Building a GD427 Cobra Replica

It’s about time I made up my mind about a gearbox to put behind the LS2. When I started to focus on a gearbox I had not a very good idea what criteria to use to evaluate gearboxes. I sort of tried to deliberately ignore (but not completely) the obvious choices in the different forums; not because they are bad or anything, but I wanted to reason my way into a gearbox choice based on technical criteria and personal preferences.

So, the first thing I did was read up on how the thing actually works.

The way I look at it, the drivetrain from engine to the wheel on the road is a divider, where you can control the division factor by selecting gears. Other than the gearbox there are is only one other factor which determines how 1 turn of the engine-output-shaft translates into a movement of the rear wheels.

This is the differential ratio, 3.54 in my case. Once the gear is known, you can calculate how many rotations of the engine it takes to let the wheels rotate once.

How that translates into road-speed and handling, which is what I am ultimately interested in, is a bit more complicated. The size of the (rear) wheels is one thing, their grip on the road and the influence of all sorts of drag are others.

It is pretty easy to create a set of graphs with the above information and the gear ratios of the gearbox. These graphs show you rpm vs (ideal) road-speed and a ‘shift behaviour’ graph, which shows the drops in rpm given a shiftpoint. Here’s what that looks like, given a Tremec TKO600 gearbox and 275/40-18 rear wheels.

Obviously this is all theoretical. The speeds will be lower in practice as drag increases with speed and how the ‘dry numbers’ will translate into actual experience is the big surprise I guess. What can be determined from the numbers however, is if the setup of engine,gearbox and differential makes sense overall and helps giving a quick visual overview of the length of the gears, an indication of the effect of the overdrive etc. The second graph above clearly shows the length of the overdrive vs the first 4 gears.

I’ve done this graphing for a couple of gearboxes and stared at the numbers and graphs for a day. Combined with my personal preferences these are the conclusions I came to:

• manual gearbox;
• in the ‘grand touring’ theme I’d like an overdrive in top gear(s) which gives me a cruising speed of around 100 km/h at 2000 rpm;
• ‘open box’, that is, the gearbox needs to be documented and serviceable (by me);

After all that, the choice boiled down for me to a Tremec TKO600 5-speed or the Tremec T56 6-speed (big surprise… not!). Having no hands on experience with either of those two boxes, I spoke to a couple of people having some experience with one or both. Apart from the obvious differences like 5 vs 6 speed and the TKO being a bit stronger on torque allowance, the main thing I distilled from the comments was that the TKO is the more ‘raw’ box, more aggressive perhaps and better suited to track (ab)use, and the T56 being a bit softer and perhaps more suited for cruising.

Making my mind up, I think I’ll opt for the TKO 600. The bigger torque number gives some playing room with the engine, and I think the LS2 has enough torque to already let me skip 2 gears, so why bother with a 6-speed?

I want to take a number of preliminary decisions on the engine, so I can move forward in that area.

Spent more or less the weekend reading up on the differences between all the different models of LS engines and their architecture in general. A while ago I bought a book on LS engines because frankly, I dont know much about the LS engines.

A friend and myself decided to exchange our ten all time favourite cars, just for fun.

Here’s mine, ordered by age:

1. Bugatti Type 57 Atlantic — 1936
2. Mercedes 300 SL Gullwing — 1954
3. Chrevrolet 3×00 (pickup) — 1955
4. Corvette Stingray — 1963 (split rear window version)
5. AC cobra 427 (duh!) — 1965
6. Ford GT 40 Mk I — 1966
7. Landrover Defender — any year
8. Ferrari Dino 206 GT — 1968
9. Jaguar XJ12 Series II — 1973
10. Aston Martin Vanquish — 2001

Runner up:

11. Wiesmann GT MF4 — 2003

Apparently the 80s and 90s didn’t do it for me.

Feel free to add your list as a comment.

“It’s been 117 days since my last confession…”

So, perhaps a little status update is in order. Although I declared the garaged finished earlier, there’s always something that needs to be finished/improved. So, this summer we place 2 new windows in the south wall.

Although not a very big job, it always takes more time than originally planned. And yet again, I forgot to take a picture of the finished project!

Blogging wise, time was spent migrating from Blogger.com to a local WordPress installation. This allows me to use all the server stuff I already have in place in the office anyway and gives me a bit more control over the blog. I also got fed up with google analytics slowing the blog down even further than I do myself with all the pictures.

WordPress also gives me a couple of extra options which would be harder to do at blogger:

• iphone specific theme;
• support for pages and posts within my ecto blogging client;
• support for both categories and tags;
• more flexible uploads which I’ll need shortly.

I have tried to keep all URLs the same where possible. All feeds and links should still work as they did on Blogger. I had to sacrifice the ‘Followers’ widget and the ‘Profile details’ which are Blogger specific items. There’s some work left to cater for Internet Explorer users (although they should use a better browser anyway).

Work on the cobra has been a bit slow, partly due to the above. All donor parts are basically done and waiting for me to put them on to the chassis. For the front-end I assemble the uprights, at first a bit baffled why the ball joints did not fit.