Overboost

[gmg31]

I've had my car for 2 years and I've suffered ongoing problems with overboost which ruined a recent track day. I have checked the usual suspects, the hoses and the linkage from the actuator and everything is ok. I rang my reliable parts supplier and he said it is most probably because my car has a sports exhaust on it. I am told there are two solutions. Change the exhaust £300. or have it re mapped but can't find anyone who wants to do this.

I would really appreciate some advice please

Gary

[HJ2]

I think there's a 3rd solution: Port the wastegate!

Tell some more about your setup, what exhaust bore etc.
Since you are allready on track, I would install a mountain chip and check that you haven a 2.25"or 2.5"exhaust now. (Also the cause could be a pre-cat eliminator or straight downpipe.)
If yes, the wastegate probably needs to be ported.

Do a search on wastegate porting on the forum, and see here:
http://wikilec.9600.org/index.php/Porting_the_wastegate

[lotusflasherman]

Big bore / sports exhaust often need the wastegate port enlarging to stop overboosting. Suggest you read the threads on here about 'porting the wastegate' before you spend any money.

[rip]

You mentioned a re-map. This can be done but it is not a good idea because:

You are producing too much boost because the larger exhaust is allowing a faster flow of exhaust gas. Even when fully open, the wastegate cannot bleed off enough excess, so the extra gas flow drives the turbo harder.
The overboost limit is there because the injectors are at their limit of how much fuel they can supply. If the boost was allowed to build further for too long, then the mixture would be too lean which would cause the combustion chambers to overheat & possibly damage your engine.

As mentioned earlier, if you want to keep the benefits of the larger exhaust, then port the wastegate to match it.

[gmg31]

Thank you all for this advice. I recently had the turbo off, I wish I had known this then. However it shouldn't be quite so hard to remove the second time. I've read the advice sheet but it doesn't say what to use to actually enlarge the gate, any views please? I'm planning to crack on next week.

[lotusflasherman]

Turbo body is cast iron so grindstones in a power drill.

www.ebay.co.uk/itm/6MM-SHAFT-GRINDSTONES-/131459712989

www.ebay.co.uk/itm/6MM-SHAFT-GRINDSTONES-/131459712987

You'll need to block off or otherwise seal all oilways and make sure you clean off all the abrasive dust afterwards or the turbo won't last long..

[HJ2]

I used these a couple of times now

[gmg31]

Turns out there may be a fourth option. I have found a local stainless steel exhaust specialist who thinks that adding a box will slow down the flow and reduce the noise. My car is very loud and failed the noise limit at a recent track day.

[John_W]

Gary,
Overboost is normal if you have a big exhaust and keep the throttle open long enough. See this post (viewtopic.php?f=31&t=14409&p=189689&hilit=Overboost#p189689) and in particular the graph to which it refers.

Adding another silencer won't make any difference to the boost creep (unless it uses smaller diameter pipework) - the exhaust I have on the car already has an extra silencer and that is the setup that was tested on the dyno and shown on the graph.

John

[gmg31]

That being the case I can see no value in keeping this very loud sports exhaust. The best option would seem to be a replacement exhaust £300 from SJ sports cars. That way the car runs as it should and I won't fail the volume test at track days.

[John_W]

If the SJ system is standard diameter (including the famous "squish") it should get you back to normal. However, if it is more free-flowing than standard (larger diameter, squish-free or unbaffled), you may still see boost creep leading to Overboost. If this does occur, you could end up having to port the wastegate.

Do you already have a Mountain chip fitted? I think I'm right in saying that these have a slightly higher fuel cut-off limit, which might just give you enough of a margin to avoid the Overboost cut-off occurring.

My car has a 2-1/4" exhaust and a Mountain chip and I have never experienced the fuel cut-out. Maybe I'm just lucky or maybe I just drive like an old woman. The only sure way to avoid boost creep with a more free-flowing exhaust would seem to be to enlarge the waste-gate.

Good luck sorting it.

John

[Elanlover]

Just my opinion but.............

I'd port the wastegate and, if you don't like the sound of the exhaust, simply replace the muffler/silencer or add a resonator midway. You can dampen the sound and keep the extra boost that the larger bore pipes can give you. It'll be a noticeable drop in power if you replace the exhaust system with a smaller bore version.

[gmg31]

Thanks for everyone's help and advice and that very helpful help sheet. I ported my turbo tonight. my turbo was off earlier in the year so it came off in under an hour tonight. I used the recommended grinding stones and it worked a treat. I'll put it back together tomorrow.

[chrisP]

Depending on what exhaust you have fitted, if it's peco or cybox you should be able to get an intermediate silencer from PNM to quieten down the noise levels with effecting power output.

http://www.pnmengineering.com/lotus-exhausts-elan.html

If you have a sports exhaust that removes the squish and or precat, you will notice a massive loss in performance if you change to the standard offering.

Chris

[tigerdog]

Congrats! Hope it worked out well. BTW, welcome! Are you the person who raised the question on the facebook lotus group?

[gmg31]

I'm sorry, I forgot to update this thread. I was too busy driving hard and smiling. Yes porting the waste gate worked a treat and I had a superb summer free from overboost.

[HJ2]

[lotusflasherman]

I'm sorry, I forgot to update this thread. I was too busy driving hard and smiling. Yes porting the waste gate worked a treat and I had a superb summer free from overboost.

That's great. I really think suppliers of sports exhaust should warn owners that it might ... er, no, will be required. What did you enlarge it to?

[John_W]

[lotusles]

I am in the process of fitting timing belt etc and would eventually like to fit a bigger bore exhaust to naturally increase bhp. Would I do any damage if I was to remove and ream out the waste gate now but not fit an exhaust until some time later?

Thanks