[dmac]

How accurate do you think the readings at the start of the runs is? It looks like under ~3500rpm, there is a loss of power both torque and HP that is fairly significant. Do you think that there is a happy medium that could have kept the bottom end from losing as much? Maybe smaller tube headers, or longbranch manifolds, and/or smaller exhaust pipes than your upgrade? I'd keep the smooth transitions.

Trying to pick your brain as to what what longbranch manifolds and a full 2.5" system would do for my planned low CR/low rpm build.

[Lee]

Originally Posted by dmac

How accurate do you think the readings at the start of the runs is? It looks like under ~3500rpm, there is a loss of power both torque and HP that is fairly significant. Do you think that there is a happy medium that could have kept the bottom end from losing as much? Maybe smaller tube headers, or longbranch manifolds, and/or smaller exhaust pipes than your upgrade? I'd keep the smooth transitions.

Trying to pick your brain as to what what longbranch manifolds and a full 2.5" system would do for my planned low CR/low rpm build.

At 225rwhp, I could floor it around 3200 and start the dyno run. At 270rwhp, I had to go to about 3500 before I floor it, or else the trans would downshift and screw the data for the entire run.
Now I'm starting the run, and just easing into the throttle. It still downshifted on me twice yesterday.

Where you see the graphs make a noted change in slope, just past 3600 rpm, THAT is when I was finally at WOT. Ignore the data at lower rpm. The car has noticeably more power off-idle and at lower rpm than what it did previously, there is NO loss!! I wish I had a manual tranny, then I could do WOT pulls from off idle.

If the cam is designed for the exhaust system you are using, then it will perform fine. My cam was designed for a really low restriction, and that is probably part of the reason I've had such gains from when the exhaust was very restrictive. If I had maximized the cam for a restrictive exhaust, I probably would not have very impressive gains with my new system.

[Will]

Originally Posted by Lee

...
If the cam is designed for the exhaust system you are using, then it will perform fine. My cam was designed for a really low restriction, and that is probably part of the reason I've had such gains from when the exhaust was very restrictive. If I had maximized the cam for a restrictive exhaust, I probably would not have very impressive gains with my new system.

Thanks Lee, but now I'm confused.

Everything I think I've learned over the years tells me that wide LSA splits with longer durations on the exhaust side are for "de-sensitizing" an engine to a restrictive exhaust. Such cams (the factory 068 is a good example) are less dependent on scavenging and therefore more tolerant of exhaust systems with manifolds or restrictive piping.

I have a book somewhere around here that briefly discusses some testing the factory engineers did back during the 455 HO & SD development years. They confirmed that a cam on a 108 LSA made more peak power and more overall power when running headers and low restriction exhaust but lost power compared to wider LSAs when you started bottling things up. This, along with the need for good idle quality and vacuum is why all the factory cams are on such wide LSAs compared to a lot of aftermarket offerings.

In a long-ago conversation with Dave Bisschop he stated that he usually recommends cams like the OF on a 114 LSA if running manifolds but 112 with headers.

Other dyno tests and things I've read & seen over the years seem to bear this out. The narrower LSA increases overlap which can contribute to reversion unless the exhaust system is good at scavenging the cylinders, in which case that actually promotes better cylinder filling and can extend the RPM range of a narrower LSA cam. The wide LSA cam with less overlap or a long duration exhaust lobe typically gets the exhaust pulse started earlier and gives more time to get exhaust gases out of the cylinder before the intake valve opens to compensate for less scavenging and lower overall flow.

So now we are getting back into the great LSA debate, but your statement and apparently your findings on the "desktop dyno" are running counter to what I've always believed to be true.

Care to comment further?

[PDC]

Lee - completely off topic - but I really like the black/red/silver bird and lettering on the early 70s car. And the YO 17” Rally’s really make the look. Fantastic car.

[Firebob]

Here's a sound bite from my 30 over 455 with Dougs, x-pipe, and 2.5" through utraflos. Not as throaty down low as the Borlas but a bit more rowdy up top.

[Lee]

PDC, thanks!

Will, I pay ZERO attention to LSA as I'm designing cams. I don't even look at what it ends up, until I'm ready to order the cam - that is its sole importance to me, it makes ordering the cam easy. Once you start using intake to exhaust duration splits bigger than 8 or 12 degrees, all the "rules" go out the window. Really now, why would the engine care where the middle of the cam lobes are? There is only gross flow going on at that point. Even is peak flow did make some difference, with asymmetrical lobes the peak could be a bit before or after the cL. The opening, closing, and the overlap events are what really need to be looked at. LSA "infers" overlap to a point, but it is different depending upon the duration spread.

It bothered me, too, several years ago. Unlearning is harder than learning! I've been able to see most of these non-traditional designs tested on the chassis dyno, several times testing a conventional design first, then installing a big split, and the results have backed up the theory. Look at the GM Performance Parts Corvette Stage 3 cam - HUGE split, designed for a low restriction exhaust 7.0 liter engine, making a wide powerband - that is what GM engineers are NOW doing.

One more thing I've observed - the big splits work best with really efficient low-restriction exhausts. The more restrictive or inefficient the exhaust, the smaller the spread that I can effectively use.

All that said, I'm still learning! And unlearning :-) You know me, and saw me follow "the rules" for a long time, in cam designs/suggestions. They usually worked pretty well. My new approach to cam designs is also working, and has been working even better than the old. We are getting WAAAYYYY off topic though, so e-mail me if you want to discuss further.

[Will]

Thanks Lee. Very interesting. I don't want to throw your thread off topic any more but am interested in your findings. I agree that LSA is merely a byproduct of getting the opening and closing events right. However, I don't understand why, if you have a very efficient exhaust system, such a long exhaust duration would be beneficial. That's the part that confuses me but yeah, we should talk about that elsewhere.

Thanks again!