Valvespring pressure and lift height

[Adaware]

How do I decide how big springtension the valvesprings should have?

In this case it is a 13mm intake valve so It's not so small compared to modelengine standards.

How about the lift?
The cylinder is 30mm in diameter and about the same stroke. I was thinking 2,5mm on both exhaust and intake (no four valve tech here), but maybe that's way of?

Equal lift on intake and exhaust valve would be preferred construction and practical wise

 

[Maryak]

How do I decide how big springtension the valvesprings should have?

In this case it is a 13mm intake valve so It's not so small compared to modelengine standards.

You need to work out the acceleration of the valve to get the force exerted by the valve when it is lifting. Have determined the force involved you then need to work out a spring which will overcome this force 130% of F.

How about the lift?
The cylinder is 30mm in diameter and about the same stroke. I was thinking 2,5mm on both exhaust and intake (no four valve tech here), but maybe that's way of?

Equal lift on intake and exhaust valve would be preferred construction and practical wise

Lift is usually around 30% of diameter - in this case around 4 mm any less and the valve is not fully open any more and there is no increase in flow through the valve.

There's a workshop practice booklet which covers this, try My hobby Store for a copy. I think it was written by Tubal Cain.

Hope this helps

 

[petertha]

Re valve lift, I cant help with design formulas, but I made the attached table of some existing engines for comparison & of similar bore. Hopefully it will help. If anything, it shows a range. There are other posts on this subject in the forum if you search.

 

[petertha]

Re valve springs, same comment. I dont have a good grasp of how to determine required force. But I noted what springs were used in similar model engine plans & kind of back-calculated the forces involved using this website.

http://www.planetspring.com/pages/compression-spring-calculator-coil-calculator.php?id=compression

For example, I think the OS-FS-61 (RC methanol glow engine) I was able to dismantle had spring specs of: wire dia = 0.028", OD = 0.25", free length = 0.6", number coils = 8.

So with a few inputs you can figure out spring force at initial compressed length, force at desired valve displacement & also when the spring bottoms out. I started building a spreadsheet like example attached. So... not really 'engineered sizing', but gave me a feel for spring forces by copying others chosen springs ;D

 

[hammer2100]

peterta,
This is why I love this site. I have been looking for a calculator just like your post. THANKS

 

[Adaware]

Very helpful, guys

My understanding is now alot better of this problem. Guess i would have to crack open the book full of formulas so i can find out how i can attack this.

 

[123RWO]

There is a mathematically correct method for calculating valve spring rate. You have to measure all valve train component masses and determine the radius of gyration of rocker arms, etc. It's not particullarly difficult, but it is really not necessary. For 12 mm valves, if you make your valve springs to give 3# force at full lift, you wll be safe at least to about 7000 rpm. I went thru the calcs. for my 2X upscale of Eric Whittle's Peewitt. If you want to, you can find the calculation procedure in "Aircraft Engine Design", by Joseph Liston. The book is old, out of print, and expensive, but answers just about any question an engine builder could ask, model or full size. I no longer have a copy, but there are plenty on the used book sites.

RWO