rjw

iTrader: (3)

The problem is that most do more than just a head swap, so it is sometimes hard to determine what added or retracted form the net gain/loss.

In my case, I am rebuilding the entire engine, pistons, rods, cam , heads.

So I won't be able to tell how much the heads are worth, power wise.

I can compare to other setups that are VERY similar, but using either stock or ported heads.

However, I am doing some stuff that many don't do.

It is always nice to hear from the few that do heads only or cam only, etc.

The bottom line is to know the capabilities of what you are NOT changing.

Many of us do agree that upping the VE in a boosted application is worth a shot, especially if heat reduction and/or efficiency are high on your list.

NicD

iTrader: (7)

Well here are some results from my combo and you guys can figure out for yourself if ported heads and a cam make a difference with FI since most of the people talking here know what stock heads/cam FI cars make.

My combo is just a stock displacement forged LS6 running about 8.5:1 compression, AFR 225s, EDC cam, LS6 intake, stock ported TB and stock MAF, T-76 turbo.

575 rwhp at 7 psi with 17 peak degrees timing 91 oct
665 rwhp at 10.5 psi with 17 peak degrees timing 91 oct

Looks pretty straight forward to me unless some dope puts too small of a turbo/supercharger on their combo and then maxes it out trying to turn it up after they increase the VE of the engine. That would indeed be a waste of money for heads, etc but that would be your own fault. As far as being worth the money to run lower boost levels and make the same power, well that's a personal decision and probably depends on how much you want to make on pump gas, etc.

TeeKay

I'm with you, PSJ. At that level of boost, it's hard to just step it up. All kinds of things can happen!

Here's the other side of the coin: NicD turned his boost up from 7 to 10.5 and got some really nice gains!

This is where common sense should step in for all of us. If you're running mild to mid boost, 7-14 psi, then you probably have room to turn it up and make decent increases. If you are running high(er) boost, like PSJ, then turning it up becomes less of an option and VE needs to be addressed for any serious gains. That's the point of my 2 step drill a few posts back. In any case, upping the VE will always yield improvement, but for a price!

Anyway, HTH, all

white2001s10

iTrader: (4)

Sounds like we're not so much disagreeing on the theory, but the issue of when the compressor becomes inefficient.

With my thinking I say to make more power, you simply up the boost.
If your compressor is already to its max before becoming inefficient, then adding heads and intake isn't going to help you anyway.
The way I'm looking at it, if you can't move more CFM through, then it's time for a bigger compressor, or take another route. My point was that upgrading the heads and intake by themselves wasn't effective.

I think you're saying if your max boost WAS 10psi, then you add heads & intake that drops boost to 7psi, then you can simply up it back to 10psi.
I'm saying no you can't because the compressor will not move the CFM to get you to 10psi. Remember you maxed the CFM at 10psi, but the CFM didn't change when the boost dropped to 7psi. The compressor is still maxed.

I'm looking at this as the compressor having a physical limit on the CFM it can move before the speed makes it inefficient... not that it has a physical limit on the boost psi it can create before becoming inefficient.

In a case of a large compressor being on a small engine, I see it as being able to hold a high boost pressure while still being efficient at moving the CFM neccessary to do this. Yes the higher psi means a higher temp, but as I said earlier I believe this is fairly proportional (boost vs temp) until the compressor speed reaches a certain point where temps jump dramatically.

Is this incorrect?

longrange4u

iTrader: (2)

I agree with you too John, I would prefer to max the CFM capability of the turbo without actully raising the psi. This could be accomplished by doing what we have all agreed that increased engine performance and lowers existing PSI allowing you to turn up the turbo to previous PSI setting increasing the CFM flow conciderably

NicD

iTrader: (7)

Are you talking about my post? I just stated what my car made with only 7 and 10 psi of boost and asked people to compare that to a stock heads/cam car with 7 and 10 psi of boost and they would have a simplified answer to this question. I am not referring to being on an inefficient compressor map area and I am not argueing "what ifs" about maxing out a turbo/blower and then adding heads/cam expecting more power because that would be very stupid. It's just a simplified example of, "are heads/cam worth it on FI when everything else is setup correctly."

TeeKay

Yes to the first part, it does just that. No, the compressor is not maxed, the engine just won't accept additional CFM as pressure is increased. The hp per psi curve gets a lot flatter for THE ENGINE (less gains for increased pressure). The blower can put out more CFM at the same pressure when the restriction of the engine is reduced. I hope that clarifies.

white2001s10

iTrader: (4)

Not clear just yet. In ref to the above part, why?

Why does the increased pressure differential at the valve not move more mass past the valve into the chamber?

I understand that just raising the temp increases the psi but not the mass, but I also think it possible that a compressor working in it's intended CFM range can produce high psi and high mass while staying in the predicted temp range. In this working range, temp and PSI both go up, but mass (density) is still being increased to the positive.

This goes back to the whole basic question of reducing this down to: "Do I boost or run NA?"

obviously I'm having a hard time giving up the understanding I thought I had.

TeeKay

Let's try this from a slightly different angle. Now, remember, we're talking about the ENGINE, not the "booster". To simplfy the discussion, our theoretical booster can supply anything we want. OK?

Start at 10 psi. We get good power (1.68 x N/A). It runs well on 93 octane at maybe 26° advance with no detonation (good tune). Now, of course, we want more (who don't? lol). So we turn up the boost to say 14-15 psi. The intake air is probably now 100° hotter and the detonation at 26° advance is off the chart. So we up the octane, add meth, or an IC or maybe drop the timing (again, good tune) to get the KR down. That works for a while, but being greedy, we want to do some more! So we up the psi again to maybe 21 psi. Now we have a handful, we are already at maybe 15° advance and running on C16 on the street, so what can we do to ward off detonation now? Add another IC or CO2, drop the timing to 9°, use rocket fuel? Remember the VE of the engine (restriction) is what's causing the PSI, the blower puffing against a fixed restriction. Without some restriction, the blower would just free-flow and there would be no psi increase.

So the only thing left to do to increase the CFM without increasing psi is to free up the flow-path of the engine. Better cam, heads, intake, and exhaust. What does that do? Well, it drops the psi required to make the same hp (CFM), and removes some of the things we had to do to run a higher psi. We can drop octane, etc. and still get virtually the same power. But now we also have the opportunity to take advantage of the extra engine flow and make more power on virtually the same tune we had before (octane, AF, timing advance). Now this works even if we don't go to the point of explosion: reducing the restriction means more power (CFM) at the same boost level and leaves us more room to move.

In answer to the basic question: Boost is gonna give you way more than anything you can do N/A. But the real question is twofold. If I'm already running some boost do I just up it or do I have to mod? What's it gonna cost? If you are in mild to mid range boost, there's probably room to just turn it up if you're willing to do the tuning and other things needed. If you are already in the teens of psi, what are you gonna turn it up to? 25 -30 psi? Cost? How fa$$t do you want to go? Hope that clears up some things.

white2001s10

iTrader: (4)

Thanks again for your time. Your last post was perfectly clear.

A. You turn up the boost and need a different tune and maybe more anti-detonate add-ons. Result = you make x amount more power.

B. You up the VE and then turn the boost up. You need all the matching parts to up the VE, and a different tune (more aggressive than tune A) without as many anti-detonate add-ons. Result = you make x amount more power.

In the end the power is about the same. One guy spends more for parts and the other has a harder tune and higher temp to deal with.

The way you explained it in your last post it doesn't say that there isn't more mass moving past the valve at higher boost, but just that there is diminishing returns due to the heat increase. I understand that part.

It was always clear that the higher VE has more "potential" to make higher power levels more easily (less boost and less temp).

My own opinion had always been that running lower boost (7psi or under) was the better option due to the diminishing returns (the heat factor).

I do think there are a few complexities that can make either of the two choices better for different combinations.
What do you think about this:

For instance you have an engine with a great VE running low boost pressure. This would mean quite a large mass of exhaust must be cleared from the chamber before the intake (at a low boost pressure) can begin to fill the cylinder. This may be especially hard for a turbo app. It seems that some instances could be very camshaft and exhaust sensitive.

What if your boost was 14 psi with a backpressure of 28psi,
then you increased VE dropping boost down to 7psi, but the backpressure remains at 28psi ?

If once the intake valve opens, the charge in the chamber is still higher pressure than the 7psi in the intake port then reversion is likely. It could reduce the time available for the intake charge to fill the cylinder along with the charge contamination.

Could it be since the exhaust pressure is so much more, that it may be more benificial to turn the boost up with a turbo, yet be better to increase the VE with the supercharger? ... generally speaking
*may also apply to a supercharged engine running restrictive exhaust.

My logic being that the higher pressure upstream of the intake valve will ensure better cylinder filling in the presense of relatively high exhaust pressures in the cylinder. This is basically the same logic that allows boost to make more power over NA.

I'm not trying to get off on another subject. I think this pertains to the discussion as I mentioned it earlier. Like most I'm still struggling with trying to understand the exhaust side of cam dynamics. The timing of the pressure pulses seems to be an unknown (combo specific) and factoring in boost just makes it harder. Please add anything you know.

Thank you again for explaining your view clearly. Hopefully it helped others as well.

TeeKay

I'll make this a quick one because we're getting more technical than I believe I can speak to.

Turbo: You definately can get reversion, that's why turbos like a "negative" overlap cam, to isolate the intake charge from the exhaust BP. The turbo is the biggest exhaust restriction so exhaust work is relatively small potatoes. Things like headers and piping, not the heads themselves. Max boost comes in as soon as the turbo spools.

Centrifugal: Exhaust flow can be optimized for best flow and you can use a cam with high duration and overlap. The SC robs power from the crank. That can be as much as 100 hp! Max boost is relative to engine rpm.

Eaton (Rootes): Much the same as the centrifugal, but boost is virtually off idle and constant (positive displacement), but looses ground on the big end due to compression heat (IC really needed). Also takes engine power to drive.

So the cam is combo specific. Tuned (LT) headers can help S/C apps, but don't mean much for turbos (don't really hurt either). Anything you can do to optimize head flow is gonna help overall.

I'll put it this way, if you heat air with compression, the mass flow will depend on both the psi and the delivered heat in the air. Excessive heat will drop the mass flow to the point where additional psi can't overcome it. IOW, anything over that, and the mass flow will decrease. That doesn't say anthing about detonation, just the mass flow. Turning up the psi is easier with a turbo, but that generalization doesn't address how the COMBO will work. The best generalization is that making the engine breathe better makes more power. Simple, huh? lol

That's about as smart as I can get. I said this would be quick. I lied!