carmudgeons.com - View Single Post - What the Mazda RX-8 should have been (new autocross car build)

John V · 05-14-2019, 07:08 AM

Minor update. I needed to do some cooling system upgrades, the car would get very hot after a single fourth-gear pull on the dyno or at the test events. The radiator was clearly undersized, and I didn't have it well ducted. The factory RX-8 ducting is actually very efficient, I've learned. The factory ducting is accomplished using the undertray, which prevents air from flowing under the radiator, and also the vertical members of the undertray, which prevent air from flowing around the radiator. I didn't install the undertray, because it didn't really fit anymore with all the work I had done. Well... I needed to make it fit. After a few hours of cutting and working to seal everything up, here is how the ducting looks. This is all using factory parts modified to fit. The openings on either side of the intercooler go directly into the radiator. There is no escape path other than through the radiator because I foamed off the sides of the radiator where it meets the vertical elements of the undertray as well as the top of the radiator where it meets the top of the core support. The bottom of the radiator is foam-sealed to the plastic undertray, which is fastened to the subframe at the rear.

Here is a view inside one of the ducts.

So now air that flows through the intercooler must flow through the radiator and there are two large flow paths around the intercooler that also must flow through the radiator. I also purchased a radiator with three times the volume of the tiny stock one, and put in a new coolant reservoir and pressure cap. After a long dyno session this weekend, no issues running hot at all. So I'm going to declare success on that.

We spent about four hours on the dyno Saturday. First we got the variable valve timing on the intake cam working reliably. We added some ignition timing and picked up a good amount of part-throttle response, which should make the car snappier on course. With the aggressive cams I have, and only 9.7:1 static compression and E85 fuel it's not likely this engine will ever preignite the fuel mixture. At some point I'd like to build an engine closer to 11:1 compression, but for now this makes plenty of power and has good response.

I also discovered a problem with my fuel system where fuel is not being siphoned from the right half of the fuel tank (saddle style with two reservoirs). We fuel starved on the dyno a couple of times at only 1/4 tank of fuel, because all the fuel was on the other side. I need to get that worked out but in the near term I'll just run the car with a full tank.

We ended up spending most of our time tuning the boost controller, so we were doing part-throttle runs. At 60% throttle we get full turbo spool, which is awesome... 60% throttle and 8psi of boost was 320 horsepower at the wheels, which is just crazy. It should make around 450 at the wheels on 14psi of boost, so I need to make sure not to turn it up that high

I'm towing the car out to Lincoln in a week and a half, which is insane because it'll be my first competition event in the car. I'm about a month behind where I need to be. But hey, if I go out there and the car doesn't make a single good run at least I'll have a weekend with friends and I'm sure I can find something else to drive!

05-14-2019, 07:08 AM	#250
John V No more BMWs Join Date: Apr 2005 Carmudgeonly Ride: Ram, MS3, CX-5, RX-8 Location: Glenwood, MD Posts: 14,753	Minor update. I needed to do some cooling system upgrades, the car would get very hot after a single fourth-gear pull on the dyno or at the test events. The radiator was clearly undersized, and I didn't have it well ducted. The factory RX-8 ducting is actually very efficient, I've learned. The factory ducting is accomplished using the undertray, which prevents air from flowing under the radiator, and also the vertical members of the undertray, which prevent air from flowing around the radiator. I didn't install the undertray, because it didn't really fit anymore with all the work I had done. Well... I needed to make it fit. After a few hours of cutting and working to seal everything up, here is how the ducting looks. This is all using factory parts modified to fit. The openings on either side of the intercooler go directly into the radiator. There is no escape path other than through the radiator because I foamed off the sides of the radiator where it meets the vertical elements of the undertray as well as the top of the radiator where it meets the top of the core support. The bottom of the radiator is foam-sealed to the plastic undertray, which is fastened to the subframe at the rear. Here is a view inside one of the ducts. So now air that flows through the intercooler must flow through the radiator and there are two large flow paths around the intercooler that also must flow through the radiator. I also purchased a radiator with three times the volume of the tiny stock one, and put in a new coolant reservoir and pressure cap. After a long dyno session this weekend, no issues running hot at all. So I'm going to declare success on that. We spent about four hours on the dyno Saturday. First we got the variable valve timing on the intake cam working reliably. We added some ignition timing and picked up a good amount of part-throttle response, which should make the car snappier on course. With the aggressive cams I have, and only 9.7:1 static compression and E85 fuel it's not likely this engine will ever preignite the fuel mixture. At some point I'd like to build an engine closer to 11:1 compression, but for now this makes plenty of power and has good response. I also discovered a problem with my fuel system where fuel is not being siphoned from the right half of the fuel tank (saddle style with two reservoirs). We fuel starved on the dyno a couple of times at only 1/4 tank of fuel, because all the fuel was on the other side. I need to get that worked out but in the near term I'll just run the car with a full tank. We ended up spending most of our time tuning the boost controller, so we were doing part-throttle runs. At 60% throttle we get full turbo spool, which is awesome... 60% throttle and 8psi of boost was 320 horsepower at the wheels, which is just crazy. It should make around 450 at the wheels on 14psi of boost, so I need to make sure not to turn it up that high I'm towing the car out to Lincoln in a week and a half, which is insane because it'll be my first competition event in the car. I'm about a month behind where I need to be. But hey, if I go out there and the car doesn't make a single good run at least I'll have a weekend with friends and I'm sure I can find something else to drive!