#project86 4.56 final drive
I’ve had shorter final drives in my old E36 and NC MX5 so it’s a mod that i’m a firm believer in. In these cars it was simple to pick up a cheap diff from another model and slam it in. In the 86 it isn’t so simple which is why i’ve held off for so long. Yes you can get a 4.3 out of a facelift car but for the price wreckers are asking for them I couldn’t justify it.
Keeping in mind pre facelift OEM is a 4.1, facelift OEM is 4.3, another important thing to note is the difference between the actual transmission gearing between auto and manual. The auto gears are very, very long! There’s a few go to options that i’ve seen some guys both locally but also some Japanese tuning shops use
I’ll go through what gearset we used and why aswell as comparison data and in car videos for all the nerds out there who love the numbers as much as I do!
Why did we go with 4.56?
Throwing in a complete 4.3 out of a facelift car would be the quickest and most cost effective upgrade however I wasn’t convinced it was enough of an upgrade to justify the $700+ spend that wreckers are asking for so I looked aftermarket. The popular options in Japan are 4.56, 4.88 and 5.1. Given our 86 is still street driven and Wakefield Park is a 350km round trip the 4.56 seemed like the perfect middle ground. A good bump in performance without making it unusable on the highway or risk running out of legs at Phillip Island or The Bend.
Which gearset did we go with?
Originally I was tossing up between MFactory and Cusco. MFactory is the more affordable option but Cusco was a factory option so there was no risk of any gear whine if the diff was built by a reputable shop. It also keeps me eligible for my Supersprint class being a factory option! I was actually lucky enough to score a complete diff out of Iain Chandlers car (i’ll link his in-car here, he still holds the NA record at Wakefield Park despite selling the car a while ago). It was a Cusco 4.56 in a near new diff built by Neal Bates Motorsport. It was perfect. We agreed on a price and I waited 4 months for the lockdowns to be over before I could drive down to Canberra to pick it up. In a perfect world it would have also thrown a 1.5 way centre in it but the price was right so it had to be done. I’ll address the LSD later down the track with a Cusco 1.5 way from Race Division.
Road driving
Spoiler… it’s bloody awesome. Initial impressions on the street were that the torque dip feels like it’s just about gone (keeping in mind we’re on EL headers, you can see our dyne chart here) and the car feels so much happier on a backroad always being in the revs. Sitting on the highway is great, I no longer have to downshift to overtake or go up hills. Combined fuel consumption is currently sitting at 18L/100km with only highway and track driving on E85. I’ll attach a table I made with RPM vs speed for both final drives below.
Track driving (Wakefield Park)
The 4.56 final drive has been great at Wakefield. For the most part the gears are the same we’re just higher in the revs and pulling gears sooner. We’re no longer dropping to second for fishhook and turn 10, this will be even clearer on a faster tyre where we can have more corner entry speed. The other notable change is pulling 5th down the straight which is something I definitely need to get used to. See the comparison video below:
The biggest benefit at WP is up and over the hill where the shorter final drives leaves you higher in the useable power band. Top speed down the straight is about the same, again once we bolt a faster tyre to the car we can maximise turn 10 and pick up a couple kmh down the straight. As you can see in the data the pink line is with the 4.56 and although our corner entry speeds are lower (due to my average driving on the day) we actually climb up to the same speed which makes me very excited to throw some fresh tyres at the car!
Green line = 4.11 - Pink line = 4.56
Track driving (Sydney Motorsport Park)
The difference at SMSP was less obvious but still a great improvement nonetheless. The gears are generally the same with the exception of T2 and T8 where we are sticking to third and saving the gear change, everywhere else we’re just enjoying the extra torque and pulling the next gear a little earlier.
The one downside I came up against at SMSP is that we’re sitting on the limiter in 5th gear for about 50-80 metres coming into T1. Pulling 6th isn’t quite right and it bogs down on the exit of T1. Downshifting back to 5th feels quite awkward. The solution is simple though… back to PVS for a tune up and a couple more revs! Once we’re on a faster tyre this problem will be even more apparent. Ofcourse the other option is to put a taller wheel and tyre package under the car.
Red line = 4.11 - Blue line = 4.56
Verdict
Once you factor in the cost of the labour on top of the ring and pinion, swapping out the final drive certainly isn’t a cheap endeavour. It’s comparable to the cost of headers and a flex fuel tune with PVS with similar performance gains albeit in different areas. I think the ticket is when you combine the 2 like we’ve done here. The car is a different beast now and I’ve fallen in love with just how good these cars are NA on track. We’re heading in to see Tyler and the gang at PVS over the holidays for a bit of a birthday with some new coil packs, spark plugs and abit of a tickle on the tune to get it singing a little harder up top, we might even throw an exhaust at it along the way.
Long story short, I would 1000% recommend putting a final drive in the car if you enjoy the character of an NA car, if you can’t afford or justify to do the tune AND the final drive? I think I would stick to the tune but it really comes down to personal preference. There’s no losing but you can find out more about our tune here!