Forum

[ric]

Presto, I'm afraid the only warnings I can find on the site relate to a couple of totally unsuitable race track type exhausts which are well known from both folklore and design to suck power away from the lower rev range, create a void of lean fuelling and move the power up to the top of the rev range where there are few benefits for a road bike, but definitely a starting point for something that's likely to be further adapted and perhaps spend its working life out on the track. It is possible that putting one of those on the modern Enfield might well leave the capabilities the stock ECU floundering. So for anyone contemplating putting a totally inappropriate exhaust on an EFI Bullet then yes, it is very wise for our Hosts to proffer advice against doing so(unless accompanied with a Powaah Commander.
On a side note with regards to the initial question posted here, in one of your replies you've intimated some experience, perhaps that resulted in an unfavourable outcome. Would it not be a kind gesture to expand on this and let the collective know so the same pitfalls are therefore not duplicated? As for me perhaps I don't know any better but just really here for the fish!

[Thack]

Ric writes: "A Pulse Air Valve, it feeds air into the exhaust manifesting itself as an air leak with the exact symptoms you have described. It's usually only active on very low and closed throttle applications."



For anyone who cares, PAV's aren't used on modern systems with three-way catalytic converters, but were very common on the previous generation of emission control. That used a two-way converter and used excess air injected into the exhaust system to provide the required oxygen to burn off the unburned hydrocarbons in the cat. My Electra X used that system.



Re. our EFI bikes, obviously there will be limits to how far the ECU can be "pushed" by different exhaust and intake systems, but normal free-flow systems you'd use on the road should provide no difficulties. To be absolutely certain we'd have to conduct instrumented tests, but as Ric says, a system which can cope with the environmental range covered by the proving tests should have not difficulty whatsoever with a free-flow exhaust.

[Presto]

That’s my point ric! So it seems as if an “electronic system capable of coping with the barometric difference between ... 17,000ft and a British coastal road” isn’t able to provide a correct fuel mixture for a simple exhaust change if that simple exhaust change is among the most popular alternative exhausts offered for these machines – (none of which would accurately be described as “totally unsuitable race track” items).

[Thack]

To be fair, Presto, I don't think we can assume that fitting that new exhaust is in any way harmful. My strong instinct is that the ECU will compensate just fine.



I think it is far more likely to be two factors combined:



1/ You won't feel much benefit from the new exhaust unless you fit a PC-V



2/ It does no harm to recommend a PC-V because you are being super-cautious and thus covering your own back; besides which, another expensive sale is good for the bank balance.



I will now mitigate all that by making one last point: the ECU runs closed loop MOST of the time. It switches to open loop (and rich) when the throttle is wide open and the engine revs are high. Under these circumstances it is running on the internal map, and it is POSSIBLE that the internal map doesn't suit the new exhaust and you end up running insufficiently rich, exactly when you don't want to.



Personally I never ride like that, but I wanted to mention it for completeness. My instinct would be that if you are one of those rare Bullet riders who frequently thrashes its balls off, a PC-V may well be wise if you've changed the exhaust.

[Presto]

Thack I agree on all points. Please don't read my remarks as if I was suggesting that 'damage' would occur. But there may well be a potential fall-off in performance if certain styles of after-market exhaust are used without an upgrade of the ECU.

[ric]

Presto, Unfortunately that's how I read your remarks. But I'm not wrong when I say there are far better exhaust options available which enhance the engines characteristics and not impede it that can be fitted to the Enfield for normal use on our public roads than systems that excel on the race track. But if people are happy fitting them to their own bikes then they're happy so there's no argument there.

[MickB]

Michael wrote: "PeteF... I have checked and checked and checked for air leaks in my exhaust system - none can be found. But I think there are other possible cuases for exhaust backfires. Mine backfires when the trhottle is closed sharply from a mid-high position. The pop occurs just as the throttle gets back to the idle position, but as the engine is still at high RPM (is this over-run?), especially when the bike (and therefore exhaust bend) is warm/hot."

I had exactly the same problem with Mr H's exhaust. I changed the stock Bosch spark plug that came with the bike as on checking it, it was very black and sooty which suggested the plug was running too cold. Replaced it with a (genuine) NGK BPR6ES (which I think is now BPR6IX). Voila, problem solved. I have ridden over 4,000 kms on this plug and the bike runs as sweet as a nut. Could be as simple as that. Mick.