Comp cams xe268h for 318
Oh yeah big time!
here is something that illustrates cam characteristics pretty good.
Consider three different cam sizes B,A,and C each just one size bigger than the last. Imagine the lowest is a 256 and then a 262 and finally the 268. Notice the huge amount of bottom-end power given up to get a relatively small amount of power at the top. This stems from the pressure loss down there, as each larger cam losses progressively more low-rpm efficiency, ......... to get that higher rpm efficiency.
The trick is to choose the right cam for the combo..... and if the Scr is already in the basement to start with, then there is just no getting around that low-rpm performance loss, with a streeter.
The E and D curves are representative of the shape of the power peaks using wide and narrow LSAs, not on the same cam tho.Just consider the shapes
The thing you want to notice is where the B line crosses the C line. I'm gonna guess that is about 4200rpm(see note#1). If I'm right, that would be 47mph with 2.76s. Now just let that sink in for a bit.................................................................. The speed limit on most city streets is 30/35 mph.
Note#1,
how I got 4200. I assumed the bottom of the graph to be 3000 and the highest point of the C-curve to be 5400, so that is a range of 2400rpm. The crossover point appears to be at about 50% of the range so. .50x2400 plus 3000=4200rpm....Ok but say the low end of the graph is 2500, then .5x2900 plus 2500 is 3950@44mph, still speeding ,mostly.
Now look what that C cam is doing to your power. Nothing but a total power loss under 47mph.And not just a little bit, but a huge bit. You can never make that up in just the engine, no matter how much compression you throw at it. You can see that, without tirespin, the B-cam will be way quicker to 47 mph than the C-cam.......... because it is putting down more power during that time.
So suppose you had that B-cam which might peak at say 4400, and by the shape of it you can carry it out to 5000. So then for low ET, you could run 4.30s to get 4900rpm@35mph. And you will outrun any C-cammed 318 with any like-sized convertor and 2.76s; no matter what it's Scr.
This is why I love a stock 318 with nothing more than 4series gears and the 2800. A freeflowing exhaust is a must and a 4bbl will get you a little more wheel spin in second.
There is another option, a new TC that eliminates all the rpm or maybe just most of it; below the crossover point,which I guesses to be 4200. Now your rpm will blast up to where the power of the C-cam is and rocket you off the line with mega-wheelspin
EDIT; This part was added later:
Now lets talk about the D and E curves.
If you have two cams with the same specs except they are ground on different centerlines, this affects the power delivery, as the curves are trying to demonstrate. Imagine the D-curve is on a 114 LSA, and the E-curve is on a 108LSA. Notice how the E-curve builds to the power peak and then falls off rapidly. This is a nice shape for a drag-race engine, with tight transmission ratios or a high-stall TC .A cam with this shape usually makes a little more power over the narrow rpm range that a drag-race engine uses.
The D-shape is generally preferred for a streeter because the power delivery is smoother and spread over more rpm, so you always know what to expect when you gas it. Unlike the E-shape which generally explodes and then pukes; Like a pipey 2-stroke motocross bike.
Off the shelf cams are usually mostly 110s now, so you don't have much choice in the choosing.
But if you did, and you also had appropriate gears, remember that a streeter is basically a two-gear race-car that traps at 65 mph....... so why put a granny 114* cam into your engine?
For low-ET with a streeter,and an automatic;
probably every last streeter in the country,is stuck with the same problem; how to trap at 65 mph. Will he use one gear or two, then you have just two or maybe three ratios to choose from, to go with your choice. Small engine will usually choose 2 gears, while bigger will use just one. 318s need two gears.
So then your trap ratio is solely dependent on your tire size and trap rpm. And the rpm range might be from 5200 to 6200 depending mostly on the cam. Lets chose a 27" tire with an 85" roll-out. So;say with 5% slip in the TC, 5200 will require a 6.13Final Drive ratio, while 6200 will require 7.31FD. Probably every streeter in the country is stuck trying to fit into this range.
Now if you have a TorqueFlite with a 1.45 second ratio, then your rear gear choice is gonna be between 4.23 and 5.04 which choices are; 4.30,4.56,4.88.
Now all you gotta do is choose one of those ratios and build your engine to hit peak power at the rpm associated with one of those gears, tune it, and go win.
This now takes us back to the D and E curves.. Since you no longer care about power after the peak, ( because you will be running plenty of gear and plenty of TC, and you don't care about power after the trap;just like a drag-racer), there is no reason to run a wide LSA. In fact, run as tight as you dare, or as you have convertor for.
Let's say you choose 4.30s which hit 65@5300
or 4.56s, which hit 65@5600.
Or say you wanna be a lil quicker and go with 4.88s. Now she hits 65@5900
For a tight LSA108 cam, you might want the powerpeak to be 200rpm lower, to spend a lil more time at or near peak power. In this way you can be in the slightly higher power part of the curve that a 108LSA cam makes. (If you had a flat-line power delivery like a 114LSA cam makes, you might be able to use any of the three rear gears,and the same TC,with a corresponding loss of low-ET)
Now comes the secret; each of these cams will peak about 300 rpm higher than the last one, and so will require a bigger cam to stay in the efficiency window......... And what does the bigger cam do? It loses low-rpm efficiency. So along with the cam change, the combo may need a TC change .
Oh yeah big time!
here is something that illustrates cam characteristics pretty good.
Consider three different cam sizes B,A,and C each just one size bigger than the last. Imagine the lowest is a 256 and then a 262 and finally the 268. Notice the huge amount of bottom-end power given up to get a relatively small amount of power at the top. This stems from the pressure loss down there, as each larger cam losses progressively more low-rpm efficiency, ......... to get that higher rpm efficiency.
The trick is to choose the right cam for the combo..... and if the Scr is already in the basement to start with, then there is just no getting around that low-rpm performance loss, with a streeter.
The E and D curves are representative of the shape of the power peaks using wide and narrow LSAs, not on the same cam tho.Just consider the shapes
The thing you want to notice is where the B line crosses the C line. I'm gonna guess that is about 4200rpm(see note#1). If I'm right, that would be 47mph with 2.76s. Now just let that sink in for a bit.................................................................. The speed limit on most city streets is 30/35 mph.
Note#1,
how I got 4200. I assumed the bottom of the graph to be 3000 and the highest point of the C-curve to be 5400, so that is a range of 2400rpm. The crossover point appears to be at about 50% of the range so. .50x2400 plus 3000=4200rpm....Ok but say the low end of the graph is 2500, then .5x2900 plus 2500 is 3950@44mph, still speeding ,mostly.
Now look what that C cam is doing to your power. Nothing but a total power loss under 47mph.And not just a little bit, but a huge bit. You can never make that up in just the engine, no matter how much compression you throw at it. You can see that, without tirespin, the B-cam will be way quicker to 47 mph than the C-cam.......... because it is putting down more power during that time.
So suppose you had that B-cam which might peak at say 4400, and by the shape of it you can carry it out to 5000. So then for low ET, you could run 4.30s to get 4900rpm@35mph. And you will outrun any C-cammed 318 with any like-sized convertor and 2.76s; no matter what it's Scr.
This is why I love a stock 318 with nothing more than 4series gears and the 2800. A freeflowing exhaust is a must and a 4bbl will get you a little more wheel spin in second.
There is another option, a new TC that eliminates all the rpm or maybe just most of it; below the crossover point,which I guesses to be 4200. Now your rpm will blast up to where the power of the C-cam is and rocket you off the line with mega-wheelspin
EDIT; This part was added later:
Now lets talk about the D and E curves.
If you have two cams with the same specs except they are ground on different centerlines, this affects the power delivery, as the curves are trying to demonstrate. Imagine the D-curve is on a 114 LSA, and the E-curve is on a 108LSA. Notice how the E-curve builds to the power peak and then falls off rapidly. This is a nice shape for a drag-race engine, with tight transmission ratios or a high-stall TC .A cam with this shape usually makes a little more power over the narrow rpm range that a drag-race engine uses.
The D-shape is generally preferred for a streeter because the power delivery is smoother and spread over more rpm, so you always know what to expect when you gas it. Unlike the E-shape which generally explodes and then pukes; Like a pipey 2-stroke motocross bike.
Off the shelf cams are usually mostly 110s now, so you don't have much choice in the choosing.
But if you did, and you also had appropriate gears, remember that a streeter is basically a two-gear race-car that traps at 65 mph....... so why put a granny 114* cam into your engine?
For low-ET with a streeter,and an automatic;
probably every last streeter in the country,is stuck with the same problem; how to trap at 65 mph. Will he use one gear or two, then you have just two or maybe three ratios to choose from, to go with your choice. Small engine will usually choose 2 gears, while bigger will use just one. 318s need two gears.
So then your trap ratio is solely dependent on your tire size and trap rpm. And the rpm range might be from 5200 to 6200 depending mostly on the cam. Lets chose a 27" tire with an 85" roll-out. So;say with 5% slip in the TC, 5200 will require a 6.13Final Drive ratio, while 6200 will require 7.31FD. Probably every streeter in the country is stuck trying to fit into this range.
Now if you have a TorqueFlite with a 1.45 second ratio, then your rear gear choice is gonna be between 4.23 and 5.04 which choices are; 4.30,4.56,4.88.
Now all you gotta do is choose one of those ratios and build your engine to hit peak power at the rpm associated with one of those gears, tune it, and go win.
This now takes us back to the D and E curves.. Since you no longer care about power after the peak, ( because you will be running plenty of gear and plenty of TC, and you don't care about power after the trap;just like a drag-racer), there is no reason to run a wide LSA. In fact, run as tight as you dare, or as you have convertor for.
Let's say you choose 4.30s which hit 65@5300
or 4.56s, which hit 65@5600.
Or say you wanna be a lil quicker and go with 4.88s. Now she hits 65@5900
For a tight LSA108 cam, you might want the powerpeak to be 200rpm lower, to spend a lil more time at or near peak power. In this way you can be in the slightly higher power part of the curve that a 108LSA cam makes. (If you had a flat-line power delivery like a 114LSA cam makes, you might be able to use any of the three rear gears,and the same TC,with a corresponding loss of low-ET)
Now comes the secret; each of these cams will peak about 300 rpm higher than the last one, and so will require a bigger cam to stay in the efficiency window......... And what does the bigger cam do? It loses low-rpm efficiency. So along with the cam change, the combo may need a TC change .