Valve Job Questions

[BeauJ]

I've got the GT-40P heads on my explorer, with a little bit of porting done myself. I'd like some new heads, but don't really have the money at this time. I've heard of some guys getting a good port/polish and valve job and getting 50 HP. But those numbers are from a N/A motor. Since I'm supercharged, would it be better or worse? I know the headers are gonna hold me back until I get some custom ones and my boost might drop a little.

I found this auction to give you an idea of the valves: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7936321849&category=33621

Any idea now much a machine shop/mechanic would charge to do the valve job and how long it would take?

 

[BeauJ]

Wow, no responses. Not even any BS ones that tell me nothing.

Maybe if it gets moved over to the Need for Speed forum, someone might answer (hint hint mods)

 

[Brain]

I haven't done any extensive porting and polishing (yet), but I'm an engineer w/ a firm grip of fluid dynamics (my specialty along with controls). Smooth walls give a thinner boundary layer, meaning that more area is allowing high velocity flow. Whatever gains you get by making the pathways smooth and less restrictive in a NA intake gets compounded by the effect of compressed (SC or TC) flow. The air is more dense (the molecules are closer together) so for a given volume of air moving past, there are more molecules to collide with obstructions.

PV=nRT....ideal gas equation (which is close enough most of the time) says that if you raise the pressure and keep the temperature the same (intercooler), that the number of molecules in the same volume also rises.

I don't know how much boost you run with the SC, but let's assume 5psi...14.7psi is one standard atmosphere of pressure at sea level, so going to 19.7psi gives you (19.7/14.7) 134% the number of molecules of a NA engine (at no vacuum). So 34% MORE molecules to smash into whatever is in the way....

If the port/polish alone get it 50HP on a NA engine, then your SC engine might get around 65-70 of them for the effort.

 

[Hokie]

I haven't done any extensive porting and polishing (yet), but I'm an engineer w/ a firm grip of fluid dynamics (my specialty along with controls). Smooth walls give a thinner boundary layer, meaning that more area is allowing high velocity flow. Whatever gains you get by making the pathways smooth and less restrictive in a NA intake gets compounded by the effect of compressed (SC or TC) flow. The air is more dense (the molecules are closer together) so for a given volume of air moving past, there are more molecules to collide with obstructions.

PV=nRT....ideal gas equation (which is close enough most of the time) says that if you raise the pressure and keep the temperature the same (intercooler), that the number of molecules in the same volume also rises.

I don't know how much boost you run with the SC, but let's assume 5psi...14.7psi is one standard atmosphere of pressure at sea level, so going to 19.7psi gives you (19.7/14.7) 134% the number of molecules of a NA engine (at no vacuum). So 34% MORE molecules to smash into whatever is in the way....

If the port/polish alone get it 50HP on a NA engine, then your SC engine might get around 65-70 of them for the effort.

Wow, I understood all of that... I'm such a dork

One thing to watch out for is removing too much material or what looks like an obvious hinderance of flow might actually be helping it. The heads are designed to inpart a certain type of flow or swirl of the fuel/air mix as it enters the compression chamber, so there are guides (peaks/valleys) to create this. These swirls or currents help with flame formation/movement during combustion and with exhaust scavaging. From reading Super Chevy in the past, it seems most head jobs are just matching the heads to the intake manifold as far as the runners go, to make sure that everything flows perfectly

Unfortunately things like this are a dying trade... machine shops are harder and harder to find as are shade tree mechanics and the understanding required to takle such jobs

 

[Brain]

Yes, alot of CFD (computational fluid dynamics - I use Fluent) modeling is done to get the "right" tumble/swirl characteristics for the "best" (read - most complete burn and lowest emissions) burn cycle, but that is only for stock configurations. One you start to boost the intake (either by SC or TC'ing it) then it is a whole 'nother ball game - one that isn't normally modeled (modelling cost a bunch of money).

My suggestion for porting/polishing heads for ANY application (NA, SC, or TC) is to not remove very much material (because it is hard to tell how thick it is to begin with), but get it very smooth. Another thing is that more than half of the improvements over stock can be attributed to removing sharp impedences in the intake tract and the exhaust tract. This means getting the intake manifold/head interface and head/exhaust interface to be free of sharp edges that aren't lined up. The manifold/gasket/head should fit together and make a smooth flowing passage without sharp impedences (sometimes the gasket has to be trimmed along with the head or manifold to make this happen). Just imagine putting a lot of work into getting the head to flow smoothly (having a small boundary layer) and then having a gasket stick up on the inside 20 times the boundary layer depth....it happens, but it doesn't have to.