simos

I would say, that 6500 - 6800rpm would be where max. output will be coming with full race cams as they won't limit flow that much. There is no point to sping engine at 700rpm,
if power output starts to fall after 6300rpm as it's doing now. Something else, than AF/Ignition tuning needs to be done, but this is of course known by you already

The increased torque is mostly coming from increased displacement. Of course the velocity in stock intake port min. CSA has been increased and gives better ramming effect at lower engine speeds.
I remember that stock S4/GT/GTS port min CSA is something like 2,4X sq in. This will start to have best usable velocity(260 - 280 ft/s) and ramming effect, when engine power is around 550hp. With that velocity range, the torque starts increasing dramatically and may also lift up high hp numbers if it stays up on high rpm area...


The Stage II "tri-flow" is very usable in street applications, where nice torque and little hp increase is needed. However, when building full race engine, the problem is to fill the cylinder in few tenths of milliseconds and this is where the overlap and good exhaust scavenging kicks in. Due to high scavenging effect during overlap, the air/fuel mixture gets high velocity immediately after intake valve opening, giving much higher VE. Therefore overlap is almost mandatory to go for 100hp/L or higher(I have been told this by local race engine builders, when I show my Stage II cams to them). I'm just thinking about your headers, which operating range very much you are now. They should work nicely in your engine, giving massive scavenging effect at high rpm area...

I like the following picture, which explains the overlap idea very well:

http://www.superchevy.com/technical/.../photo_03.html

IIRC the overlap/exh. scavengin effect for filling the cylinder is around 50 times higher than Helmholz pulse, provided by intake tubes. So, don't worry much about your ITB tube lenght

It's going to be very nice and powerfull build, I'm really hoping the JE pistons will last loong

drnick

^^that hood ornament must be a slight distraction when driving..

simos

It's sad to say this, but the hood ornament isn't mine neither the car
Found it from internet, hopefully the real owner doesn't mind, sorry..

If you have ever been in Europe, you might guess from which country the ornament is. I'll give you a hint, the car is having yellow fog lights

ptuomov

I thought one should shift well past the peak hp rpm to maximize average hp that the car operates at. As a very crude rule of thumb, you want the hp to be about the same before and after the upshift. Since the graph is truncated, I don't know where exactly that would be but I am guessing close 7000 rpm. With Stage 3 "full race" cams you'd have to shift even higher, I think.

Have I misunderstood this thing as well?

John Speake

Wouldn't you want to change up at max power and with the right gearing, hit max engine torque in the higher gear ?

James Bailey

Kibort in 3 , 2, 1 ......

Cheburator

I think you nailed it, John. It certainly appears that way if you look at where you drop in the rpm range with each shift in the user manual for the 5spd GTS... Looking at the new engine, I will need to shift at higher rpm, however, this is still somewhat negated by the fact that I am making more torque across the rev range anyway

Mr James Bailey, how dare you?!

Actually, where is Kibbort - I finally give him the numbers that a stock-ish motor (size wise) with ITBs makes and he is nowhere to be seen?

Imo000

I would first find out from JE what is the maximum piston speed that these pistons can handle. Then calculate the redline RPM.

ptuomov

Because my 5th grader math says it would make the car slower than shifting in a way that the power at pre-shift rpm is equal to the power at post-shift rpm. (This assumes a power curve that has one peak and no local peaks and valleys.)

Let me give you a counterexample of how shifting to the peak torque is clearly not optimal. Suppose that the engine makes peak torque at 2000 rpm, the gear ratio factor is 3/2, and the peak power is at 8000 rpm. You wouldn't want to upshift at 3000 rpm to land at the 2000 rpm peak torque after the shift, right? You'd shift somewhere around 9500 rpm or whatever would make the pre-shift hp equal to the post-shift hp.

In any case, that's how I think about it, but I've been wrong about a lot of stuff on many topics...

John Speake

Well your first theory is fine as long as you have "max power" over a wide rpm range. Or you decide to over rev the engine into the area where the power drops off more gradually.... and that can give problems !

For your second, you've chosen a most unlikely scenario...... which doesn't help your theory...

ptuomov

I intentionally picked an absurd example to demonstrate that the shift to max torque rpm rule can't be the optimum (theoretically). I agree that it's the most unlikely scenario for a motor, intentionally picked to be that.

I think that optimality condition of pre-shift power equaling post-shift power is pretty general as long as (1) shift is instantaneous and (2) first two derivatives of the power curve exist and the second derivative of the power curve is negative. I think...? The power curve can be very flat or very peaky, you still want to shift in a way that pre-shift and post-shift hp is the same.

IcemanG17

Tuomo
Your "well beyond redline" theory for max acceleration might be true on paper....................BUT I would NOT try it in the real world..... Redlines are placed where they are for a REASON..... We all know what happens when you consistently rev a 928 engine HIGH....things like this happen


This was a bone stock S4 engine...BUT with the rev limiter moved to 7000.....it literally sucked an intake valve....

When I drive on track, for hours per day, I shift at 6000rpm....on paper I could get a bit more acceleration if I push to 6250 for sure...possibly 6500 but i have not dyno'd that high to find out....why....ENGINE LIFESPAN..... When I shift at 6000 I drop no lower than 4400....plenty of acceleration there....just past torque peak

Dennis K

This is a nice advance if JE has come up w/ a coating that will live in an Alusil bore.

Congrats on your build. Looking forward to the development.

ptuomov

I agree with the things you write here. The redline is there for a reason. What Alex's redline will be depends on how much weight he saved from the pistons and how much risk he's willing to take.

Let's back up a little bit. This conversation started from the camshaft selection. If Alex is setting his personal red line at say 7000 rpm, I don't think he would benefit from longer duration cams. Even with the current Stage 2 cams, I am guessing that this engine would be optimally shifted at the lesser of 7000 rpm and the redline. If Alex's redline is below 7000 rpm, fitting those BMW intake trumpets there becomes a real priority, not Stage 3 cams.

Now, take all my writings with a grain of salt, I don't race cars so I don't know anything about any of this beyond my 5th grade math.

GregBBRD

I'd personally think that a bigger camshaft in Alex's engine, without a significant change in the ability to turn the engine more rpms might not be positive.

I have seen, several times (in actual applications), that one needs to be really careful about trading torque for horsepower, especially on a vehicle with relatively "long" gear ratios, like our 928 transmissions have. This seems to be especially true, when increases in airflow and camshaft lift and duration are "limited" by other factors, inside the engine.

Increases in airflow, with the virtually "automatic" reduction in velocity (that comes with bigger valves and ports) usually always moves the power up and reduces torque.

Bigger camshaft profiles, obviously, tend to do the same thing.

While Alex's piston weight (I'm not sure J&E could build as heavy a piston as Porsche supplied, if they tried) and the intake system seem like they would be a great "fit" at over 7,000 rpms, I'd tend to agree with Alex that the crankshaft and the rods might not be as good a "fit" for high rpm use.

A stock 928 head flows enough air to support Alex's current power output, so any head port work (and bigger valves) might be hurting him, in his current RPM range, not helping....which is always something to consider. (Keep in mind that most 928 cylinder heads that I've tested, on my flow bench, which have bigger valves in them, flow less air than stock, with less velocity....until the camshaft lift gets really high.

The great improvements that both Mike Simard and Colin have made, in the available selection of camshafts to 928 owners (we were very "camshaft" limited in reduced base circle camshaft selection prior to these guys making camshafts) made high rpm engines, with bigger cams and more airflow very possible.

As some may know, I recently had lightweight billet crankshafts (with the rest of the crankshaft designed for higher rpms) built in both 5.0 and 5.4 length strokes that use a much stronger rod with a "smaller" bearing (also for higher rpm use.) We are getting ready to build one of these engines, with throttle bodies, and see if we can make a 928 engine that spins freely to 8K but still has a usable rpm range..even though we know that rpm range will be higher than stock.

These 928 engines, while designed over 25 years ago, are still being evolved, by people like Alex.