J-Rod,



This is a VERY good diagram to look at to understand what is going on... only problem is you need to include cylinder pressure in that as well to know anything. The intake and exhaust pressures have to be relative to that since that is the link that connects them.

I've been looking at those diagrams for a long time now, and understanding them certainly helps.

FWIW I'm going to bet that picture comes off of a turbo application.

Remember guys that boost PSI is just backpressure in the intake tract, if the supercharger is turning the same RPM it's producing the same airflow and if there is less boost there is more air entering the motor, meaning more HP.

Bret

 

[QUOTE=Adrenaline_Z]I wouldn't compare N/A motors to boosted motors when it comes to overlap.
Not sure if you're making that parallel, or taking the discussion toward another
induction type?

I'd also suggest to stay away from the term LSA when comparing overlap. There is not enough information contained in LSA values alone to determine overlap, unless we're talking about a specfic set of lobes.
[quote]
Nope, I was just showing what my friends in Australia are doing which is counter intative to many of the folks in the states. I'm not trying to steeer the direction away from the discussion at all. I was simply using it as an example of unconventional thinking. If you told the average forced induction guy on ls1tech you were going to to a narrow lobe sep cam witha big supercharger, they'd look at you funny and tell you it would never work...

I'm not sure what you mean by not enough information. I agree that duration @ .050 is only one point on the cam, but duration and LSA are enought to at least tell you your overlap at .050. From there its pretty easy withthe lift numbers to figure out the intensity of the lobe...

 

Ask an ye shall recieve. But, in answer to your question to the best of my knowledge the motor in question is a N/A BBC.

Combustion Pressure Graph

The combustion pressure graph is only 1 of 20 different ways of displaying the data. The combustion pressure graph allows the tuner to determine in a single engine test the peak combustion pressure, location of peak combustion pressure and the combustion temperatures for every cycle of the entire test load/speed range. The correct ignition timing can be established using the combustion pressure graph, energy release graph and numerical data for the entire range of engine speeds.

Adjustments to the air/fuel mixture may also be required at certain engine speeds. As the dynamics inside the engine change with changing rpm and engine modifications, so to does the air/fuel mixture ratio. Powerband flat spots and detonation zones, related to the air/fuel mixture ratio, show up immediately on the combustion pressure trace.

The numerical data can be used to determine how well the cylinder was filled with the air/fuel mixture (volumetric efficiency). Everytime a modification is made to the engine which affects cylinder filling i.e. camshaft, intake, carb, header, turbo, supercharger etc. the impact on how much air/fuel mixture is retained in the cylinder is indicated under the heading volumetric efficiency. The more air/fuel mixture that is retained in the cylinder the greater the potential for power. Sometimes an increase in cylinder filling does not result in an increase in power due to changes in some other aspect of the engine which causes a reduction in the amount of energy released from the fuel and/or how efficiently the energy was applied to the crankshaft. The pressure analyzer data indicates how well the cylinder was filled and if any other aspects of the engine, related to combustion energy and efficiency, need to be corrected to realize the power gain.

The combustion pressure trace can also be used to prevent engine damage. Detonation spikes will show up on the combustion pressure trace at the first sign of detonation. Sometimes detonation may start out by occurring mildly only every 3 or 4 cycles. Knowing when detonation is just barely begun allows the tuner to avoid making any changes which could lead to more severe detonation on the next test. The pressure analyzer data can also be used to effectively tune a commercial knock sensor so that the racer knows how much knock is OK and how much is too much.



Combustion Pressure

In the example above the scale on the left side shows the pressure (psi) in the cylinder. The scale along the bottom shows crank position in degrees. -90 indicates the piston is at 90 BTDC, 0 indicates the piston is at TDC and 110 indicates that the piston is at 110 ATDC. The blue curve shows the actual compression and combustion pressure in the cylinder. The lower pink curve shows what the pressure in the cylinder would have been if combustion had not occurred. The upper pink curve shows the combustion temperature in degrees Celsius (multiply by approximately 1.8 to convert to Fahrenheit). The software allows the user to scroll back and forth through the data in a manner similar to using a VCR.


Energy Release Graph

Some of the biggest power gains can be made by looking at the energy release graph. This graph indicates when combustion starts, how fast the mixture burns and when combustion finishes relative to piston position. It should be noted that the crankshaft rotates several degrees between the time the spark occurs and the time when any measurable increase in pressure occurs in the cylinder.



Energy Release

The pink curve shows shows how much of the mixture has combusted (left side scale in %) relative to crank position in degrees (bottom scale). In this example combustion starts at 16 BTDC. By the time the piston reaches TDC, 22% of the mixture is combusted. 75% of the mixture is combusted by 20 ATDC and combustion is completed by 85 ATDC. The blue curve shows that combustion is occurring at the quickest rate at about 5 ATDC.

Significant power gains are made by burning the entire air/fuel mixture as quickly as possible and by positioning the combustion process appropriately around TDC. Almost every engine modification has an effect on the burn rate, but without using a pressure analyzer it is impossible to know how the burn rate is affected. Many fast-burn combustion chambers allow the tuner to reduce the ignition timing, suggesting that the burn rate is improved, but in most cases only the first 50% of the mixture burns more quickly, not the entire mixture.

Although the big block engine in the energy release graph above is making 896 HP at 7454 rpm, a 10% increase in power is a realistic goal simply by getting the last 20% of the mixture to burn more quickly. Many tuners strive for a 1% increase in power thinking that there may only ever be 2 or 3% more power available. Our pressure analyzers show the tuner where to look for more power and how much more power is realistically achievable.

 

I guess that would depend on the efficiency of their exhaust system and
how well the exhaust port handled the flow of gas out of the port...among
other things.

You could probably get away with a tighter LC with a less than optimally tuned exhaust
runner/system, or a relatively restrictive exhaust.

Didn't know you were from Australia; very cool.

 

I'm wondering if Bret is referring to the cylinder pressure during overlap,
as opposed to combustion pressure.

IE: Intake port - cylinder/chamber - exhaust port pressures during overlap.

 

Santino,

Yes I was referring to all three of them on the same graph during overlap.

I honestly don't think those Aussies are that crazy in their thinking about anything, they are some smart mates down there.

Bret

 

I'm a Texan. Born and bred here. I just have friends down there...

 

You have friends?

 

huh??

 

[QUOTE=00slowredz]thats ok when i did his i come up with 15 not 7.5

 

Not that it really matters, but the big "hole" down low is not from overscavenging. The waves generated by individual megaphones are different than those generated by straight pipes. Megaphones can generate a stronger and longer suction if built right (not a simple task) but following that suction a megaphone generates a much, much more intense reversion pressure wave than a straight pipe. At lower rpm the reversion wave ends up around the time of valve overlap and you get a big hole in the powerband. Never figured out a way around this.

Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analyzer mfg.)
www.tfxengine.com

 

cars that are "undriveable" run tighter lsa's 108-110 huge lope at idle
114-116-121 are all very "mild" cams and will pull great vacum at idle

 

Depends on duration. Stock GN cams are on a 106 LSA but idle very smooth.

 

Does overlap effect driveability?

 

Not the most educated comment ever.

Overlap is more an indicator of driveability. That being said I have done cars on 108 LSA with 250-260 duration and drive very well, idle at 1000, no surge no nada

Also have done cams with 116 LSA that sound about as nasty and narly as anything else

 

I think mines on a 116 and is about as nasty as they come!

 

Perhaps this thread should have been titled "Overlaps Effect on Power."

In some generic book I have on wave tuning there was a chart that listed 30 degrees of overlap as the max for street cars. Any thoughts on this? Presume good headers true duals, mufflers, and then with and without cats.

 

I believe mine is more than 30* and I won Pump Gas drags. Car drove good. made silly power . But my gear/ tire helped allot by letting highway cruise rpm be around 2K

 

im sorry i didnt make myself clear above...
what i meant was for a given duration a tighter lsa will be more radical than a wider one.

for example you can have a 190/204 on a 108lsa and act like a granny car. but put a 260/260 on a 108lsa and watch out... however same cam on a 116-118 duration will be much milder (at idle) in sound and pull better vacum.

sorry if i confused anyone with my first statement...

 

first off, i hope that you are talking advertised duration, which would be around a 212/224 @ .050, if not then thats total b.s.

second, if you have to idle at 1000rpm to be smooth that is not necessarily a "street cam" and probably not very great vacum either.

third, people can argue what a "street" cam is or a "driveable" cam all day long like " i drive 50 miles a day with a 3000hp pro stock under the hood... so its streetable!" im talking about sounds completely stock, no idea there is a cam in there stock. sorry if there is any confusion, if you have to reprogram the computer or change the idle just to run, its probably not what im talking about.

not flaming anyone just trying to clear up any confusion i might have started!