As you can tell from my current setup in my sig, i'm thinking about getting a 6lb blower but not really keen on changing the cam or pulling the stock heads off. I just want to get some feedback from you guys as to it being dangerous or not?

 

Well, I dont think its going to be "Dangerous" but some things you might want look at is your cam. IMO your cam has to much overlap and think other will agree. Your choise in pulleys is good though. 6psi of boost is pretty safe for the most part but then again, dont be suprised if you blow somthing. Just one of those things you have to be prepared for when you have forced induction.

 

Yeah, I realised an overlap of that amount is not good for any forced induction but what i'm wondering is will I be making lot's of power and be running risk, or will I be running less power and still running high risk?

See my point? It's obviously going to be risky but is it all worth it? What other things could I do that would allow me to keep my present cam? I've seen/heard people running blowers on a cc306, anyone shed some light on this?

 

Anyone else? Please?

 

more over lap = less cylinder pressure= less power= safer

 

There have been some long threads recently about cam selection. Maybe you should try a search? Anyway, adding the blower with your cam will work fine, it just won't give you as much of a hp increase as a more optimized cam. You may surprised at the relatively small performance gain from 6psi of boost with stock heads, As far as risk goes, it's there and has little to do with the cam per se.

What "mean LT1" suggested true to a point. More overlap does decrease cylinder pressure, which is why it makes less hp (on a blower application). Higher dynamic CR (DCR) is the culprit. DCR is largely a function of the intake valve closing point. Yes, it's true that a larger air charge resulting from less overlap will also contribute, but the effect is not as great as changes in the intake closing point unless we are talking about extreme examples. But all else being equal, he is correct that more overlap will have somewhat of a protective effect, but it is also what limits the power gain.

Again, 6psi on stock heads may not be worth it. The dyno gains are more impressive than the real world performance increase because they are restricted to the upper part of the rev range.

Rich Krause

 

Thanx for the info man. That does seem to make alot of sence but my expectations are not that high. I'd want to be somewhere in the 400 to 450 hp range.

 

Umm... No, more overlap means less cylinder pressure at LOW RPM, at HIGHER RPM the cylinder pressure will increase thus why you loose power with a 112lsa as apposed to a 114lsa at low rpm, but gain power top end, becuase your cylinder pressure increases with a 112 lsa while a 114lsa falls off & acctually has less MAXIUMUM EFFECTIVE compression.

WIth a lower lsa it builds higher cylinder pressure, narrows your powerband & increases maximum torque, it also decreases piston to valve clearance, thus all these things lead to DETONATION & they are all the things you just don't want with a blower.

THat is the wrong cam for your application, you're better off running 11:1 compression with a 117lsa cam.

 

The guy that posted before you has been here since 69, hes word seems to cry more weight, hehehe...

But I completly understand.

 

69 means 99, this isn't a word, I think we had tests about this stuff in school & I can explain to you how an internal combustion engine works all day long, its not straight & simple like you think, just when you think you understand, there are so many variables, I only post what I know.

 

There's some confusion about here about LSA and cylinder pressure. To talk sensibly, you have to be very specific. There are two closely related concepts that first need to be understood. The first is what I will call "cranking compression" (CC). This s function of the bore, stroker, and combustion chamber volume. The same things that determine static CR. But it takes into account cam timing as well. Actual compression of the air charge doesn't start until the intake valve closes, as the piston is moving up toward TDC. Once the intake valve closes, cylinder pressure starts to rise regardless of any combustion (i.e. engine being cranked at a low rpm, not running). The CC is much less than the static CR. My ccurrent motor has a static CR of ~8.5:1 and a CCR of ~6.5:1. The difference is even greater for a cam that has a later intake closing point.

Notice that when discussing CCR, overlap does not play into it at all. Overlap has an effect only on a running motor. When the motor is running, the mass of air the cylinder takes in will change as the VE changes. Well setup NA motors will exceed 100%VE at certain rpm ranges. When VE exceeds 100%, the dynamic CR (DCR - what the cylinders would see under the same conditions prior to combustion) increases. It is dynamic CR that ultimately effects the tendendcy toward detonation. A high DCR causes increased cylinder pressure prior to combustion and thus possible detonation.

How does LSA play in? Well, LSA is a main determinate of overlap. NA motors depend on overlap along with ram effects in the intake and exhaust to get VE over 100%. The more overlap (the narrower the LSA) the more pronounced this effect will be, but the narrower the rpm range over which it occurs. So, in an NA motor, a cam with a lower and LSA and more overlap will generally have a narrow window where the VE will exceed that of a similar combo with a wider LSA. This the narrower LSA motor will have more tendency to detonate (at the sweet spot of the rev range), and a narrower but higher power band.

It's very different with a forced induction setup. The thing to keep in mind is that when under boost, VE will far exceed 100%. Increasing overlap here will decrease VE because the intake charge will get blown out the exhaust. This also happens NA when you are out of the power band, one reason a narrow LSA (more overlap) is peakier. So, under boost, the DCR will be much higher than in an NA motor, even with a low static CR. That's a big reason why detonation is such an issue with blowers. The ram effects of a narrow LSA are trivial compared to the effect of a manifold pressure that is always positive.

Conclusions: if you hold the intake closing point the same and increase overlap by narrowing the LSA on a SC setup you will lower DCR and decrease the tendency to detonate and make less hp. On an NA car, increasing overlap will increase the DCR over a small band of the rev range and lower it elsewhere. The peak power will be higher but the power band will be narrower.

Of course, all of these statements apply only over a relatively small range. IOW, if you had an LSA of say 100 degrees the thing might not even run, etc.

Rich Krause