive for this type of block for this type of application?

[383Malibu]

...If the above statement is true is it worth the extra cost in the block and the jesel belt drive for this type of block for this type of application?

Everything is relative. The extra wall thickness of the aftermarket blocks is worth something by itself and the shorter pushrods will have less deflection, but the cost of the block and the new raised cam Jesel setup is a significant price to pay. We built a sbc with an aftermarket short deck (8.75") block and a .441" raised cam (Aurora style). The pushrods are 7.890" x 7/16" x .165" wall and still had .040"+ deflection (400#/1200# springs). But, since we didn't build another motor with longer pushrods, I can't say how much we gained.

[panic]

[Dan Barton]

YES!

[cmw]

Shorter push rods are stronger, But if you had the strongest push rod, it does not mean that you cured the deflection problem. Wall thickness, heat treat and push rod ends all effect the performance of the push rods. Have you ever seen push rods that rub the block, heads or both and there was clearance around the push rods. The push rod as I call it is doing the "Hula" and when this occurs the push rod gets like a jack hammer that twist and turns and causes tons of problems. I have been building special push rods that address resident frequency problems in push rods with vary good results. Just remember bigger push rods with more wall thickness does not mean it will cure your push rod problems
I believe push rods are one of the biggest problems that can cause failures and horsepower loss.

[455-4+1]

So in a class like mine where there is a lift restriction (flat tappet, max 0.500" measured at the valve) even though the engines have a limited rpm range during the race (5800-6400) there are obviuos gains to be had by decreasing pushrod deflection, against a class with no lift restriction

The lift is measured at a nearly static condition yet the engine is operating at over 6000rpm. (still vey low compared to most race engiens but high enough) The duarion of our races is around 60 seconds also so any small cumulative gains that may not be seen in a 7 second race may show up here

Does anyone know how much potential lift could be lost via an inadequate pushrod in this type of scenario ?. A weak pushrod and an adequate one would probably measure very close while turning over by hand to measure lift, how do they fare at 6000rpm

Is this high enough to show up and real limitations ???

[1989TransAm]

I would think you would not only loose lift but effective duration as well at the valve. I have always thought the idea is to transmit the cam lobe profile to the valve. Anything along the way such as the pushrod, rocker studs/shaft and rocker arm needs to be as stiff as posible.

[MadBill]

I don't know the specifics of either HG's or 455's rules, but there are applications where pushrod flex is very valuable, e.g. rules-limited cam lift, where 'pole-vaulting' the rocker 0.060" or more above static max lift can be worth 40 HP or more. Even Cup motors rely (or used to) on this to some extent...

[panic]

[OldSStroker]

I don't know the specifics of either HG's or 455's rules, but there are applications where pushrod flex is very valuable, e.g. rules-limited cam lift, where 'pole-vaulting' the rocker 0.060" or more above static max lift can be worth 40 HP or more. Even Cup motors rely (or used to) on this to some extent...

Yes, you can use basically uncontrolled loft by using a pole-vault style pushrod, or you could design the cam profile to loft the valve in a controlled manner using a stiff pushrod. I'd rather have some control of what's happening, but you have to do lit right, as Panic said.

455-4+1, (should we call you 452?) do you use a (lift rule) cam with dwell when the valve is .500 up? If so, this may cause lots of strange behavior with weak pushrods. I'd opt for the stiffest pushrods I could fit in the engine, and lift rule lobes which needed stiffness to get the valve up fast and hold it there. If you don't use dwell, why not?

Do you have a rocker arm ratio rule? If not, more rocker and less lobe is probably the way to go.

[455-4+1]

452 is fine (username was from another board, kept it the same so I didnt have to rember too many things, stood for a 455 pontiac with a Nash 4+1 box in my street car at the time)

I currently dont run a lift rule cam, Had one ground over here in NZ at the last minute as we had some issues and couldnt run the lobes I wanted.

Ended up short on duration and having to run more lash than what I wanted to come in under the rules. (waste of any decent ramps) I had decided to go with 1.65 rockers and a smaller lobe, got everything done and rockers are obviuosly greater than advertised (my fault, didnt check)

I would like some help on a new cam and will start a new thread soon seaking some advice.

I just noticed that my old pushrods came up with quite different net lift values between using checking springs and my installed springs. (even with crappy flat tappet springs) I have some new pushrods which are alot "better" than what I was using. Just thought I may have been leaving a bit on the table with the old ones

[Alan Roehrich]

I think Performance Research makes long (or tall) lifters, I know they make them for flat tappet applications, I'd be they make them for rollers as well. You can use a longer lifter to get you to a shorter pushrod without changing blocks in order to try your theory out.

[HG]

Wow theres some interesting stuff in there!!

Mad bill wrote:

e.g. rules-limited cam lift, where 'pole-vaulting' the rocker 0.060" or more above static max lift can be worth 40 HP or more

Thats a scary thought what sort of lifters were used in these applications and how did the camshafts take these events? My application is not limited on camshaft so I can run a roller cam with as much lift as I want, shifting between 9000 and 9200.

Alan Roehrich wrote:

I think Performance Research makes long (or tall) lifters, I know they make them for flat tappet applications, I'd be they make them for rollers as well. You can use a longer lifter to get you to a shorter pushrod without changing blocks in order to try your theory out.

Do you know how much extra these types of lifters weigh, as this could be a downfall in a competition/super stock type application. If you think the extra weight is minimal it might be worth a go. Good sugestion. Do you know how long they are?

CMW wrote:

Shorter push rods are stronger, But if you had the strongest push rod, it does not mean that you cured the deflection problem.

I understand that pushrod deflection is unavoidable particularly when using radical cam profiles with high spring pressures. I just had the theory that running a shorter pushrod may provide some measurable results by reducing flex in this part of the valvetrain.

The push rod as I call it is doing the "Hula" and when this occurs the push rod gets like a jack hammer that twist and turns and causes tons of problems. I have been building special push rods that address resident frequency problems in push rods with vary good results.

How have you adressed these problems? If you don't mind me asking. I have just identified the pushrod as a major problem as you also stated in your post, and threw it out there to see what other people thought.

Dan Barton wrote:

YES!

Dan is this a yes my statement is correct or a yes it is worth the extra cost?

Thanks everyone for the replies and as I thought it is making for some very interesting conversation.

[HG]

CMW

i just read through my post and realised that some of my replies to you read a bit harsh. It's not meant to Sorry

Should hit that preview button before I submit.

[455-4+1]

HG, great topic

I posted here including my application to add another "range" to the problem. Our engine is no where near your operating range but we do have the issue of limited lift dictated by class rules. With such low lift limitations every thou counts so to speak

This made the pushrod issue very important in our case.

Just thought it might look like I was hijacking the thread a little
Not my intention, just trying to get some opinions on the same topic but for different reasons

Its an area I never really gave much thought to until confronted with it

Any suggestions or info greatly appreciated

Thanks