If this is not advanced tech, please move. I searched but couldn't find any info about this.

I'm looking into the GM 845 cam.(Part# 12370845) It's specs are as follows:
Duration 214/224, lift with 1.6 RR's=0.521"/0.543" on a 112 LSA

Anyways, I'm changing only my valvetrain over to 1.6 RR's and LT4 springs soon. No cam yet, but I'm looking at the 845 when I put a cam in. These LT4 springs should be fine for the intake duration and lift of the 845 cam, but not so good for the exhaust lift. (since the LT4 springs are usually used with the Hotcam 218/228 0.525"/0.525" 112 LSA)

Is there anything wrong with keeping the LT4 springs on the intake valves and just replacing the exhaust springs when I put the new cam in?

Seems to me that you would want to optimize the springs for the cam lobe that it is working with.

Thanks,
Dan

 

I have thought about mixing springs on the intake and exhaust and don't see any reason why it wouldn't work. Never tried it though.

Rich

 

Its no different then shimming intake and exhaust springs different, but 1: you'd be hard pressed to fine a set of 8 and 2: I'd replace them all, beef springs are just added valvetrain stability.

 

Thanks guys. I guess I don't want to "overbuild" my valvetrain for the cam. The stiffer the spring, the more wear and tear on the components. If I can get away with the LT4 springs on the intake, and the proper exhaust spring, I'd be happy with that. Even if I had to buy a set of 16 springs, and only use 8 for the exhaust. To be honest, I don't see why this isn't done more often.

Any thoughts?

Dan

 

Exhaust and Intake valve springs have different masses. One reason why sometimes you see Ti intake valves and Hi Temp Steel Alloy exhaust valves on a motor. You can also match the springs to the lobe profiles on the cam, if the intake and exhaust have the same spring then you could theoretically put a more aggressive lobe on the exhaust side of the cam.

It's not a problem but a set of 977 comp springs or beehives that can handle the lift would work much better.

Bret

 

I like the idea of the beehives. They don't require any head machining right?

I'm only going to use springs if they don't require head machining as the heads are staying on the car.

So you think the LT4's would be "o.k". for the intake duration and lift of the GM 845?

Thanks Bret.

Dan

 

May not matter much at all, but on out racing Karts (Briggs and Stratton 5hp) we actually do use a different rated spring on Intake and Exhaust. Engine live past 10k RPM and make plenty of power. May not apply to a V8 though

-Shannon

 

I think it applies more often than folks suspect. I recently saw an example on a V8. I should have taken a picture.

 

Yeah you should have. I want to see that.

Bret

 

In theory you would think that the lesser spring would go on the exhaust because of MASS.

However, if you are worried about valve float you would put a bigger spring on the exhaust because there is more chance of hitting the exhaust valve then there is the intake valve if you valve float.

This is why you would keep the same spring on the exhaust/intake. Kind of a compromise, like if the spring is good enough to keep the intake valve from floating, then you should put the same spring on the exhaust because chances are if you over rev, the exhaust won't float as quickly as the intake because of it's mass being less, cuz if it does, you're prolly not gonna like it. Or say you set everything up and go out for a boot and find that at 6500rpm you floated the valves and you felt it, well chances are you prolly didn't float the exhaust if the intake just started to float and you felt the power drop.

Also, another thing to think about, although I don't want to think exactly which cylinder's valve corresponds to the other, but if one cylinder is opening the intake valve, isn't the corresponding cyclinder just closing the exhaust valve?

If that is the case, you would loose power or have an unbalanced valve train by having non matching springs, because the reason why you don't really loose power by going to beefier springs is because while one spring is being opened by the cam, the other cylinder's exhaust valve spring is closing and thus giving back the force, so kinda of like one spring is helping the other open as one is opening the other one is closing.

Now you put a lesser valve spring on the exhaust, while the intake is opening say on cylinder #1, the exhaust valve is closing on cylinder #6 (or whatever) but that exhaust spring doesnt have the same mechanical energy to compensate for the other spring, so now your engine has to help to open that other valve. However, this can work the oppossite, the intake is closing while the exhaust is opening and thus offering back that loss on the last event, however this would prolly result in an unbalanced valve train and I don't know the gains/losses will be inflicted and I don't want to find out.

There are other factors involved like the closing spring has lost some of it's energy through heat, so it doesn't quite make up for giving back the same mechanical energy as it took to open it because as metal bends (spring), you loose energy through heat. But this becomes even more important now if you have a lesser spring plus the spring is exhausted on it's cycle.

Of coarse you also loose a bit of power with bigger springs because of rolling resistance on the lifter rollers/rocker rollers, that coupled with heat but bla bla bla.

I can go on

 

Interesting thoughts Bunker, and thanks for the reply.

I'm not sure that the spring pressure back to the cam, when the valve is closing, is very significant to the overall power output of the engine. It's there, but does it matter, I'm not sure. I know what you are saying, but the amount of force that is available to the cam from the power stroke of a cylinder is HUGE compared to what a spring might be able to put back into the cam. I'm not sure that a slightly stiffer spring would make a significant difference in how much "power" it gives back to the cam's rotation when you look at the overall power output of the engine.

When you mention an unbalanced valvetrain, wouldn't that happen naturally with equal rate springs on intake and exhaust lobe anyways? Since the lobe profile is very different for exhaust and intake, the spring forces exerted on the valvetrain and cam will be very different. The intake spring will put much less force back into the cam than the exhaust spring since the lift and duration of the intake is typically less than the exhaust lobe.

My #1 reason for asking about this is valvetrain wear/abuse. I don't want to put springs in that are fine for intake valves, but overkill on the exhaust, or vice versa.

Maybe it's just a mute point and there is no reason to try different springs. I really do appreciate your thoughts on this, and agree with what you are saying. I'm just not sure if it's significant. Hmm.... fun to think about.

Dan

 

Yup it was night time, I've though of this before and remember thinking about different exhaust/intake lobes making a difference as well, and I am sure in some ways this effects some power output in regards to drag as well.

Have you ever tried to put rockers on one side of the engine and not the other and taking the spark plugs out of the other side so you could turn the engine.

The result when I tried it was I could not spin the engine over, its like the pistons hit the valves, it just stops and if you put a lot of force it will keep trying to go the other way. So the effects are there regardless as one spring does help the other one, it might not be perfect, but I would think keeping every spring the same, if you think about it, going a roller timing chain helps horepower and electric water pump from just a small drag, so the force to open a spring is pretty important I would say as well.

It would also be a piece of mind in regards to also the other thing I was saying where it is more important to keep the exhaust from floating then it is the intake as the exhaust is open when the piston is going up and the intake is opening so if you float, you hit the exhaust.


As for wear and tear, I'm running 150lb on the seat, almost 400 open, then added a hydra rev kit which adds 80lb open at the lifter, so I've got almost 480 on the roller of the lifter and 400 of it on the plunger of the lifter.

 

Ok so if spring pressures being different on the intake and exhaust cause problems with balancing out forces on the camshaft shouldn't the intake and exhaust durations of the cam be EQUAL to balance the forces in time as well? (Since duration is basicaly the time measurement of valve events) Screw correct valve events......

How about the acceleration rates and 3rd 4th and 5th orders of velocity on the cam? Shouldn't they be equal to make sure the cam doesn't twist?

Seriously, getting the right spring to control the valve for the lobe acting on it, and the masses it acts on is much more important. I would say the same about valve events too. If you want to cut down on cam twist and deflection then you start moving to a larger journal cam and add rocker arm ratio to gain lift rather than lift at the lobe. Kind of like a LS1

The point is different springs aren't causing widely different loads on the cam and loosing you huge amounts of power. Look at the duration example of that, I would think that the timing of such events is much more important than the unequal loading and yet it doesn't seem to effect anything.

Bret

 

Thanks Bret, that follows what I was thinking as well, especially regarding the differing cam durations with the same valve springs. The forces aren't going to balance anyways, so may as well go with springs that would control the valve properly. If the intake and exhaust lobes are quite different, as is the case in the GM845, or Crane 227cams (both of which I'm considering), may as well use springs that are optimal for intake, and another that is optimal for the exhaust.

Dan

 

The biggest thing is that the valve masses are so much different. I've seen 28g difference in valve mass from the intake to exhaust valve. Considering that's a 30% heavier Intake valve that's a ton more mass that you have to control or don't have to control in the exhaust valves case. If you can still control the valve with less spring pressure you gain benefits from reduced valvetrain deflection.

Bret