MYTHBUSTERS: Modified head makes no difference with SC???

[Kevinedwards05]

I started this thread because of the information that I have been given by some members on this site. If I find something hard to believe, I will research it to find out if that person is sure of what they are talking about....I'm guilty of it myself...yes, I think I have good info on something and type it in here only to be blasted by another member saying that I am wrong....Oh well, I'm human and subject to be wrong.

Recently I was told that installing larger valves really would not make much difference on the supercharged LSJ because the supercharger is supplying all the usable boost. Hmm...well, something to that effect anyway. Fact is, our supercharger is pushing about 12 psi of boost. If I port, polish, and install larger valves, I will lose boost due to the larger CFM of the modified head. Install a good set of cams, and it drops even more. Now, although boost is less, Flow is better, resulting in greater horsepower. Forget the cams for a moment and put my modified head engine pushing 8 psi of boost against a factory stock motor pushing 12 psi, and I will walk all over him.....Boost is relative to flow. Your still getting blown, but more efficiently since you have worked the head.

Okay....someone go ahead and take your jabs....I'll back up the research with my information sources.

[ion20redline04]

word... good job.. i like this guy someone get him a cookie... that statement puts more ideas into my head

[clevelander2002]

that makes total sense, which is exactly why boost doesnt always equal horsepower, right?

[ion20redline04]

i wonder what it would take besides sleeving the block to push 30psi to a redline... if you could get it dun omg the power

[pclear9061]

Originally Posted by ion20redline04

word... good job.. i like this guy someone get him a cookie... that statement puts more ideas into my head

[Sp00ner]

Actually, when it came to the larger valves, I/WE told you that there were more variables involved.

Adding larger valves reaches a point of diminishing returns. At some point, there is not enough room around the valves for more air to flow in. Prior to that point, you have the standard flow dynamics to deal with. Top end vs. Bottom end. I spent about 3 months researching valves, lifters, and cams before I rebuilt my small block. The larger and larger you want to push the valves, the more clearance required around the valves to allow the air to flow around the lip of the valve and into the combustion chamber. With 4 valves in an 86mm round combustion chamber, you don't have much room to play with. Without knowing what the inside looks like, or flow testing the valves, you just have to guess if something will work or not. Again, concerning the valves, we do have a pumped up valve train, including the sodium filled valve stems. So chances are, to increase the size of the valves, you would need to remove the head, have the combustion chambers ported out, get the right valves, and hope that the stock ones don't look nearly identical. There are really only 6 'types' of valves out there. They differ primarily in size, material, and finish. Just speaking of the valves.

[clevelander2002]

damn physics

[Sp00ner]

I'm also assuming that you're using the 8psi level as an example. Because what you're saying is that you're gonna increase flow through the engine by 33% from a port and polish and larger valves...

[Kevinedwards05]

Originally Posted by Sp00ner

I'm also assuming that you're using the 8psi level as an example. Because what you're saying is that you're gonna increase flow through the engine by 33% from a port and polish and larger valves...

Yeah Spooner, you're assuming correctly, and you weren't the one that gave me the errant information. Basically this information was given to show that yes, there is a difference. You are definitely going to reach a point of diminishing returns once you get to a certain size, but it's up to each person how far they want to go with it. This motor is capable of putting out about 1200 hp in it's drag racing form, so keeping that in mind, what's to keep us from reaching 400 hp on our daily driver by having some good performance parts installed? There is a lot that can be done to the head on this vehicle. Porting and polishing alone will give you much better flow, so you will see a drop in boost just with that mod. However, you will gain HP. How much is still to be determined. I am going to try and find a used car to drive around for a while, and go ahead and have some head work done on mine. The valves that I will use are still to be determined, but if the larger sodium filled ones are not available, I will be using titanium valves. I am still researching what would be the best port shape for this head.

[rip_vw32]

What you're missing is that with the larger combustion chamber it's going to read a lower psi in the comp. Our SCs are not limited by rpms, but by that stupid little bypass valve which is comp controlled, correct? So with the greater space and lower psi readings you should reach the 12 psi right? I mean, the bypass kicks in at 12psi of boost (at sea level), so with the lower reading (map vs bar vs maf) we should still see around 12 psi with greater power increases. I guess what I am trying to say is under a straight SC application on a carborated engine, your're blowing max psi all the time, but in our cars the comp limits it right?