More info from Chevelle Tech by someone I tend to believe a lot more. Camshaft Expert, Harold Brookshire. (UDHAROLD)

(Post by a Chevelle tech Guy)
"Hello guys,

Did any of you happen to read the article in the May 2005 issue of Popular Hot Rodding about "Exhaust Science Demystified" written by David Vizard.

He brought up a very interesting point about an additional cycle in the combustion process.

"With a race cam and a tuned-length exhast system, negative pressure waves traveling back from the collector will scavenge the combustion chamber during the exhaust/intake valve overlap period. 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 travelling down the bore will pull in one-eight 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 but 717cc but 717+71.7cc=788.7 cc. The result is that engine mow runs like 385 cubic inch motor instead of a 350. That scavenging process is, in effect, a fifth cycle contributing to total output. " (Vizard PHR pg 57 May 2005)

I think this a neat way of looking at things. I am probably very roughly guessing here but does the 5th cycle increase the volumetric efficiency of the engine? What do you guys think?"

Question by TAFF:

"Is there still a reflected wave back up the primary pipes from the collector ,or is this wave cancelled out by the pre turbine pressure? "

YES! BUT,

Harold Brookshire's comment:

ROTFLMAO!!!!!

Getting those negative pulses is veeerry, veeerry hard..... Between each negative pulse section is a POSITIVE pulse section, putting more exhaust BACK into the cylinder. To get exhaust tuning to really work, you need either a 180° crank or else 8 individual exhaust pipes, with 8 iindividual intakes to go with them.
It is much more effective to get the exhaust away from the cylinder head, rather than to try to get it to come back and do extra work. What he is talkiing about I spent YEARS theorizing on in the 1950s-1960s. All the experimental work by Honda, etc, show individual exhausts with individual intakes, and 8-speed transmissions to work with the extremly narrow power bands.
Yes, the Iskenderian 5th-Cycle was all about this.
Ask yourself this question: How much charge can you garauntee that will enter the exhaust, and how much will be pulled back into the cylinder, and how much will be lost down the exhaust pipe? Remember, Isky claimed the 5th Cycle was for COOLING the engine down, which it did. The extra fuel cooled down the combustion chamber---Remember, we're talking NITRO cams here..... In the 50s, 98% pure Nitro melted many a Chrysler 354 or 392, and extra overlap helped save the engine by cooling it down.
The next article will be on how much more air/fuel will get in the cylinder when using a supercharger!!!"

Chevelle Guy:

"So Harold, are you saying that Vizard maybe overstates the benefits a bit? To tell the truth, I'm never too sure what to make of his articles. I read of them I can find and sometimes I think he's way off base and others he seems to know what he's talking about."

Harold Brookshire's comment:

"Don't believe everything you read on the internet.....
Sometimes things have a simpler explanation than that offered....
Cylinders reach max cylinder capacity at BDC obviously, and so do the intake port reach maximum velocity. BDC is the point where you have max cylinder volume, and max rate of filling.
And THEN the piston starts up on the compression stroke, into the 'Miller Cycle'. Cylinder volume is decreasing, but what about cylinder filling? Has the port velocity instantanously stopped flowing into the cylinder and now started flowing OUT?
No, the port velocity has INERTIA, because it has both weight and a direction of velocity, and so it continues to fill the cylinder, but at an ever decreasing rate. The trick is to close the intake valve before the intake port velocity has dropped to 0 and began to reverse-flow. On my old 288/296R6 roller, even in on 102° ATDC, the intake valve closed at 69° ABDC, which is 38.33% of the total compression stroke. It was still filling at that point, which makes it an 1980 version of the Miller Cycle. At various times I have mentioned the Miller Cycle in UD ads.
As far as the thing about blowers, they must ALWAYS have more pressure in the intake runner than there is back pressure, or you have hot exhaust gases flowing into a fuel-rich port, and a nice blower explosion.....
More fuel and air in a cylinder causes more torque and BHP. YOu can put it in there by inertia-ram, as I do, or by supercharger/turbocharger. Either method works...."

Tom Vaught

ps Ford has done a LOT of testing on Miller Cycle with Boosting (turbos/ superchargers over the years).

 

Tom, no, I hadn,t read the Vizard article 'til now,although I have heard about it. I used to read his articles when he lived over here though, they were very good.
Thanks too for the Harold Brookshire post, I too have heard good things about his work,hence the reason I asked him (via Bullet cams) for a turbo cam for my combo.
I 've been kicking my questions around for a while, I've read a lot of turbo books and done a fair bit of research but haven't come across a definate answer. However ,this discussion has cleared up most points. It appears to me from what's been said, that if I do build a tuned length header system and it works,all well and good. If it doesn't actually improve things, then it doesn't look as if I would lose anything either, the turbo sytem can still work efficiently. Thanks.

 

How are you gonna know if your tuned system is doing anything above and beyond what a short system will accomplish. Now you have an extra 20 inches by 8 tubes of tubing that doesn't need to be there. Also, tuned headers work when the system has room, but I was reading that once you force a tuned system to make it's way around a ton of obstacles, it loses the tuning ability really fast.

 

A Pontiac Buddy (Firebird) went 6.80s at over 200 mph in the quarter with short length headers. How fast do you really want to go Taff?

Tom Vaught

 

BOS, my tuned length headers will be a short system,probably around 21" primaries. The tuned length headers I made for my 9 sec. n/a racecar are short too,about 22",and they seem to work well. These 22" primaries were the 2nd best specs, the 1st specs being around 38" (IIRC) which I couldn't fit in. Calculated via Pipemax.
Tom, I don't need to run as quick as Marty did, but to be anywhere near competetive in the class I want to run in I'll need to run 7.90s at 170. Don't forget I'll be running a little 421 cu in Pontiac up against 540/572/600 cu in BBCs and BBFs so I'll need all the help I can get! The leveller in the playing field are the real street tyres that must be used.

 

&gt

Show them what a Traditional Pontiac can do, lol.

Tom Vaught