RPM - What does it really mean?

[Bruceynz]

Hi Guys,

I have been laying low for a while, I have been thinking and thought would start a debate over RPM range, when you buy an intake it is rated from idle to 5500 eg performer, then the performer rpm 1500 to 6500 and cams are rated something similar.

Lets start with intakes - What's this RPM figure mean, if we look at cams we can say at 50thou its 224 degrees, so now I know that every cam is based to that spec, but how is the RPMs worked out on a manifold, lets use performer, at 5500 does it stop flowing, is this size for a 273, 318, 340, 360 or 408?? Lets move to the performer rpm, it says 1500 to 6500 what happens under 1500rpm (I have an airgap and my car idles fine) if you take a look at a motor trend article they will show on the flow bench the performer rpm has poor flow but then they dive in to telling you that its a very good intake and makes more power than the higher flowing intakes, so that means flow of an intake is not the whole story? Its seems something so trivial but to me seems such unknown. So back to my questions what's it all mean? How does an RPM rating apply, how do you really know the intake you choose is the right one for your setup, bigger always sounds better but is it?

Cams - Same deal, you can look at a cam something like a 274, its operating range is say 1200 - 5700rpm, but in a decent engine it makes max HP at 5200rpm, (look at the comp cams dyno graphs) once again what's it all mean, are those operating ranges like a frequency audio response curve where they measure the 3db points and it gives the operating range of an amplifier, I dunno eh, it seems simple but what manifold you you pair up with a 274, if you go off cam specs the performer rpm, if you go off actual dyno run then a performer would be fine or would it?

Looking forward to your thoughts on this one, its just something I have thought about for many years and never really knew the answer too, sure you can bolt stuff up and it works but what are you leaving on the table. Do those numbers really mean anything?

Thanks
Bruce

 

[Oldiron440]

That’s one very broad question.
Theres things like volume and viscosity come into play but all parts from the cylinder heads, intake manifold, camshaft and compression all will alter torque and horsepower curves individually so you want to match all these parts up. You want all these parts to maximize the curves at the same time producing larger torque and horsepower numbers operating in the same RPM range….

 

[Oldiron440]

Now that I’ve said that you can manipulate where to power is produced by advancing and retarding the camshaft. The camshaft has the most control over the torque and power curves, if you advance the cam it brings the power in earlier retard the cam brings it in later, so you can increase or decrease the RPM range by moving the cam timing.
The cam duration is what determines the RPM range, I have found that using a camshaft with ten degrees more duration than is required for the range I want then advancing the cam about 4 degrees Will bring it back to the range I'm looking for plus the additional duration. Duration is what determines how long the valve is open determining the amount of air flow in and out of the engine.
You mentioned engine size camshafts are not one size fits all, if you compare a 318 to a 408 your looking at 90 cubic inches in difference. If your looking at cams for a 318 with 224 duration at 050 you will need a cam with 240 duration at 050 for the 408 to just be equal to the 318 camshaft in optimization.
The larger the engine the the larger all the components need to be.
Compression comes into play here because duration bleeds off with the increased duration so you can only go as far as your parts allow. This is kind of a rambling summary of how things work together so I hope it helps.
I think I’ve had a thread on Cams and combinations theres even dyno sheets of my work.

 

[jasperjacko]

as old iron said, bigger engines will make a cam and intake act smaller because of the amount of air they can move with each stroke.

 

[Duke5A]

Everything has a generalized operating range from the manufacturer to help in the aid of choosing compatible parts. A number of factors goes into this, but the biggest one is displacement. Everything needs to be matched within some sort of reason or the entire combination will suffer as a result. Following the recommendations of whomever makes the parts will get into a ballpark of a happy combo. The real wizards (not myself) can take that down from a ballpark to a pool table and get squeeze every last bit out of said combo. Lots of variables and moving parts to all of this.

 

[Aspen500]

I've experienced the wrong part on an engine. Until last year, I had a 750 vacuum secondary carb on my car. Then I switched to a Brawler 850 double pumper and it was a night and day difference. Should have done that from the start over 10 years ago. In other words, a 750 on my particular engine (500 ci) didn't match the cam, heads, exhaust, gearing, etc. A VERY knowledgeable guy recommended that carb and I'm eternally grateful!

 

[Bruceynz]

So it seems there really is nothing in the specs, the only true answered are the combos on the dyno or seat of the pants at the track for proven parts that work together?

 

[M_Body_Coupe]

Ohhh...this post reminds me of a little "intake compare" I did once...a while back, but I still have the pics on my web server, too bad I can't get the darn LINK functionality here to work..LOL

Anyways, OK, so why do the intakes "come" with an RPM range?

Well, most of it deals with the intake runner cross-section area and therefore the speed of the air (or the ability to sustain a particular speed and therefore provide a given flow). For that reason, you will find that single plane intakes generally have higher RPM ranges, while dual plane intakes lower, and are therefore most often recommended for off-idle applications.

So why does that even matter?

As the guys have already pointed out, this is always part of a "package" and the intake itself has to provide enough flow capability to support the head's ability to flow the air past the intake valve and into the chamber. The bigger the engine displacement, the more air is needed....BUT...the higher the RPM the more air is required as well for the same engine displacement.

And when do you get higher RPM? That's the domain of longer duration cams (bigger number @0.050")...so as that valve stays open longer, the waves run up and down the intake runners and cause all sorts of things to happen, but eventually they transmit the vacum signal that's being developed in the cylinder to the carburetor. The fancy intake tuning is about balancing all that's going on within that runner and providing an intake that will support the engine's desired RPM it "wants" to live in.

Pics are worth a thousand words, let's see if this direct URL reference will get resolved once I post my reply...if it does, I will add all the remaining photos.

This is the TOP view of a series of small block Mopar intakes => http://darcio.no-ip.org/mopar/intake/intake_compare_top2.jpg

OK, looks like that worked, so here are the remaining pics (sorry, you have to click on the link to see 'em):

2) top front view

http://darcio.no-ip.org/mopar/intake/intake_compare_top_front.jpg

3) top rear view

http://darcio.no-ip.org/mopar/intake/intake_compare_top_rear.jpg

4) front view

http://darcio.no-ip.org/mopar/intake/intake_compare_front.jpg

5) side view

http://darcio.no-ip.org/mopar/intake/intake_compare_side.jpg

Now, what these should show you is the difference in runner sizes, the plenum size as well as the runner AOA (Angle Of Approach - that being the angle the runner makes as it meets the head, basicaly, the steeper that angle is the more of a charge/ram effect the moving mass exhibits - this is way oversimplified, but that just about maxes out my understanding of the intake runner physics).

 

[BudW]

To aid others, the pictures, in order:

my computer was giving me difficulties on pulling up the pictures, so I thought everyone else might be having problems, as well.

 

[M_Body_Coupe]

...my computer was giving me difficulties on pulling up the pictures, so I thought everyone else might be having problems, as well.

Thanks BudW....I suspect it may be the https vs http thing (secure vs unsecure, the secure one pretty much being the standard web interface these days, meanwhile my web server is a tiny app which does not implement https).

Anyways, appreciate the help!!!

 

[Voeltagear]

This is a great thread. I have few ideas to add from my perspective and experience. I'm open to feedback so don't hold back. It's all in good fun.

On the intake question, a quick way to determine the useful power band is to look at the runner length and cross section. Shorter runners = higher rpm and cam overlap. Longer runners = lower rpm and less cam over lap. Larger cross section = more cam duration. Smaller cross section = less cam duration.

For camshaft selection, look at what rpm your are going to be operating under a load. If you're idling along pulling 5000 lbs then look for a combination of duration and lobe separation that gives you high cylinder pressure at low RPM. This would be the "RV cam" group. You'll have ground pounding tire turning power but it really works best with heavier vehicles. You'll run out of tire shredding torque before your little A body or F body can get 50 feet from the stop light. These also get about 10mpg no matter what you do for your carb/tune.

If you have something slightly lighter than a full size van pulling a car hauler look for an rpm range that will get you started into the torque power band between 1500 and 2500 rpm, I've seen as high as 3500 be a daily driver but your dealing with a lot of heat and A/C and touring become worrisome. This is the "street and strip" group. You'll have plenty of time to roll those tires as you burn rubber away from the stop light. The duration and overlap should give you good enough vacuum at idle so you can still use power brakes and HVAC controls. Cylinder pressures ramp up more slowly so you can advance timing earlier and get more snap when you hit the gas. Pair with a stall converter or standard trans for the most street fun.

For drag race applications you need to narrow down your choices to the RPM range that gets you across the finish at the top of your power band. Tire size, trans gearing, diff gearing, all play a role in decision making. Realistically you're using 2500-3500 rpm of power band. Vacuum signal is not you concern. RPM is your focus. You'll have a ton of overlap and duration because you don't care about getting moving in traffic, you're staging at your ideal rpm to launch. Tire burning isn't a concern for show but a concern for 60 ft times. The max tourque for your tire/suspension setup without "blowing the tires off" is your target. And that has a whole pile of data that goes into decisions.

 

[AJ/FormS]

As for a streeter
the basics are to match your parts for your particular combination.
If you have an automatic and plan to run 3.23s, then there is no good reason to have parts that are good for 6500, cuz yur only ever gonna get there but once on the way to the speed limit, and the tires are gonna be smoking anyway.
If your combo is gonna spend 95% of it's life just cruising around at 50mph or less, then you might as well set it up for that.
With good heads, you can run the rpm up to whatever, until your cam chokes or the lifters pump up.
Most of the time , a guy says he wants 500hp when he only needs 300. So he spends his wad on something that he couldda spent maybe half as much on, to have the Same fun.
On street tires, pretty much anything over 300/330hp is overkill.
Well OK that's what happened to me, lol. I built an engine that takes my combo to 93mph @3457 pounds in the Eighth, yet the vast majority of the time is cruising around at 30/40hp....... or even less. I was such a dope. lol.

What do the numbers mean?
They are guidelines to help you pick matching parts.
If you have an auto with a 3000 stall TC; you are not much concerned with any rpm below that.
If you have a 4-speed, you should be somewhat concerned with low-rpm operation.
If you are running 4.30s in a hot streeter, then you are more concerned with the higher rpm numbers.
But on the street,nothing "bad " happens when you go outside the operating range; except maybe for cams with not-enough spring-pressure, lol.
There are , however some things every streeter needs to contemplate;
1) the 1-2 split
on Torqueflites is pretty wide, which really only affects small engines. The split is 59% so if you outshift at 5500, the Rs will fall to ~3250, thus establishing your powerband as from 3250 to 5500, or 2250rpm. But if you have a lil 273, she better have some grunt down there to pull you back outta that hole. Whereas a stroker has gobs of power down lo.
2) the starter gear and stall.
You can trade these around some, to get the cruise rpm, but if the end results in a low amount of torque ending up at the rear tires, when starting out from a stop, acceleration is gonna suffer. Once over about 20 mph this problem rapidly diminishes.
3) kickdown for passing
If you pick the wrong combination of gears, you can end up with no passing power, or you could run outta rpm halfway around the slowpoke. Again, this is more applicable to smaller engines.
4) if you have a regular 4-speed
You gotta pay particular attention to your low speed gearing/ low-rpm operation, cuz not enough of either of those, will forever piss you off. A good starter gear, IMO is ~11:1 Under 9 to 1 sucks, and over 13 sucks just as much. But 11/1 is right on. So therefore, with a 2.66 low with the regular 4-speed you need 11/2.66 =4.10s.. The smaller the cam you have, or the more cylinder pressure your engine makes, or the more displacement you have, the smaller the starter gear can be, down to ~8/1 minimum.
5) meh, that's about all I got