Caamshaft Selection

I need some help with a cam selection for my Chevy Nova SS. I am a street driver not interested in racing. I want a fairly comfortable driver on the street. The engine is the original 350 bored 030" over. I am down to two cams, the Comp Cams XE262H and the XE268H. Comp recommended the XE268H.

The heads are World Products S/R Torquer’s with 67 cc combustion chambers, 2.02” intake and 1.60” exhaust valves.
The stock flow on the head at 28” H20 is: 221 cfm for the intake at 0.500” lift, and 166 cfm for the exhaust at 0.500”.
The engine builder performed some mild pocket porting on the heads such that the exhaust to intake flow ratio should be about 70-75%.
The 350 cu. In. engine is bored 0.030” over, balanced, and the compression ratio with new hypereutectic pistons is about 9.6:1.
I am looking at the Edelbrock Performer manifold that is designed for peak torque at about 3800 rpm and peak hp at about 5300 rpm.
The Performer RPM is designed for peak torque at 4500 rpm and peak hp at 6200 rpm which may be a little high for me. Although dyno test results from many magazines show excellent numbers with this combination.

The rear end gear ratio is 3.31:1 with a 27” tall rear tire for a 60 mph highway rpm of 2500-2600.
The transmission is a standard, 4-speed Muncie M20 wide ratio.
With the relatively high gear ratio and considering that this will only be a street car with a good share of city driving, I will need a lot of low end torque and realistic fuel mileage. On a full speed run, I plan to shift at 5600-6000 rpm. I want good low end and midrange torque. I also want a a noticeable idle sound, nothing overly radical.
I am concerned about too much cam and detonation. The 268H will build less cyl pressure and be less likely to detonate. The 262H may be better suited to my rpm range.

HELP PLEASE!!

What would I do? I'd pop for the short cam, Performer intake and finalize the squish at 0.035" to 0.040". There won't be any lump to the cam, but then I'm way past trying to impress other people with a lumpy cam that won't pull the hat off your head at low r's. Should make excellent cylinder pressure, especially if you leave provision to change the advance of the cam once you have it running and establish a baseline. The tight squish will guarantee no detonation and full spark advance with pump gas.

I'm not a fan of cams that slam the valves open and slam them shut like the action on the XE cams. I think they leave more chance for the lifter to wipe lube off the lobe with the pressures involved and are very harsh on all the valvetrain. You can comprare ramps from cam to cam by subtracting the duration at 0.050" from the advertised duration and dividing by 2. The result will be the length of the opening or closing ramp in degrees and will allow you to see how agressively the cam moves the lifter. Example 1: 240 degrees @ 0.050", 290 degrees advertised. 50 divided by 2 gives a ramp length of 25 degrees opening and 25 degrees closing. Example 2. 240 degrees @ 0.050", 280 degrees advertised. 40 divided by 2 gives a ramp length of 20 degrees opening and 20 degrees closing, a much more agressive lobe. It may look better on the dyno, but is it going to be too harsh on the drivetrain compared to the softer-hitting cam? Will it last as long before the cam wears through the case hardening and goes soft? These are the questions I ask myself and since I'm not concerned with the very last horsepower I can wring out of a motor, I prefer to use longer ramps. Take a look at some of the GM muscle cams from years past. They used ramps up to 35 degrees and if I haven't gone senile, it seems like we hardly ever had a cam go soft in the old days. Of course springs will make a difference too, so maybe I'm not being fair.

CAM BREAK-IN
Anyway, use only stock springs to break in the cam, changing them to the springs you're going to run after you have several hundred miles of easy runnin' on the motor. Fill the crankcase with racing type oil that has sufficient zinc content to protect the cam. (Read the article in Car Craft March 2006 concerning the removal of zinc by the oil companies). Add one bottle of GM Engine Oil Supplement, available at any GM dealer. Prime the oil galleys. Adjust the valvetrain and spark advance so that the engine will crank right over. Bolt on a carb that is primed with gas and ready to run. DO NOT, REPEAT, DO NOT GRIND THE STARTER TO START THE MOTOR. IF IT DOES NOT START RIGHT AWAY......STOP!!!!!...... FIND OUT WHAT IS WRONG. While an associate monitors oil pressure and water temperature, bring the r's up to between 2,500 and 3,000 and hold it there for 20 minutes. Shut down, replace oil, filter and GM EOS. Run easy for 300 miles. Replace oil, filter and springs. Enjoy.

Here's an addition. I too have a 350 with 4226 world heads. Has alum dualplane,headers, 600ed carb. I too am changing cams. Check out the dyno charts on the comp website. The 262 non roller out dynos most all of the other grinds including rollers to about 280s. Also check out the vacuums @850rpm. Interesting results from the source-comp. I might add that have a .620 hyd roller in my ford and it rocks to 6k. Am surprised a small cam works so well with the sr head.

Quote:

---

Originally posted by Richard Quattro
I need some help with a cam selection for my Chevy Nova SS. I am a street driver not interested in racing. I want a fairly comfortable driver on the street. The engine is the original 350 bored 030" over. I am down to two cams, the Comp Cams XE262H and the XE268H. Comp recommended the XE268H.

The heads are World Products S/R Torquer’s with 67 cc combustion chambers, 2.02” intake and 1.60” exhaust valves.
The stock flow on the head at 28” H20 is: 221 cfm for the intake at 0.500” lift, and 166 cfm for the exhaust at 0.500”.
The engine builder performed some mild pocket porting on the heads such that the exhaust to intake flow ratio should be about 70-75%.
The 350 cu. In. engine is bored 0.030” over, balanced, and the compression ratio with new hypereutectic pistons is about 9.6:1.
I am looking at the Edelbrock Performer manifold that is designed for peak torque at about 3800 rpm and peak hp at about 5300 rpm.
The Performer RPM is designed for peak torque at 4500 rpm and peak hp at 6200 rpm which may be a little high for me. Although dyno test results from many magazines show excellent numbers with this combination.

The rear end gear ratio is 3.31:1 with a 27” tall rear tire for a 60 mph highway rpm of 2500-2600.
The transmission is a standard, 4-speed Muncie M20 wide ratio.
With the relatively high gear ratio and considering that this will only be a street car with a good share of city driving, I will need a lot of low end torque and realistic fuel mileage. On a full speed run, I plan to shift at 5600-6000 rpm. I want good low end and midrange torque. I also want a a noticeable idle sound, nothing overly radical.
I am concerned about too much cam and detonation. The 268H will build less cyl pressure and be less likely to detonate. The 262H may be better suited to my rpm range.

HELP PLEASE!!

---

Nice combo.I think you would be happy with either cam.For better low end torque go with the smaller of the two cams.Also like Denny said maybe check out the hyd. roller cam profiles.After reading this I am not really sure why you need our help.You are very close on everything I.M.O.

I would suggest checking out Lunati's Voodo cam #60102. It will outperform the CompCam 262 and has comparable stats: 219/227 degrees@.050 and .468/.489 lift. Harold Brookshire designed the Extreme Energy cams for Comp Cams, and later he designed the Voodo series for Lunati later. They are a better design and do not have such an extreme downramp that the XE series has. According to Harold, the Voodo line of cams out performs the XE series. If I were looking for a cam I would choose a comparable Voodo cam ove an XE.

Thanks for all of the help. I really appreciate the time all of you took to write your response. I may not have clearly explained what I am after, so I'll try to be more explicit. I have a few more questions because as you can see, I am still a novice when it comes to proper engine building. My racing days are long since over and I am trying to put together a fun cruiser. Extracting every last hp out of the engine is NOT my primary concern. I want low speed drivability, high torque, dependability, and a slight sound. I will sacrifice sound for the right combination of parts though. My engine builder seems knowledgeable and we have done a lot on a budget. He builds a lot of circle track engines and has been able to obtain high quality / low budget used parts. The engine has full roller rockers which should ease the strain on the cam lobes a little bit. However, I am not interested in too big a cam anyway. He originally suggested a Crane WG1170, grind 962H with an advertised duration of 272 degrees intake and 288 degrees exhaust, 0.450" I, 0.460" E and a duration of 224/224 at 0.050". The LSA is 114 degrees. I wanted a smaller LSA, but similar lift & duration. The fast acting cam is a consideration, but that is why I have narrowed it to two of the smaller choices. The engine is fully balanced and the builder says that it will be good up to 7000+ rpm. None the less, I plan to shift between 5600-6000.
I have looked at extensive dyno tests by many magazines on similar set ups as mine. With the XE268 cam, most of them show max hp at 5600 rpm and with the XE262 max hp is at about 5200-5300. However, in every test, max hp within 5-8 hp remains 400-600 rpm after peak. I also noticed that heads with smaller combustion chambers and more valve shrouding such as the Vortec heads push the torque and hp curves up 200-400 rpm.

Again, my builder likes the XE268 with the 4-speed as does Comp Cams. As far as bleeding off cyl pressure with the bigger cam, there are only 3 degrees of separation between the two cams for intake valve closure. Both of these cams show tremendous bottom end with good high rpm performance. In fact on the dyno tests, the torque and hp output between the two cams (all other factors being equal) only varies from 1 to 6 ft-lbs and 1-4 hp between 2600 and 4200 rpm.

When you refer to tight squish, I assume you are talking about head gasket compression. The Speed Pro hypereutectic flat top pistons come up to about 0.015 below deck height. I thought that a thicker head gasket would reduce running compression and reduce the chance of detonation.

I have read so many books and articles in an attempt to educate myself and make intelligent choices, I am more confused than ever.

With all of this, I have several pointed questions:
1. Can you explain why a tight squish provides less of a tendency to detonate? Is it because a greater swirl is obtained?

2. Why would a small combustion chamber (Vortec) raise the hp and torque curves of the engine with all other factors equal?

3. I am looking to keep shift points between 5600-6000 rpm. I noticed that with the long horsepower range on the dyno charts, these cams pull hard 400-600 rpm past peak. That means that a cam that peaks at 5300 could be shifted at 5800 before power really drops off. Conversely, a cam that peaks at 57-5800 is still pulling hard at 62-6300. That is faster than I care to rev. I don't know how to judge these powerbands for my application. Any suggestions?

4. I have read countless dyno tests on both cams from testing conducted by magazines and Comp Cams Powerbands can vary significantly from one test to another. This seems to be dependant on head design. Any comments as to why powerbands can fluctuate so much?

5. My engine builder says he can lower the powerband about 200 rpm by advancing the cam 4 degrees. This will also advance the point at which the intake valve closes, thus raising cylinder pressure. Could advancing the cam lead to detonation? Any other comments on advancing a cam?

I think #2 is not the truth ,I think it moves lower?

1. Squish (also called quench) is the measured distance from the crown of the piston to the flat surface of the cylinder head at top dead center. The tighter this dimension, the more aggressively the fuel/air mixture will be "jetted" across the chamber, leading to superior mixing and the elimination of dead spots in the chamber. The area between the crown and the head is also a dead spot where combustion does not occur, so the smaller and tighter that area, the more power will be produced and the cleaner the emissions will be.

2. I have no idea. Ask your engine builder.

3. Richard, I think you're very confused and that you need to step back and reevaluate your goals. In one breath, you're telling us that you want a "driver" (here's your quote: "I want low speed drivability") and in the next breath you're telling us that you are planning on shifting at some insane rpm's (here's your quote: "I am looking to keep shift points between 5600-6000 rpm").

4. It's also dependent on the dyno used, the dyno operator, the shop header design used, the fuel, the fuel curve, the ignition curve, the components used in the build, the clearances used in the build, the ambient temperature and humidity at the time of the test, etc., etc.

5. Your builder is right on the money. If you have the squish optimized and are running less than 11.0:1 c.r., then you can run on pump gas with no detonation. It's all about cylinder pressure. You can run more cylinder pressure on the same gas if you optimize the squish. With a deck height of 0.015", you would want to use a gasket that would compress at between 0.020" and 0.025" in order to yield a squish of 0.035" to 0.040".

"I thought that a thicker head gasket would reduce running compression and reduce the chance of detonation"

Not so. A thicker gasket will INCREASE the tendency toward detonation. Even though it will increase static compression ratio slightly, a thinner gasket will reduce the squish dimension and reduce the tendency toward detonation.

Here is a good explanation of quench:

www.chevyhiperformance.com/techarticles/94138/


Maybe this will help you understand this better.

Once again, I thank all of you for your input. It is clear that many of you have a lot more experience than I do. I have received excellent info on quench, engine break in, operating parameters to consider…
I am leaning toward the smaller of the two cams. These cams seem to develop a great deal of low end torque while maintaining a wide power band. Their rated ranges are as follows:
XE262H: 1300-5600 rpm
XE268H: 1600-5800 rpm

My head selection may not have been the best. I read several books by David Vizard and others who liked the S/R Torquers as a good economy head. Perhaps the Vortec head would have been a better choice as they seem to have better "fast burn" qualities due to their combustion chamber shape. The S/R heads only have 170 cc runners that flow approx 221 intake @ .500" and 166 cfm exhaust @ .500" in stock form. These heads were touted to have excellent swirl characteristics and high velocity runners with relatively unshrouded valves. My builder performed some mild pocket porting to improve the flow and swirl. These heads were rated to work well up to the 5500-6000 rpm range which seemed to be ideal for my situation. Some people feel the bigger of the two cams may be better suited to these heads?????????????

After I purchased the heads, I saw a few tests where CHP ran the Vortec head against the S/R and the Vortec out performed it. No surprise. However the vortec compression was approx 9.25:1, where the compression of the S/R head with its larger combustion chamber of 67 cc would have been slightly lower. My static compression with flat top pistons will be about 9.5:1. The pistons are 0.010” below deck so that with a 0.040” compressed head gasket, the squish will be about 0.050”. Therefore, head performance may be improved slightly over the CHP test. It still made 417 ft-lbs torque @ 3600 rpm and 379 hp @ 5200 rpm with the XE262H cam It was still up at 377 hp at 5600 rpm. That’s plenty for me. Remember, I am accustomed to the original 1971 smogger engine with headers that ran a 13.60 quarter mile (with street tires) on its best day ever. The q-jet still had the limiter caps on it.

In any event, power isn’t my prime concern. I want to be able to drive the car in traffic, open it up from time to time, and have a slight performance sound, if possible. I am never going to noticwe the difference between 10 to 20 hp. Both of these cams seem to make decent low end torque and are touted to be drivable. I was just trying to find the best choice for my set up and driving habits. I am sure some of the Lunati cams would work well, however they seem just a bit more radical.

I have not been successful in obtaining torque numbers below 2000 rpm for a set up similar to mine. I guess this is because dyno’s don’t work well below 2200-2500 rpm. I don’t know how much torque will drop off below 2000 rpm. Again, with a 3.31:1 gear ratio, Muncie M20 wide ratio trans, 3100 lb car weight, and an RPM of approx 2500@ 60mph, that should help me in the low end areas.

I am also wondering about carburetion and intake manifolds. I have an Edelbrock Performer #2101, and have access to a Performer RPM in great shape. I thought the Performer would be the better choice, however in all of the dyno tests I have viewed, the rpm seems superior above 2400 RPM. Again, no data in comparing the two below 2400 rpm.

My builder has a Holley #4150 model 0-4777S, 650 double pumper he will sell cheap. Holley recommended the Street Avenger. I originally thought to use the original Q-jet which will fit on the Performer manifold, but not the RPM. I know little to nothing about carbs.

After reading this long writing, any comments on:
1. Cam choice
2. Intake Manifolds
3. Carbs
4. Torque numbers below 2000 rpm
5. Anything else.

Again, I truly appreciate the help!

If the pistons were 0.010" in the hole on my motor, I'd use this gasket.....
http://www.sdpc2000.com/catalog/576/...ad-Gasket-.htm

Quote:

---

Originally posted by Richard Quattro
Once again, I thank all of you for your input. It is clear that many of you have a lot more experience than I do. I have received excellent info on quench, engine break in, operating parameters to consider…
I am leaning toward the smaller of the two cams. These cams seem to develop a great deal of low end torque while maintaining a wide power band. Their rated ranges are as follows:
XE262H: 1300-5600 rpm
XE268H: 1600-5800 rpm

My head selection may not have been the best. I read several books by David Vizard and others who liked the S/R Torquers as a good economy head. Perhaps the Vortec head would have been a better choice as they seem to have better "fast burn" qualities due to their combustion chamber shape. The S/R heads only have 170 cc runners that flow approx 221 intake @ .500" and 166 cfm exhaust @ .500" in stock form. These heads were touted to have excellent swirl characteristics and high velocity runners with relatively unshrouded valves. My builder performed some mild pocket porting to improve the flow and swirl. These heads were rated to work well up to the 5500-6000 rpm range which seemed to be ideal for my situation. Some people feel the bigger of the two cams may be better suited to these heads?????????????

After I purchased the heads, I saw a few tests where CHP ran the Vortec head against the S/R and the Vortec out performed it. No surprise. However the vortec compression was approx 9.25:1, where the compression of the S/R head with its larger combustion chamber of 67 cc would have been slightly lower. My static compression with flat top pistons will be about 9.5:1. The pistons are 0.010” below deck so that with a 0.040” compressed head gasket, the squish will be about 0.050”. Therefore, head performance may be improved slightly over the CHP test. It still made 417 ft-lbs torque @ 3600 rpm and 379 hp @ 5200 rpm with the XE262H cam It was still up at 377 hp at 5600 rpm. That’s plenty for me. Remember, I am accustomed to the original 1971 smogger engine with headers that ran a 13.60 quarter mile (with street tires) on its best day ever. The q-jet still had the limiter caps on it.

In any event, power isn’t my prime concern. I want to be able to drive the car in traffic, open it up from time to time, and have a slight performance sound, if possible. I am never going to noticwe the difference between 10 to 20 hp. Both of these cams seem to make decent low end torque and are touted to be drivable. I was just trying to find the best choice for my set up and driving habits. I am sure some of the Lunati cams would work well, however they seem just a bit more radical.

I have not been successful in obtaining torque numbers below 2000 rpm for a set up similar to mine. I guess this is because dyno’s don’t work well below 2200-2500 rpm. I don’t know how much torque will drop off below 2000 rpm. Again, with a 3.31:1 gear ratio, Muncie M20 wide ratio trans, 3100 lb car weight, and an RPM of approx 2500@ 60mph, that should help me in the low end areas.

I am also wondering about carburetion and intake manifolds. I have an Edelbrock Performer #2101, and have access to a Performer RPM in great shape. I thought the Performer would be the better choice, however in all of the dyno tests I have viewed, the rpm seems superior above 2400 RPM. Again, no data in comparing the two below 2400 rpm.

My builder has a Holley #4150 model 0-4777S, 650 double pumper he will sell cheap. Holley recommended the Street Avenger. I originally thought to use the original Q-jet which will fit on the Performer manifold, but not the RPM. I know little to nothing about carbs.

After reading this long writing, any comments on:
1. Cam choice
2. Intake Manifolds
3. Carbs
4. Torque numbers below 2000 rpm
5. Anything else.

Again, I truly appreciate the help!

---

I would rather have the S/R torquer heads myself.The S/R heads are a much better bang for the buck.To build a Vortec head the same as the S/R heads you will spend a lot more money.Ifyou can find a vac. carb instead of the double pumper you will pick-up a little gas mileage with out any lose of power.

Thanks;

When I refer to torque, I am comparing it to the original smogger engine which was a mild set up. The car was comfortable to drive at low speeds. I am not trying to build a monster, I just wanted to increase the hp of the engine while maintaining something close to the torque the engine had when it was bone stock. In stock form in 1971, the L48 engine was rated at 270 hp with the quadrajet. My goal was to end up with 325-375 hp with the rebuild and still maintain torque levels that were close to (or greater than) stock form. Thus the following changes:

1) World products S/R/ Torquer heads mildly pocket ported.
2) Engine was bored 0.030" over, rebuilt, balanced.
3) All fasteners replaced with ARP fasteners.
4) Hypereutectic pistons at 0.010: below deck for a 9.5:1 c.r.
5) camshaft change
6) intake manifold change.
7) headers & dual exhaust.

Hopefully these changes will yield a bit more power without sacrificing low end drivability. Thats all.
Thanks