I posted this in lt1 tech and didn't get any responses. I thought that this might be somewhat advanced tech. If this needs to be moved please do so as i would like to learn more about camshaft selection. Thanks!!

I am going to take the "smart" route in cam selection. I am going to have my heads ported and then make a custom cam that will work with my heads better than an off the shelf cam such as the cc306, 230/236, etc.....

I understand how to select the lift numbers for the cam by looking at the peak flow numbers on the heads, but i have a queston concerning duration. What determines what duration i should have the cam ground at? I know that it move the rpm band around but there has to more to it than that. If that is all that duration did wouldn't just about all cams be the same? I have also heard that the duration split will be changed a lot by the split of the head flow numbers (i/e)

IN THEORY: my heads flow with adequate lower lift numbers, and keep increasing until .600" where the flow numbers flaten out at the high 200's (285/22x) I am pretty sure that i will want a cam that lift #'s will be right around .600" where my flow peaks at, is that correct cause i never see anyone running cams with those high of lift numbers? Now if my heads were flowing these numbers, what would be the optimal duration (i/e) for an automatic car with a forged 355cui motor and several bolt on's that will be running a 3200-3600 stall? How much would you change the duration if i was to build a 383cu motor instead?

I am sorry about this being so long but i really appreciate the good feedback from the knowledgeable members on this site

thanks guys,

thomas

 

TTT guys

 

Nope.

 

I'd pick the duration that gives you the powerband you want, and then have them grind the most lift they can safely run on that duration.
It's cr@p to say that if your heads level out in flow @.6 that you shouldn't lift past there! If you lift to .65" then you'll just have that much more time at peak flow, since you said your flow #'s level off (not drop).
Of course, to get over .6" lift you'll need to check that the heads will allow for that much, and get some pretty nice springs. Longer valves might also be of use to allow more room to avoid retainer/valve guide contact...

You've basically got the whole thing about duration down - it moves the powerband up or down. The reason that all cams aren't the same is if you're making peak torque at 6000rpm, then your power at 2000rpm is going to suck!
Invest in a good program like Engine Analyzer (not the DD2000 - it's way too basic) they will allow you to put in all the specs of your heads, cam, headers, etc. Then you can play around with different cams and see what your powerband, idle vacuum, cranking compression would be. It's a pretty accurate program, too

 

where can i get that program??

 

OUCH! just found a price on http://www.auto-ware.com/software/eap/eap.htm for $495

 

I think you're looking at engine analyzer pro. It's a few steps beyond engine analyzer, but alot more input.

look on www.performancetrends.com and you can dl a demo version of the regular engine analyzer. I think the price is something like half or less than that of EA pro

 

as stated above, run the duration to get the powerband you want. FYI, EA has been VERY inaccurate in power bands for me. never tried the pro version.

also, lift.. run as much as you can with the duration you chose, ASSUMING your ports don't stall and get turbulent.

 

EAP is very good. I can get it within 1% of actual dyno numbers.

$500, yep it costs that much to play.

Camshaft design with that takes a lot of work, but you can do it alot faster than with real cams on the dyno. On top of that having the help of Comp Cams is even better.

I agree with these guys here, pick the duration you need and then get the most lift possible. It's even more complex than that. The flow numbers are important and low lift numbers below your peek cam lift are very important. Cam's have a lift area measured in Inch Degrees, that area and the intake valve closing point are going to determine all your other camshaft specs. Everyone tries to simplify cams, they are complex and everything you change on them changes everything else.

What you need to tell us is the valvetrain and bottom ends max rpm. Your tranny gearing, then we could get a little closer. The most accurate rule I have found for cam selection is that certain inch degrees of valve lift are required for a certain RPM range even without putting flow numbers into the equation.

Bret

 

Well, I would be running this in a forged 383 shortblock with the heads flowing around what i posted at first. I would not like to take this motor past 6500 rpms unless if you think the bottom end will handle it?? I will have a 3000-3500 converter with 3.42 gears. I am thinking about running the 236/242 xe grind with the most lift that i can run on that cam. Can someone give me a relative number as to how much lift i could put on there? I don't think that i would want to run over .600" of lift would I? my heads aren't going to flow that well. Don't you want to match your lift on the cam to the flow #'s of your heads??

thanks for all of your help guys, i really appeciate this. I am a 19yr old college student trying to fill my youthfull mind with all the knowledge that i can!!

thomas

 

This is for a LT1 based engine?

236 242 is way to big for 6500rpm.

Your running a Hyd Roller Right?

Post those flow numbers up here again.

Bret

 

A couple of things to keep in mind. First of all, lifts over 0.600" are not practical for a street motor. Most people don't want to replace their valve springs every few thousand miles, right? The springs will simply not last when used at very high lift, especially if high rpm is also used.

The general idea of the highest lift/duration is a good one for street high performance cams. This maximizes total area under the lift curve for a given duration, thus allowing lots of flow without the problems associated with very long durations. But there is (of course) a limit to how steep the lobes can be. This limit is based on the ~350lb open spring pressure limit imposed by using hydraulic lifters. Trying to use more pressure will collapse the lifters. The spring pressure limits how steep the lobes can be and still allow the lifter to maintain contact with the lobe. Much higher pressures are possible with solid rollers.

The heads have a major influence on what cam will work best, but it's more than the flow numbers. The starting point is the rev range and the intended use. I get the impression he is building a street car. In that case the intake ports should be 185-200cc to preserve intake velocity at low and medium engine speeds. Intake ports this size will max out at ~6,200-6,400rpm. If he is after a maximum performance street engine, that's the rpm range to aim for. A cam with ~230 degrees intake duration would fit the bill.

A suggestion based on the partial info he has provided would be Comp Cams #3111/3122 lobes on a 112 lsa with a 108 degree intake center line. The duration at 0.050" would be 230/236 with .560/.570" lift using 1.5 rockers. An excellent exhaust system will be needed to make a cam like this work.

My own preference is to maximize mid range torque and my personal choice would be CC lobes#3194/3196, which are from the Xtreme Energy line. These would give 230/236 @ 0.050 with".584/.585" lift.The revs would need to be kept to <6,000 but there would be more midrange, especially if a bit smaller head was used.

Good luck.

Rich Krause

 

Bret,

here are the flow numbers again:

29x's/low 21x's with adequate low lift numbers. don't remember all of the specs but they were decent

Rich,

you think the grind that you said, 230/236 will work? i though that you could put a bigger cam in a 383 and have it be just as steetable as a 355. i like this to be a weekend driver/race car. I will have higher stall converter and 3.42 gears along with this. I want to not have to rev higher than 6400-6500 max and still make excellent power.

thanks everyone,

thomas

 

Twells, the cam you wanted (236/242) is too big for your needs and is more suited for an engine over 400-427ci.

The 230-236 XE HR is perfect.

 

Is that (xe 230/236 hr) the same cam that rich was talking about?

How is the 236/242 too big? isn't that what trey is running in his stroker with stock heads??

thomas