I get the impression that the standard VL37 turbo on a t-jet is a touch on the small side and cannot maintain full boost pressure as the revs rise hence the slightly disappointing top end performance (compared with the low and mid range anyway). I’m wondering if anyone who as a standard t-jet has fitted a boost gauge and can tell me at what revs the boost starts to drop away (assuming it does) and the peak pressure value. The test must be done at full throttle to have any meaning.

I have no intention of modifying my car but as an engineer I’d just like to know out of curiosity.


Thanks

It's not the turbo itself, it's the wastegate. As the revs rise the turbo builds up boost, but eventually the pressure gets too much for the wastegate, and it opens, letting the boost pressure out, and that's why it drops off at the higher RPM, I'd say the drop starts at about 4.5k RPM

I know you don't want to modify, but I thought I'd let you know the solution to this is a stiffer spring in the wastegate to hold it shut better at higher pressures, however since the standard actuator/wastegate is a solid unit, you can't change the spring, so you'd have to buy the forge adjustable actuator and put a stiffer spring in, that's what I did, and it holds onto the boost nicely now at high RPMs.

Surely the wastegate has closed-loop electronic control to ensure a constant boost pressure throughout the rev range (assuming full throttle), why would it deliberately reduce boost pressure at the top end? It must be the case that the turbo is flow rate limited and therefore incapable of producing maximum boost pressure at the high flow rates required above a certain engine speed. Or to put it another way, if you plotted the boost pressure (y axis) the turbo could make against engine rpm (x axis) it would drop at higher rpms.

My theory tallies with the fact that mapping a t-jet with the standard vl37 doesn’t produce great results. You can increase the maximum boost pressure the ECU will allow but if the turbo is maxed out your not going to see any power increase (at very high rpms). I bet those that have re-mapped t-jets with a vl37 produce their new peak power at quite low revs, maybe 140bhp@ 5000rpm Vs the standard car’s 120bhp @ 5500rpm; but they probably get good improvements in low end torque.

In a closed loop system a stiffer spring just means that the ECU bleeds less of the wastegate control pressure off to atmosphere thus increasing the pressure on the wastegate to open and giving the same boost pressure as before. I read that the reason wastegate springs are up rated in tuned cars is to prevent the exhaust backpressure inadvertently opening the wastegate without the ECUs say so.

Some of the things you're saying are going a little over my head, my only experience comes from modifying my own car, seeing the results with the boost gauge and assembling/disassembling the forge actuator.

The wastegate itself is not controlled by any electronics, it only works by the air inside the actuator, there is nothing electrical to operate the piston or the flap.

It would be interesting to see some before & after Boost Pressure/RPM graphs for T-Jets mapped with the VL37 turbo, I have such a graph for my car but unfortunately that was with the VL36 turbo.

jameshobiecat said:

Surely the wastegate has closed-loop electronic control to ensure a constant boost pressure throughout the rev range (assuming full throttle), why would it deliberately reduce boost pressure at the top end? It must be the case that the turbo is flow rate limited and therefore incapable of producing maximum boost pressure at the high flow rates required above a certain engine speed. Or to put it another way, if you plotted the boost pressure (y axis) the turbo could make against engine rpm (x axis) it would drop at higher rpms.

My theory tallies with the fact that mapping a t-jet with the standard vl37 doesn’t produce great results. You can increase the maximum boost pressure the ECU will allow but if the turbo is maxed out your not going to see any power increase (at very high rpms). I bet those that have re-mapped t-jets with a vl37 produce their new peak power at quite low revs, maybe 140bhp@ 5000rpm Vs the standard car’s 120bhp @ 5500rpm; but they probably get good improvements in low end torque.

In a closed loop system a stiffer spring just means that the ECU bleeds less of the wastegate control pressure off to atmosphere thus increasing the pressure on the wastegate to open and giving the same boost pressure as before. I read that the reason wastegate springs are up rated in tuned cars is to prevent the exhaust backpressure inadvertently opening the wastegate without the ECUs say so.

Click to expand...

Stiffer spring means more pressure is required to open the wastegate/higher peak pressure can be obtained and as you said less leakage. Also there is no constant boost pressure as the boost pressure depends on the output of turbo at a given time and the volume of gases spooling the intake turbine(through exhaust turbine), but it will remain at its peak for as long as it can.

I know of cases on the Abarth Forum where wastegates have failed and need to be replaced and this is a fault that has electronic implications as it comes up with errors on the EMS/Fault codes. However sometimes it has failed and there are no fault codes.

The Vl37 will have a peak output that it can put out as a volume of air (cm3) in a given time at a peak turbine RPM, at higher revs the consumption of this volume of air will increase due to the increased RPM of the engine and breathing requirements, which is why the boost pressure (PSI) will decrease higher up the rev range past a given point (To my understanding anyway, please feel free to prove me wrong) As the volume of air supplied will stay the same but the demand will increase.

The Vl36 (AGP turbo?) has the same outside appearance but different internal which seem to allow an increase in the peak volume displacement?.


The low RPM torque figures for the base T-Jet are partly to do with the CAM setup and the aggressiveness of it, for example the Bravo 150 T-Jet and AGP both have peak torque from 3000RPM to something like 5750rpm, but the less aggressive cams in the T-Jet Sporting produce max torque from low in the rev range, 1750rpm if memory serves me well. So the increase in low range torque is purely down to the increased boost pressure that the map allows.

The SS with the same cams as the AGP produces more torque over the same rev range (boost pressure is increased from 1.3 Bar to 1.5 Bar I think, through a different turbo and there are different injectors so assuming more fuel as well) this should prove the higher boost pressure creates more torque

DlightSwitch said:

Some of the things you're saying are going a little over my head, my only experience comes from modifying my own car, seeing the results with the boost gauge and assembling/disassembling the forge actuator.

The wastegate itself is not controlled by any electronics, it only works by the air inside the actuator, there is nothing electrical to operate the piston or the flap.

It would be interesting to see some before & after Boost Pressure/RPM graphs for T-Jets mapped with the VL37 turbo, I have such a graph for my car but unfortunately that was with the VL36 turbo.

Click to expand...

I think that the wastegate is partially electronically operated as the standard T-Jet can overboost in 4th and 5th Gear even with the standard spring.

Your right that the wastegate is a mechanical device opened by the boost pressure but the ECU can bleed off pressure from the control line to prevent the wastegate from opening. In this way electronic control of the boost pressure is achieved.

Closed-loop means that the ECU uses readings of boost pressure from the MAP sensor to bleed off pressure from the wastegate control line as required. If it were open-loop there would be a map telling the ECU to how much pressure to bleed off at a given rpm and throttle position but this is less accurate and less adaptable to changes in temperature etc. Limp modes use a known safe open-loop map so the car can continue (with reduced power) with a faulty sensor.

Looking at some boost vs rpm graphs on google it looks as though most turbos are ‘undersized’ in that they cannot produce maximum boost at high rpms. Seems more pronounced in petrols than diesels (which tend to have the boost pressure electronically regulated [horizontal line] for a much larger proportion of the rev range). Perhaps this is why diesels are so tuneable. The t-jet must be at the more extreme end of the undersized spectrum.

Superchips have before and after power and torque curves for the t-jet but no boost plot. The max power seems to occur around 5250rpm tuned which is higher than I would have thought but lower than standard. Power gains with the standard vl37 are only 10bhp but torque is up a useful 40ish Nm @ 3000-3500prm again supporting the undersized theory.

I know that the overboost increases from 15 PSI in standard T-Jet to 18PSI which I assume is the natural limit of reasonable efficiency from the standard turbo, goes from 18PSI to 21PSI in my Abarth. (Sport mode only though) and mappers must just allow this as standard and maybe a 20PSI peak somewhere, that would make a reasonable difference.

The SS starts at 21PSI as standard and then also has overboost I am pretty sure and thats also in sport mode.

Ah so in the broken Abarths the control line has been stuck open/damaged somehow? which means minimum pressure built.

Also you wouldnt want a turbo producing maximum boost at maximum RPM as it will not often get there or stay there for long in a higher powered car, also a thing to look at is the RPM/air flow as a diesel will rev to 4.5k roughly and petrol 6.5k, I would assume the petrol is "breathing" a lot more at 6.5k than the diesel would be at even 4.5K

TallTom said:

Stiffer spring means more pressure is required to open the wastegate/higher peak pressure can be obtained and as you said less leakage. Also there is no constant boost pressure as the boost pressure depends on the output of turbo at a given time and the volume of gases spooling the intake turbine(through exhaust turbine), but it will remain at its peak for as long as it can.

Click to expand...

I think we need to make a distinction here between ‘facilitating’ and ‘causing’ an increase in boost pressure with regards to the wastegate spring. My understanding is this; changing the wastegate spring for a stiffer one in a standard car will ‘cause’ no increase in bhp as the ECU will continue to regulate the boost pressure to its desired value. However, if a re-mapped ECU is allowing very high boost pressures the wastegate may open without the ECU telling it too, this is where the stiffer spring comes in to ‘facilitate’ higher pressures.

As for the constant boost pressure, it appears that petrols have ‘undersized’ turbos and thus very little wastegate opening occurs and so boost pressure is largely down to turbo output (though max pressure is still regulated electronically). Diesels on the other hand appear to have ‘oversized’ turbos which run at ECU regulated maximum pressure for most of the rev range.

The above would explain why bolting a vl36 on to a t-jet will increase power without a re-map. It can get closer to the maximum pressure for more of the time. More wastegate action will happen at lower rpms but the top end will be improved.

I’m not convinced that the cams are the reason that the bravo/AGP produce peak torque at higher rpms, surely it is because their larger turbos don’t reach max pressure until then? I’d have thought the different cams facilitated the higher flow rates produced by the larger vl36 at high rpms.

Thinking about it a bit more on the way home I have concluded that we must be missing something here.



If it were only the physical capacity (max flow rate) of the turbo that limited the boost pressure at high rpms then how would a remap of a car with a vl36 increase peak power? Only three things contribute to the sizeof the bang in an SI engine; mass of air in cylinder, mass of fuel, and sparkt iming. Fuel is proportional to air mass as the ECU will always be aiming for(close to) stoichiometric combustion , and there isn’t much you can do with spark timing.



If the current theory (that only turbo size limits boost pressure for much of the rev range) were correct a re-map would increase lowdown torque (where electronic boost limiting is happening) but above around 3000rmp the power and torque traces would be identical to an un-mapped car (with vl36).



So my revised theory is that the turbo pressure must remain electronically limited for much of the rev range BUT that the electronic pressure limit varies with rpm. I still maintain that the t-jet turbo is ‘undersized’ in that it reaches a point where its flow rate limits boost pressure earlier than on many cars but I no longer believe this happens for the majority of the time.
Perhaps it is 'optimally' sized in that the turbo output pressure (without boost limiting, i.e. no wastegate) falls in line with the electronic boost limit hence only marginal gains from re-map.





Now if only I knew why it is desirable for boost pressure to fall at higher rpms.



P.S. I didn't know t-jets overboosted in 4th and 5th.

So to totaly contradict my last post I have found this graph of an Audi 1.8 TT turbo pressure:



It shows the ECU boost limit as a horizontal line for much of the rev range but the actual boost well below this from about 3500rpm upwards because the turbo just can't move enough air. If this is the case, could it be that the size difference between the vl37 and vl36 is so vast that the vl37 hardly troubles the wastegate where as the vl36 boost is limited and therefore can be raised by a re-map.

The cams create a different lift profile and therefore change timings allowing more ideal detonation conditions and allowing more
Fuel and air to be injected before TDC and ignition, anyone going for more than 190 out a T-jet has changed the cams to the Abarth version

Air displacement increase of the larger turbo may increase lower pressure relative to rpm of the turbo turbine maybe?

Whilst the cam profile dictates the valve timing and lift i'm under the impression that this is less significant on force induction cars and I just can't see it being responsible for low end torque increases when the slow engine speed means there is ample time to fill the cylinder with air. I'm convinced its boost related.

Perhaps the abarth has equal or even greater torque at 1750rmp than the t-jet but it keeps on giving more until 3000rpm long after the t-jets vl37 has either run out of puff or the ECU has limited the boost. I can find a graph of the t-jets torque but not abarths.

Anyway, back to the original post, has anyone got a boost pressure gauge or boost graphs for a standard t-jet?

jameshobiecat said:

Whilst the cam profile dictates the valve timing and lift i'm under the impression that this is less significant on force induction cars and I just can't see it being responsible for low end torque increases when the slow engine speed means there is ample time to fill the cylinder with air. I'm convinced its boost related.

Perhaps the abarth has equal or even greater torque at 1750rmp than the t-jet but it keeps on giving more until 3000rpm long after the t-jets vl37 has either run out of puff or the ECU has limited the boost. I can find a graph of the t-jets torque but not abarths.

Anyway, back to the original post, has anyone got a boost pressure gauge or boost graphs for a standard t-jet?

Click to expand...

Hey just seen this thread I have my t-jet standard graph, I can't remember if it has the boost on it. I would upload it now but I'm just on my phone.

James, a lot of your conclusions are true from an engineering perspective. The reason the VL37 exists and boosts in the way it does is simply the designed practical purpose of the car spec. The very small internals speed up very quickly to get that very low boost RPM where 95% of driving is done. The fact that the boost cannot be maintained at higher RPM is a limitation of the design, but not an important one due to the designed usage of the car. Of course the wastegate is also electronically regulated and in standard trim this limits boost a lot in the higher rev range. This again serves a practical purpose of keeping low charge temperatures so it is ready to boost well again and decreases wear through lower turbo speeds. People who map or add a tuning box to this spec will find a more sustained boost, but at the cost of high charge temps and ultimately a short turbo life. It is simply unsuitable for sustained high RPM driving.

The VL36 and 38 turbos have very much larger turbines and compressors which naturally can only create their higher boost at a higher RPM. When combined with the "Abarth" cams, the effect is exaggerated, but the higher duration and lift removes some of the restrictions on peak performance too, all at the considerable expense of daily driving usability.

Obviously you can have both of these driving characteristics from a turbo to a point, but then it comes down to the obvious cost factor for FIAT.

I'm afraid I only know boost numbers on an Abarth spec engine and turbo as that is the car that has a gauge in it.

Heres the standard graph for my T-jet.

when my t jet was standard. it ran 11psi as standard.
as i had a boost gauge fitted before and after remap.
After remap i was running 21psi.

hope that helps

Hi Friend, how are you?

You know what table was modified to raise your boost to 21psi? I try a lot of combinations, but no sucess.

Please, can you help me?

Regards

techardware said:

Hi Friend, how are you?

You know what table was modified to raise your boost to 21psi? I try a lot of combinations, but no sucess.

Please, can you help me?

Regards

Click to expand...

Table..

You want a time machine