What Cam should I use? - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 08-19-2013, 04:55 PM Thread Starter
 
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What Cam should I use?

I'm trying to figure which Cam to get and thought I could benefit from the wealth of knowledge on this site. I'm new (read my story in member introductions) and I'll try to give as much details as I can, but I don't know every detail of this car.

I have a 67 GTO with stock engine 400 / 335 hp and stock manual transmission. It doesn't have factory air, but I'm adding A/C. I'm also adding 1 5/8" headers and 3" collector and 3" cut outs with a crossover. I want an improved Cam that will give me some lope and lift, but won't stall out with the A/C on. Also, I'll be ordering from Summit Racing. Any suggestions? What lift and duration can the stock valve springs handle?

Thanks for your help, Steve
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post #2 of 6 (permalink) Old 08-19-2013, 09:36 PM
 
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If you have never had headers on these cars, you may want to think twice, especially with a manual trans. They can and will be a pita. I had a cheap set on my 400 and hated them with a passion.

I then purchased a set of ram air manifolds from ram air restorations for my new 455. On the dyno we only lost 8 or 10 hp compared to full length headers! And believe me, the car is much nicer to work on and maintain with the manifolds. With cast iron heads, factory intake, qjet, and cast ram manifolds, I ran a best of 13.7 in the qtr on all season radials... so if this is a street car your pontiac will likely provide plenty of oomph without headers.

Ok, will get off my soap box now... I am sure plenty here are happy with their headers and will give you some suggestions.

Oh yeah, just ran into a guy at the local car show with a 67 chevelle with a small block and 4spd, and he was also cussing his brand new ceramic headers, which are now full of clearance dents. And that is with the little chevy powerplant!

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post #3 of 6 (permalink) Old 08-20-2013, 08:17 PM
 
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"I want an improved Cam that will give me some lope and lift, but won't stall out with the A/C on."

Steve,

Your 335 HP engine has the factory "067" cam - Duration: Int 273 Ex 289 Lift .406" on both intake and exhaust. Lobe Separation Angle (LSA) 113 degrees A step up to the factory 360 HP engine used the "068" cam - Duration: Int 288 Ex 302 Lift .406" on both intake and exhaust. LSA 116 degrees. More on LSA below.

As you can see, the difference in the cam is the durations, but the lift stays the same.

Now if we step up to the factory 1968 Ram Air I engine, still rated at 360 HP, it used the "744" cam. Int 301 Ex 313 Lift .406". LSA 115.5 degrees

Pontiac plays with the duration, yet maintains the same lift on these cams until you go to the Ram Air IV "041" cam do you get .516" using 1.65 rockers (.469" using 1.5 ratio rockers). And duration was slightly upped to Int 308 Ex 320. LSA 113.5 degrees This is pretty radical for the street due to the long durations. You also need all the "good" parts to go with it, ie heads, intake, exhaust, etc..

The key is more about Lobe Separation Angle (LSA) and Lobe Centerline (LC) with LS being your biggest concern. LSA determines where in the RPM range maximum torque is developed in that a tighter LSA of 106-110 degrees will build torque sooner in the lower RPM range AND give you that rougher idle sound. HOWEVER, here is what could be a big problem for you when selecting a cam. Your engine has 10.75 advertised compression. Using a tighter LSA angle, as many cam grinders offer the 110 degree LSA cam, you will increase your dynamic compression higher than with your factory cam.

The above factory cams were ground on a 113 - 116 degree LSA as it reduces dynamic compression. Say what? Static compression ration is advertised at 10.75 for the 400CI, but the cam tells the valves when to open and close. The Pontiac grinds allow for later closing of the intake valve which results in a drop of compression, or what we call the dynamic compression ratio. The example I have is this, a 400CI engine with 9.5:1 actual compression (static) using the factory 112 degree LSA has a dynamic compression ratio of 6.68:1. Going to a tighter 106 degree LSA opens the intake sooner to allow more mixture in and raises the dynamic compression to 7.08:1.

My suggestion. Be careful of the tighter LSA with your higher compression ration. (Not so much a problem with lower compression as this will trick the lower compression to think it higher) Be weary of trying to get that lumpy idle by using a tighter LSA for the reasons mentioned earlier. Talk with a good cam grinder and they will ask for all your specs, engine, trans, gear ratio, intended use, etc.. Don't just select a cam because a buddy has one -it may not work for you. I think the "068" cam is a good safe choice. More lift? With stock heads, might not be a wise choice because the heads will only flow so much air in stock form and going higher won't gain you much -and you have to make sure your valve springs will work. If you have head work done to increase flow, then that's another ball game.

Just my opinion on it.
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post #4 of 6 (permalink) Old 08-20-2013, 09:25 PM Thread Starter
 
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Originally Posted by PontiacJim View Post
"I want an improved Cam that will give me some lope and lift, but won't stall out with the A/C on."

Steve,

Your 335 HP engine has the factory "067" cam - Duration: Int 273 Ex 289 Lift .406" on both intake and exhaust. Lobe Separation Angle (LSA) 113 degrees A step up to the factory 360 HP engine used the "068" cam - Duration: Int 288 Ex 302 Lift .406" on both intake and exhaust. LSA 116 degrees. More on LSA below.

As you can see, the difference in the cam is the durations, but the lift stays the same.

Now if we step up to the factory 1968 Ram Air I engine, still rated at 360 HP, it used the "744" cam. Int 301 Ex 313 Lift .406". LSA 115.5 degrees

Pontiac plays with the duration, yet maintains the same lift on these cams until you go to the Ram Air IV "041" cam do you get .516" using 1.65 rockers (.469" using 1.5 ratio rockers). And duration was slightly upped to Int 308 Ex 320. LSA 113.5 degrees This is pretty radical for the street due to the long durations. You also need all the "good" parts to go with it, ie heads, intake, exhaust, etc..

The key is more about Lobe Separation Angle (LSA) and Lobe Centerline (LC) with LS being your biggest concern. LSA determines where in the RPM range maximum torque is developed in that a tighter LSA of 106-110 degrees will build torque sooner in the lower RPM range AND give you that rougher idle sound. HOWEVER, here is what could be a big problem for you when selecting a cam. Your engine has 10.75 advertised compression. Using a tighter LSA angle, as many cam grinders offer the 110 degree LSA cam, you will increase your dynamic compression higher than with your factory cam.

The above factory cams were ground on a 113 - 116 degree LSA as it reduces dynamic compression. Say what? Static compression ration is advertised at 10.75 for the 400CI, but the cam tells the valves when to open and close. The Pontiac grinds allow for later closing of the intake valve which results in a drop of compression, or what we call the dynamic compression ratio. The example I have is this, a 400CI engine with 9.5:1 actual compression (static) using the factory 112 degree LSA has a dynamic compression ratio of 6.68:1. Going to a tighter 106 degree LSA opens the intake sooner to allow more mixture in and raises the dynamic compression to 7.08:1.

My suggestion. Be careful of the tighter LSA with your higher compression ration. (Not so much a problem with lower compression as this will trick the lower compression to think it higher) Be weary of trying to get that lumpy idle by using a tighter LSA for the reasons mentioned earlier. Talk with a good cam grinder and they will ask for all your specs, engine, trans, gear ratio, intended use, etc.. Don't just select a cam because a buddy has one -it may not work for you. I think the "068" cam is a good safe choice. More lift? With stock heads, might not be a wise choice because the heads will only flow so much air in stock form and going higher won't gain you much -and you have to make sure your valve springs will work. If you have head work done to increase flow, then that's another ball game.

Just my opinion on it.
That is some great info Jim, thanks. I felt a little like a 1st grader sitting in a Calculus class, but good info nonetheless.

I spoke with a technician from Summit Racing and gave him all my specs and told him what I was trying to accomplish. He recommended these...
http://www.summitracing.com/parts/cc...22-4/overview/

Any thoughts?
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post #5 of 6 (permalink) Old 08-21-2013, 08:50 PM
 
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Originally Posted by smac8876 View Post
That is some great info Jim, thanks. I felt a little like a 1st grader sitting in a Calculus class, but good info nonetheless.

I spoke with a technician from Summit Racing and gave him all my specs and told him what I was trying to accomplish. He recommended these...
http://www.summitracing.com/parts/cc...22-4/overview/

Any thoughts?
Yes, understanding cam specs can be a brain puzzle. I tried to simplify and even I am by no means an expert.

Kits are great in that it matches everything together. The cam however, has that 110 LSA that may aggravate your already high compression (you don't want engine "ping/spark knock"), but, the "short" duration figures may balance this out. Again, no expert by a long shot as cams come in so many flavors and sometimes it can be hit or miss. I used a Comp Cam with 110 LSA. My engine machinist told me I was going to be unhappy with it as he felt it too big. Very untrue, it proved to be a great selection for my combination. Loads of torque, fast, and with a 3 speed manual, 3.55 gears and tall tires I could coax about 16 MPG steady highway driving at 60 MPH. I was happy with my choice -which my engine machinist would have not chosen, but he was really a Chevy engine builder to start with.

Why not call Competition Cams directly? See what they recommend and then see if Summit has a corresponding kit. I like Summit and use them myself, and no doubt they have good people. However, I might just want to get it right from the horse's mouth at Competition Cams. Jim Butler Performance is another good Pontiac site that may be also able to help. There are a number of great Pontiac engine builders who can also help and you can post to get further references/opinions.

Also, throw a post on the technical forum, if you have not already, and see what feedback you might get from a member having a car combo much like yours. Lot of good members here with years of experience. You may get too much info and be even more confused! HaHaHa.

Just don't rush into a selection. Take your time to select a cam your comfortable with. Remember, a Pontiac won't sound like a nice rumpity-rump Chevy with a hot cam. Pontiacs have a sound all their own. Listen to some of the U-Tube videos on Pontiac cars to get some "sound" advice. Some have that great rough sounding idle, but these are usually the big/high dollar engines that are built for big power, speed, and need big cams to get it.

Hope this helps.
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post #6 of 6 (permalink) Old 08-22-2013, 07:40 AM Thread Starter
 
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Thanks Jim. I'll contact competition cams and Jim Butler Performance.
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