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Double Valve Springs. An Inconvenience ?


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#1 PACINO

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Posted 22 July 2020 - 06:30 PM

I have an original cylinder head but with slightly larger valves (35.7 / 30.9) and double valve springs. I am thinking that these springs may not be bringing me any benefit, and perhaps a loss of power and premature wear of the camshaft. What is your opinion ??

Cheers

#2 nicklouse

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Posted 22 July 2020 - 06:36 PM

depends on what your camshaft is.



#3 Moke Spider

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Posted 22 July 2020 - 07:06 PM

With Valve Springs, I only ever fit - in terms of total poundage - whats needed and appropriate.

 

As you are already thinking, higher poundage only increases wear rate.



#4 PACINO

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Posted 22 July 2020 - 08:03 PM

Thank you guys for the answers. My camshaft and the rockers are the standard.

Moke, with "increases wear rate", do you mean the camshaft lobes ??

Regards

#5 Moke Spider

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Posted 22 July 2020 - 08:19 PM

Thank you guys for the answers. My camshaft and the rockers are the standard.

Moke, with "increases wear rate", do you mean the camshaft lobes ??

Regards

 

Sorry, I should have explained.

 

All parts in the Valve Train, Cam Lobes & Followers, Sprockets & Chain, Rocker Shaft, Rocker Tips.



#6 PACINO

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Posted 22 July 2020 - 08:47 PM


Thanks a lot MokeSp. I want to remove the double springs. Now the question; If I remove the inner spring and only mount the outer spring on the cylinder head, it would be fine or I need to buy specific single coil springs ?

#7 nicklouse

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Posted 22 July 2020 - 09:04 PM

you will need the correct single springs.



#8 Moke Spider

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Posted 22 July 2020 - 10:03 PM

Thanks a lot MokeSp. I want to remove the double springs. Now the question; If I remove the inner spring and only mount the outer spring on the cylinder head, it would be fine or I need to buy specific single coil springs ?

 

It depends on what the out springs are that you have now. You'd really want to test the poundage of them to be sure as there's seldom any indicators as to what they are.



#9 Curley

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Posted 22 July 2020 - 10:49 PM

The way I understood it is, if the rocker opens the valve then it's the spring that closes the valve. Increasing the spring pressure - adding a second spring - increases the amount of pressure needed to open the valve. Either because the valve is heavier or you want to increase the velocity the valve closes.

 

Too much spring pressure and the cam, followers, pushrod, rocker shaft, rocker and value stem all work harder than they need to be, receiving greater forces as they need to work harder to overcome the springs. Hence the advise to use the least amount of spring pressure needed to do the job. Too little spring pressure and the values can bounce off the head when closing, damaging the seat and making noise; sometimes called valve slap (not to be confused with piston slap) or valve float.

 

There is also the potential for spring crush on high lift cams and/or higher ratio rockers - but you shouldn't have to worry about this on a stock setup. An increase in valve lift means the gap from the back of head (spring seat) to the back of the spring cap is shortened at full lift (open) meaning the spring needs to take up less space under compression.

 

As MikeSpoke says, it's almost impossible to tell what springs you have just by just looking at them. Ideally you'd want to take em off and measure the lengths and pressure. Then again I have seen someone blow compressed air into the cylinder via the spark plug hole and change out the springs, collets, and caps with the engines still in the car  :ohno: never tried it myself!

 

MED made a video about setting up race heads, buts the principles are the same for a standard engine:


Edited by Curley, 22 July 2020 - 11:22 PM.


#10 PACINO

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Posted 22 July 2020 - 11:16 PM

Thank you guys! Hello Curley, fantastic answer. Good idea about compressed air. I already did it a few years ago without lifting the cylinder head with the help of a friend. What we did was put a tool through the hole in the spark plug to prevent the valve from falling on the piston.

My double valve spring set is the standar.

https://www.minispar...|Back to search


It says: 160lbs on the nose at 400" for standard Cooper S.

How many lbs are the correct to valve springs mounted on a 1275 engine ?

Regards, Luis

Edited by PACINO, 22 July 2020 - 11:29 PM.


#11 Mini Manannán

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Posted 23 July 2020 - 07:13 AM

I'm interested here too. I inherited double valve springs with my engine and they've always been refitted. I've wondered what the poundage required for a 266° cam would be but never had a definitive answer.

#12 GraemeC

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Posted 23 July 2020 - 07:37 AM

OzOAP's conical springs are a great upgrade and suit a variety of cams due to their design.

Drop him a PM - I'm sure he'll tell you more about the technical reasons why they're superior to the 'normal' double springs.



#13 Ethel

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Posted 23 July 2020 - 09:10 AM

The traditional way to keep valves from falling into the cylinder is to push rope through the spark plug hole to fill the gap between the piston & valves.

 

The cam opens the valve with kinetic energy, the spring has to have enough force to accelerate the valve gear so it keeps contact with the cam, so the speed of rotation & the steepness of the lobe have an effect.

 

Springs have a natural frequency that they vibrate at when free, the stiffer the spring the higher it is. If that frequency harmonises with the frequency of the valves opening, the forces combine and the amplitude grows - like pushing a kid on a swing. That's valve bounce.

 

Using two springs, with different frequencies, allows one to damp the other. Done correctly, it allows a lower total spring rate at higher rpm.

 

It's very complex, but as punters we usually only have a handful of options to choose from.



#14 Moke Spider

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Posted 23 July 2020 - 09:56 AM

The traditional way to keep valves from falling into the cylinder is to push rope through the spark plug hole to fill the gap between the piston & valves.

 

The cam opens the valve with kinetic energy, the spring has to have enough force to accelerate the valve gear so it keeps contact with the cam, so the speed of rotation & the steepness of the lobe have an effect.

 

Springs have a natural frequency that they vibrate at when free, the stiffer the spring the higher it is. If that frequency harmonises with the frequency of the valves opening, the forces combine and the amplitude grows - like pushing a kid on a swing. That's valve bounce.

 

Using two springs, with different frequencies, allows one to damp the other. Done correctly, it allows a lower total spring rate at higher rpm.

 

It's very complex, but as punters we usually only have a handful of options to choose from.

 

Yes, there certainly is merit in the lower poundage double spring set up, but, as you've also mentioned here, it's difficult for us mere mortals to work out.

 

In this short video, you can see how valve Springs behave and also gain an appreciation as to why it's necessary to have a gap between coils of 40 to 50 thousands at full lift, without which, the spring would soon break. Also observe how much the Valve Stem flexes

 

 



#15 PACINO

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Posted 23 July 2020 - 11:38 AM

Cheers Ethel, good explanation. I'm going to Kep the rope system through the spark plug hole.

With my configuration: Pistons, camshaft and standard rocker arms, but with 35.7 / 30.9 valves.
My fear is that the camshaft lobes are suffering accelerated wear as the engine stretches in revolutions less than before, and the oil shortly after changing it comes out quite black.

The question I ask you is; do you think the double valve spring kit (standard) is too much for a Mini 1275 (in this case 1293cc) engine ??

Regards, Luis




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