Thinking about engine limits and swaps.
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From: Orlando
Thinking about engine limits and swaps.
In the other thread when mentioning 420 lb-ft torque from a turbo 3.0 engine was talking about the VM turbo diesel. The point was really that to get that kind of number from a petroleum based engine requires direct injection since the required chamber pressure exceeds detonation limits for natural aspiration (NA).
With direct injection you can talk about compression ratios above 12:1 but as much of the power from a boosted engine comes from the increase combustion chamber volume resulting from lowered compression as the boost itself. This is even more important with a gasoline engine.
Trying to avoid academia but there is one set of conditions that we strive to achieve and that is the maximum chamber filling volume with ignition causing maximum combustion pressure happening at a specific number of degrees past TDC. ANYTHING else is a kludge to achieve this.
Now despite being called explosion machines, compared to the engine speed, how fast the fuel burns is a fixed speed (FPR-flame propagation rate) for a particular fuel (octane is one measurement) in a specific mixture of an oxidizer (air) and a specific pressure.
A SWAG for gasoline at STP (standard temperature and pressure) figure 170-200 ft/sec. At cruise the pressure is lower and the FPR decreases so you need more advance. In the bad old daze this was the responsibility of the vacuum advance.
Now to this kludge you have to account for rpm. If it takes a fixed number of milliseconds to get near full combustion and you want that to occur at 7 degrees ATDC then for 3,000 rpm you need 38 degrees of advance for 5000 you need more, and 2000 less.
Add in the fact that while 91 PON has the same btu/lb (power) as 87 but burns slower and is harder to ignite so needs more advance and you get some of the idea of the complexity.
Bottom line: it ain't easy but is more complex than difficult. That said if I were to add boost , the first thing to do is to lower the compression.
Second, without direct injection you are going to be limited to about 260-270 lb-ft of torque and you can get that from a NA Pentastar 3.6.
So for about the same time, effort, and cost (not going to be cheap, figure $5k-$7k) as a turbo on a XF 3.2 , I'd just drop in a 3.6 Pentastar
and get as much power on 87 PON and can get parts at any FLAPS.
For me the choice is easy but for now am happy to stay stock and reserve first for malls and parades. Now if one with a blown engine and otherwise good should appear...
I am a fan of diesel (my RV has a TD) but that is too complicated a swap particularly if you need to add Urea. Diesel also costs about 20% more than 87 PON around here.
For me I believe in N/A gas engines for the street. I also believe in engines that run well on 87 PON. Without direct injection this limits the effective compression ratio to about 10.5:1 and torque to about 260 lb-ft.
DOHC with VVT i&e adds a very broad torque range almost flat from 2000-6000 rpm. 4 valve allows a center spark plug essential to conventional high compression without detonation. From a flame front propagation standpoint, the three valve two plug design is not optimal.
(this is why the SBC went from a straight plug design - and indexing was popular - to the angled plug heads - and that is in the dim past).
90 degree V6s are popular for just one reason: cost. V6 and V8 engines can use the same production line and a 60 degree V8 makes no sense (a flat 8 now...).
Finally if I lived in the EU, a later Merc engine swap would make sense. Living in the US, a Chrysler engine based on a Mercedes design makes more.
With direct injection you can talk about compression ratios above 12:1 but as much of the power from a boosted engine comes from the increase combustion chamber volume resulting from lowered compression as the boost itself. This is even more important with a gasoline engine.
Trying to avoid academia but there is one set of conditions that we strive to achieve and that is the maximum chamber filling volume with ignition causing maximum combustion pressure happening at a specific number of degrees past TDC. ANYTHING else is a kludge to achieve this.
Now despite being called explosion machines, compared to the engine speed, how fast the fuel burns is a fixed speed (FPR-flame propagation rate) for a particular fuel (octane is one measurement) in a specific mixture of an oxidizer (air) and a specific pressure.
A SWAG for gasoline at STP (standard temperature and pressure) figure 170-200 ft/sec. At cruise the pressure is lower and the FPR decreases so you need more advance. In the bad old daze this was the responsibility of the vacuum advance.
Now to this kludge you have to account for rpm. If it takes a fixed number of milliseconds to get near full combustion and you want that to occur at 7 degrees ATDC then for 3,000 rpm you need 38 degrees of advance for 5000 you need more, and 2000 less.
Add in the fact that while 91 PON has the same btu/lb (power) as 87 but burns slower and is harder to ignite so needs more advance and you get some of the idea of the complexity.
Bottom line: it ain't easy but is more complex than difficult. That said if I were to add boost , the first thing to do is to lower the compression.
Second, without direct injection you are going to be limited to about 260-270 lb-ft of torque and you can get that from a NA Pentastar 3.6.
So for about the same time, effort, and cost (not going to be cheap, figure $5k-$7k) as a turbo on a XF 3.2 , I'd just drop in a 3.6 Pentastar
and get as much power on 87 PON and can get parts at any FLAPS.
For me the choice is easy but for now am happy to stay stock and reserve first for malls and parades. Now if one with a blown engine and otherwise good should appear...
I am a fan of diesel (my RV has a TD) but that is too complicated a swap particularly if you need to add Urea. Diesel also costs about 20% more than 87 PON around here.
For me I believe in N/A gas engines for the street. I also believe in engines that run well on 87 PON. Without direct injection this limits the effective compression ratio to about 10.5:1 and torque to about 260 lb-ft.
DOHC with VVT i&e adds a very broad torque range almost flat from 2000-6000 rpm. 4 valve allows a center spark plug essential to conventional high compression without detonation. From a flame front propagation standpoint, the three valve two plug design is not optimal.
(this is why the SBC went from a straight plug design - and indexing was popular - to the angled plug heads - and that is in the dim past).
90 degree V6s are popular for just one reason: cost. V6 and V8 engines can use the same production line and a 60 degree V8 makes no sense (a flat 8 now...).
Finally if I lived in the EU, a later Merc engine swap would make sense. Living in the US, a Chrysler engine based on a Mercedes design makes more.
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