The way I read it, you are both saying the exact same thing.

 

I was just thinking the exact same thing....

 

"In the turbo cars especially, the exhaust pressure is gong to be greater than the intake pressure, sometimes by a factor of 2:1. Ten pounds of boost, in other words, results in twenty pounds of exhaust pressure. So overlap, even a little, forces hot gasses BACKWARDS into the intake path, increasing sensitivity to detonation. I don't have Vizard's cam amd valvetrain book with me here at work, but that's where I got the above viewpoint"

I'm having a tough time agreeing with those statements.

If you have that much backpressure occuring at high RPM, you better take
the brick out of your tail pipe...or select a bigger diameter exhaust pipe
to handle the flow and power.

If there is reversion occuring at mid to high RPM, there is something very wrong
with the tuning of the motor, and/or engine part config.

 

when he said "not a huge amount" i took it as a moderate amount, not "just a touch"

 

I think the best way to get everyone to agree is that a slight amount of overlap is ok/needed in a turbo'd engine...Unless you're at the crazy levels of 20+psi and looking for every single bit of extractable hp...Like the previously mentioned setup with a !!!!!!!!!!!!126!!!!!!!!!!!! LSA<---That's friggin nuts!!

 

turbo setups are wierd and need pretty specific stuff depending on application, setup and the other billion factors. few more varriables thana blower.

 

I didn't realize the purpose of starting a thread was to get people to gree on something. You should probably stay away from the impala forum if this level of disension bothers you!

Incidently, Lingenfelter has a new cam out for turbo cars, the GT7:
208/230 duration
121 LSA
undisclosed advance/retard

 

I'm not saying I like everyone to agree all the time, but if they're saying the same thing, why not??? And the purpose was to truely find out some runner lengths on stock intakes, but I'm not complaining, it's turned into some interesting conversation from my point of veiw...

Chris

Out of curiosity, what are the rest of the specs on that LPE cam?? Lift, rockers, Intake centerline, etc.?????

 

No you are right on all of those effects............

The problem is with centrifigal blowers low rpm TQ can suffer if you stick with late IVC and since you are usually dealing with low compression levels there is only so much you can do. So if you make the IVC earlier (raising DCR) and you want to put some duration in there you are going to have overlap, no way to get around it.

I don't shy away from duration on blower cams, for the most part big blower cams are 224/236 range, which in the case of a blown SBC, especially with extra cubes is not always enough. The more duration you get with valve events in the right area is going to cause you to get overlap.

Turbo cams are interesting also since you do have other things to deal with that you don't with a centrifgal blower, one of the big things is exhaust backpressure another is turbo spool up and how fast the boost comes in compared to a centrifigal blower. A turbo doesn't have a linear boost curve if it's done right it comes on early and hangs there for as long as possible.

The backpressure is interesting and it's not a bad idea to have more boost on the intake side than you have backpressure on the exhaust side.

As for that LPE cam, it's not a bad idea I like where they are going with that vs. the standard thinking in turbo cams.

Remember LSA, advance and duration are just numbers if you get caught in a range then you are not going to find new avenues to find power. You have to give the motor what it wants not what you think it wants.

Bret

 

Well, I'm not sure they're agreeing on the fine points yet, but I see your point. Sometimes restating something clarifies your meaning for your listeners/readers.

I didn't bother with the lift numbers on the Lingenfelter cam since they were fairly standard (mid 5's) with 1.7's, and none of the other specs were listed.

 

I'm used to a site called ChitownRacing.com and all they do is argue and fight over there....mostly since it's a street racing board, but it's nice to come here and relax, learn, and inform others....As for these *New* turbo specs, I think it'd be great to try one if I could get enough backing to do a dyno test of three different *Turbo* Cams and compare them for HP ratings....Too bad I don't have the money or the motor done yet...

Chris

 

The problem is that "brick" is what's supplying your intake with boost. The turbine housing on a fairly average turbo setup may narrow from 3 inches to 3/4 of an inch where it dumps onto the turbine in order to spin the turbine fast enough to turn the compressor, nevermind the work needed to spin the turbo.

A race setup can be designed to have much less backpressure, but the cost for efficiency at high rpm is big lag time. In a street car, you sacrifice more backpressure at high rpm in order to get more boost in the midrange and to shorten lag time. If you're running a manual transmission or a big TC, you can size the turbine housing larger in order to reduce backpressure and thus increase horsepower at high rpm.

Have you ever stopped to think about how much horsepower it takes to run a turbo? The STS turbo setup, which supplies only 5 psi of boost, costs about 35 horsepower to run. That means you need 35 hp worth of energy going down the exhaust pipe instead of out the crankshaft.

 

Well the old addage of "Horsepower is never free" is true even for turbo's....Just something to think about on the next build

 

"The problem is that "brick" is what's supplying your intake with boost"

Now we're getting into a totally seperate subject .

My initial statement in response to the previous post simply meant, if the
exhaust is backed up due to restriction, the charge isn't entering the cylinder.

In order to increase cylinder filling, the exhaust flow must be free flowing.

The potential difference in pressure between the exhaust runner and
plenum = manifold pressure; or a closed intake valve and the plenum (depending
on the point in time in which you sample).

25 pounds of manifold boost means nothing if it's not getting into the cylinder.

The S/C is simply pressurizing the plenum at a higher rate than the pistons
can draw from the plenum.

If the pressure behind the intake valve is sitting at 15 PSI, the charge will
rush in proportional to the lower pressure created by the piston drawing down
on the intake stroke.

At the point the intake valve begins to open, the piston isn't moving much,
that is why some overlap coupled with a fast moving exhaust pulse down the
header primary will help lower the pressure within the chamber and help suck in
more intake charge. This will also push out any residual exhaust gas which
would otherwise displace the fresh charge.

I checked out the LS1-GT7 cam. It seems fairly standard other than the LSA.
There's no sign of valve timing anywhere!

Someone find these IVO, EVC specs and post them!

..................Intake........Exhaust
Lobe lift:......0.326”........0.321”
Rocker ratio:..1.7:1........1.7:1
Valve lift:.....0.554”........0.546”

Duration at 0.050: 208/ 230
Advertised (total) duration:" 261/ 282

Lobe separation angle: 121.0
Centerline:

 

even still the big difference is the huge variation in durations....usually they're somewhat close to each other....I'll get IVO,IVC,EVO,EVC #'s for you tomorrow...Gotta go home and play on the computer, the only problem is that no-one has a centerline #...That would probably change it around a bit...

Chris