Post by 19_Chevelle_69 on Jan 16, 2005 21:49:15 GMT -5
The purpose of this thread is not to tell you which cam you need for your engine, but to help you understand what factors you need to consider when selecting a camshaft that will work the best for your combo. Opinions expressed are derived from my own experience building engines, and you are free to disagree. Any specific numbers, specs or recommendations in this article are based on smallblock chevy engine theory.
INTRO
Everyone has seen and heard the lumpity-bumpity choppy idle, black smoke puffing, barely running street cars and someone always says "They got a '3/4 race cam' in there."
In the words of Al Borland from "Home Improvement".... "I don't think so, Tim."
What on earth is a 3/4 race cam? Have you ever heard anyone try to explain this? Go ahead and ask the guy the next time you hear it, you won't get an explanation that makes any sense. Give Competition Cams, Crane Cams, Isky, any one of these manufacturers a call and tell them you want a 3/4 race cam for your car. Let's see how far that will get you...LOL
The street car mentioned above runs like that because there is so much valve overlap, and exhaust reversion back into the cylinders that it's trying to run on its' own exhaust at low engine speeds. This cam is WAY too much for the engine of a street car. The saddest part is that many people go out and buy this type of grind for the lumpy idle alone, and are disappointed when the car gets 2.5mpg, and has NO low end power for the street. Back in the old days, the 327cu/350hp Chevy had such a wicked idle due to it's 11.0:1 CR, and solid lifter lobe ramp design. The camshaft companies have fine tuned the profiles so much now, that you can expect MUCH better power and driveability out of them over the ancient muscle car blueprint cams, while still getting a SMOOTH idle, and enough vacuum to run the power brake booster....
FACTORS
Vehicle Use
Cam selection relies first on knowing the intended use for the vehicle for 90% of the use it will see. If you are building a 327cu-350cu smallblock chevy for your street car, a cam with about 225* duration will be plenty if you'll also be going to at least a 2000rpm converter, or will be rowing through the gears of a manual tranny.
More than that is a waste, and will leave you unhappy with your choice. This is due to the normal RPM range of a street engine being in the range of idle-5000 or so.
Compression Ratio
Compression ratio is another factor in the cam choice. If you find your combo has a high static compression ratio (CR), you can slightly retard the cam to let you bleed off your Dynamic (or running) compression ratio to get it into the more manageable 8.2:1 (DCR) area for a street car on pump gas. Bigger cam = more overlap = more dynamic CR bleed off.
Heads
Stock heads are a tad on the restrictive side unless you are fortunate enough to get a set of the factory Hi-Po ones, or have dropped a set of new generation Vortec heads on your engine. A big cam with restrictive heads is like trying to run a marathon while breathing through a straw. You're fighting a losing battle. At the very least, even a decent D-I-Y home Port & Polish job can give you gains of up to 15HP on an otherwise stock 9.5:1CR 350cu. Adding a bigger cam without first adding better intake and exhaust flow is beating a dead horse. The whole purpose of the bigger cam is to increase flow, and it can't do that pulling air through a straw, and pushing it out of a pea-shooter exhaust system. Which brings us to......
The IN's and OUT's -- Flow
An air cleaner that can flow around 900-1000 cfm is great. Most SBC street engines have 600-750cfm carbs on them (more is a waste in most cases), which means the air filter can outflow the carb. No restriction...that's good! A HUGE carb on your smallblock actually can hurt performance by lowering the throttle response and giving you a bog when you mash the pedal down.
Dual plane intakes will help you increase power simply by design. They create a good harmonic resonance in the intake that smoothens out pulses, and helps your mixture flow more evenly into the ports.
Headers. 1 3/4" headers are considered "big tube" headers for a smallblock. A street SBC will behave itself with a 1 5/8" tube header, without robbing the bottom end power due to exhaust reversion. Back them headers up with a nice free 2.5" dual exhaust system with an H-pipe in the middle, and you will be on your way to getting the most you can out of your engine, in the WHOLE RPM range, without disappointment.
As a BONUS, you will also get surprisingly good fuel mileage for your cruiser.
Let the question period begin......
INTRO
Everyone has seen and heard the lumpity-bumpity choppy idle, black smoke puffing, barely running street cars and someone always says "They got a '3/4 race cam' in there."
In the words of Al Borland from "Home Improvement".... "I don't think so, Tim."
What on earth is a 3/4 race cam? Have you ever heard anyone try to explain this? Go ahead and ask the guy the next time you hear it, you won't get an explanation that makes any sense. Give Competition Cams, Crane Cams, Isky, any one of these manufacturers a call and tell them you want a 3/4 race cam for your car. Let's see how far that will get you...LOL
The street car mentioned above runs like that because there is so much valve overlap, and exhaust reversion back into the cylinders that it's trying to run on its' own exhaust at low engine speeds. This cam is WAY too much for the engine of a street car. The saddest part is that many people go out and buy this type of grind for the lumpy idle alone, and are disappointed when the car gets 2.5mpg, and has NO low end power for the street. Back in the old days, the 327cu/350hp Chevy had such a wicked idle due to it's 11.0:1 CR, and solid lifter lobe ramp design. The camshaft companies have fine tuned the profiles so much now, that you can expect MUCH better power and driveability out of them over the ancient muscle car blueprint cams, while still getting a SMOOTH idle, and enough vacuum to run the power brake booster....
FACTORS
Vehicle Use
Cam selection relies first on knowing the intended use for the vehicle for 90% of the use it will see. If you are building a 327cu-350cu smallblock chevy for your street car, a cam with about 225* duration will be plenty if you'll also be going to at least a 2000rpm converter, or will be rowing through the gears of a manual tranny.
More than that is a waste, and will leave you unhappy with your choice. This is due to the normal RPM range of a street engine being in the range of idle-5000 or so.
Compression Ratio
Compression ratio is another factor in the cam choice. If you find your combo has a high static compression ratio (CR), you can slightly retard the cam to let you bleed off your Dynamic (or running) compression ratio to get it into the more manageable 8.2:1 (DCR) area for a street car on pump gas. Bigger cam = more overlap = more dynamic CR bleed off.
Heads
Stock heads are a tad on the restrictive side unless you are fortunate enough to get a set of the factory Hi-Po ones, or have dropped a set of new generation Vortec heads on your engine. A big cam with restrictive heads is like trying to run a marathon while breathing through a straw. You're fighting a losing battle. At the very least, even a decent D-I-Y home Port & Polish job can give you gains of up to 15HP on an otherwise stock 9.5:1CR 350cu. Adding a bigger cam without first adding better intake and exhaust flow is beating a dead horse. The whole purpose of the bigger cam is to increase flow, and it can't do that pulling air through a straw, and pushing it out of a pea-shooter exhaust system. Which brings us to......
The IN's and OUT's -- Flow
An air cleaner that can flow around 900-1000 cfm is great. Most SBC street engines have 600-750cfm carbs on them (more is a waste in most cases), which means the air filter can outflow the carb. No restriction...that's good! A HUGE carb on your smallblock actually can hurt performance by lowering the throttle response and giving you a bog when you mash the pedal down.
Dual plane intakes will help you increase power simply by design. They create a good harmonic resonance in the intake that smoothens out pulses, and helps your mixture flow more evenly into the ports.
Headers. 1 3/4" headers are considered "big tube" headers for a smallblock. A street SBC will behave itself with a 1 5/8" tube header, without robbing the bottom end power due to exhaust reversion. Back them headers up with a nice free 2.5" dual exhaust system with an H-pipe in the middle, and you will be on your way to getting the most you can out of your engine, in the WHOLE RPM range, without disappointment.
As a BONUS, you will also get surprisingly good fuel mileage for your cruiser.
Let the question period begin......
