general engine question

[ncage]

Hi everyone i have a general engine question here. Say you have to engines. One being smaller and the other being larger but they have the same hp/torque specs. Is is there any advantage to one over another? For example:
I test drove a mazda speed3 today. Its a little turbo 4cly. It has almost identical HP/Torque specs to the 3.5L 6cyl engine in the Altima. BTW, i LOVED the speed 3. It was very fast. Anyways, i digress. With my example someone might say turbo lag. Well this is the first turbo car i have ever drove and while i did notice a turbo lag it definitly wasn't a big deal for me. Turbo lag just isn't that bad. You might think gas mileage? Well the EPA difference on these cars is 1MPG difference. So that definitely isn't a difference. You might think the 4cyl would take premium and the 6cyl wouldn't but they both take premium. Anyone have any pros/cons of each? The same situation happens with 6cyl & 8cyl sometimes and we are not even dealing with the "turbo" factor.

Ncage

 

[sniperruff]

you'll have to look at what rpm those engines generate their peak power. having a turbo means you have more parts in your car that can fall apart. i'd take the VQ any day.

 

[996GT2]

In many cases, when a small-displacement engine makes the same power/torque of a larger engine, forced induction in the form of supercharging or turbocharging is used to boost the amount of air/fuel that enters the combustion chambers. Turbocharging can give an engine a very non-linear power curve if there is a single big turbo used, so there will be a rush of power at around 3000-4000 RPM when the turbo kicks on. If you keep the engine under that speed, the car can feel relatively unresponsive and slow because the turbo (s) are not making enough boost to give the engine a real performance increase from a non-turbo engine of the same displacement. In fact, a turbocharged engine at low RPMs is likely making LESS power than a similar non-turbo engine because the compression ratio in forced induction engines is often lower. Some cars have gotten around this "lag" problem by using a sequential turbo setup (FD RX-7) or by using two small turbos (Skyline GTR). However, the extra piping and parts needed to implement these setups greatly add to the complexity of the engine and can reduce reliability and increase maintenance and repair costs. Porsche has tried to solve this problem with Variable Turbine Geometry in their latest turbo 911s (in effect giving one turbo the ability to act like several of different sizes), but this technology is again very complex and can make a car less reliable.

Superchargers do not have the same problem with power delivery since they have zero lag by design, but they do rob some power from the car when beginning to accelerate. This is because some engine HP must be used to drive the supercharger pulleys (the Ford GT, for example, is estimated to lose about 100HP at launch due to parasitic losses from the supercharger). In addition, forced induction engines MUST use premium fuel in almost every case to prevent knocking. They also can be more expensive to maintain than an N/A car to to the complexity of a forced induction setup in a modern car. Also, highly tuned turbo engines often need a turbo timer to idle the engine for a few minutes after the driver leaves the car to prevent overheating problems.

If both the small engine and large engine are normally aspirated, then the smaller engine is likely tuned to make power at a higher RPM. As a result, it's low-end torque will suffer. Compare the engines in the BMW M5 (E39) and Ferrari 360. The BMW's big 4.9 liter V8 made 394 bhp @ 6600 rpm and 369 lb-ft of torque @ 3800 rpm. The Ferrari's engine, on the other hand, was a relatively small 3.6 liter V8 that made 395 bhp @ 8500 rpm but only 275 lb-ft of torque @ 4700 rpm. This means that the Ferrari's engine won't come alive until very high RPMs, and it will likely require more shifting due to a lack of low-end torque. A similar case arises in many performance-tuned Honda engines such as the F20C (found in the early model S2000). This engine made an astounding 240HP from only 2 liters, but it made that power at an astronomical 8300RPM (redline was 8900) and only produced 153 lb-ft of torque, this time at a very high 7500RPM. As a result, many testers said the car felt "uninspiring" during everyday drives where revs did not go above the 6000 RPM VTEC engagement. This was also partially the reason Honda decided to replace the F20C in the S2000 with the 2.2 liter F22C for the 2004 model year.

EDIT: Added some more stuff

 

[ncage]

Originally posted by: 996GT2
In many cases, when a small-displacement engine makes the same power/torque of a larger engine, forced induction in the form of supercharging or turbocharging is used to boost the amount of air/fuel that enters the combustion chambers. Turbocharging can give an engine a very non-linear power curve if there is a single big turbo used, so there will be a rush of power at around 3000-4000 RPM when the turbo kicks on. If you keep the engine under that speed, the car can feel relatively unresponsive and slow. Superchargers do not have the same problem with power delivery since they have zero lag by design, but they do rob some power from the car when beginning to accelerate. This is because some engine HP must be used to drive the supercharger pulleys (the Ford GT, for example, is estimated to lose about 100HP at launch due to parasitic losses from the supercharger). In addition, forced induction engines MUST use premium fuel in almost every case to prevent knocking. They also can be more expensive to maintain than an N/A car.

If both the small engine and large engine are normally aspirated, then the smaller engine is likely tuned to make power at a higher RPM. As a result, it's low-end torque will suffer. Compare the engines in the BMW M5 (E39) and Ferrari 360. The BMW's big 4.9 liter V8 made 394 bhp @ 6600 rpm and 369 lb-ft of torque @ 3800 rpm. The Ferrari's engine, on the other hand, was a relatively small 3.6 liter V8 that made 395 bhp @ 8500 rpm but only 275 lb-ft of torque @ 4700 rpm. This means that the Ferrari's engine won't come alive until very high RPMs, and it will likely require more shifting due to a lack of low-end torque.

Wow Great Reply. Thanks. I guess you seen this a lot with older generation engines and newer ones. For example. I currently have a 2003 F150 with 5.4L. It gets about 265HP but but here is the big BUT. It puts out 350 lb of torque. This is how i used to think of the small/big engine topic. The bigger engine would always just produce more torque but of course this isn't always the case. Just as a side not the year i got my ford the dodge blew the ford away when you looked at just HP numbers but my ford had close the torque of the dodge. Anyway the nissan engine (3.5) has hear the HP of my truck engine but not near the amount of torque. I kind if felt this dilema when i test drove a Civic SI. Eventhough the civic had 200HP the torque sucked. I think 140 lb or something like. At 65-70 you really had to get the RPM high to have a lot of passing speed. I am not saying the Civic SI sucked. It handled great but i think the engine could be improved a lot.

 

[ncage]

Originally posted by: sniperruff
you'll have to look at what rpm those engines generate their peak power. having a turbo means you have more parts in your car that can fall apart. i'd take the VQ any day.

Ya i was looking at nissans but the QA problems at nissan have me really worried. Look at JD power. Initial quality ratings are just above VW (which is one of the worst).

Read here:
http://www.businessweek.com/ma...ent/04_20/b3883053.htm

ncage