If you’ve ever seen Vyv at a show, or on the MX5 pages, it’s probably not escaped your attention that the car breathes through a set of home-built GSXR ITBs. Now, I posted some blog entries about these waaay back when I built them, which you can find here and here, but I never actually finished the story. After they got running in Part 2, I kept on tweaking and improving them, but always neglected to actually document what had been done.
After part 2, I got the vacuum manifold and catch can fitted, though the PCV system never worked (it seems these ITBs don’t draw enough vacuum to close the valve at idle, meaning a complete loss of brake booster and incredibly high idle speeds, so I had to get rid of the PCV system). I also fitted a manual idle speed control in the form of an original Mini choke cable connected to a secondary throttle cam that comes standard on these GSXR bodies. This directly pulls on the throttles and means I can make adjustments for cold start, or even cruise control entirely at will! In the below image, the secondary cam can be seen just under the throttle cable, and works by pushing directly onto the throttle quadrant itself.
After some more tuning, I then proceeded to thrash the car for a year of drifting and autotesting, discovering and fixing other minor problems along the way. Here’s a little video from the initial shakedown, the noise of these things is something I just can’t get over!
Now, if you’ve seen the first part of the Vyvolution chain, you’ll also know that I’ve started building up a turbocharged engine as a personal project, and to take the car to a reasonably competitive power level. But I LOVE N/A, ever since I fitted the ITBs I’ve felt the character of the car come alive and if I could, I’d keep an N/A version of this car alongside the turbo. So because the turbo engine won’t be ready until after the winter season, and because I loved it N/A so much, I felt that before I took this engine out, it needed to be ‘completed’. To me, this meant a target of 150whp and a noise that made me squeal with excitement.
The dream list would be a set of nice high compression pistons, a dirty head skim and porting out a spare head, but that’s a lot of work for an engine that’s already running in the car and is soon to come out, so internal work is off the cards. Instead I decided it was time to finally finish off the intake, a set of trumpets and a proper sausage filter to replace the awful paper filters, and fit something I’ve always wanted: some high lift cams with a massive duration.
Cue Tom Hay and Piper Cams. I’d asked on Nutz what experience people had with big profile cams, after buying a second hand set of Skuzzle cams that turned out to be scrap. Tom dropped me a message saying that they were looking to do some more social media stuff, especially within the Jap car and drift crowds, and would I be interested in following an order through, right from day 1, through the factory, back to me, installing into the car and then a shakedown at a drift day? Of course I said yes!
The spare cams out of the new turbo base engine were dusted off, wrapped up and sent out for Piper to work their magic.
Meanwhile, I set about stripping down the intake and considering the improvements:
After a scare with a particularly strange looking cylinder 4 runner, compression came up good across all cylinders and the problem was promptly ignored. (In all seriousness, we couldn’t work out how this runner had gotten so filthy with no PCV upstream, no signs of a leaky injector and good compression. It’s possible that a leaky stem seal is the culprit, but no evidence of this was seen and hey, if it makes compression, I’m just gonna send it!) Audience suggestions are greatly welcomed…
Once off the car, the silicone hoses that make up the runners were each shortened by around 15mm. Much more than this and there wouldn’t be enough silicone to seat nicely on the ovalised intake manifold ports. This should be enough, however, to give me some extra space for the trumpets and filter. Another advantage of the shorter runners is that they won’t be as susceptible to collapsing under vacuum, a problem I had with the original setup in its first iteration. I also replaced the perished O-Rings that came with the vacuum manifold, and built up a template to use for air filter hunting.
Another big factor in runner length is the powerband. Short runners are great for top end power, while longer ones help power down low. It all has to do with pulse tuning effects, and the time it takes for a pressure wave created by the intake event to travel down the runner, reverse when it hits fresh air, and make its way back again. A quick calculation showed that my setup, originally tuned with long runners in mind for maximum usable torque, is still pretty much in the 5000-6000rpm range, exactly where I wanted it! (Note that runner length is specified as two numbers, one twice the size of the other. This is because the first order wave, ie. wave goes out and back once, usually requires a far longer runner than is practical, and so second or third orders are far more commonly used). If I were to do another ITB setup I would probably shorten the runners further to have a car that absolutely screamed at the top end, but a nice wide usable range is more beneficial for this setup.
Below is a good look around the intake in its entirety, complete with DVD-player bracketry and genuine 2014 Mercedes F1 throttle position sensor.
I also made up a cardboard template for a Ramair filter that seemed like a good bet, to check for clearance. The issue I am having is that most filters are too long, and would hit the clutch reservoir, as well as having clearance issues to the fuel pipes and brake prop valve (Which really needs rid of anyway…) This Ramair is a little shorter than most and fits to the clutch res, buuuut it seems a little more space is needed to the brake valve if I am going to avoid needing to locally dent the filter.
These photos were actually taken before the runners were shortened, but even so, not a lot of space available for a filter here!
Some local ‘adjustments’ were made to try and gain a bit more space on the brake side. This proportioning valve is what controls the brake bias in the MX-5. From factory, they put barely any force at all to the rear brakes, and are pretty big and bulky, and IN MY WAY. So this bodge will do for now, but the valve will be removed altogether and replaced by an adjustable bias valve in the cockpit in future.
With the intake shortened and a little extra space made it was back into the car for a test fit with the new trumpets to see how much clearance had been gained. Quite a bit!
Reset all the calibrations for the throttle pot
Likely remap the idle, which may be very sensitive to the new intake length and breathability
Test drive the new intake to check for weak links and leaky joints
Get some cams fitted!