[Torbjorn968]

Trying to figure out how much weight I can loose in order to reduce optimal power requirements for a 944/968 hybrid (S2 with 968 engine)which has been a slow project for a couple of years. PCA has 2 suitable classes, one which would be perfect with the stock engine for the car (this would be the wise choice ofcourse) and one which will require a bit more than 300 hp from the engine. The first class allowes 5 kg/hp using end of race weight including driver. 50 kgs is added to rated weight for cars without ABS. Power is calculated with a 12 % powerloss. I would think it is more like 20% on our cars loosely based on 265 engine HP on rolling road with RS Barn cams and a ported cylinder head after mapping of my own car. Using a Driftbox (which is used for rating in PCS) I got 211 RWD where power is calculated from accelleration with a a total car weight given ti the driftbox.



If all this is correct I need to put the car on a serious diet. It is now 1040 kgs with carbon seats and not much in it, or 2290 lb. Interior is stripped out and the front is glass fibre with motorcycle headlights. The engine will be balanced and balance shafts removed which gives me the following list of preliminary possible savings:



Weight today 1040 kg 2291 lb

Balance shafts -9

Battery -10

Passenger seat -4

Flywheel -22,7

Wiring loom -2

Dashboard -10

Heater package -10

Charcoal canister -2 ?

Rear fenders -5 ?

Spare tire well -2 ?

Carbon luggade floor -1 ?



Empty weight 962,3



ABS allowance 50

Driver 100

Gas at end of race 10



Rated weight 1122,3



Class 8 c

HP allowed (rated) 280,575

RW Hp allowed 246,906

Real engine HP 308,6325



Any further ideas of savings or more accurate weight for the heater package and dashboard. Tried to search the forum for numbers but did not find it. Think flash has given me a heater package and dashboard weight somewhere.



And where do I find more news about the RS Barn 300 hp engine?



All the best and a happy 2011!



Torbjörn

Partial Post: Please Login or Register to read the full post.

[aslet]

Do you have a lexan rear hatch? I don't know what you are allowed to do, but if you have a roll cage you can also do carbon fiber doors... That takes out some big weight. It would depend on class rules.

Partial Post: Please Login or Register to read the full post.

[flash]

it really makes me nuts that people still use a fixed percentage for power loss - do they really think that if you had a 500 hp engine that you would lose the same percentage of power? - it's not like you would suddenly have a drag increase from the drivetrain - the components have fixed reciprocal and frictional masses



i've gotten sucked into that too, but it's really crazy - they should just measure what you have at the wheels and call it a day



that being said, in stock output level, on a 6sped car, with stock 17" tires and wheels, we lose about 15% of the power to the drivetrain (about 35hp)



only a 4kg loss in the passenger seat? i had a 4 way power seat and swapped for a recaro sportline and still lost 10kg



the flywheel will only get you 11kg, even if you go with the super light one



i don't thing the balance shafts are that heavy either - nor would i remove them, as the work to rebalance the engine properly will be horrifically expensive



in a race car, there would be a lot more to lose in the wiring, but if you plan to keep it streetable, then maybe not



the dash is definitely heavier than that, especially given the deletion of the airbags

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

Passenger seat at the moment is carbon so therefore 4 kgs to save there including the brackets.



I have a good source for reasonable cost of rebalancing the engine and a friend who is part owner who dreams about a balanced free rewing engine. They are quite heavy at 20 lb or so including everything if I remember correctly. I would not do it without rebalancing after driving my car with a snapped belt for the balance shafts.



The wiring will only do basic stuff with the most advanced being the standard tail lights, indicator lights and headligths. How much would you say we can loose here? And then ofcourse the underseal....



Car is an 97 S 2 and think no airbags. 45 lb for the dash?



All windows are plastic except for the windshield and the quarter windows. Doors GRP etc.



With 15% powerloss things could then look like this.



Weight today 1040 2290,748899

Balance shafts -9

Battery -10

Passenger seat -4

Flywheel -11

Wiring loom -4

Dashboard -20

Heater package -10

Charcoal canister -2 ?

Rear fenders -5 ?

Spare tire well -2 ?

Carbon luggade floor -1 ?

Underseal -5 ?



Empty weight 957



ABS allowance 50

Driver 100

Gas at end of race 10



Rated weight 1117



Class 8 c

HP allowed (rated) 279,25

RW Hp allowed 245,74

Real engine HP 289,1058824



//TL

Partial Post: Please Login or Register to read the full post.

[flash]

it's not just a case of rebalancing like you would normally do on an engine - it is the issue of having to recalculate all of the harmonics, and figure out how much weight you have to remove from the other items to achieve a nominal condition over the operating rev range - i could have done that in college, but i am way too old and feeble to remember how to do that anymore - once you've done that, then you get to do the traditional balance job



basically, if you are removing that much rotation mass, which was set to cancel harmonics, there is a corresponding amount of mass that must be removed from the crank, rods, pistons and flywheel - failure to do that will result in the harmonics beating the crap out of everything - it likely won't feel like much either, which makes it a real bummer when you find out that your bearings are toast and you get to rebuild it all over again



the shafts only take about 7hp at max rpm, and less down low, but save the engine a lot of wear - not worth screwing with - there are better places to lose that weight

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

Balance shafts will be 2012, this year a standard engine will be used so there is time for me to come to senses <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />

45 lb for the dash a reasonable number then? Rear quarter panels worth replacing with grp?



//TL

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

Maybee I'll just get the supercharger kit. With moderate weight savings I can then have 350 engine Hp. If you have a hint of price send a PM or a mail to torbjorn@linderson.com! With the wide powerband it would solve a lot of things such as gearing.



//TL

Partial Post: Please Login or Register to read the full post.

[aslet]

What can you do in your class? It sounds like you are/ will be spending a lot of money on your car. To make a real racecar, you need to start with, well, a racecar. You can make a 968 into a real fast and good car, but it's not a tube frame super low weight car...



Would you stay in the same class with the sc kit?

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

Classes intended are in the Scandinavian Porsche Racing. There are four classes:



- 6,5 kg/hp

- 5

- 4

- 3,5 kg/hp



Weight is end of race weight and horse power is rear wheel horsepower * 1,125 to account for losses. Rear wheel hp is meassured accellerating with a Driftbox.



"Sensible" gentlemens mods to Porsche Cars with Porsche engines etc.



//TL

Partial Post: Please Login or Register to read the full post.

[savvas968]

very interesting Bob.



7 bhp at max rpm,does not sound too bad when you are chasing few hp here and there after exhausting all the usual, easy to do items



the static weight is appx 4,5 kg and they do rev at twice the engine speed.

provided of course the harmonics issues are sorted out ,and when we are talking on a car for road and track use.



Seem to recall that they were there to cancell unwanted vibrations at steady 3000 rpm where most engines spend most of their life on the highway.



this 7 bhp how were deduced,was this figure calculated in any other forum,was at the wheels or engine dyno,can you please give some add info?

i bought a balance delete kit but not used it yet on the 968,but i am planning to.Any input is welcomed.

Partial Post: Please Login or Register to read the full post.

[flash]

it is a number that has been passed around for years - i can't remember where i first saw it - i don't have any math to back it up



the harmonics would be a problem though, and your engine won't likely last long without them

Partial Post: Please Login or Register to read the full post.

[savvas968]

thanks .

Partial Post: Please Login or Register to read the full post.