At what point is a cam advanced too far? What characteristics impact a cam being too far advanced, simply intake center line or do other factors play a role as well?

Around here we see most cams with 4 degrees or less of advance but usually on a 112 or 114 lsa. 112 with 4 degrees advance puts the icl at 108 which really squeezes down p to v and usually puts dcr into the ranges we desire. My question is, for arguements sake, what would be the ill effects (if any) of a cam with a 116 lsa and 8 degrees of advance?


Thanks.

 

That completely depends on what advance you are refering to, and how is the 116 lsa attained. Are you asking about the advance ground into the cam (An ICL of 110 and an ECL 114 has an LSA of 112 with 4 deg. advance)? Or are you simply refering to installed ICL? But the crucial question is how is the LSA attained? Is the ICL and ECL 116? Or is the ICL 114 and ECL 118? or is the ICL 112 and the ECL 120? All of the specs I listed will have a 116 LSA. So in short a cam that is INSTALLED with too much advance may risk P to V contact.

 

Sorry for being vague in the initial post. Let's look at both scenarios, ground in advance and installed advance. For instance:

If a cam was ground with ICL 108 and ECL 124 that would make 116 LSA with 8 degrees advance ground in.

Lets compare that to a cam with ICL 116 and ECL 116 for a cam again with 116 LSA but 0 advance ground in. Now lets say we install it advanced 8 degrees.

Both cams should have the same p to v right?

Both cams should also have the same power curve right?

Are there any differences in how these cams would act or is it a matter of advance is advance whether you grind it that way or install it?

 

Correct, if the first cam was installed "straight up" and the second installed @+8 deg., then tho P to V clearance will be the same (both on a 108 ICL). But the characteristics of the two cams will be completely different. You can mimic the same intake valve events with installation, but remember, the lobe advance is ground into the cam, you can't control what the exhaust valve does, it can't be undone by moving it around. A cam with both centerlines the same, will always be a "zero advance" cam, and conversley, a cam with 8 deg. advance will always be an "advanced" cam, and they will behave accordingly. That is why it is SO important to get ALL the valve event numbers when determining a proper camshaft. So many people just say, "it's 245 deg @ .050 on a 112 LSA", that can mean anything in the cam world. The two cams you gave as an example are perfect, because the LSA is the same on both, and the installed position CAN be the same on both, but they will behave totally different.

 

Either way if it's ground in or installed at a particular ICL it's all the same, unless it's a LT1 which is about the only motor that seems to care if it is or not, due to electronics.

You can think about it a few ways....

No matter how it moves in a pushrod single cam V8 engine if you move one of the centerlines you move the other, otherwise gotta grind a new cam with a different LSA. Everytime you advance the cam another degree the ICL drops that degree but the ECL increases a degree. So it's a balance between getting the intake events correct and getting the exhaust events correct.

You can either set your Centerlines or you can set your LSA and advance.

Bret

 

Lobe seperation is lobe seperation I guess, the only way to tell for sure if an ICL is too far advanced would be to first off check for P to V clearance, and second run it on an engine dyno and see what happens to power output, starting with the most possible advance without P to V contact, and then retard a couple of degrees at a time, until you get close to P to V on the exhaust side. That would be an interesting experiment. Would need a modified belt drive though to accomodate that much of a timing swing.

 

keep advancing the cam untill you get max cylinder pressure, then leave it alone. that will give you your greatest dcr, not necessarly where the motor will run it's best, but be a trq. monster.

 

Cranking cylinder pressure and dynamic (running) pressure are two different scenarios COMPLETELY. I have seen this often mis-construed all over this forum.

Take and run an engine at idle with a compression gauge installed in one cylinder with the plug wire grounded and compare it to your cranking compression number. Next snap the throttle to WOT with the compression gauge still in the cylinder and record your MAX reading.

What do you get? Why?

 

You are confusing dynamic compression RATIO,a fixed number,determined by the IVC event, with the effects of lobe overlap. DCR is computed, and set in stone, not variable, based on the intake valve closing event and dynamic stroke (swept area after valve closes). Running cylinder pressure, which is hugely determined by the LSA, more accurately the overlap, of a given camshaft is continually variable, with rpm, based on the pumping ability of an engine. Just for your information, CRANKING cylinder pressure, and the DCR of an engine, are the exact same thing, just different ways of measuring it. For instance, a DCR of 8.6, gives 199 psi. (give or take a couple psi.) of compression while cranking.

 

I don't think I am confused......isn't that what I was saying? Wait a minute, now you have me asking you what I just said, now that is confusing.lol

Curious to know your math between the 8.6:1 DCR number related to the 199 psi you calculated.

And then would you be assessing that 200 psi cranking pressures will run on pump gas fuel?

https://ls1tech.com/forums/showthrea...8&page=2&pp=20

See this thread in another forum - post #29 I believe.

 

Yes, most new engine pump close to 200 psi. on cranking compression check, regardless of the static compression ratio. For example a '00 4.3L V-6 pumps 210 psi. at 9.0:1 static comp.. And an '06 GTO pumps 210 psi. at 10.9:1 static compression ratio, that is because both engines have nearly identical DCR. The reason it works is because 200 psi. will not cause detonation. When you introduce lobe overlap, and in the midrange of engine RPM, the cylinder pressure goes to 400 psi., this is when detonnation comes in. The more lobe overlap you have, the more cylinder scavanging in the midrange. That is why it is SO important to take into account the entire camshaft profile when setting your DCR.

 

Also lets make sure we are both on the same page here. We are talking the differences between RATIOS and PRESSURES right?

 

Don't try and correlate exactly cranking compression and DCR... it has more to do with ring seal than that.... It can easily be a 8.1 DCR and 200+ psi.

 

I understand that, ring seal, atmospherics, rod length, all play a roll in DCR. I was trynig to make a simple point, not reference an exact phsycal law. My simple point is that a compression RATIO, and cylinder PRESSURE are two different things, but related when the engine is cranking, not running.

 

Yes, I think we are on the same page.

I am still curious with your math for calculating how a DCR of 8.6:1 (in general) will generate a blanket 199 psi cranking compression. If this it what your saying I am unclear to your reasoning.

Is it not possible you could have two dimensionally different engines with different cylinder bore swept volumes because of different strokes and diameters as well as different cylinder head chamber volumes that could both compute a dynamic 'ratio' of 8.6:1 based on the chosen valve timing events yet create completely different pressures when squeezed at that ratio even at a cranking RPM?

 

No no no, I was just using that as a quick reference to bolster my position, by no means a rule. I have read you other posts, and seems like you know what you are talking about. I think we are both saying the same thing, I know I have difficulting conveiying what is going on in my head sometime.

 

Something isn't right here. I am not sure where you get the idea that I have a problem with the difference of compression ratio and cylinder pressure. Yes. They are different but there is a relationship. Since the engine is nothing more than a air pump its cylinder pressure generated is affected by atmospheric pressure which can differ considerably based on many weather variables such as water grains, air density, humidity, and the barometer and of course the ratio DCR at which it is squeezed.

So my point would be that even with the engine cranking or running the cylinder pressure is affected by atmospheric pressure, boost pressure, or most importantly the MAP (Manifold Absolute Pressure). My previous post mentioned the cranking pressure, running pressure at idle, and a WOT blip cylinder pressure measured with a compression gauge or electronic pressure transducer installed into the sparkplug hole will all yield different values. Correct? Why is this? Is it DCR, MAP related, or is it the VE of the engine? Or all of the above?

Food for thought.

I would think when you say that the compression ratio is 'unrelated' to cylinder pressure when the engine is running is maybe a little off base. The DCR is still the same as it was when cranking but now we are operating under different MAP conditions. Open throttle plate, closed throttle plate, cylinder filling events at peak torque (fifth cycle stuff), etc, etc. Whatever the air pressure goin into the engine is at a given point will always still get compressed at that engines specific DCR therby affecting the overall cylinder psi. Of course, how well the engine moves air (VE) impacts all of this as well.

I think if you install a compression gauge into an engine a do the simple tests I mentioned above you will see that even with a 'fixed' dynamic compression ratio of whatever that the MAP and VE of the engine under different conditions has an enormous impact on the cylinder pressure created at that specific and camshaft set DCR. Even while doing a cranking only test the results will vary based upon throttle plate location thereby affecting MAP and therby affecting cylinder pressures recorded.

210 psi out of a 4.3 (W) engine is alot, too much I think. I have this same truck, well a 99, at the shop. Just built the engine and installed and have about 400 miles on it (shop truck). I will stick a gauge in it just for grins and post my findings. It has the factory roller cam in it still, the only thing I didn't replace. Don't like to do camshafts (rollers) on these late model emissions tested vehicles unless they are tore up. One less variable to contend with if they don't end making through the I/M lanes. If it is anything like the 350 Vortec or TBI truck engines I have tested in the past your numbers are way high.

 

This test would only serve to tune 1 engine, but you couldn't really project those learnings to other combinations. When you advance or retard a cam, you're not just changing 1 thing - you are actually changing a whole myriad of things, some of which offset others. The end result from advancing or retarding the cam is the net total change in performance after certain things helped and others hurt.

It would be more interesting to me to see a dyno test that holds overlap, EVO, and IVC constant, while moving the overlap period forward or backward. For instance, dyno these three cams:

224/224-112, +4 (baseline)
230/218-112, +7 (advances overlap period 6 deg)
218/230-112, +1 (retards overlap period 6 deg)

I've got my theories about what would happen, but I would like to hear other peoples' thoughts or see it on a dyno.

Mike

 

helicoil, I completely see where you are coming from now, after reading your last post about 10 times. We are in complete agreement on the pressure/ ratio aspect. And I don't know if you were being facetious or not when asking about actual running cylinder pressure (forget about the DCR) vs. cranking cylinder pressure. I have actually done that test, and I had a 5.7 (I don't recall the specifics) it pumped I think 175 or so psi. on a cranking test, but when running at idle it pumped I think 90 psi.. And the only explanation I can come up with off the top of my head is due to the fact that during the cranking test the throttle body was wide open, and while idling obviously it was closed. And if your shop truck doesn't pump between 195 and 210 on a cranking comp. test (throttle wide open of coarse) there is something wrong with it. I guess what I was trying to say from the get go is that most street engines have a DCR of between 8.0 and 9.0, but the cylinder pressures are all over the board due to thousands of other variables, largely (valve events, intake, exhaust flow) relating to the pumping efficiency of the engine. And I think alot of people confuse a comp. RATIO and cylinder PRESSURE as being one in the same, I have learned through our correspondance that you are NOT one of those people.

 

Wow, where to start. I really feel like participating here, but don't know where this is going.

So many rules of thumb seem to be given heavier weight in the past few years.

I think many people get hung up on a number and maybe miss out on what the concept is, or I'm the one confused.

DCR or Dynamic Compression Ratio. Mathematically calculated, based on zero rpm (weirdness starts) is the point where the intake valve closes, finally able to trap the inducted charge for compression. I like to think of it as dynamic displacement as it shortens the stroke in essence.
This figure ignores inertia effects of cylinder fill and assumes the charge is of equal density in the cylinder. As the piston rises from BDC it assumes the charge is immediately pushed back into the intake in proportion to the swept displacement before the intake valve seals. This is no doubt true if you were to rotate the engine over by hand.

It seems to be a rule of thumb that tends to correlate to octane requirements and low end throttle response of a particular engine. What its not is very Dynamic to me as people just focus on a cranking compression number at some universal? cranking rpm, unthrottled.

Some 4.6 Fords have 205-210 psi cranking compression, hot. Are they a low end powerhouse? Are they high rpm horsepower makers? Yes? No? Well if you've had the opportunity to run one in a low gear up to the rev limiter you might come away unimpressed, depending on what you're used to. Clearly 281 inches is a bit small for a 3600+ pound car, regardless of gear. It certianly doesn't drive like an LS car. If you concede the above is an accurate account then how do you explain the performance difference?

I know this is over simplification and common sense, but my point if I have one is that regardless of the number, performance is going to vary widely between applications.

As rpm increases to some level your engine is going to continue to improve its VE% because its able to contain more mass flow in the cylinder as the intake continues to fill as the piston rises. The charge will compress and stack up in the lower level of the cylinder due to piston rise but the charge having mass will continue to flow due to inertia.
Naturally this results in more power. This is the dynamic part. Your displacement and VE effectively changes throughout the operating range. If your intake duration and or closing point is too short or early for a particular rpm you will miss out on available fill time and power will fall off.

Anybody who drove an L98 car can attest to its excellent low and mid range power. The extra long intake runners were "tuned" to produce a maximum ram effect at a relatively low rpm due to resonance and velocity. On the flip side an LTx car has a very short intake path and it subsequently peaks at a higher rpm and lower rpm power suffers. Both of these examples would use vastly different camshafts with the intake closing point in different points to maximize the performance of its induction system.

The LSx manifold has a fairly long runner and would fall between the two above examples. Its modern cylinder heads allow the LSx to have a broader powerband with a higher average power level than its predecessors.

Wow, sorry.

All 350ish engines could conceivable be ground with cams that produced the same cranking compression but would behave very differently when operated under something more than starter power.

Intake closing point is just one small detail. I can build an engine with 14.3:1 compression and 180 psi cranking pressure. Do you think it will run on pump gas? Yes? No? Why?