critter

iTrader: (9)

Several of us understand that and agree. The question then is, where does 279 CFM come from???

Cstraub:
IIRC, you came up with with the same number as DenzSS. Can you explain what it means?

critter

iTrader: (9)

Aha. I understand and have used those calculations/forumlae. Now, I will guess that 279 CFM is the necessary flow at peak piston velocity. Close???

If so, that would seem to make 320 CFM unnecessary, except that, while we know it is 320 CFM @ 28" H2O, we don't know what it is a max piston velocity, other than that it is probably less than 28".
Are you planning to publish your program?

As a side note, have you ever looked at a plots of piston velocity and acceleration with +/- 0.5" rod length? The difference is hardly noticible to the naked eye. Makes me wonder if the long rod/short rod thing is just another "speed secret" to keep attention away from the real speed secret.

Cstraub

iTrader: (1)

Critter,
That is Denzss part of the discussion, my part will be later in cam selection. J-rod has posted most of the formulas that I used on scratch paper to figure stuff. When he is ready, he will give you the formula.

Chris

Cstraub

iTrader: (1)

Critter,
I posted before your last. . .your very close, Piston speed has to do with it. . .and remember to change direction, you have to stop. . . .

Chris

DenzSS

Here you go Critter....Yup, you're in the right ballpark.

Velocity=black
Acceleration=red


Stock Rod length LS1: 6.100" rod (rounded from 6.098")


5.600" rod length


A little different aren't they. It is a real speed secret. Most of the time, you just don't have enough deck to slide in a decent sized rod so you have to minimize it as much as possible.

critter

iTrader: (9)

Thanks. Now it makes more sense. I haven't tried to look at numbers yet. I actually am working, but just check in here while a compile or download is running.
Not really. Not by any meaningful amout, though it is hard to see that without the plots overlaid. However, I don't own a dyno, so I can offer no definitive proof. It may be possible that a 0.002% difference in piston velocity shows up on a dyno, but I doubt it. I'll try to overlay the plots and compare them to mine later.

DenzSS

Look at the dip in the top pf the acceleration curve. Notice that the center of the "dip" in the acceleration curve corresponds to where the velocity curve hits zero at BDC. That is "jerk". Enough "jerk" and you're engine will tear itself apart. Jerk increases friction, friction creates heat, heat = power.

BTW, piston velocity isn't the killer. Piston acceleration is the real culprit.

Look at the graph..as Cstraub noted, it has to stop to change direction.

critter

iTrader: (9)

That is all true, but are you saying that the 5.6" rod in your example will tear the engine apart?

BTW, I dropped a decimal - I meant a 0.100 delta, +/- 0.050. For instance, a 6.1" vs 6" rod in an LS1.

While is is obvious that as the rod length to stroke ratio gets smaller (or stroke to rod length gets larger), nasty things happen, like increased side load, friction, and jerk. I was assuming a reasonable range of variation.

The main thing I had in mind re: "speed secrets" is things like the oft mentioned dwell at the top.

DenzSS

Nah, I'm not saying it will tear it apart. That is really a fairly small amount of jerk. I wouldn't spin it to 10K.

As far as the long rod/dwell argument goes, I buy into it. Anything that gives me more time to build cylinder pressure is a good thing.

Most of this stuff is useless to the average enthusiast.

BADFNZ

iTrader: (19)

I have a general question, that may not have an answer. When does overlap become too much? I know overlap=power, especially as CR rises, but I also know too much overlap is a bad thing and a drivability nightmare.

I have the VE program that J-Rod posted in the first cam discussion thread, and according to it my TR224 has 0 degrees of overlap. This cam seems to work well in my car and many other LS1's so maybe Thunder Racing knew what they were doing when designing this camshaft. But if overlap was raised by changing some VE, would more power be had?

racer7088

Well you can ghetto figure it as (346/2)CID*6800RPM/1728 CID per one Cubic Foot= air engine takes in at that RPM at 100 % VE or 680 CFM.

Now we see there are 8 cylinders so 680/8 = 85 CFM per cylinder but.......

The cylinder is only open say in a heads cam car for 224 out of the 720 degrees of the 4 cycle engine so it's only open .311111 of the cycle or roughly 31 % of the time in this case.

The actual airflow therefore has to occur during the short time the piston is traveling DOWN the bore and the little extra time it has before velocity is overcome by rising cylinder pressure from the upward motion of the rising piston in the bore.

85/.311111=273cfm average during this shorter but real period of intake port flow.

I could run MORE cam and slow the air down by running a longer duration cam but at some point the cam will both open too early on the exhaust stroke and close too late on the intake stroke so yes there are limits. Cams over 280 are getting more and more rare in racing as heads have gotten better and better.

racer7088

Overlap certainly does not = power. The right amount can help you some the wrong amount especially too much can totally kill all high rpm power. Especially if you have exhaust restriction or poor exhaust ports.

racer7088

""Buddy Reher of Reher-Morrison backs this up in a discussion he had on flow benches.
Textbooks would lead you to believe that an exhaust to intake flow ratio of 80 percent is ideal - yet a typical Pro Stock head has exhaust ports that flow less than 60 percent of the intake runners. You can improve the exhaust flow tremendously with about 40 minutes of work with a hand grinder - but the supposed improvements will just about kill the engine's on-track performance. I know because I've been there.""

Reher is very knowledgable on 2 valve PS engines but this is misquoted out of context quite a bit. Buddy is not stupid. The problem is that you can not get any amount of quality airflow per bore size with 80 per cent exhaust flow on a 2 valve. You CAN on a 4 valve easily and it's done any and every day.

It's not the exhaust flow that is killing the power it's the sacrificing of the intake flow that is killing the power. Also you need to really know some stuff about flowing exhaust ports that is not common knowledge before you really know if they flow more in the first place.

80 per cent exhaust to intake flow 4 valve stuff can make more HP/CID and more TQ/CID again any and every day than any 2 valve heads stuck with 60 per cent exhaust flow. Some of this 80 per cent stuff makes 5 HP per inch and turns 20,000 rpm. The thing is that you can get great intake flow without having to sacrifice the exhaust so much on the 4 valves AND you can use shorter valve timing as well!

racer7088

If in the above case I really was taking in 273 cfm average flow when the intake valve was open and I had a 273 cfm intake port then it would mean I had to have similar intake drop to the flow bench or 28 inches of water or two inches of mercury or around 1 psi less in the cylinder to achieve that flow. So I would be down around one psi of density as compared to atmosheric outside or hopefully in the manifold. With a 320 cfm port I would have denser air coming into the port at a lower velocity and it would slow down and back up into the intake at a lower rpm than the higher velocity but more restrictive port did. At some point you can have too much size in the head or duration on the cam or both and again the engine get's lazy down lower due to no more inertial supercharging and the later intake valve closing point starts hurting you then. Well I better hit the bed since I am still waiting for our power to be completely hooked up but the head shop stuff is operational now. Look forward to more discussion as I have been so busy till now I couldn't get in on this stuff.

BADFNZ

iTrader: (19)

I should have said "Overlap=power until you reach a certain limit". I plan on milling my heads soon and I'm thinking I might want a cam w/ a little more overlap to aid in the filling of the cylinder.

Will Trevour Johnson still do heads for ya'll at HPE Dallas?

racer7088

Don't really know yet although I would like to talk to him possibly. We have several head sources and our own work but I always keep an open mind if I can use something better!

J-Rod

Cool to see you jumping in Erik, good luck with the shop. I'm looking forward to going to eat @ Chuy's with you and Chris sometime..


On the subject of rod length. Lots of testing has been done on the benefits of a longer rod. One of the things I was talking with the Aussies about was that on the dyno one of the things they have seen is that motors with a longer rod with more dwell time top and bottom are much less sensitive to intake runner length than short rod motors.

DenzSS

I can run a few simulations for you guys if you'd like to see different rod/stroke combinations.

racer7088

Did I eat there before with you and Chris? I think I know who you are but I didn't know all this time?

racer7088

Rod length matters somewhat in that it affects timing events but I still don't think anyone would give up much or anything at all to get rod length. It's just what it is and then you build the engine.