Cam advance/Retard – What’s Really Needed?

[David Vizard]

If I could meet the guy that cam up with the intake center line positing method of setting the cam I would shake his hand and thank him for such.
When I was kid (and that is not as long ago as Ed Iskenderian but it was back in the ‘50’s) there were several ways of setting up the timing of the camshaft but virtually all were flawed other than the intake center line method. For instance Aston Martin give n intake a valve opening amount at TDC as a means to set a known reference position. This only works for a stock cam in a stock engine and only then if Aston Martin got it right during the engines development.
Then there was the deal where the intake was set such that at TDC overlap the intake was set to be open about 0.050 thousandths further than the exhaust. This was better than the AM method but still made some assumptions that are incorrect. The most obvious and the most important of these is that the cam advance/retard is not a mechanical function but and airflow function.
In simplified form here is how it goes:- For given intake/exhaust flow curves there is an optimum balance between each during the O/L period. Finding this is important if the intent is to squeeze every single lb-ft/hp from thje engine. That’s my job so I get to do this on a very regular basis.
When the cam company sends out a cam for a typical Detroit V8 application the almost always say to set the cam into the engine at 4 degrees advance. That is the intake center line has to be 4 degrees smaller that the LCA. i.e. 108 LCA 4 advance is 104 degree intake center line.
This is a number which comes up time and time a again. But that does not mean that it is universal. When I was doing all that testing back kin the mid to late ‘80’s for Harvey Crane the test engines had heads with good three angle valve jobs which gave a low lift discharge coefficient of about 0.68 to 0.7 for both intake and exhaust. This is sort of t a typical number for a 3 angle valve jof as measured on my bench.
With that the 4 degree advance was pretty damn close time and time again with all the cams tested in all the engines with nall the heads.

The problem comes when we want to deviate from what everybody does in an effort to get a power advantage.
For years I had a Serdi seat machine in my shop so I got to try a multitude of seats in an effort to produce better results. Over the years one of the factors I have found that influences torque iis that the faster you can present breathing to the cylinder and take it away at the end the better the torque and power is. But as you may expect any breathing capability has to be presented to the cylinder AT THE RIGHT TIME and here is where so much confusion comes into play.
OK guys got to go here will pick this thread up later.

DV

[hoffman900]

David,

I'll take a stab.

I see advance/retard as a time issue. Let's go back even further...


An engine works off of pressure differentials. On the intake stroke, the descent of the piston away from TDC causes the atmosphere in the cylinder to depress to lower than atmosphere. The ascent of the piston on the exhaust stroke (during the pumping phase) raises the atmosphere in the cylinder to that higher than the atmosphere.

Two things we need to keep in mind:

* the piston does not move in a linear fashion.
* the faster the piston moves, the stronger the effects of intake and exhaust ramming.

At some point, the piston is moving fast enough to start to drive the ramming process. This will (hopefully) have the effect of increasing intake port pressure just before the intake valve closes and lower the cylinder pressure even further as the exhaust valve closes.

Really the entire premise of the valve motion is to maintain positive intake flow (flow from the intake port) into the cylinder and maintain negative flow from the cylinder out the exhaust. The reverse for either is bad for power production.

The pressure differential is what moves the mass. At some point however, somewhere in the system, the air goes sonic, at this point mass flow cannot increase despite a greater delta of good pressure differentials. At some point the pumping losses exceed the amount of force created and power starts to nose over.

Harold Brookshire's design theory was that it wasn't choke flow that caused the issue, but the failure of the exhaust to fully evacuate. The build up of residuals eventually diluted the mixture enough that power started to nose over.

I've seen two examples along those lines but contradict each other.

Example 1:

A limited development engine (valve lift, compression, ported limited) wasn't responded to cam changes / any development. After header R&D, the engine began to respond to changes. The peak power wasn't much more, but the power curve was able to be extended by almost 1000rpm. By maintaining a very slow fall off after peak power, the engine was up almost 30hp at that 1000rpm later number (because power was falling off a lot slower) then it was before. Thus, the engine can now spend most of its operating range at close to peak hp. In that case, I side with Harold.

Example 2:

A more open rules engine, but limited in valve area by the architecture. Header R&D resulted in 10% up high and 12% down low gains, peaks are the same, the shape of the curve is still the same. In this case, I don't believe the exhaust dilution was the issue after peak, but the lack of intake valve size is the limiting factor.

Okay, so where am I going with this...

The 4* advance rule of thumb is bogus. A cam should be designed so the intake center line should be ideal at the spec'ed number. The exhaust center line the same.

The centerlines should be spec'ed out to what I posted above. Give me the most amount of mass flow at a given engine speed.

IE: Why design me a cam at I:106 E:112 and tell me to install it 4* advance, when a I:102 E: 108 is what the engine wanted?

I personally see the 4* advance rule of thumb more of a crux for people not using camshafts designed for their application.

Where advancing / retarding is handy is during development. Like the exhaust changes above. Both those applications want/should get a different camshaft. The airflow flow curves remain the same. They will want different overlap periods than before. This is easy with DOHCs, not easy with SOHC/OHV applications. You can change the centerline of one or the other (advance it or retard it), but the engine may really want different splits than whatever the fixed LSA is.

[wyrmrider]

Sort agree with bob however sometimes core availability determines where the ICL and ECL have to be which is near where they "always have been" 4 degrees advanced from straight up
and what is "straight up" with split durations and asymetrical grinds? or 4 degrees advanced- you have to start where the cam grinder says to start and go from there- then the motor will tell you what it really wants

[joespanova]

Thanks for the reply to my L/C for cam guys thread....................BTW.
Interestingly:
" For instance Aston Martin give n intake a valve opening amount at TDC as a means to set a known reference position. This only works for a stock cam in a stock engine and only then if Aston Martin got it right during the engines development"
This was EXACTLY the method Jay Allen ( of Camshaft Innovations ,which is no longer around ) wanted everyone to use , to setup initial cam install.

[bigjoe1]

Most cans like to be advanced 4 degrees or more. I have seen where as much as 8 degrees works VERY WELL- Without an engine dyno, it would be very hard to pin piont the best setting.



JOE SHERMAN RACING

[gottago]

Most cans like to be advanced 4 degrees or more. I have seen where as much as 8 degrees works VERY WELL- Without an engine dyno, it would be very hard to pin piont the best setting.

That's the kicker, without a dyno most all the little guys are fumbling in the dark when it comes to the best install setting and thus have to take cam manufacturer/sales department recommendation. All the variables that come into play when choosing a cam should equal out to using the recommended install settings for that cam but it doesn't always play out that way.

It takes a fair bit of time and work to swap cam timing especially in some confined engine compartments so getting as close as possible first shot is where most average street guys end up. Getting the exact right cam for the combo would help but from my experience most street rod guys are a little off optimum one way or another. Under which circumstances would you want to deviate from the manufacturer recommended 4* advance? What dictates that? Shouldn't there be an optimal open close point for best breathing on any particular cam and that's it?

[Walter R. Malik]

Most cans like to be advanced 4 degrees or more. I have seen where as much as 8 degrees works VERY WELL- Without an engine dyno, it would be very hard to pin piont the best setting.

JOE SHERMAN RACING

X2 ...
Most people have little idea where it actually is at; considering production tolerances.

[joespanova]

"The 4* advance rule of thumb is bogus. A cam should be designed so the intake center line should be ideal at the spec'ed number. The exhaust center line the same.

The centerlines should be spec'ed out to what I posted above. Give me the most amount of mass flow at a given engine speed.

IE: Why design me a cam at I:106 E:112 and tell me to install it 4* advance, when a I:102 E: 108 is what the engine wanted?

I personally see the 4* advance rule of thumb more of a crux for people not using camshafts designed for their application. "
I've wondered this very thing for years?
My guess is since 90% of cam business goes to amateur's , you can only get a cam "close". You have a suggested C/L install..................but why not grind / pin the cam "straight up" and let the end user play with moving the cam instead of putting the pin on at some predetermined advance......................I never understood that.

[bigjoe1]

Many-times, the grind will NOT FIT on the camshaft blanks that are available. It is as simple as that


JOE SHERMAN RACING

[David Vizard]

OK, I’m back here.
Read some of the responses and to be honest they sound very complex and I am still mulling over what was said.
Meantime let me continue what was found on the dyno. One of the things that I have explored in detail is the effect of low lift flow increases. A few years back while doing a 383 build for my Chey performance book I ported a set of 200 cc Dart heads and upped the intake valve size from 2.02 to 2.08. Using the best 45 degree seat cutter I had in my extensive Serdi collection I cut a seat and surrounding chamber that had excellent low lift flow right up to the 0.300 mark. The intake numbers looked like this:- 0.050/37 – 0.100/75 – 0.150/112 – 0.200/147 – 0.250/187 – 0.300/203. After the 0.300 lift point the gains from the seat leveled out and by 0.500 the flow figs were more or less about that of a conventional seat in a strong 23 degree SBC port. Final flow was 317 at 0.700 lift.

I worked out what I needed for a cam for the 12/1 383 this cam was intended to suit. Without going through a ton of data I think it was on a 108 LCA and had some of comps Solid roller profiles having 250/254 at 0.050 duration . With the rockers used the valve lift was about 0.700 intake and 0.680 exhaust. When I got the cam back from comp the timing card said to set the cam in at 4 degrees of advance. The reality was that I knew form the high low lift f;low figs that the cam would want to be way retarded form that number but I went and set it in at 4 advance anyway. Why? Because this engine – as usual – was equipped with a Jesel adjustable belt drive and I knew that someday I would fing the data from testing at 4 advance compared with what was actually needed would come about. At least I was right on that last point because here we are looking at those results.

This motor made 532 lbs-ft and 643 hp. Not bad for a relatively cheapo 383 that can be built at home in a clean shop with a good set of tools. The build used a 2 piece seal block with a Scat cast steel crank, a set of the through bolt budget scat rods rod’s, ICFON pistons with TS ring, a Moroso pan, the 200 dart heads I ported (great starting place those Pro1’s) a Super Victor Jnr and an AED 950 carb.
Now most folk would made a comparison of this with other 383’s and say that this was a pretty successful build and take it off the dyno. Well let’s do that cam advance/retard and see where we go.

The cam was progressively retarded until the best setting was found. That best setting was 1 degree retarded. Torque went to 555 lbs-ft and 668 hp. That’s a 25 horse increase that was their for the taking – at least it was for anyone prepared to invest in a Jesel belt drive.
The lessons to be noted here are that the cam companies advice on the cam setting is for a typical, (you could also say average here) engine. As soon as you start experimenting with factors outside of common practice you must allow for a domino effect where an improvement in one area means the optimization of some other affected factor.

It’s lunchtime now but I will continue this later.

DV

[gottago]

Wow a split duration cam DV. lol That is a substantial increase from only a 1* degree change. I may have to invest in one of those adjustable belt drives.. Otherwise I'd have been off the mark again using my high tech 3 position cam gear.

(you could also say average here) engine. As soon as you start experimenting with factors outside of common practice you must allow for a domino effect where an improvement in one area means the optimization of some other affected factor.

I'm interested in even some generalizations for just what some of the factors are that create requirements for either advance or retard. In the example above increased low lift flow larger intake valve with same exhaust side flow allowed a slight retard. Could ask if a slight change in lsa due larger valve diameter would hit the same mark? Or is retard advance the only option left at some point?

[plovett]

Without going through a ton of data I think it was on a 108 LCA This motor made 532 lbs-ft and 643 hp.

The cam was progressively retarded until the best setting was found. That best setting was 1 degree retarded. Torque went to 555 lbs-ft and 668 hp. That’s a 25 horse increase

DV

Was that 1 degree retarded from the original 104 intake centerline (105 degrees) or 1 degree retarded from "straight up" (109 degrees)?

If it was a 105 degree ICL (versus 104) that made the 25 hp difference that would be startling.

JMO,

paulie

[hoffman900]

Two things I think everyone needs to ask themselves;

Where are my lobe centers at under running conditions?
How much is my cam twisting under load and how much are the rear cylinders deviating from the front?

It's easy to talk about timing events in a static engine, but under running conditions, where are things?

Really only those with spintrons and in cylinder pressure measurements are the ones that can answer that.

[joespanova]

Sounds to me like he was saying 5dg back from where Comp suggested.

[GARY C]

Sounds to me like he was saying 5dg back from where Comp suggested.

If it's the one I am thinking of that is correct...109 IC