Motor Physics Question
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Road Captain
Joined: Jan 2008
Posts: 640
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From: Greensboro, NC
Motor Physics Question
i know this might be out of the realm for most, but i figure i can catch a few insightful responses. when looking at a motor build with the same displacement (107), compression ratio (10.5:1), stroke, and bore, what would be the preference and why for head volume and piston type between these two:
(1)flat top piston with larger head volume (86cc)
(2)pop up piston (11cc) and smaller head volume (75cc)
(1)flat top piston with larger head volume (86cc)
(2)pop up piston (11cc) and smaller head volume (75cc)
Last edited by rock92; Sep 19, 2009 at 12:31 PM.
Well, if the only difference between the 2 builds is the pistons being flat top or domed, then the compression ratios wouldn't be the same. A domed piston would increase your compression ratio given the same head used in both set ups.
Road Warrior
Joined: Nov 2008
Posts: 1,955
Likes: 7
From: The Great Northwest
i know this might be out of the realm for most, but i figure i can catch a few insightful responses. when looking at a motor build with the same displacement (107), compression ratio (10.5:1), stroke, and bore, what would be the preference and why for head volume and piston type between these two:
(1)flat top piston with larger head volume (86cc)
(2)pop up piston (11cc) and smaller head volume (75cc)
(1)flat top piston with larger head volume (86cc)
(2)pop up piston (11cc) and smaller head volume (75cc)
Road Captain
Joined: Mar 2006
Posts: 683
Likes: 3
From: Tallahassee, FL
OP stated that the heads were different, keeping the compression ratio the same. Question was, given this, which is preferable. I would think the flat tops would be preferable due to the torque generated, at least for low end. You need torque to get hp. The domed piston setup probably is better at higher rpms. Just my humble opinion.
Ultimate HDF Member
Joined: Feb 2006
Posts: 5,939
Likes: 3
From: TX
Your example does not create "like" conditions. I ASSume you've been you've been looking at someones online compression calculator, but it appears you have listed the pistons with opposite chamber size to create similar resultant compression ratio.
"Generally speaking", for what we are doing, it really does not make a huge difference on which way ya get the the desired end.
**However**, some piston dome configurations compliment chamber design. Is it a measurable or intelligible difference? For street motors... not really.
There is also a caveat to domed pistons in certain configs too. Depending on chamber size & shape, valve size, dome size, and cam profile (not just lift, but timing as well), one can experience piston to valve interference "challenges".
Which is "better"?
Simple!
It depends....
Talk to the folks "spec'ing" your engine package. If they are competent, they will have the experience to make the proper recommendation for you.
Clear as mud?
"Generally speaking", for what we are doing, it really does not make a huge difference on which way ya get the the desired end.
**However**, some piston dome configurations compliment chamber design. Is it a measurable or intelligible difference? For street motors... not really.
There is also a caveat to domed pistons in certain configs too. Depending on chamber size & shape, valve size, dome size, and cam profile (not just lift, but timing as well), one can experience piston to valve interference "challenges".
Which is "better"?
Simple!
It depends....
Talk to the folks "spec'ing" your engine package. If they are competent, they will have the experience to make the proper recommendation for you.
Clear as mud?
Last edited by PhilM; Sep 19, 2009 at 02:06 PM.
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Road Captain
Joined: Jan 2008
Posts: 640
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From: Greensboro, NC
for those providing the light hearted responses, thanks for the entertainment value ;-)
philm: thanks for the well thought out response. most of what you described relates to 'mechanical fit' of the components and you bring up some good points. you also make reference to the performance differences and that was more of what i was looking for. i was just wondering along the lines of ignition and flame propagation as to whether there was a difference.
i have been conversing with tman and that is where this stems from. he offers two different configurations for the 107 and its a package deal, so the mechanical fit issues you addressed should be managed. no science projects, just two different ways to get there. fyi, he prefers the domed option because it requires less to be taken out of the chamber area on the heads.
philm: thanks for the well thought out response. most of what you described relates to 'mechanical fit' of the components and you bring up some good points. you also make reference to the performance differences and that was more of what i was looking for. i was just wondering along the lines of ignition and flame propagation as to whether there was a difference.
i have been conversing with tman and that is where this stems from. he offers two different configurations for the 107 and its a package deal, so the mechanical fit issues you addressed should be managed. no science projects, just two different ways to get there. fyi, he prefers the domed option because it requires less to be taken out of the chamber area on the heads.
Ultimate HDF Member
Joined: Feb 2006
Posts: 5,939
Likes: 3
From: TX
TR's a sharp cat.
If you are considering one of his packages, listen to what he tells ya.
One thing about his 98 & 107 packages.... He really likes the RevPerf cylinders. Been there done that... stick with the bored stockers!
Gonna get ya Stage 3 Elite HO?
If you are considering one of his packages, listen to what he tells ya.
One thing about his 98 & 107 packages.... He really likes the RevPerf cylinders. Been there done that... stick with the bored stockers!
Gonna get ya Stage 3 Elite HO?
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Road Warrior
Joined: Nov 2008
Posts: 1,296
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From: Cheltenham, UK
This question certainly brings back fond memories of a 4th year thermo lecture with a guest prof. As "wannabe" performance engineers we all attended the lecture and were promptly lost in about 5 minutes.
We were fortunate enough to cone away with some basic summary point, namely:
stroke vs bore affects torque and top end performance
top end chamber geometry (including number of valves, valve location, plug position, piston shape and TDC clearance) was all about rate and even spread of flame propogation and the ability to expel exhaust gas.
That was enough to make me realise that I didn't have a clue and this is best left to the experts.
We were fortunate enough to cone away with some basic summary point, namely:
stroke vs bore affects torque and top end performance
top end chamber geometry (including number of valves, valve location, plug position, piston shape and TDC clearance) was all about rate and even spread of flame propogation and the ability to expel exhaust gas.
That was enough to make me realise that I didn't have a clue and this is best left to the experts.
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Extreme HDF Member
Joined: Apr 2006
Posts: 10,449
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From: Roswell, GA
This question certainly brings back fond memories of a 4th year thermo lecture with a guest prof. As "wannabe" performance engineers we all attended the lecture and were promptly lost in about 5 minutes.
We were fortunate enough to cone away with some basic summary point, namely:
stroke vs bore affects torque and top end performance
top end chamber geometry (including number of valves, valve location, plug position, piston shape and TDC clearance) was all about rate and even spread of flame propogation and the ability to expel exhaust gas.
That was enough to make me realise that I didn't have a clue and this is best left to the experts.
We were fortunate enough to cone away with some basic summary point, namely:
stroke vs bore affects torque and top end performance
top end chamber geometry (including number of valves, valve location, plug position, piston shape and TDC clearance) was all about rate and even spread of flame propogation and the ability to expel exhaust gas.
That was enough to make me realise that I didn't have a clue and this is best left to the experts.
Go with the builder's recommendation for what you want to accomplish from the build and possible future modifications.
Thread Starter
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Road Captain
Joined: Jan 2008
Posts: 640
Likes: 23
From: Greensboro, NC
TR's a sharp cat.
If you are considering one of his packages, listen to what he tells ya.
One thing about his 98 & 107 packages.... He really likes the RevPerf cylinders. Been there done that... stick with the bored stockers!
i know..having a difficult time digesting both sides of this equation; revperf and its oddities of the all aluminum and whether the new cleaning and rings are the solution or the exceptional wall 'thin-ness' with the bored out cylinders..im still out to lunch on this one
Gonna get ya Stage 3 Elite HO?
If you are considering one of his packages, listen to what he tells ya.
One thing about his 98 & 107 packages.... He really likes the RevPerf cylinders. Been there done that... stick with the bored stockers!
i know..having a difficult time digesting both sides of this equation; revperf and its oddities of the all aluminum and whether the new cleaning and rings are the solution or the exceptional wall 'thin-ness' with the bored out cylinders..im still out to lunch on this one
Gonna get ya Stage 3 Elite HO?