Leon Posted October 22, 2012 Share Posted October 22, 2012 "Every S30 cam will have some overlap." All I can say at this point is: "Muahahahahahahahahahaha!" Some more than others. Quote Link to comment Share on other sites More sharing options...
Xnke Posted October 23, 2012 Share Posted October 23, 2012 Yeah, do the math on piston speed in an L28...you'll find most of the recommended max pistons speeds for cast pistons fall into line about 7500RPM. Will good cast pistons take more? Probably. Will I risk it on my engine? Nah, 7200RPM gives me plenty of go for the street. Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 23, 2012 Share Posted October 23, 2012 Xnke, from what I'm guess you're looking at figures pointing to around 3800 fps (feet per second). From most of the people I've talked to, and literature I've seen on the subject that's about the figure you'll find for OEM cast pistons with factory cranks. The OP in this thread is having his crank dynamically balanced, hopefully by someone who knows what they're doing. And hopefully he's smart enough to be buying NEW cast pistons, and hopefully hyper cast types at that. I've met and seen plenty of people with experience building bottom ends suggest that the limit for a setup like this should be more around 4500/4600 fps and be "just fine". Remember, main issue with a stock L28 revving high is due to crank balance issues, not cast pistons deciding to just fly apart. My only point is that even cast pistons (done right) can be WELL beyond the RPM territory most of us will make power. Plus the OP is also building a 2.4, which should net him a good 500-600 RPM at the top of the RPM range to meet the same mean piston speed, plus he'll have longer rods keeping acceleration and side load forces on the piston lower. Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 23, 2012 Share Posted October 23, 2012 Wow, whats the piston speed on an L20A with L20B rods running 11,500? Or an L28 running 9,800? Quote Link to comment Share on other sites More sharing options...
franky240z Posted October 23, 2012 Author Share Posted October 23, 2012 Xnke, from what I'm guess you're looking at figures pointing to around 3800 fps (feet per second). From most of the people I've talked to, and literature I've seen on the subject that's about the figure you'll find for OEM cast pistons with factory cranks. The OP in this thread is having his crank dynamically balanced, hopefully by someone who knows what they're doing. And hopefully he's smart enough to be buying NEW cast pistons, and hopefully hyper cast types at that. I've met and seen plenty of people with experience building bottom ends suggest that the limit for a setup like this should be more around 4500/4600 fps and be "just fine". Remember, main issue with a stock L28 revving high is due to crank balance issues, not cast pistons deciding to just fly apart. My only point is that even cast pistons (done right) can be WELL beyond the RPM territory most of us will make power. Plus the OP is also building a 2.4, which should net him a good 500-600 RPM at the top of the RPM range to meet the same mean piston speed, plus he'll have longer rods keeping acceleration and side load forces on the piston lower. Well gents, after lots of research and my 'total goal' considerations i'm going for a set of +1mm cast pistons. The whole rotating assembly is going to be dynamically balanced, the second 290F type cam, 40mm carbs and a long manifold nimso type exhaust. Having thought about it and looked at the costs/done so so much research it seems cast will more than do for the intended road and occasional track use with a limit of 7200rpm that it will see once in a while. The whole engine will be new components and everything will be matched as far as possible. If it does go bang(hopefully not!). looking at lots of dyno runs it seems that on similar modded L28's 40mm carbs will still make 220+bhp with them flattening out at the very top end(7300rpm where I guess the size restriction affects ultimate bhp). They should suit my old school modded 2.4 fine, if they do cost me a few bhp i'll get down the gym to shift a few Ib's to improve my own power to weight Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 23, 2012 Share Posted October 23, 2012 Wow, whats the piston speed on an L20A with L20B rods running 11,500? Or an L28 running 9,800? Answer: Very much A lot higher than we're talking, that's for sure!!! Only a mere 5100 fps. Quote Link to comment Share on other sites More sharing options...
JCan Posted January 9, 2013 Share Posted January 9, 2013 Wiki D - Are you going to make me say it.... (sweating...). Ok. I looked up my cam angles and found to my surprise...on my mild + cam the intake starts opening a full 62 degrees prior to the exhaust valve closing. Madness!! I guess that the piston only moves +/- 14% of its vertical distance (cosine of the angle of -31deg to 31 deg) as the crankshaft moves from the exhaust stroke to the intake stroke. I am also guessing that momentum is the primary force at work during this time at higher RPMs. This fogs my perception of the difference between a medium cam and a mild cam. As I look at progressively more agressive cams I notice that the duration increases only by 8-10 degrees with each step and now I believe the only reason for the additional timing (overlap) is to accomodate for larger and larger valve lift as cams get more agressive. I guess one must close a valve and there is probably a maximum valve closing rate (thus causing the valve duration to increase) My perception of reality has been shattered... I'll never be the same! Quote Link to comment Share on other sites More sharing options...
Leon Posted January 9, 2013 Share Posted January 9, 2013 Every S30 cam will have some overlap. Wiki D - Are you going to make me say it.... (sweating...). Ok. I looked up my cam angles and found to my surprise...on my mild + cam the intake starts opening a full 62 degrees prior to the exhaust valve closing. Madness!! I guess that the piston only moves +/- 14% of its vertical distance (cosine of the angle of -31deg to 31 deg) as the crankshaft moves from the exhaust stroke to the intake stroke. I am also guessing that momentum is the primary force at work during this time at higher RPMs. This fogs my perception of the difference between a medium cam and a mild cam. As I look at progressively more agressive cams I notice that the duration increases only by 8-10 degrees with each step and now I believe the only reason for the additional timing (overlap) is to accomodate for larger and larger valve lift as cams get more agressive. I guess one must close a valve and there is probably a maximum valve closing rate (thus causing the valve duration to increase) My perception of reality has been shattered... I'll never be the same! I think I originally brought it up (I probably should have said "almost any cam for a naturally aspirated automotive engine", really), but nevertheless, you are correct about momentum being the driver of valve overlap. The larger overlap of a more aggressive cam takes advantage of the increased ram effect (momentum) at higher rpm. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.