Can anyone explain to me, or tell me where to look for the following info. How can i tell by reading a Cam spec sheet, what rpm band a camshaft is designed to best work? This is for a sbc. I have all the figures, and understand the very basics of lift, duration etc, but not how this relates to engine rpm, and performance. Thanks, and Merry Christmas to all.
do you want to understand the effects of lift/duration/overlap on intended rpm range, or are you trying to estimate the rpm range based on specs? the first part is easier to explain using some basic camshaft theory the second part takes time and experience along with knowing the details of the engine it's going in. I have a 292*degree cam with a 108* lobe separation angle. Should be a fairly lumpy grind with healthy overlap, but since the mill's been stroked to 415 inches the idle is only "rumbly" with a bit of some rich fumes out the pipe. If i put that same cam in a 273" mopar, it would be one rowdy bitch. So to answer the second question: refer to the manufacturer. rick
The closing angle of the intake valve is the easiest way to estimate where in the rpm range the cam will make power. The earlier the intake is closed (lower number) the lower rpm the power will come on. This kind of cam, though, will not make as much power at higher rpm as a cam that closes the intake later will make. There's a lot more to cam selection, of course. Higher compression will help make up for the loss of low end power with a later-closing cam, for example. But in starting your search for YOUR ideal cam, the intake closing is where I'd begin.
All of the above is very good. To confuse things further, what else has been done to the engine, how heavy is the car, and what is its intended use?
RPM range is as dependant on the cylinder heads and intake runner size as it is on the cam. Heres a couple rough rules of thumb for cam selection. In terms of cam specs, duration has the biggest effect on where peak power and peak torque occurs. Lobe seperation angle has more effect on the shape of the power band, narrower making it fatter between the hp/tq peaks, and falling off faster after peak HP. It also has a big effect on cranking compression, and idle. Tighter lsa makes for a cam that makes more torque, has more cranking compression, less manifold vacuum ,idles rougher, and falls off faster after peak hp. Basically, its all about manipulating the intake closing point. More duration later intake closing point. wider lsa, later intake closing point, less overlap. Tighter lsa, earlier intake closing point, more overlap. To give you an example, when I selected the cam specs for the 327 in my chevy II, I ran computer syms on two different cams, one designed to meet the compromises I had laid out out for reasonable part throttle fuel economy, and one for an all-out "saturday night special" type cam. Because I build my stuff on the ragged edge of available pump gas, both cams had to have the same cranking compression, so they had to have virtually the same intake closing point. One cam, the one I wound up using, is a custom grind, 240@050 intake, 246@050 exhaust, 114 lsa. The saturday night special cam was 256@050 intake 263@050 exhaust, 106 lsa. These two cams both made the same cranking compression, they also made peak power at virtually the same rpm, which makes sense, as the intake closing point is virtually the same. But the characteristics of the two cams is completely different. The wide lsa cam will pull around 14" of vacuum at idle, the bad boy cam would be down around 8". The wide lsa cam will idle virtually like a stocker, the tight lsa cam uh well, not so much. most guys are going to assume that the bad boy made way more high rpm power on the syms. Not so, where the bad boy cam buried the wide lsa cam was in the middle of the power band, between peak torque and peak hp. All syms were run on Performance Trends engine analyzer, which is pretty accurate. Guys always say "refer to the manufacturer". I tend to disagree with this. I have called the Comp cams "tech" line in the past. The guys on there have no clue what they are talking about. I have had the catalog in front of me, and they are reading verbatum from the catalog, and thats pretty much where thier knowledge ends. Also, every single street/strip type cam in the Comp catalog, with the exception of the "Thumpr" line is on a 110 lsa, so they are not likely to reccomend a cam with anything other than a 110 lsa, even if thats what your combination requires. I know I am harping on Comp cams, but with the way they have the magazines in thier pocket(due to thier advertising budget) Thats where most guys are going to go. In my experience, the tech lines at Comp are inhabited by sales people, not techs.
O boy this sounds like it's getting complicated fast.. To answer the quetions I can , here's what i have 383 sbc Static comp should be in the 10:1 area, with flat top pistons dart pro one alum heads 64cc Edelbrock air gap intake Holley 670 street avenger carb This is backed by a T56 manual six speed trans, and 9" ford with 4:11 gears. Car is a 53 Stude coupe, listed wieght is 3730lbs Purpose of car is strictly street. Cam I interested in has lift .548 and duration 286 Does that help any? Would any other info help?
How heavy the car is and it's intended use have zero effect on the engines performance, it's torque curve and or rpm range. A cams overlap, and as was pointed out, valve opening and closing points dictate this. Other modification to the engine will influence, either enhance or counteract the power curve of the cam. Over lap and exhaust valve closing have a big effect on the power curve. More over lap and a later closing exhaust valve will move the powerband up in the RPM range. A later closing exhaust valve and more overlap will allow fresh intake gasses to escape out of the exhaust port, lowering combustion pressure and power loss, especially at lower RPM. Raising your static compression will counteract this, but there are limits to static compression, especially on pump gas. If you want an engine with a lot of torque in the lower RPM range, less overlap and a sooner closing exhaust valve will produce this but it will trade off high RPM power. If you are looking for high RPM performance than more overlap and a late closing exhaust valve will give you this. Of course you need a good flowing exhaust and intake, matched compression to take full advantage of this. There is no such thing as a cam that will give you max power all over the powerband, cam timing is always a compromise. You are trading power in one RPM range for another. That is unless you have one of those fancy, new, hightech variable timing and variable lift set ups like for instance BMW has developed. The you can have your cake and eat it too.
Completely and utterly false, and a common misconception that is repeated over and over in the magazines. Overlap occurs at the end of the exhaust stroke and beginning of the intake stroke. Back to basics here, people. the four strokes of the internal combustion engine, in order. Intake, compression, power, exhaust. Overlap occurs almost 180 degrees away from the compression stroke, has ABSOLUTELY NO effect on cylinder pressure. Cylinder pressure is ALL ABOUT intake closing point.
You are talking about compression pressure, not combustion pressure. Compression pressure is dictated by the closing of the intake valve, combustion pressure is dictated by the amount of burnable gases in the cylinder at time of combustion. When your intake valve opens and the fresh gas fuel mixture begins to enter the combustion chamber, (which contrary to common conception do not get sucked into the cylinder, atmospheric pressure pushes them into a vacuum created by the descending piston) some of those gasses will escape through the still open exhaust port. The escaping exhaust gasses create a vacuum in the exhaust port due to the inertia they carry down the exhaust system, so you can use this negative pressure wave created by the escaping exhaust gases to start intake flow before the piston has started down the bore. The escaping exhaust gasses create a pressure pulse in the exhaust system which travels down the exhaust system until it reaches the end of the exhaust, it then literally bounces of the atmosphere and travels back up the exhaust system, if by the time the pulse created has traveled back to the combustion chamber and the exhaust valve is still open these fresh gases will get pushed back into the combustion chamber and out of the intake port. This can literally be seen at the carb as something known as gas stand off at certain RPM's, a lite area of what looks like fog over the carb. These pulses, which occur in the intake tract as well, are what is used to create ram charging and will enable an engine to reach well over 100% volumetric efficiency, I have built naturally aspirated dragster engines which produce in the area of 115% to 120% volumetric efficiency using these pulses, something which other wise is only possible with some type of super charging. These exhaust pulses are only effective on an exhaust sytem that has virtually no obstructions, such as mufflers, and the length and diameter of the exhaust is tuned to work with the cam overlap and a specific RPM. If your exhaust system is not tuned to these pulses or you run obstructive mufflers the overlap and late closing exhaust valve will cost you power, especially in the lower RPM's. A to obstructive/badly tuned exhaust system will cause exhaust gases which have just been expelled into the exhaust port to create over pressure in the exhaust port and will push exhaust gases back into the combustion chamber once the piston has passed top dead center and since these gases are entering the cylinder at the same time as the fresh gases should be entering the cylinder through the opening intake valve you will have what is called intake charge dilution. Both scenarios, fresh gases escaping through the exhaust port or intake dilution will decrease the amount of fresh gases that are in the cylinder at time of combustion, which will decrease cylinder pressure at combustion and thus less power.
You will need a enginering degree to understand all the intricate details of camshaft profiling . Too many variabels.... Best to ask a cam grinder for ther recomendations for your spasiffic intended use. Give them as much info as you posably can for best results
duration is 286 advertised at .050 lift intake duration is 226. , exhaust.548 Basicaly I'm trying to figure out where this cam works best. What I would like is a cam that works best at low rpm, say 1500-2000 rpm ans up to the 4000-5000 area. I forgot to mention, it is a hyd roller cam. I would prefer low end grunt to high end screaming. Like i said street car.
Not me, I have only built 800 hp naturally aspirated 509ci single 4 bbl street/strip motors. I wouldn't know anything about intake or exhaust resonance... However, you are correct about one thing, I assumed you were talking about cylinder pressure, even though you said combustion pressure, as it is such a common mis-conception that increasing overlap decreases cylinder pressure, when in fact, the opposite is true, if the duration remains constant and the lsa is tightened, cylinder pressure is INCREASED as is overlap, as the intake closing point is earlier. Guilty as charged.
We need the lsa to give any sort of meaningful answer to your question. Is this a comp cam? If it is, the lsa will be 110. A cam with a 108 lsa will perform way better in a 5.7" rod 383 and give a BUNCH more mid-range torque. Other than that, the cam specs are not far off what you would want.
There's a million different opinions on cam selection. I'm not a camshaft engineer, so I leave the selection up to somebody that is. My favorite is Cam Motion. I've had them custom grind several cams for me, and have been very satisfied. Give them a call, they will ask more questions than you have answers for, and the cam they grind will do what they say. It don't cost much more to do it right.
Call me green but 120% volumetric efficiency? How is that even possible? I would like to hear more and learn.
Your stylin. Should be about as good a match for what you want as you are gonna get. Excellent cam choice, dead on the money.
Falcongeorge has pretty well answered you questions. Since you have the rear gears for leverage and the 6 speed, I would use a cam with about 242 to 244 deg, 108 LSA and 106 installed CL. Peak torque should be around 42/4400 and pull hard to 6000. you have the heads and manifold for it and you're not going to putt that thing around at 1500 rpm. My $.02
I'm inclined to agree with you, if it was mine, I would run something bigger and spin it tighter. But for the parameters he laid out, (not what mine would be, but its not my car.)the cam he has is dead on.
Another thing to keep in mind is he hasn't told us WHICH Pro 1's he has. If they are the 180cc intake ports, they are right in line with his rpm expectations on a 383.
Pretty much explained it in the post you quoted. It has to do with pressure waves that are present in the intake and the exhaust tracts. They bounce back and forth in the tracts and if the exhaust system length and diameter as well as intake tract length and port size are tuned to work with the cam over lap and timing you can achieve a ram charging effect. This is only effective in a fairly narrow RPM band but can be used to either increase peak power to over 100% volumetric efficiency or can be used to broaden the RPM range at which peak Torque is present. It takes a lot of math, some experience and most of the time a lot of money to have an engine built like this. Every thing has to be tuned exactly right to work efficiently. Formula 1 engines use these principles to produce over 100% volumetric efficiency since they are not allowed to run super charging. Dodge experimented with this for a while in production engines, that is where the Dodge Ramcharger got it's name from. But on production engines with restrictive exhaust systems, intakes a.s.o. it was only marginally successful.
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