jcorenman

Promises, promises...

mark kibort

Bob... you are a funny guy!

Well, what does a crank and lightweight parts really do? provide more HP? thats not the real goal.. as greg says, its to couteract driver deficiency due to the fact that the 928 doesnt have computer aided traction control. (as Bob says ) and now you get less peak torque and possibly the same hp if you make all sorts of mods to intake, cams and heads.

the truth of the matter, is that with this light weight version of the crank, which is a shorter stroke than the real 6.4L cars, all you would get is a real expensive 5.4L that wont make any more measurable HP. However, it might last longer and with Gregs new pistons, it wont burn as much oil as the stock GTS.
However, if the intake, cams and heads are re-worked you get a lower peak torque engine with possibly near the same HP as a mostly stock top end equiped 6.4L. The other truth here is that there is no problem with 400ftlbs of torque and a 928 . its not dangerous, and its not something to attack by designing a new engine to remove some of accelerative forces from the 3000rpm to 4000rpm range. More truth, is that if you have a problem with 400ftlbs at low RPM, you certainly will have a problem with 300ft-lbs.

This would be a classic example of tightening the nut behind the wheel before you dump $15k on a new engine to produce high HP and lower torque than the traditional strokers.

mark kibort

Bob, i think is stray "spewage" as you call it, is just discussion about a goal , made clear by the "inventor" , to make an engine's performance a little different to help with "driveability". in fact, the more you get the RPM up in the engine , the greater the forces on alll sorts of other areas of the driveline.
I think its great to have different flavors of cranks and strokes for the 928 engine. If we had to fit a class that limited displacement, the 5.4 liter GTS crank would be ideal for a foundation for that effort. however since there seems to be no draw backs for moderate HP 6.4 liter engines, i dont see the need for the street or race track to get the same HP at higher RPM just to reduce peak torque.

My information is just reflection of what has been discussed and my experiences over 20 years (over 200 documented race days) of successfully racing the 928 with no engine issues. (and building a few that are still in great running condition today!)

Now, robs engine design by Greg is very interesting. 5.8 liters, and twisting up to 7500rpm for near the same HP as my car sounds like fun! I dont think you end up with any safer oiling situation due to the engine spinning so much faster, but thats a guess on my side. you also loose peak torque and without a gear box change to closer gears (like a GT3), the net result is a lower average HP. in otherwords for his car to be as fast as mine, it has to make near 420rwhp because of the average hp available going down with reduced peak torque values.
If the cam changes and head work does more than im giving it credit for , heck, it might make as much hp as the 6.4 liter engine made when Mark anderson owned it. (i.e. 520rwh ) and only make less torque. I dont know. but it will be exciting to see.

Looking forward to the dyno day of Robs new Jewel of an engine!

ptuomov

There are probably a number of 928 GTS owners who either have an engine problem or need to change the R1 rods. At that point, some of them might want to go for a better crankshaft with stock stroke. The S4 crankshaft and early rods are really nice pieces as long as the rpms are kept down, but the GTS crank and rods are frankly poorly designed, very expensive paper weights. It makes sense to me to offer a stock replacement crankshaft and rod combination with better bearing options. Also, you want to test it in a much more stressed setting before you offer it to the general public. My guess is that this testing is one of Rob's and Greg's motivation.

mark kibort

Now that makes a ton of sense, and is really not the message Greg was conveying. and since the engine intake , heads and cams really don't support higher RPM power, much has to be done to take advantage of the changes of weight and balance. I think the best mod to date , is still going with the 6.5 liter stroke option and helping he engine breath at 6-6500rpm will yield massive gains, as Anderson has seen, and you don't get into the expense of dealing with a shorter life, higher RPM (hgh strung) engine.

a great contrast is the mustang boss 302 motor 8krpm vs what Rob has now in his GTS 6.4 liter. (6500rpm )

Interestingly, the Boss 302 still makes as much as the stroker 6.4L makes in the low RPM range as well. all out of a 5 liter engine. 4000rpm both are around 300hp. the GB intake boosts that to 350 with few other changes (I think rob said slight hotter cams and alpha N tune) with a stock set of gears, the width of the mustang HP curve allows you to stay squarely in the peak HP range.. which is optimal for racing.

mark kibort

It doesn't matter if you are on racing tires or on ice.. if you can floor the car, then you are putting max force to the transmission on the input shaft, clutch, and short shaft. on a race car, you get very little wheel spin if you are driving it correctly , no matter the HP or torque. our cars operate from 4000 to 6500 and mark's more like 4600 to 7000. Greg is not pulling my leg.. he wants to reduce mid range torque as he says its basically dangerous. I disagree. And if this is the goal, there are other ways to detune mid range torque. heck, all I did was retard my cam timing a bunch and lost 20ftlbs of torque and gained about 15 rwhp, and im not a "tuner".

as far as catching air....... no, that's really not the force that will kill the transmission, although it can be one to consider if you catch air, get runaway RPM to be brought down on road contact when you land. The main killer as I mentioned before , is a mismatched rev downshift. this can be in excess of 50% more torque than the engine can put out alone, and this is what can break drivelines and input gears . CV joints and shafts can break too, but they should be rated at whatever forces it takes to break the tires free . with race rubber, this force is much greater but lower than what can be achieved in the higher gear mis rev match downshifts. (the force to break input shafts is multiplied by the gear box so it gets much worse at the faster speeds, but the tire traction force on final stage gears and CV parts is a given value, regardless of speed.

catching air is like dropping the clutch on a standing start or speed shifting ... max force is =engine torque + deceleration rate of engine inertia. this brings engine speed down, by use of KE of engine components.

mis RPM matched gear change is break away traction force (torque) / gear ratio.. This is the road bringing engine speed up via use of car's KE.

you see the later has a much greater potential to do damage.

ptuomov

Why is verbal abuse by some not only tolerated by celebrated here? At least there were no threats of physical violence this time.

In terms of the actual event, breaking a single rod in a race engine isn't very informative event on its own. If the computer says rod X is a better rod for the application than rod Y, yet rod Y lasts for a decade and rod X breaks shortly, it's certainly a time to go and check the computations. Still, if the computations check out, forward looking I'd go with the rod X that the computer says is better. Deviating from the fully computerized state-of-the-art analysis of the rod because of a single failure would amount to nothing short of superstition.

The same way if someone breaks a single A-beam rod and doesn't break a single H-beam rod, that doesn't mean that the A-beam rod isn't superior design in every other way except manufacturing cost in machining operations. The A-beam rod is still superior, and the H-beam rod only exists because it was cheaper to machine back in the day by aftermarket rod manufacturers. The difference today between the designs is small, of course.

Since its 2015, I think the appropriate weight we should put on computer simulations and computations is high compared to anecdotes from practical testing. Furthermore, I think it's best to apply that sort of weighting consistently across the board, no matter which way it happens to point in a particular debate or argument.

mark kibort

Greg,

you have as much experience and knowledge racing as I have building engines. your arrogance exceeds only your repair rate charges.

You have challenged me on many topics and rarely answer with anything more than anecdotal evidence. that's fine, because this IS a discussion board. you should take it back a notch, because, believe it or not, when you are civil, you do provide good information here.

However, I do understand. yes, things break... its the nature of this game. BUT, the old adage still can reign true. if something is working, don't mess with it. you might, on occasion, over think the situation, and without ALL the information , it can cause you to go too far one direction. (you know trying to fix one potential problem and creating another.) its human nature and leads to some expensive issues in auto racing and building.

You can make mistakes, the manufacturer can make mistakes and in designing things, without every single bit of information , you can misjudge a design. this all is a given. F1 teams, with millions of cash, still make mistakes at ALL levels. your changes to Marks Rods were YOUR choice, not corrillo... its possible the design change was not as good as the prior rods. I don't know..... but someone does!

Mark didn't over rev that car.. Ive raced for 20 years and never over reved a car... the only way you can do that is if you miss a shift. fortunately, our 928 gear boxes are too crappy to allow for that, and the shift pattern doesn't allow that to be too likely.... plus, there Is rev limiters that keep revs from going over redline. misshifts are the only thing that can drive revs over the limit.... so that's all a guess on both our parts .
that's one of the reasons I film every single time im in the car. cheap data acquisition. If you were serious about being a builder, is a simple thing to add to the costs of building all these high performance engines for top clients. data aqu. is available for relatively cheap prices these days too.

Greg, you are not being polite, but I understand your defense, because its your livelihood and reputation. im only here for one set of reasons, to be a part of the 928 community, learn, and contribute by my experiences and knowledge. (and be entertained ) so, if being polite is calling me basically, "20% less than a dumb ***", is your idea of being "polite" then, you have a little work to do on your manners. You should also realize that ive been racing the 928 platform for more than 20 years. more than anyone here, and some of my experience, information and opinions should be of value to you, as yours and others are to me.. since they are not, who is the dumbass?


Well said. My thoughts exactly!

James Bailey

Mark you got all spooled up on your theory of downshifts over stressing the driverline...no one bothered to challenge that so in the spirit of Debating 101 you now assume it as FACT . However the force available is ALWAYS limited by the traction on the tires..... you never ever get more than that and in reality get much less on deceleration versus acceration due to rear suspension geometry which is part of the reason why you can spin the tires easier in reverse .
Also trying to imagine a track situation where you would not be on the brakes as you were downshifting which unloads the rear and also uses part of the limited tire traction especially if in a corner. The anti-squat built into the rear suspension is what causes wheel hop when you floor a car in reverse.
I suggest you rethink this theory....

mark kibort

Jim, you might want to "rethink" your objection. while it is true in a downshift that the there is weight transfer, lowering the friction coeff of the rear tires (and your reverse analogy/comparison is correct for rear wheel drive cars only), it only lowers the the forces transferred to the driveline compared to a straight line traction release acceleration.

I already produced values of traction under threashold braking. if those numbers are applied to the driveline , through the gear box you will see where the factual information comes from.

You seem to be missing the critical point here that a application of force from the engine is different than the application of force from the entire mass of the car and its velocity. Even if we halve the traction force to break the tires loose, the force on a high speed (3-4th gear) miss RPM downshift that gets a mere chirp, is much higher than can ever be put on the driveline by a dump clutch standing start. WHY??? because of the reasons I laid out pretty clearly in my last post.

So, in the spirit of the debate:
Jim, the force on the driveline is NOT "always limited by the traction of the tires. the force is dictated by the engine on acceleration . On deceleration, its dictated by the (as you say, "lower traction force"), on the tires, and the reduction % of the gear box. the greater the reduction, the less the forces to the driveline. (think about that for a second). This is why a downshift missing the RPM match on 2-1st gear is much different than a mis RPM match shift from 4th to 3rd. Make sense? If it doesn't, lets debate why.

edit: so Jim, you tell me the traction force of a rear race tire at 1g deceleration for a 3000lb car with 50/50 weight disti, and Ill tell you whats reflected to the driveline. and to prove my "theory" (which is not a theory at all, it is factual), ill show you why mis matched downshifts (which MOST guys who are more regular DE or beginning racers do, while Ive coach them, several times a lap until that technique is fixed), create forces that can far exceed the forces that only the engine can produce on the driveline.

RFJ

Comon Mark you know James is right,apply a little common sense when your on ice down shifting and you will get it,your smart.

mark kibort

hahaha... yes, on ice, he is right.
one other thing that he must think about, is the aerodynamic forces on the rear of the car near 100mph on that mismatched RPM downshift (adding to the friction) to make a "chirp"" of the speed on a 4th to 3rd downshift.

this isnt as intuitive as Jim is making it out to be. a little deep thinking, will lead him to the light.
I can hear Jim scouring the internet or punching in the calculations as we speak now!

GregBBRD

True on ice but not true on pavement.....that makes total sense!

What difference does the surface make???

The limit of rear tire adhesion is the limit of the force that can be generated to the drive train.....it's a really simple thing.

mark kibort

EDITto be more clear)

Its not that simple Greg.. read what Im saying again. (its not a simple thing)

sorry, i wasn't more clear... "true" in that the force on ice would not be something you would worry about, but and yet, the limit of tire adhesion is the limit of the source of the force. but the the amount of force reflected to the driveline, can VARY based on friction value and vehicle speed (conversely, as on the upshift, the engine power , torque and Spinning inertia (aka KE) is the limit for its source of the force on the drive-line)

So, the limit of adhesion is NOT the limit of force that can be applied to the driveline. does that make sense?

so yes, the surface has no bearing on the formula for reflected torque to the driveline. the values will change as you know depending on what gear you are downshifting to and from, as reflected to the drive line.

Here is an example you should be able to understand...... If i have a real sticky set of F1 tires and it takes 400ftlbs out of the engine to get a chirp out of them in 1st gear... if im going 240mph in 5th gear with my 2:2:1 ratio and i downshft to 4th and drop the clutch to get a chirp, what is force reflected to the driveline? well, lets say , 1st is 10:1 and 4th is 3.0:1. the 1st gear chirp was 4000ft-lbs of torque at the wheels, 400ft-lbs at the driveline, the 4th gear chirp at speed would be (4000 / 3.0:1 = 1,333ftlbs of torque at the driveline). THATS 3 X the force applied to the driveline Greg!!

There is NO limit of the force that i can apply to the driveline for a given force limited by the adhesion of the tires. it can go up and up based on the gear ratio i drop the clutch to.

So, your statement is incorrect and now you have to look at mismatched RPM downshifts from 4th to 3rd in an entirely different light. it might be a cause of some major transmission and possibly engine forces that can cause failures or wear.

Imo000

This is some alternate universe physics laws you are trying to explain there Mark. The coefficient of friction between the tire and the pavement is ALWAYS the limit. ALWAYS!