T
Tom Sherman
Guest
"jim beam" wrote:
> Tom Sherman wrote:
>> [email protected] aka Jobst Brandt wrote:
>>> Tom Sherman writes:
>>>
>>>>>>> ... A problem with high power engine tuning is that torque may
>>>>>>> fall off more quickly to either side of the peak. This requires
>>>>>>> more gears to keep the engine in its power range, power being
>>>>>>> torque times RPM. So more gears may be an indicator that the
>>>>>>> engine has a narrow power band rather than a desirable feature,
>>>>>>> other things being equal.
>>>
>>>>>> That is why Otto cycle engines should have variable valve timing
>>>>>> and lift.
>>>
>>>>> That sounds good, but you cant get there from here. Highly tuned
>>>>> engines get higher peak performance but the eds of the power band
>>>>> can't be made to rise equally, so the torque curve has a narrower
>>>>> or more arched form. Variable valve timing may get rid of overlap
>>>>> at the end of the exhaust stroke and beginning of the intake
>>>>> stroke, but that doesn't do much good for broadening the torque
>>>>> curve. There isn't much power there to be gained...
>>>
>>>> Better tell that to Honda Motor Company, Limited!
>>>
>>>> Having driven Honda's with variable valve timing and lift for the
>>>> last 14 years, I can verify that in real life the engines behave
>>>> like a normal engine tuned for midrange power, at less than 5000
>>>> RPM. This includes excellent fuel economy for the size and weight
>>>> of the car. However, the difference is that instead of the high end
>>>> power dropping off, the engines pull strongly all the way to the
>>>> fuel cutoff at 7000+ RPM.
>>>
>>>> The engines are also quite durable. My 1994 Si never used any
>>>> measurable amount of oil, and had excellent compression at 160,000+
>>>> miles, despite being driven almost abusively hard on a regular
>>>> basis. Nothing was ever done to the engine for maintenance, beyond
>>>> oil/filter changes, scheduled valve lash adjustments and scheduled
>>>> timing belt and spark plug replacements.
>>>
>>> I don't know to what car you are comparing the performance here, but
>>> in the days of old, auto magazines used to publish power curves by
>>> which one could compare two engines of the same displacement. For
>>> instance, my 1992 station wagon corners and brakes better than the
>>> sports car I drove a few years back. I believe your endorsement of
>>> the car but it doesn't resolve the problem of what did what. With a
>>> power curve of the engine next to one of a competitor would be best.
>>>
>> I have driven the VTEC (variable valve timing and valve lift
>> electronic control) Honda's back to back with standard engine
>> versions, and at lower RPM operation, the engines behave in a very
>> similar manner. However, the boost in high end power is very
>> noticeable, such as passing in 3rd gear at highway speeds.
>>
>> Unfortunately, the costs of the engines (standard and VTEC) are hard
>> to compare, since getting the VTEC engine means also getting a higher
>> "trim" level.
>>
>
> i have direct comparison of cam profile effects. two hondas, identical
> motors, one with the stock soh ["low rev"] cam, the other with a soh zc
> ["racing"] cam.
>
> the stock cam is the better at low rpm's and best suited to normal
> driving. by about 5krpm, it's ok, but it's starting to run out of puff.
> with the zc cam otoh, below about 4.5krpm, the thing works ok, but it's
> no great shakes and certainly has a hard time keeping up with the stock
> cam motor. above about 5k however, it really lights up and leaves the
> other motor standing. the difference is dramatic.
>
> your later model motor, with the vtec head, marries these two profiles
> on the same cam and switches between them as required - the benefits of
> the low rev cam and the high rev cam. its a great [relatively low cost]
> solution to an otherwise insoluble problem.
>
In the 1.6L VTEC engine used in the 1992-95 Si/EX, the change in cam
profiles at about 4800 RPM was quite noticeable as a "bump" in power,
while the 1.7L VTEC in the 2002-05 EX has a seamless transition - except
for the wider than normal power band, one would never realize the VTEC
was there.
--
Tom Sherman - Holstein-Friesland Bovinia
"And never forget, life ultimately makes failures of all people."
- A. Derleth
> Tom Sherman wrote:
>> [email protected] aka Jobst Brandt wrote:
>>> Tom Sherman writes:
>>>
>>>>>>> ... A problem with high power engine tuning is that torque may
>>>>>>> fall off more quickly to either side of the peak. This requires
>>>>>>> more gears to keep the engine in its power range, power being
>>>>>>> torque times RPM. So more gears may be an indicator that the
>>>>>>> engine has a narrow power band rather than a desirable feature,
>>>>>>> other things being equal.
>>>
>>>>>> That is why Otto cycle engines should have variable valve timing
>>>>>> and lift.
>>>
>>>>> That sounds good, but you cant get there from here. Highly tuned
>>>>> engines get higher peak performance but the eds of the power band
>>>>> can't be made to rise equally, so the torque curve has a narrower
>>>>> or more arched form. Variable valve timing may get rid of overlap
>>>>> at the end of the exhaust stroke and beginning of the intake
>>>>> stroke, but that doesn't do much good for broadening the torque
>>>>> curve. There isn't much power there to be gained...
>>>
>>>> Better tell that to Honda Motor Company, Limited!
>>>
>>>> Having driven Honda's with variable valve timing and lift for the
>>>> last 14 years, I can verify that in real life the engines behave
>>>> like a normal engine tuned for midrange power, at less than 5000
>>>> RPM. This includes excellent fuel economy for the size and weight
>>>> of the car. However, the difference is that instead of the high end
>>>> power dropping off, the engines pull strongly all the way to the
>>>> fuel cutoff at 7000+ RPM.
>>>
>>>> The engines are also quite durable. My 1994 Si never used any
>>>> measurable amount of oil, and had excellent compression at 160,000+
>>>> miles, despite being driven almost abusively hard on a regular
>>>> basis. Nothing was ever done to the engine for maintenance, beyond
>>>> oil/filter changes, scheduled valve lash adjustments and scheduled
>>>> timing belt and spark plug replacements.
>>>
>>> I don't know to what car you are comparing the performance here, but
>>> in the days of old, auto magazines used to publish power curves by
>>> which one could compare two engines of the same displacement. For
>>> instance, my 1992 station wagon corners and brakes better than the
>>> sports car I drove a few years back. I believe your endorsement of
>>> the car but it doesn't resolve the problem of what did what. With a
>>> power curve of the engine next to one of a competitor would be best.
>>>
>> I have driven the VTEC (variable valve timing and valve lift
>> electronic control) Honda's back to back with standard engine
>> versions, and at lower RPM operation, the engines behave in a very
>> similar manner. However, the boost in high end power is very
>> noticeable, such as passing in 3rd gear at highway speeds.
>>
>> Unfortunately, the costs of the engines (standard and VTEC) are hard
>> to compare, since getting the VTEC engine means also getting a higher
>> "trim" level.
>>
>
> i have direct comparison of cam profile effects. two hondas, identical
> motors, one with the stock soh ["low rev"] cam, the other with a soh zc
> ["racing"] cam.
>
> the stock cam is the better at low rpm's and best suited to normal
> driving. by about 5krpm, it's ok, but it's starting to run out of puff.
> with the zc cam otoh, below about 4.5krpm, the thing works ok, but it's
> no great shakes and certainly has a hard time keeping up with the stock
> cam motor. above about 5k however, it really lights up and leaves the
> other motor standing. the difference is dramatic.
>
> your later model motor, with the vtec head, marries these two profiles
> on the same cam and switches between them as required - the benefits of
> the low rev cam and the high rev cam. its a great [relatively low cost]
> solution to an otherwise insoluble problem.
>
In the 1.6L VTEC engine used in the 1992-95 Si/EX, the change in cam
profiles at about 4800 RPM was quite noticeable as a "bump" in power,
while the 1.7L VTEC in the 2002-05 EX has a seamless transition - except
for the wider than normal power band, one would never realize the VTEC
was there.
--
Tom Sherman - Holstein-Friesland Bovinia
"And never forget, life ultimately makes failures of all people."
- A. Derleth
