Engine off boost
Discussion
everywhere it is said that the turbo installation destroys the response to the accelerator is created a lag (which tends to decrease since the 2000s) there is some truth, off boost the tc motors have less rv and losses charge of fresh air in the intake ducts and restrictions by the turbo in the exhaust duct. logically the response of the engine in the off boost regime is less good than the equivalent displacement na without a doubt. however when is it for motors with a minimum cylinder capacity of 3l and more. the off boost response before the turbo starts should be correct because of the generous torque produced by the displacement? a 3l turbo off boost can therefore respond better than a 2l na for example etc.
if I put a big turbo that starts at 3500rpm on an ls2 style engine below the boost threshold the engine should still go stronger than a 3l or even 4l?
if I put a big turbo that starts at 3500rpm on an ls2 style engine below the boost threshold the engine should still go stronger than a 3l or even 4l?
Murci.sv said:
and in this waiting period is there no torque? the response of the accelerator is linked only to the torque available at the moment you accelerate if I'm not mistaken
Yes, but all you're saying here is that a big, torquier engine will perform better than a smaller, less torquey one. There is still a delay before full power is produced, whether that's the engine grunting away on its 'natural' torque to get above the boost threshold of the turbo, or the lag as the turbo spins up to operating speed (threshold and lag being often, and irritatingly, confused). It's the non-linear response that most people dislike(d) about the old-school turbo setups, not just the relative lack of punch below the turbo's operating range. And however big your engine, if it's got a turbo attached to it it won't perform as well off-boost as the same engine would without the turbo, because it has the (momentarily useless) turbine clogging up the exhaust, as well as often other compromises like the shape, position and length of the exhaust tract, valve events, fuelling response and compression ratio that often come with turbocharging.
Turbo engines generally run a lower static compression ratio than normally aspirated ones, to allow for boost without pressures and temperatures becoming too high leading to detonation.
For example, a Ferrari 458 Speciale has a compression ratio of 14:1, whereas the following 488 Pista is 9.6:1.
Compression ratio is a fundamental factor in developing power, so in my mind the disadvantage of an off-boost turbo engine compared to a nat-asp one has always been mainly due to this.
However in general I think you're right that there isn't a huge difference between nat-asp and turbo engines of similar sizes at low revs, i.e. below the boost threshold for the turbo motor. The main difference is that the nat-asp engine power increases fairly gradually with increasing revs, whereas the turbo engine has a much more noticeable transition around the boost threshold, and then is more level thereafter.
And however big your engine, if it's got a turbo attached to it it won't perform as well off-boost as the same engine would without the turbo, because it has the (momentarily useless) turbine clogging up the exhaust, as well as often other compromises like the shape, position and length of the exhaust tract, valve events, fuelling response and compression ratio that often come with turbocharging.
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precisely I did not compare with a naturally aspirated engine of equivalent displacement but slightly smaller.
linear throttle response? it all depends on the engine itself, take a sharp NA Lamborghini style, Ferrari it goes through phases where the torque can increase significantly in a Zone especially without variable valve timing. let's say you press at 2000rpm the engine gradually climbs then at 4000-5000 it starts pulling really harder. or have a torque curve that rises a lot so the response is not totally linear but rather progressive unless you have a rather flat torque curve IMO
samoht said:
Turbo engines generally run a lower static compression ratio than normally aspirated ones, to allow for boost without pressures and temperatures becoming too high leading to detonation.
For example, a Ferrari 458 Speciale has a compression ratio of 14:1, whereas the following 488 Pista is 9.6:1.
Compression ratio is a fundamental factor in developing power, so in my mind the disadvantage of an off-boost turbo engine compared to a nat-asp one has always been mainly due to this.
However in general I think you're right that there isn't a huge difference between nat-asp and turbo engines of similar sizes at low revs, i.e. below the boost threshold for the turbo motor. The main difference is that the nat-asp engine power increases fairly gradually with increasing revs, whereas the turbo engine has a much more noticeable transition around the boost threshold, and then is more level thereafter.
I agree, na has no boost threshold it's obvious but many have little torque at the bottom sometimes even with a fairly large displacement so that if you push at these speeds it does not happen great things to say nothing then the power gradually increases.For example, a Ferrari 458 Speciale has a compression ratio of 14:1, whereas the following 488 Pista is 9.6:1.
Compression ratio is a fundamental factor in developing power, so in my mind the disadvantage of an off-boost turbo engine compared to a nat-asp one has always been mainly due to this.
However in general I think you're right that there isn't a huge difference between nat-asp and turbo engines of similar sizes at low revs, i.e. below the boost threshold for the turbo motor. The main difference is that the nat-asp engine power increases fairly gradually with increasing revs, whereas the turbo engine has a much more noticeable transition around the boost threshold, and then is more level thereafter.
unless you have a huge cubic capacity, the engine response at low revs will be rather soft, well nothing hard-hitting
Finally, isn't the throttle response during the "turbo lag" phase or before the boost threshold at low revs the same as that in an NA which has little torque at low revs before its boost band? power. if we compare a 355 berlinetta and an f40 we see that there is no torque before 4000rpm in this case turbo or are they the same in terms of throttle response and torque delivered??
at 2000 rpm in the f40 you're not going anywhere and that's true.
but if we look at the curves for a 355 berlinetta with more displacement but a sharp engine you will not advance more at 2000rpm than the f40
at 2000 rpm in the f40 you're not going anywhere and that's true.
but if we look at the curves for a 355 berlinetta with more displacement but a sharp engine you will not advance more at 2000rpm than the f40
These are all somewhat simplistic comments on what is a very complex situation.
The OP is correct in that his large engine may still pull reasonably well - but only in comparison to something considerably smaller. The problem he is likely to be creating is however that his 'big turbo', will give the sort of engine response that got most of the early turbo installations such a bad rep.
If his 'big turbo' comes on boost at around 3500 rpm, the amount of power the engine is producing will drastically change - hence the 'bloom'in heck' moment and potential over steer etc etc. That sort of Jeckell and Hyde behaviour does not make for pleasant motoring. Hence so many of the more mainstream manufacturers trying to have sequential turbocharging, i.e. one small and one larger turbo and a hangover or combination system like the old Toyota Supras, or much smaller turbos which come on boost significantly earlier - but without that thump, like the old Nissan 300zxs.
Of course for mainstream manufacturers, things like fleet economy figures are now major drivers and hence things like turbo lag are much less apparent - the level of engineering in producing what are effectively composite engines, where the turbo is necessary for pretty much all operation apart from idling. This is of course nothing like a bolt on job and requires a hige amount of research and so is simply not possible for individuals - which takes us back to 'bolting on a turbo'.
The OP is correct in that his large engine may still pull reasonably well - but only in comparison to something considerably smaller. The problem he is likely to be creating is however that his 'big turbo', will give the sort of engine response that got most of the early turbo installations such a bad rep.
If his 'big turbo' comes on boost at around 3500 rpm, the amount of power the engine is producing will drastically change - hence the 'bloom'in heck' moment and potential over steer etc etc. That sort of Jeckell and Hyde behaviour does not make for pleasant motoring. Hence so many of the more mainstream manufacturers trying to have sequential turbocharging, i.e. one small and one larger turbo and a hangover or combination system like the old Toyota Supras, or much smaller turbos which come on boost significantly earlier - but without that thump, like the old Nissan 300zxs.
Of course for mainstream manufacturers, things like fleet economy figures are now major drivers and hence things like turbo lag are much less apparent - the level of engineering in producing what are effectively composite engines, where the turbo is necessary for pretty much all operation apart from idling. This is of course nothing like a bolt on job and requires a hige amount of research and so is simply not possible for individuals - which takes us back to 'bolting on a turbo'.
[quote=Pete54]These are all somewhat simplistic comments on what is a very complex situation.
The OP is correct in that his large engine may still pull reasonably well - but only in comparison to something considerably smaller. The problem he is likely to be creating is however that his 'big turbo', will give the sort of engine response that got most of the early turbo installations such a bad rep.
you say compared to a considerably smaller motor but it just depends on the torque delivered and nothing else.
if you start from a 5l engine that you add a turbo that starts say at 3000rpm, before this threshold the engine is not transformed into 1.6l it will have more torque than many 3l engines and more.
I have seen several miata owners add a turbo and found the engine response to be better at low revs since before the boost threshold and despite the fact that the compression ratio is reduced because the turbo provides a flow rate (even very low) at n any diet.
the real big difference between turbos is na of the same displacement, it's the lower compression ratio in the case of old turbos. but if it remains decent, that the car has a long enough intake manifold, there will remain torque linked to the displacement. if we observe curves a 2l na has much less torque than a 3l turbo before the boost. so the case of the lag or nothing happens depends essentially on the torque available instantly at the given speed if you accelerate strongly in a turbocharged car the pressure in the intake manifold will drop to the atmospheric level (0) as quickly as in a Na engine then gradually the turbo will wind up. Na will have a more linear or progressive response than turbo obviously. but when we look at the torque values, we realize that a sharp na (f355, 360 etc) does not have more torque than an old 3.0l turbo let's say off boost at 2000-2500rpm in both cases not much is happening and the regime must be accelerated. yet we are not talking about lag for Na, but in a way you have to get the power
The OP is correct in that his large engine may still pull reasonably well - but only in comparison to something considerably smaller. The problem he is likely to be creating is however that his 'big turbo', will give the sort of engine response that got most of the early turbo installations such a bad rep.
you say compared to a considerably smaller motor but it just depends on the torque delivered and nothing else.
if you start from a 5l engine that you add a turbo that starts say at 3000rpm, before this threshold the engine is not transformed into 1.6l it will have more torque than many 3l engines and more.
I have seen several miata owners add a turbo and found the engine response to be better at low revs since before the boost threshold and despite the fact that the compression ratio is reduced because the turbo provides a flow rate (even very low) at n any diet.
the real big difference between turbos is na of the same displacement, it's the lower compression ratio in the case of old turbos. but if it remains decent, that the car has a long enough intake manifold, there will remain torque linked to the displacement. if we observe curves a 2l na has much less torque than a 3l turbo before the boost. so the case of the lag or nothing happens depends essentially on the torque available instantly at the given speed if you accelerate strongly in a turbocharged car the pressure in the intake manifold will drop to the atmospheric level (0) as quickly as in a Na engine then gradually the turbo will wind up. Na will have a more linear or progressive response than turbo obviously. but when we look at the torque values, we realize that a sharp na (f355, 360 etc) does not have more torque than an old 3.0l turbo let's say off boost at 2000-2500rpm in both cases not much is happening and the regime must be accelerated. yet we are not talking about lag for Na, but in a way you have to get the power
Don't forget that with modern computer-controlled ignition and fuel injection the engine's performance can be optimised at every stage for whatever the manufacturer wants to achieve.
It's a world away from slapping an aftermarket turbo on the side of an old carburettor/distributor engine.
It's a world away from slapping an aftermarket turbo on the side of an old carburettor/distributor engine.
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