Question for the forced induction experts

[99-LS1-SS]

I know I read somewhere that a car with a blower and factory manifolds will have X amount of boost but if you put headers on the car the boost goes down but you don't necessarily lose power.

Is this correct? Will someone take the time to explain this in detail. I know it has to do with the breathing and efficiency but I would like to know the physics involved.

Thanks.

[NYG8GT]

but I would like to know the physics involved.

Thanks.

Are you sure about that? lol

I've never been great at explaining things myself, but I understand why it works by thinking about it. The biggest areas of improvement that help to increase power are reduced pumping losses and less heat from a less compressed boost charge. Reduced pumping losses pretty much explain themselves, and the cooler charge temps mean a larger volume of air due to its increased density. So even though boost pressure may be lower, the total volume of air in the charge will be increased. Which, as you know, more air + more fuel = more power. So you gain power through a few different aspects. There are probably a few more, but it's too early for me to think too deeply about it and I gotta get out the door to work.

I'll leave the physics explanation to someone else, that's eons beyond my realm of understanding. This could end up being an interesting thread though :)

[norm8332]

+1

[esinger]

Hi Brandon, I haven't experienced "Headers" doing that, but "heads" will. I dropped from running 26 PSI to 22-23 PSI just from having my heads ported/polished.

A friend told me that the power is in the "Heads" and I truly believe him.

[99-LS1-SS]

The way I understand it Eric, is it's the same principle as cleaning up the heads. Your engine becomes more efficient because it's able to move more air. It just happens that your heads were your choke point where the LS3 heads are pretty damn good out of the box.
That's what I think anyway. I wish a airflow physics expert would chime in....

[norm8332]

Brandon, I did loose about .5 lbs (9.5-9.0 at the time) when I replaced the entire exhaust system.

Edit: I was told this is because of the restriction some pressure was remaining in the cylinder because of back pressure. I am not an expert as you know. maybe someone else will chime in to confirm this.

[J Wikoff]

I agree with what you were told, norm. The supercharger is pushing just as much air as before, but now the exhaust gasses left in the cylinder are at a lower pressure/mass, allowing a larger mass of fresh air/fuel into the cylinder. More mass moved into the cylinder means lower pressure in the intake manifold.

[-Ray-]

There is a book out by Corky Bell that will answer a lot of questions.

[g8gxp]

I lifted this in part from The Superchevy site. Does not address headers + forced induction but the exhuast science remains the same. Basically the exhaust scavenging and temp efficiency is improved, reducing pressure (boost) on the front end. There was a whole bunch more regarding mufflers and crossovers but i left that out.

Exhaust Science Demystified

Although the mode of function of an exhaust system is complex, it is not (as so often is believed, even by many pro engine builders) a black art. To help appreciate the way to get the job done I will go through the process of selecting exhaust system components for a typical high-performance V-8 in a logical manner from header to tail pipe. Although the entire exhaust functions as a system, we can, for all practical purposes, break down many of the requirements that need to be met into single entities. Fig. 1 details the order of business. But before making a start, it is a good idea to establish just why getting the exhaust correctly spec'd out is so important. This will allow realistic goals, improved component choice, and a more functional installation.

The V-8 engines we typically modify for increased output are normally categorized as four-cycle units. Although pretty much the case for a regular street machine, this is far from being the case for a high-performance race engine. If we consider a well-developed race engine, the usual induction, compression, expansion (power stroke) and exhaust cycles have a fifth element added (Fig. 2). With a race cam and a tuned-length exhaust system, negative pressure waves traveling back from the collector will scavenge the combustion chamber during the exhaust/intake valve overlap period (angle 5 in Fig. 2). To understand the extent to which this can increase an engine's ability to breathe, let's consider the cylinder and chamber volumes of a typical high-performance 350 cubic-inch V-8.

Assuming for a moment no flow losses, the piston traveling down the bore will pull in one-eighth of 350 cubic inches. That's 43.75 cubic-inch, or in metric, 717cc. If the compression ratio is say 11:1, the total combustion chamber volume above this 717cc will be 71.7cc. If a negative pressure wave sucks out the residual exhaust gases remaining in the combustion chamber at TDC, then the cylinder, when the piston reached BDC, will contain not just 717 cc but 717 + 71.7 cc = 788.7 cc. The result is that this engine now runs like a 385 cubic-inch motor instead of a 350. That scavenging process is, in effect, a fifth cycle contributing to total output.

But there are more exhaust-derived benefits than just chamber scavenging. Just as fish don't feel the weight of water, we don't readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons! If enough port velocity is put into the incoming charge by the exhaust scavenging action, it becomes possible to build a higher velocity throughout the rest of the piston-initiated induction cycle. The increased port velocity then drives the cylinder filling above atmospheric pressure just prior to the point of intake valve closure. Compared with intake, exhaust tuning is far more potent and can operate over ten times as wide an rpm band. When it comes to our discussion of exhaust pipe lengths it will be important to remember this.

At this time a few numbers will put the value of exhaust pressure wave tuning into perspective. Air flows from point A to point B by virtue of the pressure difference between those two points. The piston traveling down the bore on the intake stroke causes the pressure difference we normally associate with induction. The better the head flows the less suction it takes to fill (or nearly fill) the cylinder. For a highly developed two-valve race engine the pressure difference between the intake port and the cylinder caused by the piston motion down the bore, should not exceed about 10-12 inches of water (about 0.5 psi). Anything much higher than this indicates inadequate flowing heads. For more cost-conscious motors, such as most of us would be building, about 20-25 inches of water (about 1 psi) is about the limit if decent power (relative to the budget available) is to be achieved. From this we can say that, at most, the piston traveling down the bore exerts a suction of 1 psi on the intake port Fig. 3.

The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events.

With the exhaust system's vital role toward power production established, it will be easy to see that understanding how to select and position the right combination of headers, resonators, routing pipes, crossovers and mufflers will be a winning factor. This will be especially so if mufflers are involved in the equation. I first started putting out the word on how to build no-loss systems as much as 20 years ago and I am somewhat surprised that it is still commonly believed that building power and reducing noise are mutually exclusive. Historically, this has largely been so, but building a quiet system that allows the engine to develop within 1 percent of its open exhaust power is entirely practical. Be aware that knowing what it takes in this department can easily deliver a 40-plus hp advantage over your less-informed competition.

Headers -- Primary Pipe Diameters
Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.

Headers -- Primary Pipe Lengths
Misconceptions concerning exhaust pipe lengths are widespread. Take for instance the much-overworked phrase "equal-length headers." More than the odd engine builder/racer, or two, have made a big deal about headers with the primary pipes uniform within 0.5 inch. The first point this raises is whether or not what was needed was known within 0.5 inch! If not, the system could have all the pipes equally wrong within 0.5 inch! Trying to build a race header for a two-planed crank V-8 with lengths to such precision is close to a waste of valuable time. Under ideal conditions it is entirely practical for an exhaust system to scavenge at or near maximum intensity over a 4,000 rpm bandwidth. Most race engines use an rpm bandwidth of 3,000 or less rpm. If the primary pipe scavenging effect overlaps by 3,000 rpm then it matters little that one pipe tunes as much as 1,000 rpm different to another. Since this is the case, then all other things being equal, pipe lengths varying by as much as 9 inches have little effect on performance. A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays.

Apart from the reasons just mentioned, there is also another sound reason why we should not unduly concern ourselves about equal primary lengths. In practice, the two-plane cranks that typically equip V-8 race engines render the exhaust insensitive to quite substantial primary length changes. Experience indicates inline four-cylinder engines are more sensitive to primary pipe length, but a two-plane cranked V-8 is not two inline fours lumped together. It is two V-4s and, as such, does not have even exhaust pulses along each bank. With a conventional, as opposed to a 180-degree header, exhaust pulses are spaced 90, 180, 270, 180, 90 and so on. The two cylinders discharging only 90 degrees apart are seen, by the collector, as one larger cylinder and accounts for the typical rumble a V-8 is known for. This means the primaries act like they do on a four-cylinder engine, but the collector acts as if it were on a 3-cylinder engine having different sized cylinders turning at less revs. (Doesn't life get complicated?) This, plus the varied spacing between the pulses appears to be the cause of the system's reduced sensitivity to primary length.

These uneven firing pulses on each bank seem to work in our favor. Evidence to date suggests that single-plane cranked V-8s, which have the same exhaust discharge pattern as an in-line four-cylinder engine, make less horsepower and are more length sensitive. Dyno tests with headers having primary lengths adjustable in three-inch increments show that lengths between 24 and 36 inches have only a minor effect on the power curve of V-8s that you and I can typically afford, although the longer pipes do marginally favor the low end.

Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.

As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length. For a motor peaking at around 6,000-8,500 rpm, a collector length of 10-20 inches is effective.

Getting secondary lengths nearer optimal can be worth a sizable amount of extra power as Fig. 5 shows. If you want to bump up torque at the point a stock converter starts to hook up the engine, you may want a secondary as long as 50 inches but something between about 10 and 24 is more normal. The shorter of these two lengths would be appropriate for an engine peaking at about 8,500 rpm whereas the longer length would be best for an engine that peaked at about 4,800-5,000 rpm.



At this point determining primary tube diameters is starting to look like a tight wire act only avoidable by trial and error on the dyno. Fortunately, a little insight into what it is we are attempting to achieve brings about some big-time simplification. Our goal is to size the primary pipes to produce optimum output over the rpm range of most interest. The rate exhaust is dispensed with, and consequently, the primary pipe velocity, is strongly influenced by the port's flow capability at the peak valve lift used. From this premise it has been possible to develop a simple correlation between exhaust port-flow bench tests and dyno tests involving pipe diameter changes. This has brought about the curves shown in the graph Fig. 4 which allow primary sizing close enough to almost eliminate the need for trial-and-error dyno testing.

[G8GT721]

why did adding the cam cause boost loss?


any time you add a cam/ported heads/cubic inch there will be some loss because the supercharger moves air, the restrictions after the supercharger create the boost. when you lower these restrictions the amount of air moving is the same, but there is less boost. boost is essentially "back pressure" to look at it in a simplistic form. but removing this it will go down...


heres what Andy said after i asked why adding a cam to a blown L99 Camaro, resulted in boost loss

[parish8]

it was said above.

boost is just a measurement of back presure. anything you can do to improve airflow will drop the backpresure in the intake. lower back pressure but more airflow.

most people pully back up to where they were before and that is where you see the good gains.

[Ktlplxm]

Just remember, boost is a measurement of restriction and airflow (resulting in pressure). Anything that decreases the amount of restriction will decrease boost. Thats why efficient engines makes more power with less boost; the airflow is still there it just isn't restricted

[who?me?]

same goes for bigger or more efficient superchargers if im not mistaken. make more power with less boost right?

[parish8]

same goes for bigger or more efficient superchargers if im not mistaken. make more power with less boost right?

i am guessing your comparing a 1900 to a 2300? it really depends on a lot of things. if you are anywhere near maxing out the 1900 then yes steping up is going to give you more power at the same boost level but if you are in the sweet spot on the smaller blower then steping up to a larger one might not help or might even hurt.

that being said i dont think it takes too much to get to the point where you are pushing a 1900 hard and the step up to a 2300 would help.

[99-LS1-SS]

What about 550 rwhp? Would you consider that to be pushing the limits of the 1900 and justifying a 2300?

[parish8]

i would only get the 1900 if your just going to bolt it on an otherwise stock car.

[norm8332]

I got mine awhile before the 2300 was available. If the 1900 can flow enough for 9psi without a severe efficiency problem with my cam, I will be pretty happy. I think it will come close at least.

[parish8]

I got mine awhile before the 2300 was available. If the 1900 can flow enough for 9psi without a severe efficiency problem with my cam, I will be pretty happy. I think it will come close at least.

yeah, i am not putting down the 1900. seems like you can really spin that thing and get some flow out of it but since there is an option for the 2300 it would be a better way to go if you plan on doing more mods on top of the blower.

[Devilish34]

What about 550 rwhp? Would you consider that to be pushing the limits of the 1900 and justifying a 2300?

Whats the price difference between the 1900 and the 2300?? $500-$700?

[norm8332]

If I wanted 550 rwhp without added hassle I'd go with a 2300 now.

[Ktlplxm]

same goes for bigger or more efficient superchargers if im not mistaken. make more power with less boost right?

Not always. Too big of a blower can hurt



Whats the price difference between the 1900 and the 2300?? $500-$700?

Depends on who you get them from. $700 would be on the low end of it. Usually closer to a grand in difference. The 2300 comes with more I believe.