who wants to talk about debate over cam advance/retard/straight up?

[Jim Ratto]

I think if we get enough guys to talk about what actually happens in each stroke, depending on cam advanced, vs. retard, vs. straight up, then we can see what works for what application.

Please no "advance gives more bottom end, retard gives more top end" general statements... 

ok here we go....

[John Rayburn]

I've always been an advocate of letting the cam grinder do what was intended, for any particular cam, and set them straight up. My combination works great and I see no reason to change it. If I wanted different results, I'd go with a different profile.

[Jim Ratto]

I've always been an advocate of letting the cam grinder do what was intended, for any particular cam, and set them straight up. My combination works great and I see no reason to change it. If I wanted different results, I'd go with a different profile.

Good point John.

I wonder how many guys even bother to check where their cams go in?

Let's make up a cam timing event....  Intake only for now...

Intake opens 63 btdc, closes 27 abdc..... so 63 + 27 + 180 = 270 degrees (all consider taken @ .050")
Let's assume cam profile is symmetrical too. So 270 /2 = 135, then subtract closing degrees to get lobe center = 108 LC

We can take this cam and juggle it's timing... it's still a 108LC cam, but what happens in the cylinder during transition from exh to intake, intake and then transition from intake to compress stroke if we:
leve it as is? compared to...
advance it 4 degrees? compared to...
retard it 4 degrees?

I know some of you, like John, prefer to install straight up. But I thought it would be fun to put some thoughts together at what is actually happening in the intake runner, port, cylinder, etc... when we change things up.

More?

[Jim Ratto]

nobody?

 

[Paul Bate]

Ok,

Natually Aspirated FK89 straight up job done ! 

I think thats the bogs dollocks (you might have to like in the UK for that one  )

Regards Paul Bate

[Jim Ratto]

so the cam we made up above, dropped in straight up...

intake valve is .050 off seat at 63 crank degrees before piston gets to TDC, traveling towards combustion chamber, and slightly open inake valve, and still fairly wide open exhaust valve. What is going on in intake tract? I am assuming positive pressure is sneaking past intake seat and up port, manifold, for fraction of a fraction of a second depending on rpm, pushing waste gas up into intake as well as out header. then as piston travels downward in cylinder, intake valve is chasing it, exhaust valve closes, we want the intake valve open as far as it will as piston reaches peak acceleration, or is that velocity? Anyway, I remember learning somewhere depending on length of rods (= angle) that occurs like @ 72 atdc....  and there should be a vacuum in cylinder, port, intake tract now.... but it's beginning to increase in psi, I think nearing 1atm around just before BDC, and at high rpm, if everything is "setup" right, even above 1atm around BDC, already "beginning" the compression stroke, if you will, before the intake valve closes 27 degrees after piston got to BDC, on its path up on compression stroke.

What if all this happened 4 degrees earlier?

This is where I get lost. It's like one too many twists on a Rubiks cube...

[Tony M]

It's not all ways about a cam profile, but a complete engine package. Comp ratio, air flow, displacement. If a cam works good set straight up, would retarding or advancing the cam help or hurt ? Does it come down to what your are using your car for ? Drag racing your not looking for low end only top end. So retarding your cam may not be the best, but advancing the cam would. As for the motor going into my car, I may want the cam retarded, not looking for total top end, need the low to mid range preformance of a ball buster, stump puller. Most steet cars need both, so setting the cam straight up is normal. Im i right or wrong ?

[Jim Ratto]

It's not all ways about a cam profile, but a complete engine package. Comp ratio, air flow, displacement. If a cam works good set straight up, would retarding or advancing the cam help or hurt ? Does it come down to what your are using your car for ? Drag racing your not looking for low end only top end. So retarding your cam may not be the best, but advancing the cam would. As for the motor going into my car, I may want the cam retarded, not looking for total top end, need the low to mid range preformance of a ball buster, stump puller. Most steet cars need both, so setting the cam straight up is normal. Im i right or wrong ?

you're always wrong....   

Kidding

I know it depends on so many factors, but what I am trying to establish here is "engine A" with the cam specs I laid out above.... what is physically happening to air flow and compression as we change WHEN the cam opens and closes valves, and we all know, advancing or reatrding it IS going to change where the peak lift occurs.
So forget if this is a drag engine, a road engine, a weekend warrior... etc.
Let's just say it is a 90.5 x 82 motor with big, big street heads, Webers, 9.5:1, header. It actually doesn't even have to be a VW engine! But since this isn't posted in Off topic...
We can look at valve vs piston events and what the airflow likes and doesn't like.... the only variables are rpm and where cam is bolted in (degree wise).
does that help?

[Tony M]

So i guess you need to build 3 engine's - all the same parts, just cams changed. 1 w/retarded cam, 1 with straight cam, and 1 with advanced cam to see the difference. or just change the washers on the cam  .

[Jim Ratto]

cool I will just push Sheep's POS out of my way and do this instead.... cool... 

[Sarge]

Back in the Dark Ages, before there were adjustable cam gears, the "tip" was to mill off about .040" from the crank timing gear woodruff key.  The "tip" was to run about 4 degrees retarded which was the way I had my 1835 set up with the Engle 125.  I had a close ratio 4:37 box and 165X15's out back.  I remember the car pulled like a freight train in third and fourth.  As for why I did it, let's just say it was a "tip."

[Sarge]

cool I will just push Sheep's POS out of my way and do this instead.... cool... 

I'm down for some pushin'

[Tony M]

Hell sheep wont care, he's out grazing. So variables are only rpm and deg of cam. Well Jim your over my head with this - need a shop with a dyno to test this out on. I havent built enough engines lately to do this. ANY ONE ELSE WANT TO JOIN IN ?

[Jim Ratto]

Back in the Dark Ages, before there were adjustable cam gears, the "tip" was to mill off about .040" from the crank timing gear woodruff key.  The "tip" was to run about 4 degrees retarded which was the way I had my 1835 set up with the Engle 125.  I had a close ratio 4:37 box and 165X15's out back.  I remember the car pulled like a freight train in third and fourth.  As for why I did it, let's just say it was a "tip."

I had one of those keyways, Sarge, tried it to advance a cam that was way out of index. Believe it or not, even it was not enough, had to make up 17 degrees. So I just jumped a tooth then backed it up with washers to get it "right".    

Hell sheep wont care, he's out grazing. So variables are only rpm and deg of cam. Well Jim your over my head with this - need a shop with a dyno to test this out on. I havent built enough engines lately to do this. ANY ONE ELSE WANT TO JOIN IN ?

yeah Sheep is on another vacation. yet again.

if we open intake valve @ 67 btdc instead of 63 btdc, if intake peak occurs @ 104atdc (instead of 108) and closes @ 23 instead of 27abdc, what will happen at
idle? earlier opening of intake on exh stroke, sends more waste mixture up into manifolds to foul fresh mixture columns? earlier closing allows less reversion though on compression stroke? So is there more psi (or bar) in cylinder during end of exh stroke or beginning of compress stroke, at idle? Which causes more reversion?

3000? earlier opening gives intake column a head start on entering chamber now... I think at this rpm there is probably a vacuum in chamber from scavenge of header, so the early opening is a good thing. But then shutting intake valve earlier than straight up position, are we cutting off the "ram effect" of late filling at BDC and a little after? Somewhere I have a book that shows psi in cylinder as piston descends on intake stroke... and you'd be suprised what a good intake helps the incoming mixture do!

5500? even more so than 3000rpm on early opening, I think mixture column is probably moving so fast that the reversion upon opening is cancelled out by scavenging of the header during the end of the exh stroke. Earlier opening is a good thing. But earlier closing is probably what "hurts top end" stifling off cylinder filling @ BDC area..... ?

7500? like 5500 but even more so.

[Jon]

we want the intake valve open as far as it will as piston reaches peak acceleration, or is that velocity? Anyway, I remember learning somewhere depending on length of rods (= angle) that occurs like @ 72 atdc....

Advance thinking on a late night... don't know enough about the topic, but I have a impression of what we are doing here is timing the cam against natures own timing; the "weight of air". Thereby it will "click" at some point, the challenge is finding where...?

But is there always fresh charge air waiting behind the intake valve when it opens?

[Harry/FDK]

Finally (learning) and understanding more.


Pls Cam Professors more input here.

Me dumbass is installing every cam straight-up on  engines. When i could taylor the engine for customer or my needs, and probably give away HP.

[Jim Ratto]

I remember back in 1994 after Sheep had his 1914 built with K8 and 8.8:1 and I woke up one night and it came to me that the cam super-influences the compression ratio. The swept volume isn't truly 100% of cylinder displacement, right? You've got valves hanging open at beginning of compression stroke, right. The longer the intake hangs open, the less swept volume gets "compressed" but then I realized (burn) after that, that if you are cramming more mixture in by leaving valve open, then is the CR higher? Or is it lower ? Anyway i called Sheep next day and said "we should put a K10 in it and raise CR more" which we never did.
One year in junior college, a frined of mine and I were in trig, and we figured out a way to compute REAL compression, based on where piston was when intake valve closed and volume left in cyl before piston got to TDC.... crap, it took us like 3 hours of sin this, cosin that, radians, bla bla bla... but we did figure it out.... wish I still had the formula written down.

I think it would be cool to see what happens on a dyno. Is it the low lift stuff that takes effect or is it where the full lift event happens vs. piston speed?

[Mike Lawless]

Are you taking into account the scavenging effect of the exhaust pulling the intake charge in before the downstroke even begins to draw when you figure dynamic CR? At certain points in the rpm range, some motors might achieve over 100% VE and that's where retarded or advanced cam timing comes into play, raising or lowering that point in the rpm scale.

My thinking is that altering cam timing is more effective on automatic transmission race cars where one is trying to squeeze every ounce of performance out. For a stick shift car, its easier just to change the launch rpm and shift points. It's doubtful one would see a realizabe difference if the cam was matched to the rest of the total package. If performance picks up dramatically from changing the timing, the cam should be changed to something that matches better as Mr Rayburn pointed out.

Back in my V8 days, I got more variance in result from changing valve lash than changing the timing a few degrees this way or that. If the track was loose, tightening the lash would soften the bottom end enough by effectively increasing the duration by a few degrees. We didn't have adjustable shocks then...at least not that we could afford. Easier to tune the motor than tune the suspension.

On my own stuff, if it's within 3 degrees either way when I set it up, I'm good with it. And I use the intake lobe centerline method of degreeing the cam.

[Jim Ratto]

Are you taking into account the scavenging effect of the exhaust pulling the intake charge in before the downstroke even begins to draw

yep

3000? earlier opening gives intake column a head start on entering chamber now... I think at this rpm there is probably a vacuum in chamber from scavenge of header, so the early opening is a good thing.

5500? even more so than 3000rpm on early opening, I think mixture column is probably moving so fast that the reversion upon opening is cancelled out by scavenging of the header during the end of the exh stroke.

I think there is an ideal "setting" to getting intake centerline or peak right. Dema Elgin taght us that intake valve should be full open at peak piston velocity. But rod length, stroke...etc... that all comes into effect. Again, seems that "sweet spot" is.....where?
I use the intake center method too, so that way you can burn  the cam card and still get it dialed in. Most of the time I spill Pepsi or assembly oil on them anyway.

There is something to getting cams "in" where the motor likes them. I was dumb and ambitious and reatrded an FK10 in a low CR 2276 in my car, and let me tell you, it was softer than an old man's tool in the gym sauna. Advanced, it made all the difference in the world. The motor grew balls and woke up.
You can see in some production motor where a juggling of cam timing brought more power. Same total time open, but occurring at different numbers. Sometimes done for emissions, but also for power. Look @ Lamborghini Miura vs. Miura S vs Miura SV or 911 engines..... those guys moved timing around to get more power (and sometimes clean tailpipe up).
Personally, I've been setting street stuff up with intake lobe center @ 104 ATDC and haven't run into any limp dick low end or "soft as bananas" idles. They seem to rip right off first crack of throttles. Problem in my last motor was it was done by 6300.... I mean c'mon.... an IDA motor laying over @ 6300??? WTF? So I cranked duration up this time around, kept lobe peak @ 104 (interesting that Mario @ Pauter said "advance our cam 4 degrees.....") and it stomps now to (letting off @) 7000. And it's getting an average of 25mpg.

[Mike Lawless]

I'm with ya there. I admit that I will typically not "leave it be" if the cam initially sets up at 3° retarded. If it sets up at 3 degrees advanced, I'll let it be every time. I've been running CB cams exclusively for quite some time, and their race cams are ground on 107° lobe centers, so anything between a 104 and 108 intake lobe centerline, and I'm good to go.

Wouldn't it be cool to see a "super slo-mo" video of what really happens in there?

Your low cr 2276 with retarded cam timing had two strikes against it for low end punch. Initially low compression, made even lower dynamically by retarding the cam. My guess is that it was more than 4°? If it was less than 4° retarded, my opinion is that illustrates in dramatic fashion that the motor may have been overcammed for the rpm range it was to be mostly driven in.

All things being what they were with that, do you think that had the cam been several degrees shorter in duration, the soft low end might not have been as pronounced?