Duratec in detail

[David]

Is that the ignition advance map?

No, it's fuel. I think more fuel is going in, not because it is producing more power, but because the fueling has become less efficient - possibly by pooling or spitting some out. The serious guys have the injectors further away from the valves, and the really serious guys have two sets of injectors to provide both max power and the response of the closer injector (to the valves).

[Edit to correct] x=rpm, y=injector time, z=throttle site

[robin]

I thought about it being fuel, but it didn't quite make sense - fuelling is, I suppose, equal to torque x efficiency. You have 4x the full throttle torque at 7000 RPM than you do at 2000 RPM, yet the fuelling is barely double.

I guess that means it is really inefficient at the low end, which doesn't matter at all, of course.

I wouldn't get hung up on the fuelling spikes.

The key thing to work out is how to increase usability on track. If you don't already hold it at very high revs into and out of the corners, you're going to have to start doing that. I don't know how practical that is in your car at present - I assume you can keep it 5K+ all the time; when I had the n/a s2 exige I found I could keep it 6K+ all the time but it was hard work and sometimes compromised gear choice in/out of corners. The sharp torque difference between 5K and 6K in your map is possibly going to be an issue for managing traction on the corner exit if you drop down out of the power band.

With cams like yours, you will struggle to make the engine good everywhere, I suspect. I would focus on getting the torque higher in the 5K-6K band. Look at overlap on the cams - a bit less overlap might ultimately hurt the top end power (which is probably outside of your RPM limit anyway!) - but it might make the sub-6K less lumpy. Next give it every last RPM you think your bottom end will survive. If you can get a 3,500 RPM power band from 5,000-8,500, for example, then assuming you have the normal R400 gearbox you'll be fine; if you can only go 6,000-8,500, then it will be much tighter - you will end up being compromised in corner gear selection as you have no slack.

gearspeeds

[mckeann]

Gear ratios in the R300 caterham are superbly close together, from memory you only drop approx 1000rpm on every gear change, i dont think a 6K plus power band will be a problem at all assuming the R400 uses the same ratios.

[David]

Thanks for doing that Robin - really useful, If you substitute 0.00109051254 (from Avon data base) for the rolling circ. (tyres are 205/55/13) and the current diff 3.62. It give exactly what I am experiencing already. It could work well down the back of KH with apex speeds I'm using - the 3.92 seems not so good but working better at the hairpin with maybe some more gear changes too. I need to digest this a bit more (working at the moment )

[David]

I thought about it being fuel, but it didn't quite make sense - fuelling is, I suppose, equal to torque x efficiency. You have 4x the full throttle torque at 7000 RPM than you do at 2000 RPM, yet the fuelling is barely double.

This injector time is per cycle - so if you double the the RPM you are also doubling that injector time so 'total' inject time is nearer what you thought it should be - or is my logic wrong when thinking about torque?

[David]

Gear ratios in the R300 caterham are superbly close together, from memory you only drop approx 1000rpm on every gear change, i dont think a 6K plus power band will be a problem at all assuming the R400 uses the same ratios.

Yeah they are the same. At the start of this project, I though getting some more RPM would let me hold gear longer but I think that was misguided. I think all that has happened is that the useful power band has just moved up and become more pronounced. But overall it's no worse than before, just keeping the rpm up might feel strange at first.

[robin]

This injector time is per cycle - so if you double the the RPM you are also doubling that injector time so 'total' inject time is nearer what you thought it should be - or is my logic wrong when thinking about torque?

Yes, I think it doesn't matter what the units are ... ms/cycle, etc., they all scale linearly - i.e. if you double the number in the map you'll also double the fuel delivered (ignoring 2nd order effects to do with opening/closing speeds of the injector). I think you're just suffering from fuel drop out at low RPM due to the air speed (or lack of it) in the inlet path and so you have to dump a lot in to get any into the bore. The fact that the AFR looks normal-ish means that the fuel is probably getting stuck in the manifold or the bores (because it is a liquid now, not a vapour) - you should avoid running the engine for prolonged periods in this condition because the fuel will wash away the oil from the bore walls.

Cheers,
Robin

[David]

Going back to the gearing - here's the SLS practice with the 3.62 diff and shows that I am between 6,000 and 7,800 (limiter) most of the time . I think another 500/700 rpm will make things work well [althought the 3.92 diff may work even better]. I suspect the issue will be when the weather is intermediate - I'll probably need to avoid the the 5000-6000 bit as it'll be too aggressive. But having 20 BHP less below 5000 might also reduce the work load in the wet and be an advantage.

Dry

Watch on YouTube

wet

Watch on YouTube

[robin]

Hi David,

If you look at the rev counter in and around all the corners you are <6,000 pretty much all the time and in the slower corners down the <5,000. The engine already sounds like it is bogging down a bit and that's the old engine. I think you will either need to fix the torque in the 5-6K range or radically change your gear selection to be much more aggressive with revs in the corner - i.e. be down another gear. I wouldn't use the longer diff unless your top speed is too low for any of the circuits you compete on. It doesn't alter the power-band-to-gear-matching problem - it just makes all your gears taller.

The extra revs will help, but you will need to force yourself to use them too!

If you want less torque in the wet you can have a wet map with less ignition advance.

Cheers,
Robin

[David]

Hi David,

If you look at the rev counter in and around all the corners you are <6,000 pretty much all the time and in the slower corners down the <5,000. The engine already sounds like it is bogging down a bit and that's the old engine. I think you will either need to fix the torque in the 5-6K range or radically change your gear selection to be much more aggressive with revs in the corner - i.e. be down another gear. I wouldn't use the longer diff unless your top speed is too low for any of the circuits you compete on. It doesn't alter the power-band-to-gear-matching problem - it just makes all your gears taller.

The extra revs will help, but you will need to force yourself to use them too!

If you want less torque in the wet you can have a wet map with less ignition advance.

Cheers,
Robin

Thanks, but I think you have it the wrong way round, the new diff I have is 3.92:1 and shorter than the 3.62:1 (in the videos) - so I hoping it may improve the situation but I will have to up the rev limit. But that torque curve does need some work

[David]

I've now got some expert opinion back on the rolling road data - the culprit for the dip in torque is the exhaust. I've got some figures to build/buy a new one, so roll on phase two of this project.

[robin]

Thanks, but I think you have it the wrong way round, the new diff I have is 3.92:1 and shorter than the 3.62:1 (in the videos) - so I hoping it may improve the situation but I will have to up the rev limit. But that torque curve does need some work

Quite right my mistake. It won't make any difference except for reducing your top speed. The distance between gears is the same (in RPM terms), but obviously they are all closer together in terms of road speed.

Cheers,
Robin

[Dominic]

As a total aside... I notice you are using the tonneau cover in the wet but not in the dry. Would there be a slight aero advantage to be gained by using it in dry too?

[David]

As a total aside... I notice you are using the tonneau cover in the wet but not in the dry. Would there be a slight aero advantage to be gained by using it in dry too?

I expect there might be, but as it is about as aerodynamic as a brick, probably very little

[David]

I think I said right at the beginning of this project that there was going to be more than one phase - the idea in my head was to build the engine and then upgrade the exhaust and inductions over the following year. But what I have learnt is that the more you push the limits, the less tolerant the engine becomes to minor deficiencies.

So I have decided to accelerate exhaust phase of the project in the hope of a less peaky power delivery.



Today the exhaust was finished. It is basically a bigger diameter and a 4-2-1 design. The dimensions have been developed and tested specifically for the CNC head and cams I'm using. There are other headers and exhausts on the market that are similar (and may have worked) but I decided the additional cost of one fabricated to the exact dimensions was not that high and would be worth it. So, this is a one is a one-off made by BTB and, I believe, a first for a metric S3 chassis.