Hi,

On a turbocharged 350 with aluminum heads. With a small amount of boost somewhere around 8 lbs and somewhere in the 8ish compression range would running 1.5 roller rockers on the intake and running 1.6 on the exhaust help power much by letting the exhaust escape easier?

 

why not just run 1.6 on both...

 

That would depend mostly on cylinder head flow (intake VS exhaust and the flow stall point of the ports) and camshaft design.

 

You really can't go too big on rocker arms for performance. Especially on a blown engine. It becomes a spring endurance issue at that point.

 

what is a good size rocker arm for a blown engine?
does it depend on cudic inches?

thanks:hail: :hail: :hail:

 

First you have to know what the lift is on your cam. Then you need to find what the coil bind height is on your springs. To be safe you want your spring height at max valve lift to be .120" greater than coil bind height. Just remember that springs don't like being mashed really far really fast. The bigger the rocker arm the faster and farther the spring is going to compress. This is the endurance issue. Too high of a stress will limit your number of spring cycles. Most engines don't live long after a spring breaks. For pure performance the more you can open the valve the better the engine will breathe at almost all RPM. Air velocity is important in wet manifolds to keep the fuel vaporized. Too slow and the fuel falls out. Too fast and the fuel can't make a corner. Dry manifolds need to keep proper air velocity to fill the cylinders and get good vaporization after injection. Port cross-section area and engine speed as well as cam duration determine air speed. For a Turbo charged engine a lot of overlap will actually hurt performance, but some is still required to properly evacuate the cylinder. Hope that helps.

 

okay. i under stand what you're saying. now for example. let's say we are talking about an L98 tpi, with 9lbs boost off an ATI. what size rockers or roller rockers should you use? can you change valve springs without removing the heads? what combo would give the most power with this setup?

 

I have been reading some bad things about roller lifters. How long should they last?

I have heard of them lasting 2000 miles and that is being good.

Any input?

thanks:hail: :hail: :hail:

 

I've heard of people using 1.6 rockers on stock heads. I personally don't think this is a good idea because stock springs weren't meant to handle that reliably. You can change springs with the heads on. You use an air compressor to keep the valve from falling down into the engine. You must be very careful though. However, the springs you get will probably need a bigger pocket, the valve guide boss cut down, and the 1.6 rockers might push the push rods into the head and scuff them.
I'd stick with the stock valve train until you can get the heads done properly.

 

ATO -
what do you mean by done properly? what work needs to be done? i've come into a little cash, and want to maximize my blower. what should be done to the head then?

 

You need to take them off and send them to a machine shop. You'll be down for about a week probably. Get the heads ported and polished. Take your intake and gaskets there too and get them port matched. Get minimal work done on the combustion chamber as I believe it disturbes the combustion process. Just enough to balance the combustion area. Relieve the area around the valve guides inside the intake and exhaust port. CHP or Carcraft got their Vortecs pocket ported(don't know exactly what that is). You can cut the spring pockets and valve guide bosses yourself. There is a tool that works in a drill that makes it real easy. Replace the press in rocker studs with screw in studs. Get good springs and retainers and balance all of them with shims. You'll need a special tool for that. You might want to consider pushrod guide plates and roller 1.6 rockers. Also make sure the mating surfaces are all flat. You could get the heads angle milled, but that would raise compression. If you have enough money left consider a larger cam than stock and roller lifters. Build a good relationship with your machinest so you can communicate what your goals are and he will know what's best for you. Gettng the heads flow matched takes a lot of time, but that also helps. I'm sure someone will say buy new heads and don't waste your time on stock heads, but that's also up to you. Even aftermarket heads can always use some work.

 

thanks for the advice! my heads and intake are already port n' polished. i just had the whole motor rebuilt 2k ago. wish i would've thought of this then. nonetheless, the motor was built to stock. i'm not going to change the camshaft. but it looks like my car will be out of service for a little while here. oh, i know the machinist pretty well. he's my school machine shop teacher, and my boss. (i just get to do crap work though, pushing a broom, oiling machines) thanks ATO!!