Discussion and questions related to the course Understanding AFR
Hi everybody, I have a 4age Toyota Mr2 engine for 24h endurance races. I would like to know what is the maximum safe egt I could reach without problems.
It is going to largely depend on the exhaust valve material and the turbine material used - the maximum recommended should be posted on their web sites.
I would expect you're using a fairly large A/R for the turbine, so that would help keep it down, too.
I assume you're asking as you're trying to work out the best economy/power balance for fuel loads/refuelling vs tyre and driver changes? Maybe you have a total race fuel limit to work to?
Yes, we have the fuel limited. It is naturally aspirated engine and I am going to check the EGT trying to reach leaner mixture as possible before to much torque be lost.
I don´t know if, for example 800º,900º, could be supported by and std. engine. Because we have to mantein std. engine. I can work on intake and exhaust manifold.
Ah, sorry, I was thinking it was forced induction. Because the hot gases aren't being held in the ports the same way as with a turbo's hot side, I don't expect any modern engine to have a problem with heat, as such, provided coolant and oil temp's are maintained.
With NA, peak power is around 12.7-13.2:1 and best power/fuel (AKA lean best power/torque) used is closer to 16:1 - exactly what your particular engine combination runs best at will need some testing - graph power to rpm for different AFRs and timing to establish what works best, and bear in mind it may change at different rpm/loads. But you will also need to check at the track so you can figure out if a little more fuel mass allows a richer mixture for power and it's quicker than a lighter, leaner running car, and how that lines up with tyre laps before changes and the the time loss in the pits. If you're allowed verniers, or even just offset keyways, you may find a small gain in changing the cam' timing to lift the torque and power peaks slightly, as the OEM has to balance the full operating range
If you need 2 fuel stops between changes, regardless, might be quicker overall to fill to less than full if the stops are needed anyway and you don't need to carry the extra fuel, but it may be worth fitting a larger fuel tank if it cuts down a stop and/or allows it to co-incide with the tyre stop.
It isn't something I've had to worry about, but I can see there are going to be a LOT of factors to take into account, however exhaust temperature isn't expected to tbe one of them.
In the interim, I would be having a VERY thorough read through the rule book to see what was, and wasn't allowed - some classes can require 'stock' internals but, if allowed, careful use of valve seat work can make a significant difference, as can re-working the throttle butterfly(ies), using heat insulation and spacers, ensuring there is a free-flowing cold air feed to the engine, lower restriction air filter (surprisingly, some OEM papar filters are better than aftermarket - don't know about that car). There are often little things that can give a 1/4 horsepower here, half there, a third somewhere else - if you can make use of them they can add up in a hurry. You may find you can use a slightly lower viscosity, and/or level, oil in the engine (many years ago, there was a 1300cc single make race series and running the oil level at minumum was worth FOUR horsepower over running it at 'full' - doesn't mean it will happen with your engine, but it's an example of the possible gains) and transmission.
But don't get hung up on engine power alone, as simple things like lightweight wheels can be worth several tenths over some heavier wheels of the same size. Heck, if you have to use the same size wheel (width and diameter), you may be allowed to use some with a greater offset to increase the track, etc. You may find you need to change discs towards the end of the race, so it may be worth giving us a small amount of lap time and fitting slightly better brakes than needed, if they last the full distance.
Thank you answer.
I want to know this temperature because there are some work I could do in the engine in order to mantein the AFR as leaner as possible. But temperature can produce some engine fails,so...
It is an endurance race but the cars go full throttle every they can.(Sorry about my english)
with out shooting my self in the foot i tune a few of these engines in dirt track speedway here in New Zealand they are foot flat with nearly no lifting for about 3-5 min that is a lot of 8200 rpm from a standard engine we run them at 850c and bounce on the limiter every lap for about 20 meters so i would comfortably say 815c should be fine for a endurance standard engine when there is no way you would be on the gas flat for an extended length of time in say a 3 hour race. you will have decel every corner for recovery
Ah, with modern engines, if I were you and didn't care about anything other than maximum power, I'd just tune it for that - the engine will be good for it, especially as your exhaust should be holding less hot gas in the exhaust port.