Decreasing compression pressure

[ejw 71]

Am building a '59 348 250hp. As it has 10:1 cr would it make sense to install longer duration 409 cam vs 250 hp stock cam and thereby reducing compression pressure and effectively decreasing the 10:1 cr. Idea is to decrease compression ratio so could be more able to run lower octane gasoline. Thanks, Ed

[dq409]

???:confused: ????:confused: ?????:confused:

[tripowerguy]

I think you have a misconception of cranking pressure vs. compression ratio. If you have 10 to 1 compression ratio you have 10 to 1 compression ratio no matter what cam you put in. If you put in a cam that has a lot of overlap or duration it will reduce the cranking pressure. This will also make it less prone to detonate at low rpm but cylinder pressure should be the same at higher rpm. I think I'm right on this if not we'll hear about it soon. :) Roy

[SS425HP]

Roy, I'll buy that! Very good description. Whish I had said that!

[jim_ss409]

While using a cam with more duration won't actually change the compression ratio it will make the engine less prone to detonation by bleeding off compression at lower rpm's. At higher speeds when the delayed intake closing could actually increase cylinder presures there is more turbulence and there is also less time for detonation to occur. But these motors are not all that detonation prone to begin with so you might be fine the way you are, assuming the carbs and timing are set right. But the modern cams are alot beter than the old ones so it would be worthwhile changing it anyway. If you were thinking about changing it you could let us know a little more about your setup, intake, exhaust manifolds or headers, standard or auto trans, stock converter, heavy car, gears, smooth or lumpier idle wanted, maybe Aubrey or some of the other guys can suggest the right cam.

[fatride]

Jim, you are right on. Ed you have the right idea..;)

[CDNpontiac409guy]

Hey... you guys don't need me here anymore:cry ;)

Yes, unless you are doing some kind of purist restoration, it's best to stay away from any of the original GM cam profiles.
To keep your smooth stock idle and sound, along with increased fuel economy & power, while retaing your existing stock valve springs.......
Chose a cam with 206 - 214 duration at .050" ( original is around 196... VERY little difference in the 340HP 409 cam too )
110 - 112 centre line separation
.470" - .490" lift. :cheers

[CDNpontiac409guy]

Just kidd'n, Michael;) ... but I think I better come up with some new material:cool:. We'll see what happens tomorrow afternoon. I may have a LOT more "material":deal

[dq409]

Cause I`m here,,, NUK NUK NUK !!!!

OZ,,,,,,,,WE LOVE YOU MAN !!!!

HEY,,,,,,,I`ve got this old cam (new in box) on the shelf,,,,,,,,

[CDNpontiac409guy]

awhhh.... DQ.... I feel so guilty..... you stick up for me.... and I gotta say:



YOU POOR B%$#@ !!!!!

What a TERRIBLE camshaft ! Makes a 62 409HP 409 cam look like a brand new state-of-the-art roller profile !
HOLY $%&#... where did you get that thing... and WHO is responsible for the design:cuss :p :p ? !

HEE HEE HEE !!!!

[dq409]

GEZZZZZ,,,Thats twice I`ve gotten the Razberry from you about this cam !!!! heeee heeee
This was in the big pile in the barn the car came from,,,,

[CDNpontiac409guy]

DQ.... in the barn ?
OH NO!... the stuff YOU wanted was in the OTHER big pile:eek:

I understand that the price of clean cast is up these days;)

[ejw 71]

jim-ss409. Taking your advice. Here is info on my '59 Elcamino you said to provide so a cam selection can be suggested. Intake-stk cast iron 348ci 250 hp, Exhaust=stk cast iron, heads-'63 409ci 340 hp 10:1 cr, transmission-'71 TH350 with stk converter, weight-stk '59 Elcamino, rear gears- 3.36. Looking for duration around 220 @ .050 valve lift and lift about .500. Car will be a daily driver. Thanks, Ed

[CDNpontiac409guy]

Ed... Sounds like you are looking for a little more personality from the cam than what I had thought. Your spec is good, but when choosing a cam with those specs you've indicated, make sure the advertised duration is not more than about 274. Anything else would indicate that it is an "out-dated" profile.

Another note.... you have "817" 340HP 409 heads ? If so, you NEED a better intake. The 340HP 409 intakes are around, and afordable. Uses a Carter AFB / Holley bore size... bigger ports right through. Would add substantially to your power and efficiency over 2500 RPM. With those gears, your highway cruise RPM is around 2900-3200.

[jim_ss409]

With the stock intake and exhaust 220deg. @ .050 would be the biggest cam I would go. The stock converter won't work well with anything bigger than that either. In fact I'd be more inclined to go with 5 or 10deg. less duration to give better botom end and throttle responce. Most experts say that 220 deg. is about the point where you start giving up botom end with little gain in top end on a 350 smallblock with stock heads and intake. They also say that you should use just a little less duration with a shorter stroke engine like a 348. Your compression is good so 220deg. would be okay. Just think of that as an absolute maximum unless you plan some engine mods. Here are some Comp Cams big block grinds. Any of the first 3 Extreme Energy grinds would be great. I'd lean towards one of the first two. Remember that they are rated at a 1.7 rocker ratio and you have 1.75 so you will get more lift. Also don't go by what is said in the first column about idle charactoristics because they pertain to how the cam would be when installed in a 454. As a rule of thumb you can add about 8deg. for every 50 cubic inches so a mild cam in a 454 would be wild in a 348. I don't know how much lift those 340hp valves will handle but I think you will be okay there but with an aggressive cams like these you should also change springs.http://www.compcams.com/catalog/092_093.html

[ejw 71]

Thanks Jim. I'll probably go with the 2nd one. Intake duration=212, exhaust duration=218. I do want good low end preformance as car will operate mainly below 3000 rpms. Ed

[SS425HP]

You might also try advancing the cam about 4 degrees. Gives more bottom end. Check valve to piston clearance, also. Just to make sure.

Fred

[jim_ss409]

I just thought I should add that these cams are for a regular big block and won't just fit a W motor but You can get Comp Cams to grind you one using the same specs. They are a really good modern spec. where some of the cam listings for W motors are years old and not as good.

[fatride]

I had very good results with the High Energy 268H camshaft in my 348.;)

[ejw 71]

CPG. Thanks for info on 409 340hp intake. Would it make sense to use stock 348 250hp intake (carb-450cfm-?/small ports) to have better low speed responsiveness? Motor will operate 95% of the time under 3000 rpms. Ed

[CDNpontiac409guy]

Ed, if your compression really IS up to 10:1, and you're using those 409 heads, yes, I think it WOULD round out the package. Would be best in conjunction with a change to the 2 1/2" outlet exhaust manifolds too ( or the 1 3/4" Belanger headers ! ).:cheers