batman's ati kit and ssm heads/cam?

[Patrick Javert]

I have heard that reducing backpressure on the exhaust side actually drops boost psi but make much more power. will this also apply to the ssm head/cam pkg. if it were done? also, I have a theory about matching cams to the sc. I think that a cam w/ less overlap, more lobe separation, slighly more duration, and more lift is the best setup for our engines w/ a sc on it. I say less overlap so that more of the intake charge stays in the clyinder instead of going out the exhaust valve at lower speeds. I know overlap helps power up higher, but isn't too smog friendly here in california. Is my theory/assumptions correct? Thanks for the imput guys.

[Patrick Javert]

come on guys. one of you out there has to know something about these things.

[TheBeast]

refer to this post

http://www.camarov6.com/cgi-bin/ulti...10&t=000510&p=

that is Magnus's car if he got the 10psi of boost....remember he already has the heads/cam

[DominusEtDeusNatus]

<blockquote>quote:</font><hr>Originally posted by Patrick Javert:
I have heard that reducing backpressure on the exhaust side actually drops boost psi but make much more power. will this also apply to the ssm head/cam pkg. if it were done? also, I have a theory about matching cams to the sc. I think that a cam w/ less overlap, more lobe separation, slighly more duration, and more lift is the best setup for our engines w/ a sc on it. I say less overlap so that more of the intake charge stays in the clyinder instead of going out the exhaust valve at lower speeds. I know overlap helps power up higher, but isn't too smog friendly here in california. Is my theory/assumptions correct? Thanks for the imput guys.<hr></blockquote>


About your question.

People who talk about boost when discussing power don't know too much. It's about how much air flow it takes to create manifold pressure. If you have more space to fill, that turbo/SC has to move more air. This is why a larger engine or higher rpms will make it tougher on the turbo/SC to produce the same PSI level. How many Supras do you see running 20psi+? How many LS1s do you see even doing 15psi?

You put some blower on your stock engine car and get 10psi of boost then you put heads and cam and headers on it and your boost drops to 6psi. Guess what though. You'll still make more power.


About your theory. SC and cams didn't just arrive last night, so SC cams are old news. Plus you're totaly wrong on your cam theory. You want less overlap and more duration? How are you going to do this? Have you any idea what overlap is? You increase duration you increase overlap. More lift? I think everyone knows that more lift is good up to a point.

You want more exhaust duration/lift then intake duration/lift for SC/Turbo/Nitrous applications. It's that simple.

[John_D.]

<blockquote>quote:</font><hr>Originally posted by Patrick Javert:
I think that a cam w/ less overlap, more lobe separation, slighly more duration, and more lift is the best setup for our engines w/ a sc on it. <hr></blockquote>

I think you have the right idea about more lobe separation, to achieve less overlap, if you are also going for more duration. Overlap increases significantly with small increases in duration or lift though, so it will be difficult to get much added duration or lift without running into more overlap.

Another thing is, more lobe separation means the intake valve opens earlier, or the exhaust valve closes later, or both. Open the intake too soon, and you get reversion from the residual exhaust. Close the exhaust too late, and you can draw exhaust back into the cylinder, or even overlap the next intake.

Pettitt's book, "How to Install and Use Nitrous Oxide Injection Systems for Maximum Horsepower", describes the intake part of this, in the Basic Engine Dynamics starting on page 62.

Davis has a book, "Supercharging, Turbocharging, and Nitrous Oxide", which describes this more fully in the chapter on Technical Basics, the section on Soft Power. Soft Power refers to the tendency for pressurized intakes to offer a number of benefits, including a softer landing for the piston at TDC, slightly pushing down on the piston after TDC, and purging exhuast from the cylinder more fully (without having to increase duration or lift). A quote from page 23, "forced induction all but eliminates the need for an excessively long duration, high-lift cam profiles because the compressor can force more air/fuel into the cylinder in a shorter length of time".

Good thinking, and good question.

Hope this helps.


p.s. Amazon had a special on the pair of books, when I went looking for the Pettitt book.

[Patrick Javert]

cool. thanks. my whole thing is i want the car to have a broad powerband. most people say that more lobe separation leads to broader powerbands. I don't think I'll get more duration unless i have to to get more lift. also, roller cams don't need as much of a lash ramp so it usually can have more lift/quicker ramp up than other flat-tappet cams. I am not looking to go much higher than 6500 rpms( my car doesn't have vtec or vvtl-i or whatever the imports have for better power out of small engines). I also want my car to idle around 600-1000 rpm so i'm not eating gas in town too much. so since centrifugal sc's and turbos make more power up high, i don't need as much overlap since more boost will be made up high, so I would just do well with a cam rated for low-mid range power and let the sc do the rest, right? also, i think i wouldn't pass smog w/ too much overlap since more unburned fuel goes out when the rpm's are lower. Am i right? i live in california.

[John_D.]

<blockquote>quote:</font><hr>Originally posted by Patrick Javert:
I am not looking to go much higher than 6500 rpms. I also want my car to idle around 600-1000 rpm so i'm not eating gas in town too much...

so I would just do well with a cam rated for low-mid range power and let the sc do the rest, right?

also, i think i wouldn't pass smog w/ too much overlap since more unburned fuel goes out when the rpm's are lower. Am i right?<hr></blockquote>

Right about the overlap. More overlap brings on that lumpy idle due to exhaust reversion at low air velocity. And will tend to put more unburned fuel out the rear, which won't be good for emissions. Also the computer will be trying to compensate, based on a slightly false a/f ratio, and will be leaning the mixture that the cylinder actually sees.

Keeping the rpms at or below 6500 will keep you from having to beef up the internals so much too. I'm thinking about bringing my shift points down to 5800-5900 just for a little extra durability after getting the s/c.

The cam grinders will generally custom grind, if they don't already have a basic cam package, specifically for s/c. At least for the v8's, I haven't checked on the v6's, since I plan to keep the stock cam in mine.