Remapping question for the techies

[ntel]

During a chat with Dave_P last night, the question of remapping came up.

I am led to believe that if the precats are removed then the ecu does not need to be remapped. However, if the main cat is removed, I understand that the ecu should then be remapped. Given that the Lambna sensors are positioned before the main cat what difference will remapping make as the sensors will be sampling the same gasses catted or decatted.

Is the remapping required to get the emmissions down or am I just talking bollox.

[stegefal]

Isn't remapping required for fuelling? Similar to when you place a race exhaust on a bike?

[ntel]

stegefal said:

Isn't remapping required for fuelling? Similar to when you place a race exhaust on a bike?

Yes, but what has that got to do with having a main cat or not?

[spend]

There is a different tune resistor that plugs into the ecu harness gubbins - that is all I have changed on the odd times when I go catless.

Dave

[stegefal]

If you remove the main Cat, surely that will give you a more free flowing exhaust?

That will effect the speed of exhaust gas flow, which is one of the facotrs an ECU takes into account when doing its magic stuff, along with other data like air flow, fuel/air ratio etc.

A remap can also improve throttle response, fuel consumption, typically top end power.

I'm sure there is someone on this site who can explain this better than me?

[shpub]

Nope. Rover V8 engines like a bit of back pressure which is why sometimes the "let's make the exhaust noisy" mods can cause a loss of power. Secondly the main cats are pretty free flowing anyway so the difference is not that noticeable. Take one look at the cross section to see. It is huge.
The ECU has no idea of the gas flow as it only gets info from the lambda sensor.

Steve
www.tvrbooks.co.uk

[stegefal]

So what effect would reducing back pressure have on power? I thought reducing back pressure increased power, by reducing the force required to expel exhaust gases?

I know there is a requirement for some back pressure, but I don't know why?

[K4TRV]

stegefal said:

So what effect would reducing back pressure have on power? I thought reducing back pressure increased power, by reducing the force required to expel exhaust gases? I know there is a requirement for some back pressure, but I don't know why?

Backpressure is required because each bank share the exhaust manifold. The length of each "header" has been sort-of optomised against the firing order. In other words after no 1 cylinder fires/exhausts, the next cylinder (don't remember the correct cylinder sequence) on that bank (in a V8) will utilise the flow of Cylinder 1 to help exhaust/fill the appropriate neighbour. Because of differing flow rates dependant upon engine speed, length of headers, is all of a compromise, but one which can be up-set by too little or too much back pressure.

Its nearly an exact science.........but each TVR is unique anyway !!

Trev McM

[joospeed]

stegefal said:

So what effect would reducing back pressure have on power? I thought reducing back pressure increased power, by reducing the force required to expel exhaust gases? I know there is a requirement for some back pressure, but I don't know why?

I've never understood this either .. you can shift the torque up and down the rev range by altering the back pressure (but like shifting the cam timing you usually lose out on outright power at the expense of shifting it where you'd prefer it such as if you have a car racing on a std greabox and shifting the torque makes it perform better even though the outright figure may be less peak), but Ive never seen an engine make *more* power with more back pressure ...

[HarryW]

I'd do a search on back pressure as this has really been done in greater detail before . Not sure I agree or understand the increased back pressure is good line though .
My motor has a bespoke reduced bore system up to the main box which I did the maths for at the time and it does not increase back pressure however it does increase gas speed which is good. Cerainly for the 4ltrs the standard system is overbore IMHO. I personally feel this is the area where confusion sets in.
AIMHO etc................

Harry

[GreenV8S]

I don't think a simple 4:1 manifold is ever going to work properly on a cross plane crank V8. There just isn't a long enough gap between adjacent cylinders so three out of four cylinders on each bank don't benefit from the overlap. The capacity gives you plenty of torque but the specific power levels are really quite poor compared to any decent engine. It's possible that adding back pressure helps make the best of a bad job, but I'm struggling to imagine why backpressure would help in this case. I suspect it's just one of those guesses that has been repeated enough times to get lodged in the collective memory. I'd be interested to know if anyone can come up with any credible explanation why back pressure would increase power, or reliable evidence that it does.

[chimtvr]

after modding my exhaust the car felt quicker it may have been my imagination but who cares as a superchips dealer i know the average driver would never notice 10%hp difference

[M@H]

From a Laymans point of view Backpressure can't actually increase power as the engine has to "work" harder to push against the pressure to get the exhaust gasses out of the cylinder. I agree with the sentiments aboout tuning the Torque curve with it mind you.. but this has got to lead to a drop in overall gains. That aside I'd rather have 200bhp at 3000rpm than 220bhp at 6000 rpm in my Chim.

Cheers,
Matt.

[shpub]

As I mentioned back pressure....

I was talking about a little amount compared to none at all.
In discussions with most of the Rover V8 gurus and in my own experience, the Rover V8 likes a bit of back pressure compared to none at all or lots more.

Not advocating increasing back pressure although some of the track day silencers do that and the performance is not quite as crisp but I was pointing out that equally reducing it also seems to affect the engine and not turn it into a screamer.

For some reason, there does appear to be a happy medium.

[joospeed]

How do you define *a little* bit of back pressure though Steve? do you have any pressure figures for instance? If there's a happy medium do you think a variable valve system to keep the backpressure constant as revs and throttle rise would be a good idea? Do you know of any V8 experts who've logged backpressure variation against load and power output? I'd love to see some traces of cars making more power with slightly restrictive exhausts.

[GreenV8S]

joospeed said:

Do you know of any V8 experts who've logged backpressure variation against load and power output? I'd love to see some traces of cars making more power with slightly restrictive exhausts.

I only know one person who did this, and it was quite an informal test (basically G versus speed before/after, but we know that's a perfectly good way of comparing power output don't we Joolz), but he did end up making more power on a supercharged RV8 with a small constant back pressure than without. Point in this case was that it eliminated blow-through and improved the bottom end torque without compromising the cam. I don't think this was exactly a typical case though! Still can't think of any credible reason why back pressure would increase power in a normal engine. But I imagine that it would be easy peasy to make some other small change while removing the back pressure, which had much more impact. For example bigger bore primaries = less back pressure but also less torque.

[HarryW]

Joolz don't know if they took back pressure readings or not, doubt it, but a few years ago V8D did back to back tests with the same engine on nearly every size and shape of manifold available on the market.
Mine has reduced bore primaries and secondaries with pproper blending of the 2-1 section, it certainly gave the car more grunt and power on before and after fitting runs within hours of each other. Whether this is back pressure or optimal size for gas speed to help some pull through (i know for the odd firing V8 with a 4-1 it doesn't quiet stack up). Proof is in the pudding as they say though .
Tim at ACT has used similar info to design and get built his improved systems for the griff with some good results too.

Harry

[joospeed]

Cool

If i get the rolling road sorted it'll something I'd love to experiment with, purely for my own satisfaction.

[HarryW]

Joolz, Not forgetting V8D did this all on their bench dyno which should have given very meaningful and repeatable results .
It made the decission and hence the courage (personal expense same thing ) needed to get mine made as a one off by JP to a smaller bore.
One off shoot was that it also highlighted that JP's RV8 head jig was of the wrong dimension between Left and right banks sorted now though .

Harry

[GreenV8S]

Any pictures of your exhaust setup Harry?