Cam and valve lift

[chrisa82]

Ok. I have a question for you guys. I build street and mild chevy v8s. Im getting ready to build a drag car and have a question about cam and valve lift. I have herd that you take your cam lift and multiply it by your rocker ratio and that gives you your max valve lift. So when im selecting valve springs to go in my heads what do i need to look at? Do i look at max lift so i will know the springs will take the cam lift? what throws me off is the calculations because there is alot of cams that would be to large for the springs. I was just hoping someone could help explain this all to me a little bit so i dont get spring i ether dont need or will take the size cam i want. thanks you guys.

[jethro2996]

I will answer the best I can, however I am only a garage builder myself.

One of the things you might be looking at is hydraulic flat tappet cams versus roller cams. Roller cams carry considerably more lift so they need their own line of valve springs. Make sure when you are looking at cams and valve springs that you are comparing like for like.

[chrisa82]

so when I pick out a high lift cam do I pick out springs that handle the same lift of the cam? or do I add in the rocker arm ratio also?

[jethro2996]

There are two types of lift to consider: Gross lift and actual lift. Actual lift is the actual size of the cam lobe. This can be physically measured as the "lumpy" part of the cam or by measuring how far it pushes a pushrod up.
Gross lift is actual lift multiplied by rocker arm ratio. When you look at a cam in a catalog, they are usually giving you a "gross" lift number. Cam manufacturers usually multiply actual lift times what a stock engine used for a rocker arm. If you go to a cam web site you will see what rocker arm ratio they used to determine gross lift on the cam card. The lunati website is quick and easy to read in this respect (plus I really like their cams!). If you are going to use a different rocker arm ratio than what is on the cam card you will want to divide the gross lift by the ratio on the cam card, then multiply times the rockers that you are using. As long as this number is below what your springs can handle, you will be alright.
When assembling an engine, however, don't always trust your numbers. Take a breaker bar and turn your engine over by hand to ensure that your springs can handle it. Also, check your valve to piston clearance, especially when you get into the bigger cams. It is very possible to have them hit each other.
I hope this helps!

[jethro2996]

One other thing. You mentioned building a drag racing car. The cam has to be chosen with many different considerations. Most novice builders like to get the biggest cam they can shove in their car, thinking this will make it run faster. The cam MUST be matched to your flow of your heads, torque converter, street or no street use, etc etc. Be careful on which cam you pick-more is not always better!!!!

[chrisa82]

Thanks for the info. Yes that helps out alot. I know not to pick to big of a cam for my car, but base it off my heads and stuff. I was just wondering because I have seen some really big cams and just wondered how it all worked. The heads im looking at is the edelbrock victor 15 degree or brodix 18 degree heads. im leaning more towards the edelbrocks.