DenzSS

Ok, how about a new discussion.

Using the following specifications(stock LS1/LS6):

Bore: 3.900"
Stroke: 3.622"
Rod Length: 6.100"


The maximum CFM requirement for a 6800rpm stock bore/stroke LS1/LS6 is
279.07 CFM.

How does this influence camshaft design both with and without ported cylinder heads?

I'm sure I'll throw in some other questions as we go. I have my opinions (as usual ), but I'll let folks weigh in for a while on this one before I get on a soapbox.

Cstraub

iTrader: (1)

The maximum CFM requirement for a 6800rpm stock bore/stroke LS1/LS6 is
279.07 CFM.

Denzz, I get the same number. Optimum valve area, measured at the seat, I am coming up with around 1.890" Now with what Denzz has posted, does it not make sense that with high flow heads that are exceeding the necessary CFM requirement that a smaller shaft would be logical. Instead larger cams are coming out as the heads get better. One point that is overlooked is "Acceration Quickness". . .how quick after as shift point does the engine reach the desired shift rpm again. With everthing big. . .sure the motor pulls. . but it is lazy. With equal power, the quicker engine to accelerate will always win.

Chris

DenzSS

Wow, I'm getting zilch on this one so far.

CHRISPY

Where did you come up with this number?

Cstraub

iTrader: (1)

Hey, I had to go to lunch!!!!!!!!!!!! Besides everyone is at the AERA show in Vegas.

Chris

DenzSS

I can post the formula after I get home. Just have to find it and dig it back up. We just wrote an application to handle it.

DenzSS

Without getting too crazy, here is something for comparison...

At 6800rpm

408 Cubic Inch LS1
4.030" Bore
4.00" Stroke
6.125" Rod

331.93 CFM max.

CHRISPY

What are the parameters in this equation? What cc's (intake and exhaust) are you using in your calcs? What cam timing? what intake? what lift? just dont understand how you are arriving at these numbers.

99 Black Bird T/A

iTrader: (4)

Ok lets say it's ~280 cfm as suggested.

I had a set of JPR LS6 S1 heads that flowed 285 cfm on intake so they should have met the maximum requirement for airflow on a 346 at 6800 rpm.

With LS6 intake those heads only flowed 239 cfm peak. Which isn't enough air for max power.

I think you have to take into consideration what the heads flow with the intake in order to make accurate cam selection. In my example a bigger than expected cam would be needed to scotch the poorly flowing JPR LS6 S1 heads.

There are of course other things to consider but the raw intake numbers are basically worthless. The JPR's lost ~45 cfm thru the intake. Many other heads tend to lose a lot thru the intake. Even great heads are going to loose some too.

soslo

iTrader: (12)

excellent question...I brought something like this up with carbs a long time ago...the typical small block 350 doesn't need even a 600 CFM carb...but for some reason, a 750 gives more power most of the time. Perhaps it has to do with the flow at non-peak values.

Example: a head may flow 280 CFM (seems like you wouldn't need more than that), but that is only at peak. So if you can get a head that flows 320 max, but 280+ over lots of lift, then you make more power.

I don't know.

Cstraub

iTrader: (1)

soslo,
has to do with air turning. . .kinda like a car in a curve. . .gotta slow down. . .tighter curve. ..slow down alot . . . long sweeping curve. . .doesn't have to slow as much. Now picture the venturi size and air entering. Make sense?

Denzz, this is getting alot of views, but not alot of comments.

Chris

DenzSS

Chris,

You're moving the discussion too far ahead and making it too complex at this point. Since I made this post to get people to thinking, I'll hold off posting the formula for a bit and just give you a few things to think about.

A few thoughts to ponder:

We aren't even discussing the cylinder head at this time. No valve angles, valve diameter, intake/exhaust cross sections, etc. All you're doing at this point is figuring out how much air you need to be able to supply to fill the cylinder at a given RPM. You should be able to come up with the equation yourself. You have a certain volume of a cylinder that needs to be filled, you know how a 4-stroke engine operates, and you know at what speed the engine is operating.

Make sense?

To correctly think about camshafts and cylinder heads, we need to tackle this first.

DenzSS

Yup. Maybe we'll get a few more bites later.

SSCamaro99_3

iTrader: (1)

Denz, ihis is interesting. I want to see where this goes, but you make my head hurt. Can we have another 4 page discussion about the valve events of the Hotcam?

Steve Bryant

iTrader: (1)

It seems to me that at 6,800 RPM'S for a normally aspirated four cycle engine operating at 100% Volumetric Efficiency (VE) you will have 3,400 exchanges of the displacement of the engine.

This means that a 346 CID (5.7 L) you will exchange 1,176,400 CI of mixture and for a 6.8 L /408 CID engine you will exchange 1,387,200 CI of mixture. Now there are 1,728 cubic inches in a single cubic foot.

If we divide the cubic inches of mixture flowing through the engine at 100% VE by 1,728 we will get the CFM requirement for the throttle body/induction system. Now there are always inefficiencies and the engine. However, my math calculates out at 680 CFM for the 5.7 L and 802 for the 6.8 L.

Mark Campbell

You are assuming that the pressure drop across the port is 28"H2O at all times. It isn't, much of the time it is far less than 28". Same thing with the carb issue. 4BBL carbs are flow rated at 1.5" Hg pressure drop (20.4" H2O) across the venturi, but most engines run at WOT with only .3 to .8"Hg across the venturi. An 850CFM carb (rated at 1.5"Hg) only flows 640 CFM at .85"Hg across the venturi. The pressure drop throught the port is continually varying and that is why you get more power from a head that flows more at 28" H2O.

DenzSS

You're getting there Steve, but you've left a few things out. I'll give you a hint on one of the problems. 4-stroke. It is also easier if you convert engine displacement to cubic feet/cylinder.

DenzSS

Mark nailed that one dead on. Pressure differentials change throughout the whole operating range. That is one of the reasons flow benches don't tell the whole story.

DenzSS

We'll get to a point here pretty soon. I'm sure more will jump in pretty soon.

J-Rod

I've been busy today. But, I have something I have been working on for "cam thread III", so I will mostl likely post some stuff in there... It relates to camming/overcamming..