Few techie questions...

[optikal illushun]

Whats the advantage or a single and dual pattern cam. which is better for a street engine, which for a strip engine?

which side of a cylinder head is usually more restrictive, intake or exhaust? will a dual pattern cam help this area out?

i have a few more but its bed time

 

[xctasy]

Without going into the whys and wherefores, the basic rule is that exhast flow in Cubic feet per miniute must be 65 to 80% of the intake flow in cfm.


If its not, there is a restriction formed. In practice, the intake manifold is between 10% and even 30% restriction to the intake flow. A great exhast system with little restriction may allow the engine to have 80% of intake flow in the exhast. In a similar manner, adding a 2V head istead of a log head may cause the exhast to flow less than 65% of the intake....simply by virtue of better intake flow causing a , um, log jam (?) at the exhast end.

When cam makers are researching the ideal cam, the take into account the total package. If its intake or exhast causes a greater than 80% exhast flow or a less than 65% exhast flow, they look at split patterns to balance the flow rates. Nitrous oxide cams are often split pattern for a similar reason... balancing the exhast to intake flow under high heat situations.


Example 1. 2V head with stock exhast.

If the intake of a port flows 200 cfm at 25 inches of water, and the exhast flow is only 125 cfm, (less than 65%) then the exhast is restrictive and a longer exhast duration or lift will correct it to up to 165 cfm (greater than 80%) if its done right.


Example 2 . Log head with stock exhast.

If the intake of a port flows 165 cfm at 25 inches of water, and the reworked divided port exhast and FSSP "header-ed" exhast flows over 135 cfm, ( thats more than 82%) then the exhast isn't restrictive at all and any longer exhast duration or higher lift exhast is wasted as you don't get any more hp when the exhast over scavanges in this situation.


There is a port window flow figure the cam grinder looks for, and the idea is to have the piston moving at maximum speed when the intake valve is opens are both open. This is how the 65 to 80% figures are defined. I don't understand it that well, but it is the critical piece to setting up camshafts. Many cam grinders go straight to this calculation, and its based on rod length and the whole cam is geared around this part.

There is a lot that can be done with the exhast system before the cam gets selected. There is no doubt that US cam grinders are at the cutting dege of there craft, and cam makers tell you what works for each combination, becasue they often have years of practice, dyno runs and customer feedback to hang there cam combos on.

 

[optikal illushun]

2V head? log head?

thanx for the assistance.

 

[xctasy]

Oppps. I forgot. You have the tecnically superior 4.9/300 engine, not our tiny log and 2v engines.

In a 4.9/300 scenario, you'd possibly look at a stock 4.9 which has the restrictive 1-bbl and small single outlet exhast. That has poor info, poor out flow, and the exhast flow would be perhaps 65% of the inlet figure.


Then you go and add a header or dual branch EFI exhast headers. You go up to an Offy 2-bbl intake, and a stock EFI cam. Result is a balanced engine, with exhast flow 65 to 70% of the inlet flow.

If you are stuck with a highly restrictive exhast, a dual pattern cam is of no benifit. If you are using nitrous or are stuck with having to run a mandatory cataylist exhast, then you don't need dual pattern.

Most companies build cams for the 80% of customers who never get into there engines. Carb, headers, and then maybee a cam is the focus of most I6 owners, and F-150 truck owners especially are often stuck with the need to keep the cat for annual inspections on vehciles which are newer than the typical 40 year old Mustang without smog controls.

Since the emission era exhast is restricted, then the cam makers tend to run cams with a little more over lap on the exhast to suit.

 

[rbohm]

8) here is some more info from ron iskanderian to help you out.


Tech Tip - 2003
Longer Exhaust Duration: Is this really necessary?

Most stock camshafts from American production V8, V6 and 4 cylinder engines manufactured today are ground with the longer exhaust lobe duration. Or, another way of looking at this is that they are ground with shorter intake durations! The former embraces the viewpoint that either the Exhaust Ports or Exhaust Pipe system is somewhat restrictive, and is in need of an assist. The latter suggests that the intake system is rather efficient and cam timing can be trimmed back a bit with out much sacrifice in power, in order to maximize throttle response and cruising efficiency.

Take your pick here. There is no absolutely correct viewpoint - because both are probably true! In a stock engine running at conservative RPM levels, for the sake of overall efficiency, fuel economy and a quiet smooth running engine, this staggering of intake and exhaust duration is quite common and appropriate.

However, High Performance is another thing entirely. Change one factor, let's say in this case, the exhaust system (installing headers and larger pipes) and you have just negated in most cases, the need for that longer exhaust lobe. Now couple this change with a different intake system and camshaft and you have really scrambled the equation. But, wait just a moment. Why is it that so many people (racers & cam grinders alike) insist on running a cam with longer exhaust duration regardless of what equipment is employed? The answer is "habit". Most of them have been somewhat successful in doing it their way and will probably never change unless virtually forced by circumstances to do so.

Before we go any further however let's review what it actually is we are trying to do with an engine when we attempt to make more power. Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!

Example #1 (Oval track racing) Here, I have often observed that the most experienced drivers are those who are most likely to run a single pattern (equal on intake and exhaust duration) cam. Why? Because such cams always, I repeat always make more torque! These veterans have a more educated foot and greater experience in feathering the throttle in the corners. They can therefore, utilize the benefit of added torque, in the lower to mid RPM range, to their advantage.

Their counterparts, the younger drivers on the circuit, generally are not as experienced and may at times actually get "crossed up" in the corners especially with a lighter car or when they are learning the ropes. In their case, a longer exhaust duration is often the more appropriate choice. It will often help them to drive better, more "flat footed" if you will, without consequence. But please for the sake of accuracy, let us be truthful. The benefit comes from an actual bleeding off of low to mid range torque, which is always what happens when Exh. Duration is lengthened, not from any improvement. The improvement, (if any) would come because of an improvement in scavenging at the extreme upper end of the power curve and would usually be marginal at best. Yet the so-called "extra power" potential of a longer Exh. Duration cam is most often why they are touted - power most people are backing away from at the end of the strait away!

Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well.

One of my favorite expressions is how "The Drag Racing mentality has infiltrated the ranks of Oval Track". Many have crossed over and made the switch in the past 10-15 years and some have brought their preconceived notions about how to cam an engine with them. A few may actually read these concepts and if they do so will at least come away with a better understanding of what they are doing. On the other hand they also could find that this information might actually help their cars to run just a bit faster!

Note: Readers may find Camfather Ed Iskenderian's Top Tuners Tip #33 "Can an Exhaust System Over-Scavenge the Combustion Chambers" to be a relevant precursor.