How far to rev this 273 commando build
AJ ..............Ron at Isky made the cam with 2* advance , giving the 273 more grunt on the bottom end . which it sure does . That is what I specifically asked for . I don't think this cam has that radical of a sound when idling . imo. sooooooooooooso if the cam is advanced 2* then lsa is at 106 is that correct ?
I didn't hunt the cam up.
Try not to confuse LSA (Lobe Separation Angle), with ICA ( IntakeCenterAngle, or sometimes called Intake CenterLine; or InstalledCenterAngle; all same thing)..
LSA numbers usually fall between 106 and 114. ICA numbers usually fall between 100 and 110. So it's easy to imagine confusion. LSA is ground into the cam and cannot be changed in your driveway. ICA can be put anywhere you want,and some have, with catastrophic results. But if you stay within 6 to 8* of straight up, most street combos with flat-top pistons will be safe; domed pistons not included.
SO
If the cam was ground on a 108 LSA and you installed it 2* advanced, then it is in at 106* . This is a different way of saying you installed your 108cam at 106* for a 2* advancement; different lingo, same end result.Usually it is written something like; 268/276/110+4. The first two series (268/276) tell the intake and exhaust duration, and the 110 tells the LSA, and the +4 tells the ICA to be 4* advanced. From that simple series of numbers, every design cam-timing event can be figured out just with a bit of math.
But
if your grinder took the patterns that he normally grinds on a 108 LSA and tightened it up to 106*; this is a whole different animal, and it will run a lil different. And if you install this 106LSA cam 2* advanced, it will be in at 104*, fully 4* advanced from the original "straight-up".
Try not to confuse LSA with ICA.
Tightening up the LSA, closes the intake valve earlier, builds more low-rpm cylinder pressure, and so that builds low-rpm torque, and usually brings a few more ponies at high rpm.The idle vacuum usually increases and you might have a slightly smoother idling engine, but not always.
Increasing the LSA closes the intake later,reduces low-rpm cylinder pressure, and so softens the low speed torque. It usually also sacrifices a bit of absolute power, to gain some powerband.
And so
a manual-trans car, with tighter ratios, running a deep low, can use a tight LSA(like 108*) cam and wham thru the gears closer to peak power, and ET lower, than if it had the opposite.
But an automatic-car with street gears, and a street-stall, needs powerband to not die on the 1-2 shift. So he might chose a wide LSA cam, like 112 or 114*. This is particularly important with smaller engines that don't have a lotta torque to start with.
When choosing a cam then,
you select the .050 duration to put the power where you need it to be. Then select the LSA to adjust the powerband to suit your transmission. Then let the cam-grinder select the lift according to how often you are willing to replace valvetrain components, or how much money you sunk into your valvegear in the first place, or whatever he is willing or able to give you. And finally you fine-tune the ICA, in the field for best results, or just install it where the cam-card says to.Moving the ICA on a typical streeter, away from the recommended install, rarely results in anything of substance......... unless you got the wrong cam for your combo in the first place. Yes;it happened to me.
If you are happy with your combo, then Congratulations, the cam is doing exactly what you want it to, and don't mess with it.
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Here's another trainwreck tip;
the more gears you have, usually the tighter the splits will be; and so the tighter the LSA can be, sacrificing powerband for absolute power. This is how small engines can compete with bigger, on the street.
Small engines, to be fun on the street like lightweight, or; lotsa gears, and modest cams, tuned to the trans. There is no way I could or would, run a 273 in my fat 3650# street-Barracuda (me in it) with a 3-speed TF and 3.23s....... But stick it in a 3000# early-A (driver included) with a tight LSA cam,a 5-speed, and 4.30s and watch her fly! The deep low rear gear lets the engine get up on the cam right away, and the tight-split manual trans keeps her on it. Whereas
3.23s would take forever for the cam to wake up, and the wide gearsplits will keep the engine off the cam for a lot of the time.
You see;
performance cams have a very a limited effective operating range, where the engine is said to be "on the cam" of perhaps 1000rpm. You can increase/decrease the operating range with engine displacement and by manipulating the LSA. But after that power is dropping off, and faster on the top than on the bottom.
But the transmissions Mopar gave us have much much wider powerband requirements, so we have had to invent ways to adapt. The hi-stall TC is one of them. And stealing off-brand 5-speeds is another. And 4-series street gears is a third. All of these help get the engine up on the cam where the power is. But only multi-gears can keep it on the cam, cuz they reduce the powerband requirement to better fit the cam's operating range. For example
The standard TF 3-speed has ratios of 2.45-1.45-1.00, and the splits are 59% and 69%. This means that at whatever rpm you shift at, the Rs will fall to those percents. Lets say you have a cam that wants to be shifted at no more than 6000. Well, going into second, the Rs will drop to .59x6000=3540 .. which being a powerband requirement of 2450 rpm. What do you do with that? You sit back and wait for the engine to get back up on the cam. And the smaller the engine is, the longer you gotta wait. This isn't fun anymore. So what do you do?.... Well next step up is a 4-gear with ratios of 2.66-1.92-1.40-1.00. And with splits averaging 73%.. So now when you shift at 6000, the Rs drop to .73x6000=4380 for a pb requirement of 1620; halleluja to the 4-speed for getting you closer to the cams pb. At 4380 your little engine will be making a good bit more power than at 3540. ...... So what's the next step?
Well you got two choices; more rear gear making it a city car. Or a 5-speed, to keep hiway travel on the table.
So suppose you found a way to bolt this one onto your 273;
3.83-2.33-1.44-1.00-.79od with splits of .61-.62-.69-.79 This ain't looking all that good cuz .61x6000 is 3660 and you are back in the basement. But are you really? Say the earlier combos ran 3.55s about the highest most streeters are willing to run. Now you have .79overdrive, so now you can run 3.55/.79=4.49s for the same hiway rpm. Lets round down to 4.30s, use 27" tall tires, and lets keep the 6000rpm shift speed for now. And lets compare to the standard TF with its 3.55s.
First the TF; You will be shifting at
53,89,130 revved out. When you shift, the Rs will fall to 3540 and 4140. On the way to 65mph, you will pass thru the powerpeak just one time and finish at 4360, seriously choked for low-ET.
Next the AX-15 with 4.30s;You will be shifting at
29,48,78,112 into overdrive. When you shift, the Rs will fall to 3660,3720,4140 and 4740 into od.On the way to 65mph, you will pass thru the powerpeak twice and ending at 5000rpm in third gear; bang on the money!..... three trips thru the powerpeak trumps powerband plus one-trip and choked, any day. This is the magic of TM; Torque Multiplication.
But lets back up a bit. Lets switch to
the Commando A833(3.09low) and regear the back to match the AX-15 and compare those two.This will take 4.42s, lets use 4.56s. Here goes; your shiftspeeds will be;
34,55,76,106 revved right out. On the way to 65mph you will pass thru the power peak twice and finish at 5150 in third gear; hey that looks pretty good! But here's what you don't see; With the Commando box,on the run to 65mph, shifting at 6000, the Rs drop to 3730&4380 versus 3660&3720 with the AX-15. I know which box I want for a 273.
Ohcrap Ididit again, wrong forum, sorry.
