is it possible to have a nice lope to the idle without having a high rpm cam, say redline of 6000?

 

Yes. The lope on a hi performance cam is due mainly to the amount of overlap. Overlap is a function of how much duration and also how close the lobe centers are. If you use a cam with moderate duration but close separation you can have enough overlap to give a lopey idle without also having to rev the **** out of it to make peak power.

 

The problem with a high overlap, short duration cam is that you compromise the torque bandwidth. High overlap kills low end torque and the early closing inlet limits top end power. Overlap and exhaust backpressure are mortal enemies, so there is a definite limit on how much overlap a street engine can take without compromising output. Think about this. Exhaust backpressure is ALWAYS higher than inlet pressure, even at WOT, so at low speeds exhaust gas migrates into the inlet port during the overlap period, diluting the fresh charge. As you throttle the engine the pressure difference increases, so dilution and operating characteristics are even worse at idle, and part throttle operation.

Headers can harness exhaust system wave dynamics by creating slight negative pressure at the exhaust port during the overlap period to begin the inlet process - usually only in a relatively narrow engine speed range, but the effects of headers are mitigated or negated by street exhaust system backpressure, so even with headers, there is a real limit to how much overlap is effective before it begins hurting average output across the rev range.

GM used this philosophy on some 70s engine, which as most know, have some of the worst operating characterics of any automotive engines ever produced. High overlap created an "internal EGR system" and eliminated the need for an EGR valve to control NOx. Trouble is, you don't need EGR at idle to control NOx, but a high overlap cam reduces idle quality, fuel economy, and low speed torque. Seventies vintage engines are certainly not an example of how to build a broad torque bandwidth engine with excellent operating characteristics throughout the range from idle to WOT.

Seems that a lot of guys are more concerned with how an engine sounds at idle than how it actually runs. Whatever!

Duke

 

i think the Comp dual patern cams with 110 lop sep, offer lope and low end, those are the grinds i'm looking at

 

Except the word "compromise" is a gift.
No low end and no top end, but it sounds mean.

That would be great for loping through the drive in, on cruise night, but make sure no one is following you on the way out. If you get my drift.

RACE ON!!!

 

I'm running a CC XE-274 (230/236/110) in my 327 and I'm pleased with it. With 3.36 gears, I can mash the throttle (700DP) in 1st gear at 1500 rpm and there's no hesitation, no complaint. It can cruise smoothly at 1500 rpm in 4th and will accelerate smoothly from that rpm. It's never been on a dyno but "seat of the pants" tells me that peak power comes around 5800-6000. Others may prefer different characteristics but having usable power where I can use it suits me.

While tighter lobe separation does increase overlap for a given duration(and the negative effects previously mentioned), it can also increase torque. Once intake velocity is adequate to prevent exhaust reversion (and it's negative effect on fuel atomization) the benefits of the tighter LSA begin to show. That benefit is more time with both valves closed (we're dealing with 720º of cam timing per cycle, If they're not closed, they're open). For a given duration, a narrower LSA means that more of the combustion energy is used pushing the piston down rather than being sent out the exhaust valve.

 

Optimum lobe separation angle (LSA) and overlap are determined by valves size, cylinder size and how well your heads breath. And the less your exh is restricted the better your engine breathes - the better the cyl savenging. Better volumetric efficeincy (VE)! A wider LSA can make up for a set-up/engine combo that has lower VE with less overlap/dilution at low rpm and a higher rpm range to make higher peak hp. Why u get a higher rpm range with wider LSA sorry I can't recall but u do. But pack a lunch before you leave to go there (peak hp). And if you have a choker exh and wheezer heads you may want a wider LSA. But to obtain highest midrange torque - the stuff that you'll feel the most and gets you to the end of the 1/4 fastest - all the cam manufactures are using/selling tighter LSA's (110 or less) for street performance.
No, I'm not a dyno expert and have never used a dyno. But I buy books of people who are experts and thats what they're preaching.
If u really want the optimum camshaft for ur engine combo there are computer programs that can figure it out for you. And here is one http://www.davidvizard.com/camshaft.html. All the theory that describes optimum LSA fills a few pages/chapters in a book(s) - too much to write here - but I don't think you need to research it with the programs now availible.
Good luck. cardo0

 

Oooopps! My bad. It was late when I posted this and can see my mistake: you would want the tighter LSA for the lower VE. Yes an engine with a higher VE needs less overlap and can have an wider optimum LSA. A restrictive exhaust needs the exh vlv to close sooner to prevent exh reversion. And optimum LSA occurs when the intake valve closes just when pressure accross the the valve is equal - no reversion flow on intake side to catch the most fuel/air charge. A lower VE motor needs a tighter LSA to trap that optimum charge and that tighter LSA creates more overlap.
So what I'm saying is that even if you have great breathing heads, if your exhaust is still restrictive you would need a tighter LSA for optimum torque.
Okay now I can sleep better tonight. cardo0

 

Hi, I just wanted your oppinion on which camshaft i should put back into my 383 after my 2400 converter upgrade. I cannot realy decide

Last time i had the summit 1105 hydraulic 224/234@0.50 0.496"/0.520" (with 1.6 rockers) 114 lobe center 1.0 overlap in. I was thinking about putting back the summit 1107 hydraulic 234/244@0.50 0.520"/0.544" (with 1.6 rockers) 114 lobe center 11.0 overlap which I had when the engine was a 350cui.

I did try the 1107 camshaft when the engine was a 350 whith the mods in my sig, but it was DEAD below 3000rpm, it was not driveble at all and the milage was horrible
When I rebuilt it to a 383, I went with the 1105 camshaft. The low end was good, but it did not have that extra grunt up high. Both of the times I had a stock converter and 2.73 gears (350cui and 383cui).

The heads have dual springs with 125lbs of seat pressure, so it can take the revs.

Do you think I can run the 1107 cam with the 2400 converter and make up for that loss of low end with the 383 or will it be just as dead as the 350 down low?
Do you think that the rhodes lifters will gain back some low end torqe? Or is it just :BS:

By the way, the car is intended to be a weekend warrior

 

Good link i've read some of he's other stuff

 

I just buy a can of Lopey from Autozone at every oil change and it makes my Vette sound like a 1962 fuel injection Corvette.