Hi to all,

 

I got a Decat cheap and worried about back pressure etc before deciding to pop it on.

 

I dont like it at all and noticed i lost 3-4 psi boost. Can someone explain how this happens with a Hybrid turbo?

 

Decat is off tonight! 

The map needs tweaking to suit the reduction in gas speed, the N75 is feathering the turbo too early, you've just provided the perfect example of how turbo diesels are different to turbo petrols.

Cheers Sep!! 

 

N75 is what opens / closes actuator ? Like how a wastegate works i take it?

 

There has never been any real evidence on this subject hence why i half expected issues. I expected either no improvement, overboost which worried me or slightly looser rev range.  Never boost loss though.

 

Sometimes you have to find out for yourself...  Good job i had a boost guage to see difference tho you can feel it.

Boost control is by fueling and N75/"vanes". Part of it could be an overall increase in efficiency (exhaust is now a bigger hole to escape through). Which might translate into MPG savings. But without map tweak, yep, unlikely to see much in the way of power.

1 hour ago, Graffiti said:

Cheers Sep!! 

 

N75 is what opens / closes actuator ? Like how a wastegate works i take it?

 

There has never been any real evidence on this subject hence why i half expected issues. I expected either no improvement, overboost which worried me or slightly looser rev range.  Never boost loss though.

 

Sometimes you have to find out for yourself...  Good job i had a boost guage to see difference tho you can feel it.

 

Yes, except a wastegate is simply a sluice gate to divert exhaust gas away from the exducer, it can't help get a turbo boosting from low RPM.

 

Either your exhaust system flows well enough or it doesn't, this cretinous idea about back pressure comes up all all the time, there is NO back pressure until an exhaust system hits a hard limit in terms of flow potential.

 

Think of it like a traffic jam caused by sheer weight of traffic, up until a critical point the traffic flows freely, above that point congestion builds and rapidly gets worse. This is why an aftermarket turbo exhaust system doesn't add power unless the standard system has reached its flow limit.

 

Not convinced? Try blowing down a 3" pipe, can you feel any resistance? Now blow down a 4" pipe, is there any less resistance?

 

Of course there isn't, it's a silly question, if there's no resistance in the first pipe there still won't be any in the second, but there is a slight difference in the speed your breath travels down the pipe, this is important because at high speed the gas has momentum, p = mv (GCSE Physics), if m stays constant and you reduce v then p reduces as well.

 

QED.

40 minutes ago, sepulchrave said:

 

Yes, except a wastegate is simply a sluice gate to divert exhaust gas away from the exducer, it can't help get a turbo boosting from low RPM.

 

Either your exhaust system flows well enough or it doesn't, this cretinous idea about back pressure comes up all all the time, there is NO back pressure until an exhaust system hits a hard limit in terms of flow potential.

 

Think of it like a traffic jam caused by sheer weight of traffic, up until a critical point the traffic flows freely, above that point congestion builds and rapidly gets worse. This is why an aftermarket turbo exhaust system doesn't add power unless the standard system has reached its flow limit.

 

Not convinced? Try blowing down a 3" pipe, can you feel any resistance? Now blow down a 4" pipe, is there any less resistance?

 

Of course there isn't, it's a silly question, if there's no resistance in the first pipe there still won't be any in the second, but there is a slight difference in the speed your breath travels down the pipe, this is important because at high speed the gas has momentum, p = mv (GCSE Physics), if m stays constant and you reduce v then p reduces as well.

 

QED.

 

So smaller pipe draws air where as with bigger pipe that effect is lost.

Hmm GCSE ??  Similar age to me then.

 

Well no need to take this any further,  with my practical experiment and your detailed explanation there in theory should never be any more questions on De-cats. 

Some folk say Yay, some say Nay and now I know for myself.

 

One more observation, no owl noise ( Thank God! ). But either way Cat is re-fitted.  

 

 Now to focus improving the Yeti's performance slightly that we bought thursday. ( 110bhp 4x4)  But thats the sensible car! And another story.   

Have you seen the monster-yeti build in Projects by BossFox? Same guy with the everlasting 4x4 CitiGo-Go project.

I'm gonna say that there is always some back-pressure. even if the exhaust gasses exit the turbo directly into atmo. Whether it's actually worthwhile trying to do something about it is another matter.

 

http://www.autospeed.com/cms/article.html?&title=PoweringUp-the-19-litre-TDI-Part-4&A=113077

 

I think that article pretty much sums up VW TDI tuning... Or at least PD tuning. It needs mapping.

9 minutes ago, vindaloo said:

I'm gonna say that there is always some back-pressure. even if the exhaust gasses exit the turbo directly into atmo. Whether it's actually worthwhile trying to do something about it is another matter.

 

Nope, the pressure of the atmosphere is 1 bar, the pressure of the exhaust gas is higher, any fluid (gas) moving from a higher pressure to a lower pressure experiences no resistance to that movement.

 

Edit: Also that Autospeed article is fine and dandy BUT he changed a whole big bunch of stuff he needn't have bothered changing, including the exhaust and air filter. On a standard turbo there is NO restriction other than the mapping, for more power you need a bigger turbo, until you hit that limit at about 220 BHP nothing you do will make any difference.

Edited 12 July, 20178 yr by sepulchrave

46 minutes ago, sepulchrave said:

 

Nope, the pressure of the atmosphere is 1 bar, the pressure of the exhaust gas is higher, any fluid (gas) moving from a higher pressure to a lower pressure experiences no resistance to that movement.

 

Edit: Also that Autospeed article is fine and dandy BUT he changed a whole big bunch of stuff he needn't have bothered changing, including the exhaust and air filter. On a standard turbo there is NO restriction other than the mapping, for more power you need a bigger turbo, until you hit that limit at about 220 BHP nothing you do will make any difference.

 

Autospeed: They did the traditional flow stuff external to the engine and manifolds. Lower resistances in the filter and the exhaust. It had a negative effect on performance without a map. But it had a positive effect on fuel economy. Probably on emmisions too.

 

A fluid moving through a pipe or pipes with boundaries and sudden step changes in section encounters resistance/losses. There are boundary effects and turbulance effects.

 

J.

3 minutes ago, vindaloo said:

 

Autospeed: They did the traditional flow stuff external to the engine and manifolds. Lower resistances in the filter and the exhaust. It had a negative effect on performance without a map. But it had a positive effect on fuel economy. Probably on emmisions too.

 

A fluid moving through a pipe or pipes with boundaries and sudden step changes in section encounters resistance/losses. There are boundary effects and turbulance effects.

 

J.

 

Ok, but the improvement in economy would come from simply changing the intercooler so standard power and torque is maintained rather than gradually lost through heat soak requiring more right hoof for a given road speed.

 

The improvements in flow you describe are negligible at best, there are members here who've had full head flow jobs on big power engines and only seen a relatively small increase in performance. Look at the final figures after the last remap in those articles, they're not noticeably better than many members here have seen just remapping a bog standard car.

1 hour ago, vindaloo said:

Have you seen the monster-yeti build in Projects by BossFox? Same guy with the everlasting 4x4 CitiGo-Go project.

 

Thats looking at 800bhp when it's done!  I would be set for a quick divorce if I started with Yeti . I can Imagine a low, fat and wide stance looking absolutely epic on a Yeti.... Im more into its off road capabilities. Its nice to drive in comfort as the Fabia ....... Well lets face it  teeth rattle boost loud vibration rooooar .

 

As per my boost issue, I knew my map would be needing tweaked to get its full potential but did not expect a mad drop in power...  Over boost is the issue most discussed with De-cats which are not mapped accordingly whereas mine did the opposite as per Seps explanation.

 

I dont think chasing the back pressure on a tdi is worth it at all... The noise however which rasped and barked reminds me of my old Mk 1 escort rs.  Love the sound but at a trade for power, neigh chance!!!! 

 

Damn issue with Yeti after owning it 4 days! Glow plug light flashing and limp mode... For Gods sake, I did not expect issues on a 2014, 30k Yeti already! 

 

Surely some minor mods are permitted. Just to make it a little more sporty. We're not quite talking Paris-Dakar (yet)

 

3 hours ago, sepulchrave said:

 

Ok, but the improvement in economy would come from simply changing the intercooler so standard power and torque is maintained rather than gradually lost through heat soak requiring more right hoof for a given road speed.

 

The improvements in flow you describe are negligible at best, there are members here who've had full head flow jobs on big power engines and only seen a relatively small increase in performance. Look at the final figures after the last remap in those articles, they're not noticeably better than many members here have seen just remapping a bog standard car.

 

I believe that if driving sensibly for economy, then the existing intercooler is adequate. It's possible that in the UK, mixture could be "overcooled" before the combustion chamber with ultimate intercooling.

 

I'd hope that reducing BP would mean at the least two things. A realisation of improved economy and lower temperatures in cylinder and in the exhaust manifold. Whether it's worth changing stuff out for that reason alone is the question. Without a big turbo though, I have to agree with you. The limits of the system are likely well above the capabilities of the standard setup.

 

J.

41 minutes ago, vindaloo said:

I believe that if driving sensibly for economy, then the existing intercooler is adequate. It's possible that in the UK, mixture could be "overcooled" before the combustion chamber with ultimate intercooling.

 

There is no 'mixture', only air, this is a diesel engine remember, the fuel doesn't even get injected until the cylinder is already full, compressed and at many hundreds of degrees centigrade, an intercooler cannot ever cool the air below ambient temperature, in practice they don't generally get that near ambient under load. A N/A engine will always breath cooler air than a turbo engine.

 

Belief is guessing, don't guess then there's no need for belief, the Autospeed article was written in Australia, heat soak is always a factor there. He changed too many things that didn't need changing so the experiment was meaningless, tuners change one thing at a time then test its effect before changing anything else. That way they know what works and what doesn't.

36 minutes ago, sepulchrave said:

 

There is no 'mixture', only air, this is a diesel engine remember, the fuel doesn't even get injected until the cylinder is already full, compressed and at many hundreds of degrees centigrade, an intercooler cannot ever cool the air below ambient temperature, in practice they don't generally get that near ambient under load. A N/A engine will always breath cooler air than a turbo engine.

 

Belief is guessing, don't guess then there's no need for belief, the Autospeed article was written in Australia, heat soak is always a factor there. He changed too many things that didn't need changing so the experiment was meaningless, tuners change one thing at a time then test its effect before changing anything else. That way they know what works and what doesn't.

 

I was thinking holistically.   Ultimately, it's the retention and transfer of heat to the fuel and vaporisation/atomisation of that fuel. There's an optimum environment for economy. In my experience with the Fabia, that's on a hot summer day trundling at 60-70 on the M25 for 80 miles with the odd faster sprint to find a nice gap again.

 

In the Autospeed article there's an element of "well maybe I shoulda just mapped it then". unfortunately, there's no follow-on with a standard car remapped. He states (with some evidence) is that the mods helped fuel economy. But until remapped, there was if anything a detriment to power.

 

J.

9 hours ago, vindaloo said:

 

Surely some minor mods are permitted. Just to make it a little more sporty. We're not quite talking Paris-Dakar (yet)

 

 

You dont know Mrs Graffiti!! The fabia was the limit.... Her words were " No more oil on the door handles, light switches, kettle, couch or in the bed ". (After fitting hybrid late at night i fell in bed after quick wash, oil on pillow, Duvet etc  NB Bed is super king and cost more than my Vrs. )

 

That video does make me twitch tho.