Coupethis

By now you have read "The Basics", spent some well earned money and are now licking your lips at what else you can do to your engine, as in, slip in a cam or throw on an intake blah blah blah. Well, sorry folks, even now, don't even think about the internals of your engine! We'll get into that stuff a little later, but there's one other important factor to think about.

When we talk about the dragstrip, its the car that can lay the most power to the ground over 400 odd metres that wins the race, not necessarily the car with the most PEAK horsepower. A car isnt just an engine, there's a gearbox and a diff to worry about too, and they effect how the power is put to the ground.


Diff Gears

Changing the diff gears for more acceleration is one of the most effective mods you can do. It's not uncommon to see half a second gain (and more) over the quarter with a set of gears.

Most older cars don't have an overdrive gear in their trans, and that poses the question, cruisability or acceleration? For me, I wouldn't go anymore than 3.5ish diff gears for the street without an overdrive gear. Some people don't mind reving their car out a bit to chase the times down the dragstrip, but for me, i like to cruise comfortably and let the torque do its thing when I wanna get up n go. At 100kmh, rpm sits at about 3000rpm depending on tyre diameter with 3.5 gears which I think is just right. If you do have an overdrive gear e.g. T5 or T700 etc., then definitely fit some short gears.

Be careful though, make sure you don't over gear the car because you might end up going slower. This doesnt matter as much for a mild 308, but as soon as you start making decent HP, its crucial. Rev limit, RPM at max power, and RPM at max torque among other things all come into the equation when choosing the right ratio. Basically, you want to gear your car so that you cross the finish line (in 1:1 gear) just before your engine stops making horsepower. At the same time, you don't wanna make 1st gear too short, or its smoke city (some people don't mind that ) It's all a juggling act.

Some handy tools to work out RPM to diff/trans gear ratio relationships:

http://www.5speedtransmissions.com/calculators.html (tyre diameter)
http://www.5speedtransmissions.com/gear_calculator.html (allow for slippage if auto)

Common gear ratios for certain type diffs:

Salisbury: 2.6 - 2.78 - 3.08 - 3.36 - 3.55 - 4.44(!)

Borg Warner: 3.08 - 3.45 - 3.7 - 3.9 - 4.11

9 Inch: Pretty well anything you want!


Diff Centre

If you can't get the two back wheels transfering the power to the ground, then all your effort is wasted. Having a Limited Slip Diff or LSD is just about compulsory for performance. People also talk about mini spools and full spools, but unless you are an experienced driver and can put up with the constant tyre chatter, not to mention the sheer danger in the wet, I wouldn't recommend them. Have your LSD rebuilt nice and tight, and make sure you use good quality diff oil like Redline.

I could talk about mechanical lockers like the Detroit Locker for the 9 inch, and the Powertrax Lockright locker for the Borg Warner, through to the expensive KAAZ LSD, but that would be getting beyond the budget nature of these articles.


Gearbox

Most people wanting to go fast down the quarter will opt for the auto. In stock form, they aren't really much fun. But set them up right, they can be a blast to drive and be really quick down the quarter.

The most common mod is to fit a shift kit, which is usually just a high performance valvebody kit that firms up the shifts. Different servo piston and so on are also added to help with the shift. People talk about Stage 1 and 2 kit, which is just the difference in shift firmness, as well as Stage 2 being able to hold 1st gear and being able to kick down to 1st whenever you want. Combined with a ratchet type shifter (B&M, Hurst), it makes for rapid fire gear selection, almost arcade-like. You can also get a full manual valvebody, but I would leave that to the hardcore racers myself.

Heat is the enemy of auto's. Even the best built top shelf auto will die if heat is not controlled, so it's important to fit a transcooler (get a big fuk-off one from B&M/TCI) and use quality transfluid (Castrol Transmax Z/Redline). Doing these two things will dramatically increase the chances of your auto living under contant abuse. This is especially important if you are going to fit a high stall since slippier converters will produce ALOT more heat.


High Stall

Another very effective mod is to fit a high stall torque converter. Another half a second down the quarter can be gained fitting one (Before anyone asks, if you don't know what a torque converter is, check it http://auto.howstuffworks.com/torque-converter.htm) The aim of a high stall converter is to let the engine reach further into its torque producing rpm when accelerating.

Normally, a stock stall converter will have a more conservative stall speed (e.g maybe only 1500rpm), which is the point where the converter locks. So, if you hit the throttle hard, the converter will let the engine spin to 1500rpm before it will let the full force of the engine at that rpm to transfer through the rest of the drivetrain. Naturally, by raising the stall speed with a high stall (to say 3000rpm), this effect is much greater since the engine is making alot more force (torque) at 3000rpm rather than at 1500rpm.

Now before you go out and ring Dominator for a 5000rpm stally, raising the stall speed has both advantages and disadvantages, and buying the biggest rpm converter you can find will not automatically equate to going faster. Choosing the "right" stall converter, not the "highest" stall converter will make you go faster.

Fitting a too-higher stally won't allow your engine to transfer it's full force through the rest of your drivetrain quick enough, which results in poor throttle response, and makes matching the stally to the engine's torque curve and selected diff ratio important. Furthermore, the "looser" the converter, the more power that is lost due to heat/friction. Without going into too much detail, I wouldn't go anymore than about 2800rpm for a mild 308. Even that may be a little high (2500rpm would be just right I'd say). Having said all this, don't be mistaken in thinking that you'll need to rev the engine to the advertised stall speed to get going. Advertised stall speed is where the converter will "flash" to at full throttle, not at all throttle inputs. But yes, a higher stall will generally feel more "loose" at part throttle.


Ok, so you've done all that you can making your relatively stock 308 perform a helluvalot better than it did when you got it, so, where to now? That's a very good question, a question alot of people new to the performance game (and some in the game for a long time!) don't ask themselves truthfully. Countless times I've seen people spend their money twice over, chop n change their ideas and are still not happy with what they've got.

It's all well n good to take my advice. But if you are keen on going FAST in the not-too-distant future, disregard everything I have said and start again. Going fast properly takes time, planning and money. In the next article, well get inside the engine a little and see where your options can lead you.

evil_ss

Another big thanx to jimmy for the second installment of '308 Path to Power'

__________________
ONE OF THE ELUSIVE MODERATOR/ADMIN TEAM
Go check out my SS at
cardomain
For all your photography and imagery needs austinphotography@iinet.net.au

SpLiT

doin a great job mate, good read i recomend everyone have a read if they are considering building a motor some very good info there.

__________________
New - VH 304 with N20 - 382rwhp without gas
SKIT58

OLD VH SL/E 308 on giggle
OVH308

355's are like belly buttons everyone has one and all they are good for is collecting dust

86berlina

Bloody awsome write up Coupe.

Cheers

__________________
I like boobs.