C_Rules

iTrader: (7)

I know that people complain about how some large cams have too high of an LSA in order for people to go big and still have a streetable car with a good idle.

then i saw a post on here with someone claiming that the new corvette LS7 cam has a 121 lsa. the corvette still runs low 11s in the quarter with the stock cam.

what are the downsides to going big on the intake/exhaust numbers on a high LSA as opposed to going smaller on the intake/exhaust with a lower LSA? is there a formula to figure out how much power you lose a per degree increase in the LSA?

when does is it get ridiculous to get a very large cam with an insane LSA just so that it can be streetable?

DONAIMIAN

iTrader: (18)

Most the people on here that complain about LSA dont know enough about cams or valve events to have a concrete reason.

engineermike

A test was published a while back on a 355 SBC with 3 cams, all 262/268 at .050. They tried a 106, 108, and 110 LSA. All 3 made within a couple of hp of each other, even at the same rpm, but the tighter LSA progressively made significantly more peak torque.

Keep in mind, however, that with cams like the stock LS7 cam, you have to look at all the valve timing events and how they tie into the intended use, not just the LSA number.

C_Rules

iTrader: (7)

okay, i definitely hear what you're saying on the second part of your reply.its how everything ties together.

As for the testing LSA's on that 262/268 cam what would happen if someone decided to make a that cam on a 121 LSA. would that be considered foolish and what would the outcome be, a high hp low tq car? how would that perform?

Manic Mechanic

iTrader: (3)

Here's a good article on the subject.

http://compcams.com/Community/Articl...?ID=1726205736

Vernon

engineermike

My guess is that it would be a total slug below about 5500 rpm, then it would wake up but still fall short on hp. With an IVC of 65 ABDC, it would have some pretty bad reversion problems at the end of the intake cycle and very low DCR. Even race cams close the intake valve before 50 ABDC. Anything beyond that kills low end, but doesn't add any hp.

Mike

C_Rules

iTrader: (7)

thanks manic & mike

SStrokerAce

iTrader: (2)

Don't know about the application but for the stuff we see on here that would be junk.

Bret

black_z

iTrader: (4)

Isnt that the truth.

04gtoBMXracer

iTrader: (26)

thanks for that link manic good reading

GIGAPUNK

iTrader: (2)

Instead of LSA, why don't we talk about overlap. I know race cars end up using a whole lot more (they generally have much more efficient exhausts, and ITbs, unlike street cars), but is there a rule of thumb (in degrees of overlap) where you will start loosing serious bottom end because you are blowing the intake charge out of the exhaust at low rpm? Presuming a good full length exhaust system and nice headers on an LS6 intake.

**Example N/A cam :**
Duration 236/242
Lift .568/.576
Lobe Separation (LS) 112

Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is 7.5 Degrees of overlap

If you're new to this you'll notice the pattern/trend here is that a bigger duration cam with a larger LSA can have the same overlap as a smaller duration cam with a tighter LSA.

SStrokerAce

iTrader: (2)

Hey, we can't talk about LSA on LS1tech.... didn't everyone know that the BEST LSA for a LS1 motor is 112°?

GIGAPUNK, your right on your thinking.

Bret

Adrenaline_Z

No, it's 111.5...but only if you have a stroked motor with an extra long rod!

Giga:

There are some diagrams of lobe shapes on identical LSA's which illustrate
the thought you have. The intake opening and exhaust closing points at the
valve determine the overlap.

For the most part, and generally speaking (*cough...*) the higher overlaps
are used in naturally aspirated engines with high RPM use. You'll see major overlaps occur in performance motor that are tuned for high RPM use.

Large overlaps can cause exhaust to reamin in the chamber, or even move
into the intake port at certain RPM's. The pressures present at the valves
upon opening, and the strength of the pulses and direction of pulse motion
will determine how much charge moves into the cylinder.

Think that fluids/gas move from high pressure to low pressure. Don't mind
the RPM values, they are just for sake of discussion (you have to cover yourself
in Internet tech land don't you know!)

Imagine the exhaust valve hanging open while the piston is starting
the intake stroke at 1500 RPM...the exhaust stroke just finished and the
velocity of the exhaust gas leaving the cylinder is relatively low.

The intake charge wants to move into the cylinder, but the exhaust gasses
are slow and lazy creating a high pressure area in the exhaust port.

Now the piston is moving down, creating lower pressure in the cylinder
("vacuum"), and the exhaust could get drawn back in while the exhaust
valve is still open. This displaces clean intake charge and robs power.

The same scenerio at high RPM:

...the exhaust stroke just finished and the velocity of the exhaust gas leaving the cylinder is relatively high.

The intake charge wants to move into the cylinder, the exhaust gasses
are fast and strong creating a low pressure area in the exhaust port.

Now the piston is moving down, creating lower pressure in the cylinder
("vacuum"), and the exhaust port is at an even LOWER pressure because
of the high velocity gas moving out. The exhaust valve is still open. This lower pressure in the exhaust port scavenges the chamber and helps "pull" intake charge into the cylinder.

2000TransAmWS6

iTrader: (12)

so, is it at your peak torque value that the pull of the intake charge is happening most efficiently?

Adrenaline_Z

Not quite. Peak torque would be the point when most charge is trapped after
IVC...and everything that goes along with efficient combustion (timing, AFR, mechanical efficiency...).

When all the "i"'s are dotted, and "t"'s are crossed, and VE is highest, that's
when peak torque occurs.

GIGAPUNK

iTrader: (2)

My understanding is that when properly designing a cam you kind of "end up" with an LSA...

If you start with the an IVC that proves to work well with your intake, then your IVO kinda gets determined by your displacement and rpm range. Which brings us to EVC...

Do you pick an EVC depending upon a desired overlap?
Or is it determined by I/E flow ratio, exhaust port volume, or something less tangible? This is where I get lost.

Old SStroker

Not quite. In the majority of cases, max VE will be close to the measured brake torque rpm, but with exceptionally well-done intake and exhaust tuning, max VE may occur nearer brake hp peak rpm. The brake torque at this higher rpm (max VE) will probably be less than peak brake torque because of higher friction torque losses above the (brake) torque peak rpm. Best IMEP should occur at highest VE if you can burn it, as Z said.

Think of the Mean Effective Pressure (MEP) as torque per cubic inch. Indicated MEP (IMEP) is what the engine actually produces internally, but the part that gets to the flywheel is the Brake MEP (BMEP), which is IMEP reduced by internal friction (FMEP). So (BMEP = IMEP - FMEP).

IOW, you could have higher IMEP (but lower BMEP) near hp peak rpm than at torque peak rpm. You might find this sort of thing in ProStock, perhaps Cup and maybe F1 engines especially now with fixed intake trumpets.

Adrenaline_Z

Ahhhhhh, so I would have been correct if my last line read:
What you're explaining is that VE may occur beyond the torque peak with
proper tuning, even though FMEP may also increase beyond the torque peak.
In other words, I shouldn't assume VE is close to the torque peak in all cases.

I would imagine that torque graph to be fairly flat, and/or even dip down and
pick up slightly once the resonant RPM occurs?

Adrenaline_Z

I would agree with that, and I think the cam designers would also back up
that statement. You don't tune an engine with LSA, you tune it with valve
events. LSA is the product of the other events.

I'll walk the plank for this answer, but I'll never know unless I try:

I believe EVO is chosen according to the tuned length of the exhaust primary
and secondary collector length in order to time the reflected pulse of the
previously exhausted cylinder.

This would allow enough time at a certain RPM for the negative pulse to reach
the port and create a low pressure area to help scavenge the chamber and
move intake charge into the cylinder.

GIGAPUNK

iTrader: (2)

But the stock manifolds (or I would presume, any unequal length header) would have such a mess of reflected pulses, that such "tuning" would probably be useless. There's got to be more to it...

Isn't there always...