Pete's RX-8 Time Attack project

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Re: Pete's RX-8 Time Attack project

Post by zippyonline » Wed Jun 29, 2016 12:02 pm

That's obviously a new pad vs the old pad - but how evenly did the pad wear on each side of the disc (i.e. piston side and slider side?). I've never in my life ever seen even wear on both faces (and this includes on bicycles with various expensive hydraulic disc brakes with opposing pistons that are 100% working) - so interesting to see how they fare in this type of situation. Image
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Wed Jun 29, 2016 12:37 pm

I won't be able to prove it with photos for a few days, but all four front pads looked pretty identical. I was careful to ensure that all the brakes were sliding freely when I put them together at the start of the season, and they were still sliding nicely when I changed the pads, so it was nice to see the even wear. The only difference I could spot was that the front driver's side pads were about 1mm less worn than the passenger's side (you can still see a little of the chamfer on the driver's side). It was also the passenger's side that started smoking at Brands, so it's clearly taking a little more energy. It could be a slight brake imbalance (I have thought it pulls fractionally to the left on heavy braking), or it could be a quirk of the DSC. All the circuits so far have been clockwise, so I imagine the DSC uses the front passenger side brake to bring any oversteer under control, which could cause a little more wear.

Incidentally, the discs have no measurable wear so far, so clearly the Yellow Stuff are the sacrificial partner in this set up. The discs do rust like crazy though (Evora grooved from Brake Depot)... Think I may have to take them off over winter, or they will turn to dust!
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Thu Jun 30, 2016 7:07 pm

Just worked out the optimal gear change points for a standard engine (assuming no rev limiter):

1st to 2nd: 9970rpm 2nd to 1st: 6020rpm
2nd to 3rd: 9620rpm 3rd to 2nd: 6970rpm
3rd to 4th: 9630rpm 4th to 3rd: 6950rpm
4th to 5th: 9220rpm 5th to 4th: 7770rpm
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Re: Pete's RX-8 Time Attack project

Post by Liam22 » Sat Jul 02, 2016 11:41 pm

Also assuming continued linear power delivery? i.e. no drop off?

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Re: Pete's RX-8 Time Attack project

Post by PeteH » Sun Jul 03, 2016 12:14 am

Nope. I used the Mazda issued power curve, and fitted a 6th order curve to it. The fit was surprisingly good, and looked like it extrapolated quite well above 9k revs (i.e. The torque dropped a lot....). Of course, I don't know what the true torque curve is at high revs, because Mazda don't specify the torque values beyond the red line (spoil sports!), but it all looked plausible.

Can't actually show the torque curve because I'm not near my PC at the moment.

I think you know your true power curve Liam? I'd be happy to put your numbers into my calcs, if you want to know your change points (although I wouldn't be surprised if you've already done it)
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Re: Pete's RX-8 Time Attack project

Post by Liam22 » Tue Jul 05, 2016 10:32 pm

Thanks Pete. Just pm'd you :)

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Re: Pete's RX-8 Time Attack project

Post by PeteH » Thu Jul 07, 2016 1:48 pm

OK, a Brands Hatch report is long over due.

Got up far too early in order to travel down on the morning of the event. It took a little longer than usual to access the circuit because we had to dodge around various "over-enthusiastic" teenagers whos' driving ambitions clearly outstripped their driving abilities :bump: . Fortunately the competitor entrance allowed us to avoid the worst of the carnage. Then had the fun of trying to find the paddock, which is not so easy when you haven't been to the track before, and you are faced with a huge car park and no signage.....

Things then went smoothly, and the warm up session was uneventful. Brands is a nice circuit, with some challenging corners, but the Indy circuit is very short. I was very interested in whether the newly fitted hoop spoiler would generate any real downforce, and to my surprise the car was now understeering considerably more than it had at Cadwell or Pembrey. Perhaps the spoiler does work! (or it could just be the peculiarities of Brands). Anyway, for session two I moved my rear anti-roll bar from its soft position to its hard position. I would also have increased the rear damping, but I couldn't remember which way the adjuster went! The bar change made a big difference, to the extent that on my first flying lap I entered Paddock Hill bend with about 180 degrees of opposite lock! It did cross my mind that the Club was positioned perfectly to be watching, if I did put it in the kitty litter.... Being worried that I had gone too far I took it easy into Druids, but I needn't have, since it understeered through. It turned out that the balance was now fairly neutral again, and I just needed to be progressive when turning in.

Lunchtime. I had the chance to wander up to Club stand and say hello to a few people. Nice to meet some members at last, and to see some great looking cars displayed.

Then the third session (qualifying), which was uneventful. It was once again clear that a 200bhp Mazda cannot hope to match a 320bhp Honda, but never mind, I'm not bitter.... :evil: . Actually this session was nearly uneventful, but not quite. We had "The Stig" (Ben Collins) in our session as a guest driver. He was in a 300bhp Fiesta, and was 1.5s faster than me (which I'm happy with), but he didn't quite get the Time Attack rules. We try to be polite to each other, since it's not a race, and we are all trying to get the best lap time. Unfortunately Ben was in full "race mode", and at one point he positioned himself about one foot from my rear bumper, threatening to dive up the inside on the way into the 95mph Surtees bend. I had a choice; turn in and risk him clipping me, or take to the grass. I decided he was good enough to avoid the collision, so I turned in. I've no idea what happened to him, because the next time I looked in my mirror he was no-where to be seen.... In the end I got 57.29 on lap 10 of 14.

Onto the final session. I set a pretty good early lap, and was pushing for a better one when, through Druids, the traction control decided to get a bit over-keen. Now I'm quite happy for the DSC to work a bit since all its doing is keeping the car on-line and pointing in the right direction. But this was a full-on throttle intervention which killed the power for what felt like an age (probably less than a second in reality). I got a bit peeved with this, so decided (for the first time) to turn off the electronics. A few things followed:
- On entry to McLaren's the back end stepped out massively. I held it balanced on opposite lock (bit of drifting, quite fun!), but had to wait an age for the car to straighten up, by which time I had lost about 20mph. Without the electronics I really have to keep it smooth.
- After that I decided I'd rather have the electronics on again. But (of course) you can't turn them back on without turning the car off, which isn't advised on the circuit! So I was stuck without electronics for the rest of the session :banghead: . For reference, my best time with electronics was 57.38, and without electronics was 57.69. It's possible that with practice I could be slightly quicker without electronics, but it would take much longer to get there, and come at much greater risk of binning it.
So in the final session I got 57.38 on lap 5 of 14

As I drove into the pits my partner seemed very agitated, shouting slightly un-nerving things like "you're on fire". Turned out that in trying to push for a time without the electronics I had put together six fast laps, and the brakes had over-heated. I knew the pads were getting low, so it wasn't a great surprise. I think in future I'll limit myself to a maximum of three fast laps in a row.

So, another great event. Another second place. And I've finally relinquished my championship lead to that bl##dy Civic :x . Note to self. I've got to get myself a Bridgeport.....

Onto the next event. Oulton Park, in association with TunerFest North, on Saturday 9th July (2 days!). I don't think we have a Club stand at this one, but if you're going do come and say hello. :thumleft:
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Re: Pete's RX-8 Time Attack project

Post by Liam22 » Fri Jul 08, 2016 9:29 pm

Good luck at Oulton Pete. Not so much of a power circuit as Brands Hatch. Always a pleasure to drive it


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Re: Pete's RX-8 Time Attack project

Post by brix79 » Fri Jul 08, 2016 10:03 pm

Good read! Love that last pic too. :D
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Fri Jul 08, 2016 10:33 pm

Yep, that's the one the pro took. :lol:
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Re: Pete's RX-8 Time Attack project

Post by brix79 » Fri Jul 08, 2016 11:04 pm

Worthy of a new avatar I think.

Already knocked it up if you want it. ;)
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Mon Jul 11, 2016 4:54 pm

brix79 wrote:Worthy of a new avatar I think.

Already knocked it up if you want it. ;)
Go on then! Those muddy rear tyres were starting to bug me anyway.
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Mon Jul 11, 2016 4:55 pm

Thought it was time for another techy bit, for those that are interested.

Next on my mod list is to improve the induction. It got me thinking about the physics. What works, and what is just for show.

The power of an engine is dictated by the mass of oxygen available to it, and the only practical method of supplying enough oxygen is to use air. We can induct that air naturally, or we can force it in (turbo / supercharger). I'm only interested in the natural method. The amount of air we can get into the engine is called the "volume flow rate". Roughly speaking, the Renesis takes in approximately 1.3 litres of air every revolution, and at peak power it is doing 8500 revolutions every minute (141.7 revolutions per second). So the engine takes in approximately 141.7 x 1.3 = 184.2 litres of air per second.

So what options do we have to increase the amount of oxygen available to the engine? We need to note that the engine takes in a volume of air, but the power it produces is a function of the mass. So if we can increase the mass of the air for any given volume then the power will go up. For this we need to look at the "perfect gas law":

Mass = Pressure x Volume / (Temperature x A Constant)

(We don't need to worry about the constant). We can see that, although the mass and the volume are connected, they are not the same thing. In fact, if we can find a way to increase the pressure of the air, or decrease the temperature, then we will have more mass and therefore more power. We have already said we are ignorring forced induction, so compressors and intercoolers are not relevant for us. But we can decrease the temperature with a cold air intake (CAI), and we can increase the pressure with a ram air duct.

I will look at the CAI first. If we assume that we have thrown away our standard air box, and we have a K&N style filter directly attached to the throttle body, it will draw in air from the under-bonnet area. This air could easily be at a temperature of (say) 60degC. The density of this hot air is found from the formula above (where mass / volume is the density), and is about 100,000 / (287 x 333) = 1.0463 kg/m^3. If we now replace that filter with a nicely designed CAI, and it takes air in at 15degC (and we assume it stays at 15degC throughout the intake), then the density of this new air supply is 100,000 / (287 x 288) = 1.2098 kg/m^3. This is a density increase of 15.6%, which translates directly into a 15.6% increase in power (assuming the MAF gets its sums right, and the engine map can achieve a proper air / fuel ratio). There are a number of CAIs for the Rex, and the Mazda OE system is also a CAI, so we shouldn't expect to see these dramatic gains. But we could see such dramatic losses with a poor choice of filter. The moral here is don't be tempted to put a simple filter on that feeds from the engine bay.

Next, ram air. We usually assume the air box is at atmospheric pressure. In reality, when the engine is at high revs it can draw enough air to cause a partial vacuum in the air box, which is bad. We can help avoid this by ensuring that the intake has few restrictions (free flowing air filter, large straight ducting, etc.). Again Mazda have done a pretty good job here, with an excellent filter, and the vFAD is a pretty effective method of minimising flow restrictions while keeping induction noise low. But there are some ram air systems available, which aim to actually create positive air box pressure. The potential pressure available is called the "dynamic pressure" of the air, and it increases with the square of the speed. The equation is:

Dynamic Pressure = 0.5 x air density x speed^2

We already know the air density, from the calculations above (1.2kg/m^3), so the maximum dynamic pressure available is 0.5 x 1.2 x speed^2. Obviously at zero speed ram air doesn't work (you won't see much benefit on a static dyno!). But at 150mph (67m/s) the dynamic pressure is 0.5 x 1.2 x 67^2 = 2693 N/m^2. Converting this to sensible units, that's 0.02693 bar, or 0.39psi. This is a 2.7% pressure increase over normal atmospheric pressure, which means a 2.7% power increase. So with a Renesis we might get about 5bhp from a ram air duct, but only at maximum speed (and again assuming the MAF gets it sums right).

So there is the potential for definite but small gains.
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Re: Pete's RX-8 Time Attack project

Post by Harrizone » Mon Jul 11, 2016 5:10 pm

I love these posts although I invariable only understand the half of it. :? However I am impressed with your knowledge of all things to do with getting the most from the car for your needs on track, and it's always an enjoyable read Pete. Thanks. :thumleft:
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Mon Jul 11, 2016 6:22 pm

Thanks. Good to know people are interested. All I need now is an equation to make me drive faster! :-k
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Re: Pete's RX-8 Time Attack project

Post by 13Black » Mon Jul 11, 2016 7:42 pm

Any ideas how to go about choosing the correct filter location etc?

Little barometers placed around the front bumper connected to a gauge to find the highest pressure area? Then logging the OEM intake's MAF values in, say, a WOT 4th gear pull and comparing that to your new intake design/location to see any increase/decrease in airflow and IAT over the rev range?

No idea how successful this would be in reality but seems to make sense in my mind.
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Re: Pete's RX-8 Time Attack project

Post by MadTaz » Mon Jul 11, 2016 7:50 pm

MAF would have to be very accurately calibrated to each intake system for that to work ;)

Conditions would also have to be the same on each pull, I would take RB's word and just accept that the RB intake flows better.

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Re: Pete's RX-8 Time Attack project

Post by PeteH » Mon Jul 11, 2016 8:26 pm

:whathesaid:

If the difference between perfect ram air and no ram air is 0.4psi at 150mph then looking for differences in mounting locations is going to be nearly impossible I think. You also need a big intake area to avoid pressure losses. I think the Rex is fortunate in having a big dummy grill in just the right location for the air box entry....
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Re: Pete's RX-8 Time Attack project

Post by 13Black » Mon Jul 11, 2016 8:31 pm

I like your thinking... please make the front of the car look like a whale shark :crazy:
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Re: Pete's RX-8 Time Attack project

Post by mrspiller » Mon Jul 11, 2016 9:00 pm

Pete , not having your knowledge of air intake capacity etc I read that Mazda pretty much had it licked so I proceeded to give the air box with a k and n panel as much air as I could . So here goes , not pretty looking and probably doesn't do much but stuck the one on the right hand side open and butchered as follows Image
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Mon Jul 11, 2016 9:27 pm

No abuse from me. I love functional! How do you keep the water out?
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Re: Pete's RX-8 Time Attack project

Post by mrspiller » Mon Jul 11, 2016 10:10 pm

Mmm... Keep it in the garage ...... I haven't thought of that or driven it in the rain but have done on a wet track with spray and never thought of it .... so some more Input needed .... I guess I'll have to look at the RB set up and change slightly


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Re: Pete's RX-8 Time Attack project

Post by shambo » Tue Jul 12, 2016 12:20 am

If we had PP intakes we could utilize tuning the length of the intake for charging by the pressure waves, as per the 787b VLI.

Only problem there is the then lower torque available low down, but for pure racing purposes that is the win factor :)
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Re: Pete's RX-8 Time Attack project

Post by Harrizone » Tue Jul 12, 2016 10:04 pm

PeteH wrote:Just worked out the optimal gear change points for a standard engine (assuming no rev limiter):

1st to 2nd: 9970rpm 2nd to 1st: 6020rpm
2nd to 3rd: 9620rpm 3rd to 2nd: 6970rpm
3rd to 4th: 9630rpm 4th to 3rd: 6950rpm
4th to 5th: 9220rpm 5th to 4th: 7770rpm
This I find very interesting. Having lost my beep many years ago (amongst other things :oops: ) Having either a light, or beep to signify the optimum revs at which to change in each gear would be extremely useful, because there is no way I am looking down at the rev counter as I am exiting a corner in traffic. At the moment I am changing just before the rev limiter comes in (if I remember) with the rev limit set in the region of Pete's 1st to 2nd optimum. ;) Obviously it would be almost impossible to achieve something like this in light of the fact that the F1 teams must have spent fortunes developing this technology to link it to the rev counter for both up and down the box.
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Re: Pete's RX-8 Time Attack project

Post by PeteH » Wed Jul 13, 2016 2:28 am

I wouldn't say F1 spends a fortune on it, but it is carefully thought through. For instance, the lights take into account driver reaction time, so the final shift light arrives at earlier revs in the lower gears because the engine is accelerating faster. It's much easier though when you have open access to all the source code control software....
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