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Discussion Starter #1
Some of the Basics: PART I

Today, the first component you should pick when you build your engine is compression ratio. Available octane ratings are more critical when building your engine due to the unleaded factor and ethanol blends. Once you have narrowed down a compression ratio choice, the camshaft should be your next selection based on the compression, head flow (CFM's), and the power band you are looking for, and then you can broaden that into horsepower & torque levels. With a cam choice in mind, you can then select your other engine parts, to include your drivetrain choices, that will compliment the cam and not fight it. If you purchase your engine parts first and they are not matched, they could turn out to be a very poor combination when selecting a cam and you may have to be willing to change some of these components or run the risk of having a disappointing engine. The best engine combination is when the desired torque & power band of all the components (camshaft, cylinder heads, intake manifold, carburetor size, compression ratio, headers and exhaust size) are matched to work in the same RPM range that you have chosen for your needs. A low RPM torque cam with a 2 x 4 tunnel ram intake is going to be a bad combination. A .550" lift with stock heads will be a bad choice. Selecting the wrong carb, intake, exhaust manifolds, or even the drivetrain components for your build may not be as obvious - until you get in the car and drive it.

Cylinder Pressure is an important aspect in selecting a compression ratio and a cam and something most do not consider when choosing their cam. The engines compression ratio (called static compression) creates cylinder pressure. The timing of the intake valve closing, coupled with the static compression, gives us a cylinder pressure called "Dynamic Compression" and will be covered later. The limits of cylinder pressure are based on the octane of the fuel you are using where the octane number indicates how much cylinder pressure the gasoline will tolerate without exploding. You want a very fast controlled burn and not an explosion. Exploding fuel is called detonation and it can destroy engines and parts if not corrected quickly.

Cylinder pressure is a result of 5 things, the final displacement of the engine, the compression ratio of your engine, cylinder head material whether iron or aluminum, the closing point of the intake valve, and the air density or altitude where you live.
You want to determine exactly what your actual compression ratio is or what it will be when building your engine. The higher your compression ratio, the higher the cylinder pressure. The lower your compression ratio, the lower the cylinder pressure. Using the factory or manufacturer's advertised compression ratio can be very misleading because the true compression may be actually lower. This is true in the Pontiac world as the factory advertised compression is actually lower in reality.

The engine's static compression ratio is determined by adding up the following numbers: the bore & stroke, the combustion chamber volume in cc's, the valve reliefs/dish/dome in the piston top in cc's, the piston's deck height or the space between the top of the piston at top dead center (TDC) and the top of the block's deck in cc's, and the head gasket bore & thickness in cc's. It seems that as a rule of thumb that an engine's compression should be between 9:1-9.5:1 to run on pump gas with iron heads, but of course this can vary depending on the build and your altitude.

Cylinder pressure equals heat and the more pressure the more the heat and the higher the octane you need to use. The type of material the cylinder heads are made of, such as iron or aluminum, will affect combustion heat and thus cylinder pressures. Aluminum heads draw out the heat from the combustion chamber much faster than the iron head and this in effect reduces the cylinder pressure. A general rule-of-thumb is that whatever the maximum compression is that you can use with the iron heads, raise it at about 1/1.5 points when switching to aluminum heads.

The compression stroke is what makes cylinder pressure. It begins right at the end of the exhaust stroke as the intake valve begins to open before top dead center(TDC), then draws in the air/fuel mixture on the intake stroke as the piston travels down the cylinder towards bottom dead center(BDC), and then begins to go up on the compression stroke. The compression stroke cannot start until the intake valve closes and seals off the air/fuel mixture which is at some point after bottom dead center (ABDC). The closing point of the intake valve after bottom dead center determines when the piston actually begins to build cylinder pressure as it travels up the cylinder on the compression stroke.

As a cam gets bigger(longer duration) the intake valve stays open longer and closes later to fill the cylinder with as much of the air/fuel mixture as possible. But, as the cam duration increases and keeps the intake valve open longer to maximize the air/fuel mixture, the piston goes past its lowest point at bottom dead center and begins to moves up higher in the cylinder bore on its compression stroke where at some point the intake valve is closed. This effectively reduces the compression stroke (or the squeezing of the air/fuel mixture with the intake valve closed) making the compression stroke shorter which in turn reduces the cylinder pressure. This is why cam manufacturers recommend a higher static compression ratio to counter the reduced cylinder pressure of the later closing intake valve on a long duration cam such as those cams having 280 degrees of duration or more. This type of long duration cam which reduces cylinder pressure may not work with an already low compression engine. Also, as cam duration increases, the power increases and the torque & power band also moves up in the RPM range.

The other side of the coin is that the shorter duration cams close the intake valve earlier after bottom dead center which puts the piston further down in the cylinder bore and closer to the bottom of the engine's stroke by the time the intake valve closes. This traps less of the entering air/fuel mixture in the cylinder but the earlier intake closing effectively makes the compression stroke (or the squeezing of the air/fuel mixture) longer and increases the cylinder pressure. This type of cam can build cylinder pressure and is better suited for the lower compression ratio's. The smaller the duration number, the lower the RPM that the torque & power band will be. Here it can be tailered for such uses as RVs, towing, stock engines, etc..

In general, a big cam = lower cylinder pressure, and a small cam = higher cylinder pressure. One cam grind will not do everything so you may have to make a compromise based on what you want your engine/car combo to do. If you want to drag race with a cam that pulls at 6500 rpm or more, don't expect the engine to operate on the street very comfortably. If you want to tow a camper, don't expect the engine to spin 6,500 RPM's.

Dynamic Compression. As noted ealier, at BDC, the intake valve is still open as the piston is rising up the bore on its compression stroke with no actual compression occurring because of the open intake valve. Compression does not begin until the intake valve closes(IVC). Once the intake valve is closed, the air/fuel mixture starts to compress. The dynamic compression ratio(DCR) is expressed as the ratio between the volume of the cylinder area above the piston once the intake valve closes and the volume above the piston(the static compression ratio(SCR) at top dead center(TDC). The DCR is what the air/fuel mixture compression ratio actually is. It is often lower than the static compression of the engine. In short, the DCR is dependent upon the intake valve closing point along with the static compression ratio - cam specs have as much effect on the DCR as does the mechanical specifications of the motor.

DCR is not an absolute, just a tool to use in better selecting/matching your cam to your compression and vice versa. The characteristics of an engine combo running at high speed changes the engines volumetric efficiency which will have a major effect on cylinder pressure(as would nitrous, a supercharger, or turbo). The DCR is more applicable to street and street/strip cars where much of the daily driving is at lower engine RPM's. A good rule of thumb is to have the engines DCR in the range of 8-8.5:1 and that can be dependent on iron or aluminum heads and altitude (I feel a street engine with iron heads should be at 8:1 or lower). Higher than this, there may be detonation problems with pump gas. Engines with “small” cams may do better with a lower SCR to avoid detonation while engines with “big” cams that have a later IVC point may tolerate a higher SCR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Several Dynamic Compression Ratio calculators can be found on line if you want to play around with these numbers.

Air density, or altitude, can be overlooked and has an effect on cylinder pressure. It is not often considered when building an engine. Air is thinner the higher above sea level you go. Less air going into the cylinders means less cylinder pressure at top dead center when the spark plug fires. It’s a lot like lowering the compression ratio in the engine. Cylinder pressure starts to really become affected when the altitude begins to get around 1200’ – 1500’ above sea level and at these higher altitudes the engine may need additional compression to increase the cylinder pressures needed to take advantage of the 91 octane gas which would not be needed at and altitude of 1,000' or less. So knowing the altitude at which you live and deciding where the car might be driven could cause some adjustments in your engine's compression or camshaft selection to build more cylinder pressure. This may also explain why you read how one guy runs 10.5:1 with no problems and a similar build has "pinging" with 9:1.
 

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Discussion Starter #2
Some of the Basics: Part II

Duration is the number of crankshaft degrees that a valve is held open. The larger the cam duration number in degrees, the longer the valve is held open. This affects both the power band and driveability. Longer duration cams improves top end power and moves the HP & torque higher up the RPM range of the engine with a corresponding reduction in low-end horsepower & torque, vacuum, and idle quality. In general the longer a valve is open the higher the RPM an engine can rev. Shorter duration cams can boost low-end torque & HP which results in better vacuum, idle quality, and throttle response but sacrifices top-end power.

Intake Centerline (ICL) is the number which represents where the intake & exhaust lobe reach the point of maximum lift. The ICL is measured in crankshaft degrees. The intake lobe centerline is After Top Dead Center(ATDC) and exhaust lobe centerline is Before Top Dead Center(BTDC).

Lobe Separation Angle (LSA) is the distance(measured in camshaft degrees) between the intake and exhaust lobe centerlines of a cam. As the LSA gets larger(wider), the intake and exhaust lobes are physically spread apart, which pushes the valve events further from each other and decreases valve overlap(valve overlap is the time period in degrees in which the intake and exhaust are both open at the same time). As the LSA gets smaller(tighter), the intake and exhaust events are brought closer together increasing valve overlap. A wide LSA would be 112-116 degrees and a narrow LSA would be 102-110 degrees. Lobe separation affects the valve overlap between the intake and exhaust valve which in turn affects the power curve of the engine, vacuum, and idle quality. The placement of the intake and exhaust lobes on the camshaft cannot be changed unless the cam is reground or new cam is selected having a different LSA. To calculate the LSA of a cam you simply add the intake and exhaust Lobe Centerline figures together and divide the sum by two. Changes in Lobe Separation Angles changes the four opening and closing valve timing events of a camshaft. LSA is not the same as duration. You could have a 280 degree cam duration on a 110 LSA or 114 LSA.

A narrower LSA will:
Narrows the power band
Increases maximum torque
Moves torque to a lower RPM
Decreases piston-to-valve clearance
Builds higher cylinder pressure
Increase effective (Dynamic) compression
Increase cranking compression
Increase chance of engine knock
Valve overlap Increases
Exhaust Gas Reversion effect increases
Idle vacuum is reduced
Idle quality suffers

A wider LSA will:
Broadens power band
Reduces maximum torque
Raise torque to a higher RPM
Increases piston-to-valve clearance
Reduce maximum cylinder pressure
Decrease effective (Dynamic) compression
Decrease cranking compression
Decrease chance of engine knock
Valve overlap decreases
Exhuast Gas Reversion effect is reduced
Idle vacuum is increased
Idle quality improves

Valve overlap begins as the piston approaches Top Dead Center (TDC) on the exhaust stroke and continues until just after TDC. This is the time when both valves are open as the piston travels in the bore. The exhaust valve is just closing, but still slightly open as the piston is pushing the flow of exhaust gasses out. The intake valve at this point is just beginning to open. The departing exhaust gas flow creates a pressure differential (vacuum) which in turn helps to pull into the cylinder the fresh air/fuel mixture without any of the intake charge passing into the exhaust system. So the exiting exhaust gases at TDC are replaced by the incoming intake charge under vacuum and fill the cylinder with a fresh air/fuel mixture greater than what would normally be drawn into the cylinder by the piston's travel alone - the exhaust pressure waves draw in the intake charge while both valves are open and fill the cylinders with a greater volume of the air/fuel mixture much like a supercharger does. However, this is best utilized as engine RPM's increase and can overcome the intake and exhaust gas reversion found at the lower RPM's which give the cam its lumpy and rough idling sound, low engine vacuum, and poor low RPM throttle response.

Typical overlap for a given application as follows; trucks/good mileage towing 10°-35°, daily driven low rpm performance 30°-55°, hot street performance 50°-75, bracket/oval track racing 70°-95°, race 85°-100°, and pro race 90°-115°. As cam overlap increases, engine vacuum decreases, idle gets lumpy, idle speed needs to be higher, and the car can become very sluggish at low RPM's and will require things to make up for the lack of bottom end horsepower & torque. Things like higher compression, low rear-end gears, and higher stall converters are required to get the engine quicker into those higher RPMs where a cam with a big overlap can begin to do its scavenging thing and increase cylinder pressure - think RA IV, 4-speed, and 4.33 gears.

Valve Lift is a function of the cam lobe lift multiplied by the rocker arm ratio. Lobe lift is ground into the cam but the actual lift seen at the valve can be changed by using a different rocker arm ratio, Pontiac used the 1.5 & 1.65 rocker arms. Typically more lift increases power and increasing the valve lift without altering duration will boost power without changing the power band. To find the lift that is best suited for your build, find the maximum cylinder head flow point (CFM's) and match the lift to that range. There is no need to use a .600" lift cam when the heads stop flowing at .500". On the other hand, if peak lift is too low the airflow will be restricted into and out of the engine and reduce power when there may have been more available.
 

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Discussion Starter #3 (Edited)
Now for the good stuff. I used my Dyno 2000 engine program to arrive at these comparisons. The Dyno 2000 is far from being sophisticated like a number of the more exacting engine analizers. It is basic and can give a ballpark idea of what to expect for your engine build. I set-up the program using the 1968 400CI with "068" cam and got it as close to factory numbers as I could. HP was real close, but torque numbers did not match factory, but were not bad. I then dropped the compression of the 400CI & 455CI test engines to a 9.25:1 compression to run pump gas. Playing around with better heads, headers, bigger carbs, etc. certainly improves the numbers, but the enclosed numbers are with factory parts; carb, big valve heads, free flowing exhaust with mufflers. Perhaps not 100% accurate by anymeans, the figures do reflect the changes of HP & Torque (TQ) based on the cam specs and engine cubic inches. Dyno 2000 begins at 2000 RPM so this number is used as a base. Then I found the best HP & TQ numbers for each cam/engine and the RPM it was reached. You can see how the power curves move up the scale as bottom end is lost as the cams get bigger and cam overlap increases. You can also see the comparison between the cubic inches.

Note the smaller cams work better with the 9.25 compression. The HP & TQ numbers begin to really dive using the "068", "744", & "041" cams. The HP & TQ numbers at 2000 RPM using the .406" lift were extremely low, so I upped the lift to .516" to get a better number. I then changed the cam lift to see how more lift changed things. The last block of numbers reflects an increase in compression, 10.75:1, which bring HP & TQ numbers right up.

So how far off are these comparison numbers? Prior to 1971, engine power numbers were recorded without any accessories, exhaust restrictions, and at the highest listed compression ratio's. Numbers were also juggled for insurance purposes - typically lower. The 1971 AMA specs for the 400CI & 455CI include both the Gross and Net HP & TQ numbers. This was the year that compression dropped, HP & TQ ratings went from Gross to Net, and rated power levels went down. The 400CI, 8.2 compression, 4Bbl, dual exhaust had a Gross HP of 300 @ 4800RPM and a Net HP of 255 @ 4400 RPM (200 @ 4000 RPM single exhaust). Gross TQ was 400 @ 3600 RPM and Net was 340 @ 3200 RPM (305 @ 2800 RPM single exhaust) The 455CI HO, 8.4 Compression, 4 Bbl, dual exhaust has a Gross HP 335 @ 4800 RPM and a Net HP of 310 @ 4400 RPM. Gross TQ was 480 @ 3600 RPM and Net was 410 @ 3200 RPM. The standard 455CI, 8.2 compression, 4Bbl, dual exhaust had a Gross HP of 325 @ 4400 RPM and Net HP of 260 @ 4000 RPM (230 @ 4400 RPM single exhaust). Gross TQ was 455 @ 3200 RPM and Net TQ was 380 @ 2800 RPM (360 @ 2800 RPM single exhaust).

WARNING: Keep in mind that when selecting a cam that you must have matched valves, spring heights, spring pressures, & retainers when going bigger lifts. Clearances between the compressed valve spring coils, valve retainer to valve guide, rocker arm cup to rocker arm stud, pushrod to pushrod hole, and valve to the top of the piston at full lift must be checked - DO NOT ASSUME and just install a big-a** cam and then wonder why things bent or grenaded. I hate when that happens.
 

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Discussion Starter #4 (Edited)
Having a little more fun, I chose another Pontiac cam that looked interesting. It is listed as the 1977 400CI, 200 HP, 8.0 compression, A/T cam used in the Firebird body. I used the same Dyno 2000 specs I had set-up earlier on the other Pontiac cams to get a fair comparison between this cam and the earlier better known high performance GTO cams. I began with the factory lift and increased it just to see how much it would improve. At the bottom of the page I included the power band using a 455CI engine to show a pretty good flat torque range that might make this a good cam selection for a 455 street cruiser. This cam dies out quick after 5,000 RPM's, but you might pick that up with a little better flowing heads, carb tuning, and headers.

The next comparison was the Comp Cams 110 LSA cam versus the Pontiac "067" cam. I chose this not only to compare the different LSA numbers, but also because the durations were very close. To even up the playing field, I corrected the lift of the "067" cam to that of the Comp Cam to .488"Int/.491"Ex which would be in my opinon a better choice for the larger 455CI engine.

Lastly, I dug up this cam sheet having an assortment of "old school" cams just to give you an idea of the assortment of cams and specs available. It provides the RPM range of the cams and also lists the requirements to run the cam just as all cam makers do. Often when selecting a cam, most are geared for the 400CI and it is said to go one step up with a 455CI - but again, you want to know what your compression will be in selecting any cam grind and match all your parts to the RPM band you want your engine to run in.
 

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Jim - Amazing thread, thanks! Did you look at area under the curve as well to see if there were even bigger differences ?
 

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Discussion Starter #6
Jim - Amazing thread, thanks! Did you look at area under the curve as well to see if there were even bigger differences ?
LOL, no, the Dyno 2000 is far from that precise or good. It just has basic parameters. For example, I can change stroke, but cannot change rod ratio otherwise I would have thrown in a 400CI with the stoker kit just for fun. No CFM input, just categorized by type of head and the given modifications already input into the program by selecting the head type which best describes your head. So again, kinda generic, but gives you a reasonable idea as to what an engine will do. It has a feature where I can input a range of different cam specs and few other minor items and then the program will process all the parameters and spit out the best 10 choices. I did this with my engine build to select the cam choice I am using, and with enough parameters, the program can run a good 18 hours to produce these choices. Now I don't know if the cam is going to work as I hope, but I won't know that until I get it back up and running and out on the street. However, all my research into cams along with the Dyno program should have my engine smoking the tires at will -unless my car hooks up like a drag strip racer. :yesnod:

I have seen other programs that are downloadable off the internet that really get down to the nitty-gritty where you can really input some exacting info. The Pontiac engine builders as well as the cam manufacturers all use this kind of program when you fill out one of their forms in selecting a cam for your build. It is really the way to go rather than guess at it OR rely on "old school" thinking of what we used to select in cams. So much has changed with the biggest thing being the unleaded with ethanol pump gas. If you want to run racing gas, then you could go with "old school" thinking, but my budget doesn't include a stash of cash to support racing gas prices at the rate I like to drive my cars. :thumbsup:
 

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Jim - I am about to remove the intake, carbs, and valley pan - cleanup / paint & rebuild the carbs. Is it possible to identify the cam, i.e. will there be a cast part number visible ? I'd love to know what I have rather than just replacing. Thanks
 

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Jim - I am about to remove the intake, carbs, and valley pan - cleanup / paint & rebuild the carbs. Is it possible to identify the cam, i.e. will there be a cast part number visible ? I'd love to know what I have rather than just replacing. Thanks
Pontiac factory cams have a single character stamped into the face surface (where the cam gear bolt goes) that can ID the cam.

Bear
 

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Discussion Starter #9
Jim - I am about to remove the intake, carbs, and valley pan - cleanup / paint & rebuild the carbs. Is it possible to identify the cam, i.e. will there be a cast part number visible ? I'd love to know what I have rather than just replacing. Thanks
The only way I have seen and read about to ID the cam is the stamped letter code on the front of the cam where the gear goes on (however it appears the 1977-79 Trans-Am/Formula cams were stamped at the rear). There is no part number on the cam. If I am not mistaken, there also may be the "CWC" stamping somewhere which is the camshaft foundry/casting company that cast many of the cams used in production cars as well as aftermarket cam grinders.

If the cam has even been replaced with an aftermarket cam, then the stamping seems to be found at the distributor gear end.

So you won't be able to identify the cam with the valley pan off.
 

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So I keep thinking about the cam choice, the rear end ratio, and the limited size of tire we can run on the GTO. Given 389 + ci (400 in my case), we should have plenty of low end torque with almost any cam and in most cases too much (just spin tires & create smoke) when driving hard. Why not select a cam that is ideal from 3k - 6k rpm, with peak HP at 6k (or 5,500), and have a car that launches better and keeps pulling to redline ?
 

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"...Given 389 + ci (400 in my case), we should have plenty of low end torque with almost any cam..."


Sorry, but this is not the case. Lots of guys have installed an 041 clone, and have been very disappointed with the vac & low rpm torque. Something like a Crower 60243 is about the biggest HFT I'd wanna try. If it has 10:1 or more CR, a Summit 2802, would probably be plenty of cam for most. It's similar to a Pontiac 744, but with more lift. I had a 744 in a '69 RA3 GTO. It was my DD for just over 60k miles. Then raced it for 2 seasons. The 744 did a great job on street & strip IMO. The 744 clone is still available, as a Melling SPC-3. The 744 was 1st used in a '66 3x2 389 engine.

http://www.wallaceracing.com/enginesearch4.htm

Lunati has a version with slightly more lift, less duration @ .050 lift, and a 110 LSA.

http://www.lunatipower.com/Product.aspx?id=1756&gid=340

If you have 9:1 or less CR, something like a Voodoo 262 might be hard to beat for a good 400 ci street cam.

Voodoo Hydraulic Flat Tappet Cam - Pontiac V8 262/268 - Lunati Power

Obviously, all these cams with more than stock lift will be safer with screw-in studs.

Cubes make a lot of difference when it comes to cam selection. For example: The 041 clone has proven, over the years, to be a good street or street/strip cam , for a 455.

Hey, a lot of this post is just personal opinion. And I'm just a nobody Pontiac freek. No quarrel with anyone who has opposing views. :)
 

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Good advice --- and if you compare the specs between the Ram Air IV "041" cam and the 455-SD cam that carried a different part number, you'll see that they're the same. What was pretty much a race only, higher rpm cam in a 400 was a killer street cam in a 455. Inches do indeed matter. :grin2:

Bear
 
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