jboweriii

.647 lift int and exh
duration @ .006 tappet lift 303 int 309 exh
duration @ .050 248 int 254 exh
lobe lift .3810 int .3810 exh
lobe separation 114.0
BBC
thanks for your time

540 RAT

Here are some common engine sizes, and the approximate minimum "adv overlap" needed for that WICKED **** idle lope/chop that most gearheads love, and turns heads at the local cruise-in:

350ci………..77*
383ci………..78.5*
400ci………..80*
454ci………..84*
496ci………..87*
540ci………..90*
572ci………..92*
632ci………..96.5*

Your adv overlap calculates out to 78*. So as you can see, putting that cam in any decent sized BBC will fall well short of having that bad boy idle. It will sound more or less like a stock cam, give or take a little. But if you are looking for that nasty Hotrod idle, then you will need a much narrower LSA (lobe separation angle), and/or more duration, in order to have enough overlap to achieve that. My 540 has 90* of adv overlap and it has just reached the threshold of a nice choppy idle, and I wouldn't want it very much smoother, or it would start to sound like a grocery getter, instead of a Hotrod. Hope this helps.

wfo76

How do you calculate this?, thanks.

540 RAT

Here's how to CORRECTLY calculate advertised overlap:

Add your intake and exhaust advertised duration (duration at .050 will NOT give you the correct overlap)

Divide that answer by 4

Subtract the lobe seperation angle (LSA) from that answer

Multiply that answer by 2, and you have the CORRECT answer for advertised overlap

howarsc

mine has 70* and I would not call it smooth.

jboweriii

its in a shafiroff 582 hhr. dyno 738hp at 6,000, 718 tq @ 4,600. its a street car. i hope it sounds like it has a cam

540 RAT

You've got a .720 lift custom roller with only 70* adv overlap? No offense, but maybe you need to recheck your math. That sounds awfully low.

DRIVESHAFT

540Rat, I think your numbers could use some tweaking.
I have experience with this cam http://www.cranecams.com/?show=brows...tType=camshaft in a 468 inch motor.
It only calculates out to only have 75° of advertised overlap, and I will guarantee you it doesnt sound anything like a grocery getter.

Droshki

? Mine works out to 56* in a 383 and people are already warning me about not being able to make vacuum, but its going to sound like stock ???

540 RAT

OK, my numbers are fine, BUT sound IS SUBJECTIVE, meaning that different people will think differently about any given thing. So if you like the sound of 75* of adv overlap in a 468, that's fine, each to his own. But it falls way short of the threshold of a wicked **** idle. Perhaps the grocery getters in Texas sound different than the grocery getters in California :-) As a point of reference, 427 Hotrod's 540 has an advertised overlap in the mid 80's and has virtually no chop/lope at idle, it does NOT turn heads at the local cruise-in, though it does sound sweet at 7,000 rpm. Either way, he still runs 9.90's with it, so it is quite fast, whatever anyone thinks about its sound. My 540 has 90* adv overlap and it just makes it into the threshold of a wicked **** idle that will turn heads at a local cruise-in.

If you like, use that "turning heads", as what determines a wicked **** idle from just another grocery getter. Be honest with yourself, and you'll know what I'm talking about..........

540 RAT

If 56* of advertised overlap is correct, and it'll be in a 383, you will not have vacuum issues. That is rather tame to be honest, sorry. And yes it will sound much like stock. It is NOT a wicked cam by any measure, but you had to know that already. ML67 runs a 548 BBC with an adv overlap in the mid 80's, and he runs power brakes with that, so you have no reason for concern with your combo. Whoever told you that you will have vacuum problems, does not know what they are talking about. Get it running and you'll see.

howarsc

3000-7200
60234
NA
285
293
255
263
.680
.680
110°

This one is similar to mine... but mine has a wider lobe sep with 1.8rockers...

howarsc

not a good rule of thumb.

jboweriii

will the grind make a difference? is'nt that what they use to tweak? thanks

Droshki

Numbers are 276/288, 113 lsa. Its what comp recommended for a supercharged 383.

"Hydraulic Roller-Street Machine with 125+ Nitrous system or small supercharger. 2200 stall with lower gears"

No I dont expect it to have that wicked sound, but I wasnt expecting it to sound anywhere near stock either.

OzzyTom

Wicked idle "sound" is subjective....

I have a 383 with about 85* adv overlap... and I have enough vaccuum for my accessories... and it sounds OK at idle I guess.



Don't know about being "wicked" though.... we have some awesome big block strokers at the local cruises that would rattle the fillings out of your teeth as they pass...

My cam is a HR with 242/240@0.050 (305/314 advertised duration with 0.594 lift) 112* LSA tends to smooth it out a bit...




PS... I have a question though,
How is manifold vaccum created in a blown application???

Droshki

Was there supposed to be sound? It just looked like a picture to me. Right hand drive at that? Ah....Oz...

This is a roots setup, not a centrifugal blow-through. If you pull your brake booster vacuum from the bottom of the carb, there is always vacuum since its sitting just above the spinning compressor blades of the blower. You can not pull your dist advance from here though, you must get that from the manifold itself, below the blower. In the manifold the vacuum does indeed go away at WOT, which you want, because at that point your mechanical advance is in.

In any event, my selection of cam was performance first, lopey idle second. When I talked with the cam companies I explained this to them that I thought the lopey idle was cool, but did not want to give up performance to obtain it. Also in discussions here on the forum about my overall setup it was determined that my Holly systemax heads were going to be the limiting factor anyway. I cant afford to replace them right now with the cost of building a 4 bolt, 4340 forged roller cam bottom end, and also my need for new headers and sidepipes. (Plus all the while your in there things that pop up when you can actually see the inside of the engine compartment). I am running 1.6 RR too though, so that will bump it up a tad. With 1.6 I am at something like 540/550 lift and from some reading I just did, this also makes the engine see about 2-4* more duration.

It was also advised that I go with a relatively wide LSA to minimize the supercharger blowing the charge out of the exhaust ports, and to not get carried away with the duration lest I lose bottom end torque.

hugie82

Also many of you did not mention compression. Compression will be a major factor in how far you can go with duration and overlap. Too little comp. you have a dog of the line. Too much comp you have detonation or race fuel required

Droshki

I'm shooting for about 8.3 static NA CR. This will put me at about 12.7 at 8 PSI and 13.8 static @ 10 PSI. Ceartinly the ragged edge of things, but I think the dynamic CR will be a bit lower. I've been too lazy to calulate it. It remains to be seen how much boost I will actually produce with a small bump up in cam size from my old engine and the larger displacement of the 383. I expect considerably less then the 10 PSI max I saw on my 355 right before a piston let go.

540 RAT

Tom that sounded pretty sweet, one of the better sounding 383's I've heard. Though it did sound like the idle was sort of high which masks the lumpy idle. It sounded like, if you lowered the idle some, it would have been quite choppy. And that is just one more example that nicely matches the reference chart above, which was created from real world cars like yours and others. I am surprised though that you can run vacuum accesories with that much overlap, since typically, 60* of adv overlap is about the upper limit for running power brakes, in a mid 300 ci small block. Maybe vacuum works different down under :-) My 540, with 90* adv overlap, pulls only 6" of vacuum at idle, which can't support power brakes. And UD Harold, of UltraDyne cam fame, has stated that a 76* adv overlap cam, in a 350 does not make enough vacuum to operate power brakes. So, how much vacuum does your setup pull at idle?

Droshki, now you see what I was talking about in reference to your 56* adv overlap vs Tom's 85* adv overlap in the same size motor. That huge difference in overlap will make for a huge difference in idle sound. I didn't know earlier that you were doing a blown motor. The charts were developed from naturally aspirated motors, so I guess I should specify that. And yes, you're probably right, it may not sound quite like a stocker. It should sound somewhat stronger, but by how much, is as the say, subjective. Some may think its cool, while some may not. That's life, but all that really matters, is that you're happy with it. I'm sure it will be a lot of fun. Blown motors are pretty cool, I have one of those also, and its a blast to bring in the boost.