JS

iTrader: (4)

Lests assume you have a cam and head setup that peaks at 6500rpm
Its make ??? HP.

Then u run that same setup w/the cam in 4 degrees advanced.
Would your peak number drop but your area under the cure be better?

I remember some people saying rpm equates to HP..
This is why the all bore motors tend to be better in terms of HP.

If this is the case then depending on the setup u would not want the same cam or adv in a auto vs stick shift car if u were looking to get every bit of power/et.

DenzSS

Bore motors are better in terms of HP because they increase the surface area upon which the forces of combustion act. Stroke increases torque but has little effect on horsepower because you are increasing the mechanical advantage the piston has on the crankshaft.

JS

iTrader: (4)

Okay then,What about taking a stick car that has the same weight,gear and H/C setup thats running a cam w/say 0 advance and leaving the line at 6000 rpm.Then taking the same exact setup at the same weight,gear and running it in an auto?

This would be a foot brake car,not a trans brake car.Wouldnt u want alittle advance ground into the cam thats in the auto for more area under the curve?I'd think the 0 advance would be too soft for the auto car ft braking at 2500?

DenzSS

Yup, given the exact same setup, I would. IMO, you should not use the same setup for the two. The auto, since it is launching at 2500, is going to require a good deal of torque to get all of its weight off the line. Not to mention, the A4 has a longer shift recovery time and cannot maintain the narrow power band afforded by the M6 combo unless it runs what I refer to as a high stall torque converter.

Without a doubt, on an A4 car with a moderate stall you should concentrate on making the most area under the curve. A high average hp/tq auto will be incredibly consistent and easy to drive at the track.

DenzSS

Here's something to add to all of this on the HP/TQ question.

What everyone refers to as volumetric efficiency actually decreases after a certain point determined by the camshaft, heads, etc. After a certain point, the cylinder can no longer completely fill and cylinder pressure drops reducing the force on the top of the piston. This reduction in force reduces the amount of torque. That is why we see the drop in torque on a dyno sheet. Since the horsepower calculation is (lb-ft X rpm)/5252 we know that if you double rpm, you double horsepower. If you double the lb-ft, you double the HP. However, that only holds true if the other variable remains constant. Make any sense?

So, if TQ decreases at a slower rate than the RPM increases we will see a net gain in HP. If TQ drops off faster than RPM rises, we see a net loss.

I'm sure this all pretty basic and probably repetative, but I just wanted to make sure we were on the same page when talking about bore, stroke, HP, and TQ.

Colonel

If I'm running a 3500 stall with a 4900 shift extension, I care almost none what kind of power the engine is making below that. Sure, I may be foot stalling to 2700 but it goes straight to 3500 and then almost instantly above that. Once the tach swings past 4900 it'll never go back below it. So, as to what advance would work best with an auto, it depends largely on the converter used.

I'm using a straight up cam that comes into it's own at 5300 according to the dyno chart. I have a converter with a 5300-5400 shift extension. Works out perfectly. Could I get a quicker 60ft time with the same cam advanced 4 degrees? In theory, yes...but I bet you'd hard pressed to actually measure the difference. It would be TINY! During the launch I'd only be in an RPM range that the 4 degrees has a small advantage for an instant. After that point I think I'd be at a disadvantage compared to the straight up cam since the converter keeps the RPMs up high so well.

I guess what I'm trying to say is DON'T cam a car for the launch. There are other ways to get a good 60ft time. For picking a cam with a given converter (since in a street car we have limitations on how loose of a converter we care to run), the shift extension would be more of a consideration to me than would be the launch. You never want to drop out of the meat of your power after a shift.

DenzSS

I'd agree with that. My comments are more of a general nature.

I'd count your 4000 as a big converter, especially since shift recovery in an auto is generally about 1000rpm above the rated stall. You're running in a tight enough power band that much of the torque issue isn't relevant. You're set up like a bracket car.

JS

iTrader: (4)

This might be true but lets say the 6sp car is 300lbs LIGHTER than the auto car.Now leaving at 6000 rpm u would think the 6sp drops very little in between gear changes?Dont u think the HEAVIER auto car is at a dis-advantage w/a cam thats low on tq (0 adv.) from the start.The 6sp car never drops below 6000 the whole run and it using the hp needed to propel it down the track.

I'm sorry but I dont totally agree,there is no way a light stick car and an heavy auto car should be running the same cam/adv. especailly if the stick car leaves at 6000 vs auto car w/a 4200 stall.

Iv_z28

My understanding is as follows:

The main advantage of advancing the cam is the possibility to run longer duration given the same LSA.

I believe we had a similar discussion few days ago using stealthy cams as an example. I don't know exact relation and don't know if anybody actually has the data to relate advance vs. duration and advance vs. LSA.

I'm currently thinking about ordering a custom cam, which would be TR224XE-R on 116LSA, advanced ~4 degrees. I would *think* that it should pick way below 7krpm, make better power/curve than StealthII and have ~stock idle in my M6.

DenzSS

JS. I agree, but you have to take something else into account.

Colonel is right in saying that +/- advance isn't a huge difference. It can be in some specific cases, but in general it doesn't make a huge difference. You will close the intake valve earlier when you advance the cam, but it may not make a large enough difference to really help. If it is a large duration cam, it may not help a whole lot.

What I think is more correct to say is that in many cases, manuals and automatics require different cams.

JS

iTrader: (4)

Thats kinda what I thought Denz...
Thanks

Colonel

JS, in the case of the 6 speed that you're talking about, you're saying that the clutch is dropped at 6000 and that the RPMs do not pull back below 6000? If that's the case then there is either ALOT of clutch slippage or some tires spinning.

And what RPM are you shifting at with the M6 to never drop below 6000? The RPM you drop back to is mainly a product on the gearing of the tranny and whatever slippage your clutch might have. The 300 lbs lighter really doesn't change that much about our shift recovery.

The drop between 1st gear (2.66) with an M6 and 2nd gear (1.78) leaves our RPMs at only 66.9% of what they were when we shifted. That brings 6800 RPM down to 4549. Yes, clutch slippage will help this out, especially if we powershift...but it IS going to drop WELL below 6000 unless we're spinning and/or smoking the clutch.

Colonel

"What I think is more correct to say is that in many cases, manuals and automatics require different cams."

Since Joel likes to read between the lines sometimes (you know you've pulled some things out of thin air on me before my friend), I'll point out that I totally agree with this statement. Nowhere have as said otherwise.

DenzSS

Yup, mainly the gearing of the tranny. The big benefit of the M6 is the relatively small changes betwen gears. A lot tighter range than an A4. I have a spreadsheet somewhere that works all of this out. I'll have to try to find that.

Launching at 3000rpm or 6000rpm, you still have to overcome the mass of the vehicle. 300# makes a huge difference in that, but has no effect on shift recovery. It is a lot easier to run good times in an A4 with a lot of torque and average power. Extremely consistent. In a M6, it takes a good driver with a lot of practice to run consistently good times and keep it in the correct rpm range. One goof and you're dropping from 11s to high 12s.

Colonel

"The big benefit of the M6 is the relatively small changes betwen gears. A lot tighter range than an A4."

This story changes as you get on up into loose converters. Check out the shift extension of a TP4600! I used to have one. I'd shift at 6400 with an internally stock engine and drop back to about 6000. Talk about a tight range!

You mentioned shift extension being about 1000 over stall with most converters...that's a little conservative from what I've seen. My SS4000 (which does NOT have a high SE for the amount of stall due to it's 21 blade stator) has an SE of 5300-5400 when shifting at 6750.) Make it a TP4000 and that number will go to about 5600. Drop the shift points back to 6400 and you're still looking at a roughly 5550 SE from the TP4000 (I point that out only because at first thought one might think that 100 RPMs added to a shift point raises the SE by 100 RPM...which it does not.)

DenzSS

We're in agreement there.

I was trying to talk in general terms, so hopefully I won't give the average guy reading this any wrong impressions.

Talk about low SE, I run a PYE3400. LOL

BigPlanTransAm

true, but nothing can launch as hard as a car with a high stall, high str converter. theres so much torque(at the higher rpm)+torque multiplication that 300lbs of weight becomes pretty negligable on launch.

JS

iTrader: (4)

Launching at 3000rpm or 6000rpm, you still have to overcome the mass of the vehicle. 300# makes a huge difference in that, but has no effect on shift recovery. It is a lot easier to run good times in an A4 with a lot of torque and average power.

This is basicallly what I'm saying.

Whats the % for a M12 to drop in between gear,Steve?
Just curious?

DenzSS

Agreed. Most folks on here don't have high stalls or the rest of the setup to deal with that, though.

Peaky power works well with the exactly right setup. Most folks, however, don't have that.

Another thing to point out. With enough torque, you don't need to launch at sky high rpms. The power is already there.

JS

iTrader: (4)

Peaky power is not suited to an auto..
Thats at least my opinion...

And I dont agree weight is mute...
STR has no effect on this or a trans braked car at 3000 would be faster than the same car runing the same setup at 3500lbs.