SStrokerAce

iTrader: (2)

Most motors now are going to shorter rods, cup, pro stock etc... Now with piston weights in these classes they can experiment with this more since there is no penalty at all in recipricating mass... And the larger cross section theory makes perfect sense.

What I was looking at more was how the effect of dwell and a longer rod has on overlap and cylinder pressure and then the effect of where the overlap period occurs plays into this... A advanced cam moves the overlap triangle so more of it is before TDC. This would seem to be another reason why a advanced cam helps make more TQ, other than the IVC effect on the dynamic compression. It's little **** to look at but there might be something there too.

Bret

Ed Curtis

You talkin' about me??

BTW.. this is all good stuff here...

I've been using a few different fast exhaust lobes with damned good success. Though this goes against a lot of peoples' cam theories, with today's cylinder heads, it does work quite well on dual purpose applications. To each his own...

FWIW, more than just "one parameter" of the camshaft duration needs to be discussed. Everyone concentrates on the .050" numbers. Probably because that's all the customer is given when they see a "cam spec" on the net. There are "other" important criteria that really needs to be evaluated before a cam should be designed and/or selected.

One often overlooked parameter is the total lobe "area". This must be considered when using a particular lobe in an application. I know a few "detail oriented" designers that always look into this before developing a design. You see, not every 224* duration lobe is the same. These common spec lobes can be totally different from each other in a lot of ways. The "lobe area" is one pretty damned important one! Even though "Company X" uses a 224* lobe and "Company Z" uses a 224* lobe, doesn't mean they are the same lobe, even with the same lift. It's really an apples and oranges comparison.

A lot (should I say nearly all) of the sponsor cam profiles never give the customer all the lobe info. (I guess I'll have to start doing that too!) The only way to verify the lobe parameters is to "doctor" it. Once that is done, you'd be surprised how they are all developed. Most go by the same idea as the catalog camshaft companies. Lobe "A" and "B" for cam number 1. Lobe "B" and "C" for cam 2. Lobe "C" with "D" with cam 3.. etc... etc... etc...

With a custom designed camshaft, you can use lobe "X" with lobe "W" and never worry about listing it as a part number. That's a better way to do things in my opinion.

As for the Futral lobes, without the data on the lobes themselves, it's all a waste of time to compare them on the net. I believe Allen can get into details about his own stuff so if you want to know how each works, call him!

Ed

Greg Good

Well, the rod length is a see-saw situation. Hypothetically speaking again, the longer rod will do two things for ya'. One is that the piston will move away from TDC more slowly. What that means is that the volume of the combustion space will expand more slowly. Keeping that space small keeps cylinder pressure high, which translates into more force applied to the crank. The second benefit of a long rod is less cylinder wall loading.

The benfits of a shorter rod are that you can take advantage of the better flowing heads we have these days, and the reciprocating parts are lighter too.

So basically, the long rod helps wring more power out of a given air/fuel charge on the power stroke and has less friction. The shorter rod benefits cylinder filling by applying more "signal" to the bigger heads we like to run.

How do you know when to run which rod? On a blower or turbocharged engine where I know cylinder filling is not tied to rod length, I want the longer rod. On a N/A engine, especially if the heads might be a tad big, I want the shorter rod.

Back to the cam deal, it's sometimes hard to predict what a cam's going to do when you move it around in an engine. I've seen guys that had the cam in at +4, then put it straight up on the dyno and gain power everywhere, more torque at the gear change and more torque up high (HP). All of the valve events are very important. A lot of guys like to focus on only one event, intake closing. That's not looking deep enough into things. Intake opening is important because when it occurs determines how much exhaust backflow you get into the intake port. Intake closing, we all know about that already. Exhaust opening is always a compromise. We want to start it late to take advantage of the power stroke, but if we start it too late the blowdown period and the rest of the exhaust phase as a whole is stunted, leaving a high residual pressure in the cylinder when the intake valve opens, blowing exhaust gas up into the intake port. The exhaust closing is the only event that I haven't done anything with as far as R&D, but in my mind I would think it should shut pretty soon after TDC so that the downward moving piston can produce a low pressure signal of higher strength.

SStrokerAce

iTrader: (2)

Ed,

I had to explain that to a customer the other day... He saw his cam specs and was pissed because they looked "common" to him. After I explained to him that the amount of lobes in those 2 durations with different journal configurations equaled up to almost 900 different combinations he realized it's not that common. The big thing is not just the .050 Durations and LSA, or even the valve events it's how the lobe acts with the rest of that parts that are being used with it and the RPM range where the parts operate. Very few guys look at the advertised and .200 numbers of a cam, and even then unless you have lift and some idea how to compare lobes it's hard to tell sometimes which cam is more aggressive and harder for the lifter and valve to follow without bad dynamics.

My idea of "shelf" cams is to have packages that are developed with a set duration range and centerlines. Then to take that package and tweak it per the customers needs and setup requirements. Rather than have XXX grind that's a known quantity to everyone. Then a guy is getting a "custom" cam, it's matched to what he has. Then again you probably do that too. We can look at a combination and immediately have an idea what cam will work best for that right off the bat. If those cams are stored on a computer file, a notebook or in your noggin your basically doing the same thing. You gotta have a place to start from. LS1 cams are interesting because there are so many lobe choices on the Hyd Roller end that you have an imense amount durations to choose from. It's not the standard groups of lobes 6 degs apart.

Greg,

The rod discussion is interesting. I did a motor that used a combination of things to manipulate the TQ curve using a combination of a larger than normal port (volume and cross section wise for the cubes), a long rod, good low/mid lift flow and a small camshaft. It was a "large" 358 SBC that was intended for street use. The long rod in the motor would have made the port feel like it was on a smaller motor (opposite of the short rod theory) to bring the rpm the HP peak occurs at up. The head also had extremely good low and mid lift flow, I've found that you'll see the high low and mid lift flow loose TQ in a situation like that if we put too much cam in it. That was the reason for the small cam, along with the fact that I couldn't get enough compression in the motor unless I went to a large dome on the pistons, I wanted to keep the IVC very early and keep the Dynamic Compression up and the burn as good as possible for pump fuel. The motor ended up peaking at 6200rpm which was suprising considering the 214* @ .050 on the cam. This motor has always liked smaller cams though even bumping the duration up another 15-20* on the intake still gets it to peak in the 6600-6700rpm range. The one thing the motor never likes it overlap, normally I would like a 106-108 LSA on something like this and it hates it in the mid range. The small overlap on the other hand at the top end seems to not be an issue at all. That's why I was looking at the effects of the dwell and rod length, the fact that the piston dwells for so long at TDC lets the exhaust polution take place easily but also lets the scavenging effect take place easily. It's a great motor to test reversion on too!

Next time I'm going to try a short rod setup so I can cut down the dwell time so it's less detonation prone because the long rod motor is very touchy when it comes to that but seems to hold up to detonation well. I also want to put a bigger port on the motor or have the rod lenght "shrink" the port size down some.

As you said it's a see-saw situation.

Bret

Ed Curtis

Another rod length to stroke discussion. Some BS in the mix but otherwise decent...

http://bbs.hardcore50.com/vbulletin/...ad.php?t=24051

Ed

SStrokerAce

iTrader: (2)

Thanks Ed,

I've gotten in enough arguements with Eric about rod/stroke combos that I would rather do this.... There is only so many times I can talk about bore/stroke and piston g's before I want to go sit on my thumb. We basically say the same things but like to argue with one another more. Jay's comments are good though, lets see if he thinks I get his question right.

I've always like David Rehers comment that a rod just connects the piston to the crank. In essence it's true but I think engine builders get bored and want to start looking at little details all the time and make something of them.

Bret

J-Rod

ttt for the new forum.

DavidNJ

I'm not sure I understand everyone's thinking here. Exhaust has quite a different role from intake. First the closing needs to occur after TDC so we are not compressing flow on the exhaust stroke. So there is no reason to lengthen it.

However, unlike the intake, the exhaust works under high pressure. About 10 atmospheres when it starts to open. And by BDC on the power stroke it is just a few psi over atmospheric. So it has to be open early enough to evacuate gases in the time available in the desired rpm range. When it opens it starts ending the power stroke.

I have always been sceptical of exhaust flow numbers. The test pressures are just so different from what is happening there (unless you have 150psi at 1600F on your flow bench).

We only have 70 deg or so before BTC, so the exhaust is no where near its peak openning when its work is really done. So getting it to fly off the seat is important. Lift and closing speed aren't. Many exhaust lobes are asymetrical with soft closings.

So what does this mean? Earlier exhaust opening events move peak efficiency to higher engine speeds. If the opening is too early or too late you lose.

Just judging from the wide range of numbers that work (people have 230 intakes listed with exhausts from 224 to 236), as long as your ok the differences are smaller.