PatriYacht

Bob, I'm n/a. Wimpy outdrives holding me back.

WAGS382

Sorry I haven't responded to my own thread in a while, but I must agree with Kaama. There are a few people on OSO that go out of their way to help us with these questions and I greatly appreciate the advice. I think cstraub69, rmduilder, articfriends, Kamma, RLW, etc have provided us with very useful info and you would have to be a fool not to listen to them. The way I look at it I am willing to make an investment to gain performance, but if I over cam I will only see the benefit at full throttle, if I get a big improvement in torque in the mid range I can run more pitch and improve my cruise speed and the top end will fall into place.

rmbuilder

Fountain 333
My remark on over scavenging was in response to post # 30 from John Splendorio.
Over scavenging occurs during the overlap period as the pressure differential in the intake tract > the combustion chamber > the exhaust tract. As overlap increases due to long duration and/or narrow lobe sep the intake charge passes the valve, crosses the combustion chamber, and exits the exhaust path prior to combustion. Forced induction increases that pressure differential in the intake tract. The passing of this unspent intake charge reduces the engines capacity to perform work/torque, reducing VE and increasing the BSFC number.
A wider lobe sep reduces overlap, limiting blow through.
Bob

rmbuilder

Ian,
Great point. Using the simple hose and bucket analogy, (the hose being the intake/exhaust column and the bucket being the cylinder) the more volume of fluid the hose flows the shorter the fill time of the bucket, reducing the duration of time the hose must remain open to achieve complete the fill of the bucket (VE). The same is true on the exhaust side to completely empty the bucket (cylinder). Dial that into your peak torque target (frequency of fill/empty cycle) that’s your VE sweet spot.
Bob

tomcat

The hose and bucket is also a good way of explaining why increases in displacement without any change in heads, cam etc. do not increase HP. It takes longer to fill the bucket so the HP peak occurs at lower RPM where there is more time. Merc's 540 bulldog is a good example. More torque, same HP as the HP500 based on the 502.

PatriYacht

Great explanations! Thanks.

WETTE VETTE

How do headers play into the equation? One question a cam designer will ask is how well does the exhaust flow? Actual tubular headers with 32" equal length primaries dumping into a 4" collector will really pick up the scavenging on a NA motor, and never do you hear that good headers hurt performance because of over scavenging. HP manifolds and short tube headers, like the CMI E-Tops don't promote scavenging as well as longer runner headers do and unfortunately this is what most boat guys have to run. To make it more confusing most motors are dynoed with great headers and then installed in the boat with less efficient exhaust. I know compared to stock manifolds aftermarket is better, but they aren't headers so the exhaust duration may need to be a bit longer to help promote a little more flow to make up for somewhat inadequate headers or manifolds. I may be wrong, but this makes some sense to me. It will be interesting to hear some responses regarding this.

Craig

rmbuilder

Good info Tom. As you make changes in displacement you must also alter duration of fill to accommodate the increased/decreased volume being filled. The same principal applies to RPM or frequency of filling the container. At 4000 rpm you are filling (@100% theoretical VE) and emptying the container 2000 times/min each. At 6000 rpm the cycle moves to 3000/min. The increase in frequency will require a longer duration of fill time to accommodate the increased demand in volume (frequency) to meet the 100% (fill) VE number in the same amount of time. Because peak torque occurs at or near peak VE that’s how duration determines peak VE and the resulting peak torque point. This example does not address inertial ramming, various other aspects of the entire column dynamics or the placement of valve events, but does give insight into the engineering behind the filling and emptying of the bucket.
Wette Vette
My remarks addressed over scavenging due to incorrect camming in forced induction motors. That’s why blower motors routinely run lobe seps of 115*. Exhaust scavenging in N/A motors via headers presents a new set of challenges.
Bob

rmbuilder

Apologies to Wags for getting his thread off track.
Bob

cstraub69@comcast

Craig,
Yes header lengths along with intake lengths do pay a part in tuning. Air moves in pulse in an engine, proper lengths are needed to take advantage of this.

Chris