DenzSS

Generally by its performance, on the dyno or the track. If you know about camshafts you can have a pretty good idea by looking at the specs. The main determinates for duration is engine displacement and "power band" (from what RPM to RPM do you want to optimize the area under the curve. For example 3000rpm - 6500rpm. )

I'm simplifying the whole situation quite a bit, but it'll work for general conversation and bench racing.

When comparing two engines that are similar (say a stock 346 LS1 and a C5R blocked 427), the engine with the greater displacement can utilize a larger duration camshaft. It has a greater volume in each cylinder to fill, so it can use a little extra time to do so.

Basically, you're looking at the engine and asking yourself "Can I supply all of the air it needs when it needs it throughout the power band I want?" If you can't, you're going to have to increase the duration to give the cylinder extra time to fill. The problem with doing this is you delay the closing of the intake valve. When you do this, you take away torque (less cylinder pressure) from the lower RPMs and kind of "give" it to the upper rpms. You shift the power from the lower end to the upper end.

When a car is "over-cammed" it has sacrificed too much of its lower rpm power for its upper rpm power. It makes HP in a very narrow range and has given up much of it low-midrange horsepower and torque.

Some racecars use very high peaky power. They build their power in the upper range, but they have the parts necessary to keep them in that really narrow power band. For example, F1 engines can spin upwards of 15,000rpm and they make around a 1000hp on a very small displacement engine. Sounds cool until you realize that they idle at 7000rpm.

Sorry, that is a rambling post but I'm trying to keep it as simple as possible. Hopefully I won't confuse you or myself.

verbs

I will be doing a little overcamming testing on my 347 motor....I think I'm going to test both the G5X3 cam on my Stg. 2.5 TEA LS6 Massengale heads as well as a G5X5 cam. Both on 112 LSA's with the same advance ground in. The difference between the G5X3 and G5X5 is about twice the difference between the G5X2 and G5X3. I will be using a port matched 78mm LSX manifold along with this package.

Cstraub

iTrader: (1)

To add to Denzss, the exhaust note is a dead give away for a trained ear. Engines that make ok power blubber and pop rattle. . .but the sound is not crisp. These engines make power but not "REAL" power. Engines that hit quickly when idling with that snap are cammed correctly. Next NHRA race, listen to a PS engine in the pits, they are almost mild at idle. . but the tone is crisp. . .now rap the throttle. . .she is on the Rev limiter in an instance.

Chris

DenzSS

Good addition and a better blurb than my rambling post.

mshiznitzh

It only gets to around Mach 1 at the rpm the intake is tuned right?

DenzSS

I'll give you guys a 500rpm incremented CFM requirement list.
Engine bore and stroke is the same as listed in the first post.

RPM CFM
------ --------
500 20.52
1000 41.04
1500 61.56
2000 82.08
2500 102.60
3000 123.12
3500 143.64
4000 164.16
4500 184.68
5000 205.20
5500 225.72
6000 246.24
6500 266.76
7000 287.28
7500 307.80
8000 328.32

DenzSS

We're not to the intake manifold yet. We're just at the valve as the A/F mix comes across the short side and begins to fill the cylinder. Make sense?

99 Black Bird T/A

iTrader: (4)

1 Cubic foot equals 1728 Cubic inches

346 CID/1728 = ~.2 Cubic feet for the engine displacement.

6800 rev's with a 4 stroke engine mean 1700 intake strokes because 6800 divided by 4 is 1700.

The 346 engine needs .2 cubic feet of air 1700 times in a minute.

.2 * 1700 = ~340 cfm

I come up with 340 cfm is needed assuming 100 percent volumetic efficiency.

I know based on what others say that is wrong. Will someone please explain where I made my error in calculating this?

Thanks

mshiznitzh

Yes, I was just asking.

DenzSS

We're going to work on splitting off the calculation from the rest of our app code so I can post the direct formula. It isn't trivial.

You are definitely thinking in the correct lines, which is mainly what we need.
You have to assume 100% VE at this point even though in reality you're probably not going to get it.

If we look at just the CFM requirement figures I posted we can see that with a stock head we cannot fully fill the cylinder with air/fuel at 6800 rpm before we even start adding in the intake manifold, camshaft, etc. We have a cylinder head which flows roughly 235cfm@28"(for simplicity's sake, we'll avoid talking about all of the different pressure related issues). So, we're going to have to crutch the intake duration. Do you see where I'm headed?

01 SS Jay

DenzSS - is it just me or does the CFM you list for 6500rpms about the same as what a stock ls1 head flows? So the head flow is optimal for our engines CFM requirements, which explains why cam only cars make very good horsepower?

DenzSS

The stock 241 heads flow more in the range of 235-240cfm, at least by my experience.

The LS6 head flows right around the 265-270cfm range.

Ed Curtis

They're out there...

The really good ones don't need to advertise...

That's been one of my "issues" with the typical catalog camshaft designs. They are called "custom" but the only thing custom about them is the price! (and the hype)

In Renegade, you have to pick the right power adder to be successful. Supercharged cars are a lot more "blower specific" for making horsepower than the engine package. Only the nitrous cars in this class are truly "cylinder head dependant" for making good power. The Michigan boys, Ron Sharp and Kurt Gallant have shown the rest of the field how to get it done in Renegade!

IMHO...

If you want to see amazing power producers, watch the NMRA Hot Street cars. They're running just under 400 cubic inches of naturally aspirated power into the high eights/low nines.

Then check out the NMRA Pure Street cars with only ported street heads, hydraulic roller cams under .500" lift and only 10 X 26 slicks that are now going 10.40's and 10.50's...

They're the REAL horsepower makers!!!

~~~~

Like I said, the good engine builders are out there. You have to look a little than just a magazine ad..

Ed

DenzSS

Yup. I had Ron do my cylinder heads.

Cstraub

iTrader: (1)

Verbs,
You have 2 shafts there that are both to big, what you will prove is one makes more power in a narrower powerband. A better test would be try both of those then put in a "matched" custom cam and see what the power curve does.

Okay, back to denzss thread. . .

Chris

Bink

346 CID/1728 = ~.2 Cubic feet (CF) for the engine displacement.

.2 CF / 8 cylinders = .025 CF/Cyl = one cylinder volume.
Two cylinders on intake every revolution of the crank in a V8 so->
2 Cyl / Rev * 0.025 CF/Cyl * 6800 Revs /Min = 340 CF/Min = 340 CFM.

I think this is right...corrections????


joel

verbs

Denz, this isn't rambling at all and is very informative IMO. This is a great thread for both experts and novice enthusiasts.

For a daily driver that doesn't race much I completely understand the benefits of running a smaller "matched" camshaft, but if all you care about is making max power when racing, what are the downsides of going bigger if you make more power in your usable racing powerband? If you put a bigger cam in the car and it gains no more power in your powerband up top where you race (say 5000-7000rpms on a heads/cam car) then it's obviously a lose-lose situation.

My goal is to make max power in that 5000-7000rpm range, so it doesn't bother me to lose low/mid range power, BUT only if I'm gaining power up top in my usable racing powerband.

I always admired the power that Cartek's smallish 224/228 cams were making on some of their cars, but one of the things I did notice was a lack of low/mid range power....but those cars ran some insane numbers at the track.

I know Cstraub and yourself have been saying all along that both the G5X3 and G5X5 cams are too big for my application so am I to take that as you both implying that I will not be gaining any more power up top and will just be losing power down low?

Cstraub

iTrader: (1)

Verbs,
I have recammed a 360 CID over headed engine with a camshaft with 20 degrees less duration and .100" less lift and the customer gained 70HP and 90#/ft of torque with in his same needed power band of 5000 to 8000 rpm.

Yes, overcamming can have a negative effect on power, even if you do want it up stairs. 5K to 7K with the heads Tooley is prepping and your CID it is too big.

Chris

DenzSS

Hey Verbs,

Actually, we're saying a couple of things.

#1
Any camshaft should be "matched" to the system, big or small. The camshafts that you can just buy off the shelf aren't really suited to that. There are a good number of decent camshafts marketed by different vendors that can be used pretty successfully in an engine with stock heads/bolt-ons/etc. Once your system becomes sophisticated and you are using custom cylinder heads, higher compression ratios, different gearing, etc you should leave the standard shelf market. You're just paying a lot of money for a camshaft that isn't designed for your specific engine with your heads, compression ratio, purpose, etc. We need to stop treating this like the old days when only a few camshaft designs were available. Now you have the ability to get much closer to what you actually need. You don't have to try to fit your engine to the camshaft. You can buy the correct camshaft for the engine and the purpose you have in mind.

#2
We are saying that you are giving up more than you need to. You can get what you're looking for out of a much smaller camshaft. There is just no reason to put a camshaft with that much duration into an 346ci engine that flows as well as these do with well ported cylinder heads. It isn't doing you any good and it is just costing you most of your midrange power and nearly all of your drivability. You can do a lot better.

verbs

Chris/Denz, you guys bring up some very valid and good points. I'll leave the camshaft talk for later when J-Rod starts his cam/overcam thread; I don't want to bog this discussion down....I shot you both a PM.