Can anyone give out a ballpark figure of an rpm limit where the heads quit breathing on small block with GM Heads?

I'm ready for a different valvetrain setup on my 383. Right now to chose from I have some 64cc heads that are on there right now. They have been bowl blended, had the chambers unshrouded to 64cc for the 2.02/1.60 valves, have a 3 angle valve cut on stainless steel swirl polished valves, and other misc things like bronze guides and .600" lift double valve springs. The machinist said the intake runners are 180cc and flow around 200 cfm at .500" lift and 210 cfm at .550" lift, with the exhaust flowing around 140 cfm at .500" lift. Or else I have a set of 487's that came of my engine when it was a 350. I thought about having those cleaned up a bit and have some 2.02/1.60 valves installed.

My question is how many rpms can these heads run before they run out of breath? I'm thinking it's time for a cam swap. Want a cam that will work better on a 383 and will work better with the engine profile and parts. Also want to have a better running engine without the need for racing fuel. Still haven't gotten the 2.73 gears swapped in the rearend so i'm loading the engine a lot by using the Comp Cams 292H. It's majorly detuned so I don't detonate. :(

This might sound stupid, but I bet your CC 292Magnum will float before your worked over heads give out..

Now if it was a CC 294S solid, I would feel different about the high rpm capabilities of your heads b/c a solid will rpm a lot higher then a hy- cam if the springs are good, imho..

Anybody else want to chime in??

pdq67

I don't know that there is necessarily an RPM limit, heads don't really "stop flowing" at a particular RPM.

Your power curve will just go flat early and stay flat. You might be at peak HP at only 3500-4000 RPM and stay there through 6500 RPM. That is typically what happens when there is a flow restriction somewhere whether it be the heads, exhaust, intake, etc.

Now if you run out of cam the power curve will drop suddenly . . . especially with tight LSA cams.

JMO

So if I put a really hot cam in my 355 will my 993 heads be only as good as the cam? I am on the quest to more power. And I'm still debating on a head swap or not.

Darn, accidently pushed the refresh button before I pushed Add Reply. graemlins/clonk.gif

That exactly what I was looking for guys!

What if I were to put on the 487's? Would that be the same? Would it help to do some work on the 487's before I put them on if I would use them?

What about a hydraulic roller cam? Would the same rules apply to it as a hydraulic flat tappet cam?

My target rpm is 6000 rpm and my max suggested HP is 500 HP according to the place where I got my rotating assembly put together and balanced. My bottom end consists of a Nodular Iron 383 Crank (Good to 600 HP), 5140 I-Beam Rods with ARP Clearanced Bolts (Good to 500 HP), and KB Flat Top Hypereutectic Pistons (Good to 500 HP). My current compression ratio is 10.78:1 and DCR is 7.85:1, if that helps out. Got to find somewhere that I can put my setup so I can put a link to it in my signature. DD2000 says i'm putting out 421 HP @ 6000 RPM and 424 Ft Lbs @ 4500 RPM. Granted the exact airflow specs probably aren't the same, and I used Small Tube Headers with Open Exhaust because I don't have any mufflers on it, just pipes to the rear axle. Don't know if that's healthy or not. Don't really know what its like either. He he, haven't gone over 4000 RPM with it yet. redface.gif I'm too scared! graemlins/sad.gif

In the days of old {1968-197X} My neighbor buddy had a nasty 68 chevy II. It was a 327, 13:1 compression, and he had an old Crane "308 Roller" is what he calls the cam, that setup with the Triple valve springs, and such. On worked over factory heads he'd turn 10,000 RPM. There was more to it than that but you get the idea.
OH.... This rediculous beast was a stret driver also. All the cam card said was "7,500 and up" HaHa, and you guys worry about your RPM range being to high for the street. LOL graemlins/clonk.gif

They will go 6500 or so, its going to take a 248+ duration @ .50 solid cam to even get it close to that with a 383.A good set of heads will go the same rpm but make 50 to 100 more hp.

IMO 487s aren't going to do much. If you are going to swap heads get some aftermarket heads or even Vortecs. Todays fast burn style combustion chambers make a lot more power than the old bathtub style chambers -- even if you port old style heads and they flow real good you will still leave power on the table with old style combustion chambers.

If you are bucks down, a salvage yard set of Vortec heads are the way to go, just make sure you get the valvecovers and rockers to go with them. You will also need a Vortec style intake.

Or if you can afford them a set of W/P Sportsman II heads are a pretty good deal for roughly $700.

Then there are some real killer aluminum heads like the AFR 195s in the $1200 range and many choices in between.

If you get new heads you will be shocked how much more power they will make.

Hydraulic roller cams are nice and will add a little bit of power over a good old hydraulic flat tappet cam. However, if I were choosing how to spend my money between new aftermarket heads or on a roller cam I'd go with the heads. Hands down, no contest. You can get a solid FT cam for the same price as a hydraulic FT cam and IMO the right grind will make as much power as a hyd roller . . . for a whole lot less $$$$.

The RPM limit has to do with the cross sectional area of the port from what I've read. Above a certain RPM, turbulence sets in and causes a restriction.

From what I've read, to be more exact, if you run out of intake port, power will go flat. Run out of exh. port, power takes a dive.
My setup takes a dive at 5800rpm. It's not the springs.

Competition Products has Canfield's for $950 right now. Being AL., you could go to a smaller cam if desired. I haven't decided what to do with mine yet. May live with it, may get a set of AFR 195's

Power wise, I've been quoted at least 40hp for a swap from your heads to a 200cc port head on a 383. In my combo, I shouldn't see as much gain, since the drop off is near my power peak anyways. So that's why I'm undecided. 20hp for $1k is a little steep.

The 487s are just 461s with a larger chamber, same ports. I have a ported set of 487s w/ 2.02&1.60 valves on a 355 with an honest 10.4:1 compression. Lunati 00012 cam 246@.050, .549 lift w/1.6 rockers, pretty close to the CC292H. They flowed 235cfm in/ 150cfm exh. Makes power all the way to 6500rpm. I also run on 93 octane with no detonation. If the heads you already have are "fuelies" the 487 won't help, wxcept to lower compression. If they aren't , then you could port your 487s and use the parts from your existings heads and save some bucks. You should also look at the intake,carb and timing curve as you shouldn't be detonating. And if you want that much cam, get some shorter gears :D !

Originally posted by malibu man:
Can anyone give out a ballpark figure of an rpm limit where the heads quit breathing on small block with GM Heads?

I'm ready for a different valvetrain setup on my 383. Right now to chose from I have some 64cc heads that are on there right now. They have been bowl blended, had the chambers unshrouded to 64cc for the 2.02/1.60 valves, have a 3 angle valve cut on stainless steel swirl polished valves, and other misc things like bronze guides and .600" lift double valve springs. The machinist said the intake runners are 180cc and flow around 200 cfm at .500" lift and 210 cfm at .550" lift, with the exhaust flowing around 140 cfm at .500" lift. Or else I have a set of 487's that came of my engine when it was a 350. I thought about having those cleaned up a bit and have some 2.02/1.60 valves installed.

My question is how many rpms can these heads run before they run out of breath? I'm thinking it's time for a cam swap. Want a cam that will work better on a 383 and will work better with the engine profile and parts. Also want to have a better running engine without the need for racing fuel. Still haven't gotten the 2.73 gears swapped in the rearend so i'm loading the engine a lot by using the Comp Cams 292H. It's majorly detuned so I don't detonate. :( =
=

I copied this answer I posted from your post in the engine section:

What RPM do you want to run to?
For a 383 if it is say 6500 you will need about 2.2 square inches with a flat tappet cam & 2.15 with a real good roller before the air gets very unhappy
I doubt you have 2.2 sq. inches at the smallest point in your heads.
The RPM point where the head dies is directly related to the size of the cylinder you are filling & the smallest crossection in the head.

The minimum area in a port is very important & is a number hardly any of the head people put in their specs??
With a fairly well ported SB head it is usually the pushrod opening.
With most it is the area of the throat under the valve or the "ski jump" at the short turn which is kinda hard to measure without pouring the port with a mold compound & disecting it.

Be nice if they would add this spec to their numbers.

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Here is an answer to more of your questions there or here,, I can't keep up :D

Originally posted by malibu man:
</font><blockquote>quote:</font><hr /> ]As long as anyone else doesn't have anything to add about my bottom end rpm limit, I guess my limit is 6000 rpm according to what i've read and what the parts place said. My current cam's power level is 2500-6500. How crucial are those rpm limits that are given to the bottom end parts? What i'm trying to say is, is that limit safe to run all the time and is a ballpark figure, or is that the max rpm allowed? I really don't have a need for a high rpm engine right now. The car isn't raceable and i'm thinking about giving the car a sleeper image with stock cosmetics. Wonder how it would hook with some new E78x14's though? :D Everythings up in the air yet. smile.gif </font>[/QUOTE]=
I think your self imposed 6000 rpm limit is a pretty good one & I would think your heads should be fine to there.

I would keep it under 6500 with the parts you have listed.
I do not use the aftermarket 5140 rods which are the cheap ones any more as I have seen a couple in two pieces that were pretty new :(
The 4340 capscrew ones are just not that much more & are a lot nicer rod.

I do not have a clue where the HP limits people put on their parts come from??
I fail to understand a "500HP" limit on say a set of rods without regard to piston weight, rpm, etc.

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Junkyard Dawg,

The problem with putting a cam in that has an rpm range that exceeds where the heads are efficient is all you do is kill your torque curve.
It will start where the cam starts working & end where the heads quit instead of where the cam quits.
Neither actually "quit" but power tends to head south very quick when you reach the limit of either.

So in effect even if you happen make a little more peak power with the larger cam, you will probably run slower due to the avarage power numbers & losses at lower rpm.

Well I found my RPM problem. Sorta.. My tach was off 500 rpm. Explains why it fell off hard at 5700. Puts the 6200 rpm runs into persective too. And my previous spring problems..

Now to find the missing HP w/o loosing too much torque.

A stock, double hump head is right at 160 cc's and the "X" ones are a schosh over 170 cc's but are kinda rare these days!!

As for your combination, I figure a good old CC 282S solid cam will build you some better midrange grunt over the 290+ cams.. OR one of Harold's/Lunati's cams in this range....

And this equates directly with going down the track faster!!

I figure the guy's are right about the Vortec heads b/c they are very eff. for no more then they are!!! AND they already have hardened seats, a better chamber design and 170 cc intake ports..

pdq67

PS., and to me any cam like a 280 and over really needs at least 10 to 1 CR. and a 290 cam and over, 11 to 1!!!

pdq67,

Sorry I haven't gotten back here in awhile. I've been doing some figuring with some calculators, namely the DCR calculator, and it don't seem like I can run that cam, although it would probably be a good cam. It seems it's too small for the 416 heads and too big for the 487 heads. DCR doesn't end up good. Does that sound right?

I've also been looking up Comp Cams Nostalgia Plus series of cams. They have a 284/291 solid cam that is supposed to be a replica of the 30-30H cam. That would work good with my current heads, supposedly, as far as DCR is concerned. Also, I would like to stay with the 2.73's in the rear end, even if I have to sacrifice HP for torque or whatever. I understand though that you need a fairly flat torque curve in order to be able to use high gears. I want to keep as much stuff original as I can. If I could find a replica to my original engine I would drop this mess with this engine and just put that one in, for the sake of it being stock looking. Really wouldn't mind riding in a stock, running 72 Chevelle again. smile.gif Aww, the good ole days. :D