cr0fty

I though I would start a debate about the most cost effective method to increase power to weight ratio (PWR) of a subaru wrx sti. I firmly believe that weight reduction is better for the following reasons: -

1.) Much of the work costs no money at all, and produces immediate increases in PWR.
2.) Better acceleration, braking and cornering - increasing power only improves acceleration.
3.) Less load on the engine and powertrain, as it is lugging less weight along.
4.) No need to remap the engine when you remove weight.
5.) Better fuel economy, vs increasing power which usually makes economy worse.

I am new to modding, and really concerned about the huge lack of information relating to all products claiming to increase bhp. When I ask dealers, they frequently have no idea about the increases to be expected - even for a standard sti.

Any thoughts appreciated!

Brit_in_Japan

Can't disagree with your theory. The only question would be how easy would it be to live with in a "lightened" state? If you have to drive it each day, how tiring would it be to drive if you removed all the soundproofing for example?

Also there might possibly be a safety question, how would your carbon bonnet/doors stand up to a crash in comparison to the standard items?

Not saying they're show stoppers, just something to consider. Of course if you're talking about a track car or something for weekend fun only, you can put up with a lot for some extra performance

mark6

I'd say that obviously weight reduction is a good cost effective way(excluding carbon fibre parts) of improving the performance of a car thats standard,but it can only take you so far.I'd say without getting too in depth that weight reduction will get you a quick good handling car for twisty track use but its power that will help you more for an everyday road car,high speed tracks and bragging on here.

cr0fty

I was under the impression that power to weight ratio was the all important figure, rather than power alone. Is that not the case then?

stilover

I'd rather have a lighter car than a more powerfull car.

Lightness means it will handle better (less mass weight to move) it'll brake better etc.

I remember just how much I could just chuck my 106 GTI around. Try the same manouvres in my STI, and I'll be waking up in hospital.

imprezarb5

didnt really consider this when buying my current STI, but would certainly consider something like a Spec C next time with its lightened panels etc. Also depends on how prepared you are to do without creature comforts such as electric windows etc. Most of the top performing Scoobys and Evos and Skylines for that matter at events such as TOTB, are completely stripped out of non essentials, some even having perspex side windows etc.
Depends how far you want to go really and also practicality depending on whether you need it as an every day car or just as a weekend track weapon

BIG GRINDER!

Power to weight is very improtant. You could strip the car leaving only the essentials but im not sure how much weight that would save.

Kev

mark6

#4 (permalink)
"I was under the impression that power to weight ratio was the all important figure, rather than power alone. Is that not the case then?"




well it is....almost,but what do you do when it can't be lightened anymore....increase the power.Also it's not just power,torque plays a very big part as well.do a search on TOTB,it will give you a good idea on what wins what,you won't find that the best power to weight car wins every event.

Varboy

stating the obvious I know but modding for power is not all about final bhp numbers, therefore power to weight can become slightly misleading

weight reduction of course is worthwhile but has a huge amount of drawbacks, the only weight reduction I have bothered with is making sure my missus isn't a passenger

silent running

Weight reduction is a fine aim - if it's not an day-to-day road car. I had my old turbocharged Clio 16v down to about 900kg and 230bhp which was a very handy 255bhp/tonne and made it very quick on the road indeed. But in the end it's difficult to live with the road noise, rattles, lack of places to put stuff and general inconvenience of a lightened car. You might think without the back seats or panel trim you'd have loads of space to put stuff, but actually it's a pain in the ****, cos if you put things down in the back, they end up flying about everywhere. If you're going to do the job properly you can very rapidly end up with a shed that's no fun to drive unless you're on a track. But apart from that kind of inconvenience, yes lightening a car is win-win, better handling, better fuel economy, better performance. IMHO.

Slowboy Racing

IMHO weight reduction is only worth looking at once you have completed mechanical mods.

X30NGO

surely though, as a daily driven car on the roads, with passengers sometimes etc etc, the most likely way to increase the power to weight ratio is to increase power rather than decrease weight, as you would want to keep most of the parts of the car due to it being your run around!

Track car, probably direct opposite. Get it as light as possible before adding power? Thats how it comes across to me anyways.

Gutmann pug

A lighter car will be kinder on brakes, tyres, mpg and pretty much everything else.

cr0fty

I guess that depends entirely on what you use the car for. In my case, I dont have to ferry 4 people around in it EVER!

The aim is to do it up to rally in around 3 years time, so for now I may as well do the weight reduction stuff and gain all of the benefits. At least thats how I see it. When you go down the road of power increases you are asking the whole chassis and car to work with power it was never designed for. Less weight = less stress on everything!

Martin2005

I think this is an intersting discussion, the theory of cheap weight reduction doesn't really hold up though, not for me anyway.

Cars are heavier for very good reasons, they are much safer than they used to be, and have more modcons. Do people really want to sacrifice weight for safety?

So therefore proper weight reduction becomes hugely expensive, alluminium bodywork and carbonfibre costs an absolute fortune, which is why only fairly expensive cars are built using these materials.

So I believe that power increases are far more ecconomically viable.

cr0fty

Interesting point. Just as your argument for weight reduction being done on the cheap can be valid, many owners will fit an induction kit or other power gain kit to their car without remapping the engine. Indeed, from my engineering background, standard drivetrain parts are designed with a factor of safety - and increasing power without knowing this is more dangerous than removing internals from a car.

There are many safe things to remove from a car that have no impact on safety - rear seats, air conditioning unit, replace front seats, sound proofing, etc etc. Of course the argument has been made that this may not be practical, and there is the fact that power increase mods mean you can still have a completely useable car, but at what cost to your car's engine if not done properly?

From the research I've done, you can replace front seats on scooby sti and remove spare tyre and rear seats and save about 64kg. Hardly a huge amount, but they can all be replaced later if needed - this amounts to an equivalent 12bhp gain for a standard vehicle at 1495kg/260bhp. This plus all the benefits of reduced weight, no need to remap and less load on the engine. This is before we look at stripping sound proofing etc.

Oh by the way, removing the back seats makes the sti sound awesome from inside

WRX_Rich

iTrader: (5)

missing all that for 12bhp is not worth it imho in a daily driver I have in the past stripped the interia and felt no real gains apart from noice

where as a remap etc you will notice the difference

craigdmcd

iTrader: (2)

As a side note, when previously talking to a rally engineer, he approximated that performance per mile was twice as dear to gain if going down the engine/power route than against handling and braking option.
As for your question, the lightening of the car has a far greater impact, especially if you go below the minimum weight and have to ballast back up to get the weight correct and balanced (ie, the Focus WRC has a fabulously light front sumpguard assembly whilst the rear one is unbelievably heavy by comparison - it's all about weight distribution)

reano

iTrader: (2)

The biggest question to ask yourself is what is this car to be used for? If daily driver then don't go reducing the weight. If you want to do mainly track racing then there are far more cheaper and better options for the track. Caterham, Ariel Atom, Etc. So for me I don't get the weight reduction thing as there are better cars out there for the track. If you want a fast daily driver with he occasion track day thrown in you have one of the best cars for that already but you probably want to upgrade the handling and brakes before the power for that.

hades

In principle, losing weight to gain performance is a much better method than tuning the engine; cars handle better, you don't need as big brakes (which in turn reduces unsprung mass, improving handling), you don't need as much power, tyre / brake wear and fuel consumption will improve. You won't end up reducing spool up by fitting bigger turbos, etc etc.

In practice, IMHO - if you're after a non-practical lightweight car mainly for track use, don't start with a heavy lump of a Subaru (as reano said, Caterhams, atoms, Elise's, lightweight kits etc will be just as fast if not much faster, more fun and cheaper to use on track). There are plenty of cars out there which start a lot lighter and several handle better out of the box too. If you're after a 4 door saloon as a daily driver, where you can use the rear seats, boot, decent for long journeys etc, then there is a real limit as to how much weight you can practically lose.

Add this to the fact that most scoob engines do noticably improve in power, torque, better spool and response, less lag and no decrease in economy for the basic mods (exhaust, remap and maybe a few other tweaks), doing those kind of engine changes begin to make sense of themselves.

Craig - for a rally/race car, there is no question you're right, you can find more time from grip / handling / braking than extra power, although there's a balancing point: a very low powered car with great brakes and handling will benefit more per £ from power increases, but most scoobs have decent power, moderate handling and on average moderate brakes.

I am reminded of playing on track days with my brother's Fisher Fury - which at say 550kg dry weight and 170-80bhp was still a comfortable match for most tweaked STi's in a straight line. On track, the handling would leave normal scoobs etc for dead round corners, the non-vented MK2 escort front disks faded much less than several people who were stopping slower on £2-3k 6 pot Brembo set ups on Cossies, Scoobs etc, and 3 hours of track time including some very sideways moments lightly scuffed up the surface of a set of Yokohama (A520s I think) it was running at the time. In contrast, my 1300kg lump at the time got through 3mm tyre tread in 25 minutes, cooked the (uprated) brakes in 4 laps, and was always over 7 seconds/lap slower on a 1 minute 30 lap . . . .

Flaps

iTrader: (1)

Would be an interesting topic for a Top Gear show this one. They buy a car each and do 0-62 then strip it and put race fuel in it and see how much it changes.

Playsatan

iTrader: (2)

They already did something similare with that v12 jag. They put it on the quarter mile, then stripped it and did it again.

corradoboy

YouTube - Top Gear Old Jaguar XJS Tests - weight and NOS

Carl2

There's a vid or an article on the net somewhere where they start with a complete car and gradually reduce weight removing internal bits, then bumpers,doors, bonnet, etc, finally they cut the roof off


For what it's worth I will have removed 150kg off my car when the next few bits go on and I'm aiming for high11s on approx 300bhp.

When I removed the 1st 50kg and fitted smaller diammeter tyres and it ran 3/10th faster on the 1/4 the next time out.

andythejock01wrx

What have you removed/changed mate ?

billythekid

Weight saving is by far the cheapest form if you are talking about a road spec car. Just remove everything thats cosmetic, trim, seats, wiring, sound system etc etc. You can take 200kg out of a modern car easily - for free.
Then if you want to spend a little bit of money you can go perspex windows, GRP body parts (a lot of scoobs have ali bonnet and boot though I guess) and light wheels / tyres. It would not take much to get a modern WRX from 1400kg down to 1200kg.

billythekid

BTW another forum I am a member of is obsessed with weight and they have a massive thread with weights of all parts of the car.

Carl2

Have to point out it isn't a Scooby, but the same applies. Car was 1240kg, is now 1130kg with interior removed for drag racing, plan on about 1070-80kg next year.

The 150kg off is with, front seat (-12kg) and rear interior removed (-20kg), spare wheel, jack etc (-15kg), carbon bonnet (-10kg), aircon removed (-23kg), lighter exhaust (-12kg), coilovers (-5kg) lightwieght wheels (-5kg each) total 102kg
There's another 50-60kg waiting in the wings to come off and that's before I start removing sound deadening/excess metal etc (which I don't plan on doing).

That's about 150-160kg off and still keeping the car looking standard (bar no interior)

Edited to add: I forgot -5kg on a lightweight flywheel and about -2kg on lightweight pulleys

andythejock01wrx

Cheers mate. That's very interesting. Must admit, there's not much there I'd want to do without on my car, which is a daily runaround (even the AC is pretty handy, especially for demisting during a Scottish winter !).

I'm thinking, how would the car come across if I was picking up a girl in it for the first time - LOL !

B9GLY

the sound deadening weighs alot so will save alot of kg's but will end up with alot of headaches!! lol