steve cowan wrote: ↑Thu Sep 22, 2022 4:59 pm
skinny z wrote: ↑Thu Sep 22, 2022 11:17 am
I think it's important to think in terms of overlap rather than LSA.
We know that the same LSA number could be very different in terms of EVC and IVO.
And I agree 100% with the carb tuning.
This is why I'm comfortable with 72 degrees of seat to seat overlap whereas I know of those that found it unfriendly on the street. That was 288/294 on a 110 (106 ICL).
Kevin,
Are you looking at degrees of overlap # because of the duration you want to use? I am trying to work out what the advantage is with more overlap/ low lift flow .
Hey Steve
I'm interested in a methodology for selecting a cam that was published several years ago. It falls inline with the 128 rule and TM. You can guess who may have published the article.
I'm going to assume it was presented so the average guy, without years of experience and experimentation (I'm certainly average but do have some experience and experimentation) can pick a cam to give the results the process is directed at. That would be maximum torque.
In it, it states that the order of importance in the selection process is (and I'll say upfront that the application is probably as finite as the 128 rule and TM):
Overlap,
LSA,
ICL,
Lift,
Duration.
Obviously items one and two determine item five. Symmetrical lobes too I would think.
I would venture a guess that this is done as the average guy doesn't have the means to determine the best IVO, IVC, EVO and EVC. I know this is how the pros might go about it.
So, having said that, I'm zeroed in on a overlap value that's worked well. 71°. And it worked well even though the engine wasn't optimized to utilize it to it's fullest. Namely the exhaust system.
How do I know? Testing and comparing. The last engine to use that cam was the strongest engine I'd ever assembled. it was also tame as a kitten on the street and knocked down some decent fuel economy. Win, win, win.
So, if I stick with an overlap of 71° (my choice) and use 128 to determine the LSA, which would be 108 in this case (and I'm right in the wheelhouse of the 128 rule spec wise) then I end up with something like 287/287/108/104. Lift would suit the heads in question which peak at about .550". (Decent valve job, 2.02"/1.6"). Lobe intensity is still very much a question mark. I could be anywhere between 287 with 242@.050" and 287 with 228@.050".
Projections are peak HP at 6300 and would require 250 CFM and a 179cc port (I've 255 without the intake and an as yet unmeasured port that started as 170cc before some basic porting). TM specs.
Right or wrong, it looks to fit perfectly into engine plan number 1. That being a strip specific car with open headers, the right converter (might have it already) and the right gears (might have that too).
So, to answer your question, I'm looking at those degrees of overlap because it suited what I was after before and should continue to do so.
As for low lift flow, if the exhaust is up to snuff, and I would think a tuned length open header would be, then that low lift flow should be enhanced. Using that exhaust induced induction has always been lacking in my builds and I'm really keen to see how that plays out. 71 degrees of overlap would work well. But that low lift flow I think is more the result of the port and valve job.
Thoughts?