VVT effect on DCR. (question)
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From: PA/MD
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From: Elswhere
I've thought about a 2/4 stroke switchable engine, but the valvetrain seems like it'd be the weak link. If the valves can't operate at more than 8000 engine rpm in a 4 stroke, they won't be able to operate at more than 4000rpm in 2 stroke mode, since they'll effectively be opening twice as many times per rpm. It could increase low end power (manuals would reap the most benefits from this) but max rpm would be valvetrain limited. This made me think of the sleeve/rotary valves. Properly utilized, you could spin the engine much higher in 2 stroke mode than a conventional poppet valve engine in 4 stroke mode. To be honest, the only reasons I can think of that major auto manufacturers haven't adopted a rotary valve system are:
1) cost of tooling
2) low turbulence from straight shot into the cylinder (surely there's a way around this one).
3) oiling/sealing. I believe someone solved these problems, patented the design, and then never spoke of it again.
4) It's not deemed necessary; they can meet fuel economy standards, reliability standards, and make enough power to get their customers killed just fine with a plain old 4 stroke poppet valve engine.
I'm really not sure why it hasn't seen any aftermarket fitment. It'd be the perfect solution for a high power, high rpm, low displacement engine made to fit into certain displacement based classes, or just a good way to increase power substantially with no drawbacks. (also, ptv clearance wouldn't exist anymore)
1) cost of tooling
2) low turbulence from straight shot into the cylinder (surely there's a way around this one).
3) oiling/sealing. I believe someone solved these problems, patented the design, and then never spoke of it again.
4) It's not deemed necessary; they can meet fuel economy standards, reliability standards, and make enough power to get their customers killed just fine with a plain old 4 stroke poppet valve engine.
I'm really not sure why it hasn't seen any aftermarket fitment. It'd be the perfect solution for a high power, high rpm, low displacement engine made to fit into certain displacement based classes, or just a good way to increase power substantially with no drawbacks. (also, ptv clearance wouldn't exist anymore)
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The only valve event that affects dynamic compression is IVC... so if you change IVC you change dynamic compression. Advancing or retarding the cam will do this (a la various VVT systems). Since IVC is the most important many engines do not incorporate variable exhaust timing. Gains are a lot smaller for the increased complexity. Or you have a cam in block with cam phasing which technically varies intake and exhaust timing since they are all on one cam, but does not vary overlap.
When you ask if you can design a cam that covers "various dynamic compressions" what are you getting at? If you take an LS1 2 or 3 and put a cam in it, then that answer is "sort of." Depending on how it is installed (ICL) will determine the dynamic compression, so advancing or retarding it during install will change it but then it will be set.
If you have VVT then of course it will vary. The issue for something like an L99 is that many people want to run high lift cams and that gives you PTV if you keep the stock range of advance and retard variation. That is of course what will limit how much of a change you can get in DCR with a cam of a given lift (keeping the rest of the engine constant). I do not know what the DCR range for a stock L99 is, but since it will change with a cam or a new bottom end I am not sure it will be all that helpful anyway. This is of course worth thinking about if you were to actually build one.
I have a feeling you know a bit of this already. Part of the problem with this thread is that there are too many variables and the questions are a bit vague.
When you ask if you can design a cam that covers "various dynamic compressions" what are you getting at? If you take an LS1 2 or 3 and put a cam in it, then that answer is "sort of." Depending on how it is installed (ICL) will determine the dynamic compression, so advancing or retarding it during install will change it but then it will be set.
If you have VVT then of course it will vary. The issue for something like an L99 is that many people want to run high lift cams and that gives you PTV if you keep the stock range of advance and retard variation. That is of course what will limit how much of a change you can get in DCR with a cam of a given lift (keeping the rest of the engine constant). I do not know what the DCR range for a stock L99 is, but since it will change with a cam or a new bottom end I am not sure it will be all that helpful anyway. This is of course worth thinking about if you were to actually build one.
I have a feeling you know a bit of this already. Part of the problem with this thread is that there are too many variables and the questions are a bit vague.
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From: Elswhere
Say a random cam, at full advance, makes 8.22 DCR; at full retard, it's down to 8.06. I want to know if you can spec a cam with a similar power curve to the original, but have it be at 8.47 at full advance, and 7.91 at full retard. The intended purpose for something like this is to increase throttle response of a FI engine out of boost (while still being able to run a significant amount of boost).
Last edited by Mr. Sir; Mar 4, 2012 at 06:59 PM.
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In that case id be looking at building the engine with a higher dcr, and spec the compressor to have a certian amount of lag in it......this was the thought I had with my setup if/when I go twin turbo...I'm running 12.5:1 scr, 8.22:1 dcr and would keep my same compression and cam (255/263 .624/.624 115lsa) and spec out the turbos to start building boost around 4,500rpms and reach full boost between 7,500, then hit fuel/spark cut just after 8,000 rpm.
Once you are running just over 4,000 rpms the engine is basically spinning faster than the time it takes for detonation to occur, so you can run higher than normal dc than you can at/off idle
Once you are running just over 4,000 rpms the engine is basically spinning faster than the time it takes for detonation to occur, so you can run higher than normal dc than you can at/off idle
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From: Elswhere
...cool. I've always wondered why FI engines don't run into knock in situations that a NA engine would.
It'd be ideal to have an 8.7 dcr out of boost and run 15-20 psi at full retard, so the original question remains. (also, if it lacked boost until 4000 rpm, it'd be little fun on the street. Which is where it will be most of the time.)
It'd be ideal to have an 8.7 dcr out of boost and run 15-20 psi at full retard, so the original question remains. (also, if it lacked boost until 4000 rpm, it'd be little fun on the street. Which is where it will be most of the time.)
Last edited by Mr. Sir; Mar 4, 2012 at 07:03 PM.
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From: Elswhere
Bump... and does anyone have some links/explanations for valve events and how they affect power/dcr? Since the collective intellect of ls1tech is obviously no match for this question, I'll just learn how to answer it myself. Hopefully.
