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Old 07-19-2010, 04:26 PM Show Printable Version Show Printable Version   Email this Post to a Friend Email this Post      #1 (permalink)
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Cam Gear Tuning for Dummies (Understanding what is happening)


Well< I've been wanting to work on this for a while and got it done last night...

let me know if some of the analogies are too specific ( i was trying to make it where anyone can understand how to tune their cam gears and what it will effect)

_____________________________________________
Well, this is a topic that although I'm use to toying with, i still often find myself going through a mental walk through for a good 15 minutes before even breaking out the wrenches, and probably another 10 minutes of staring at and marking the way my particular cam gears effect the cam's relation ship to the engine's crank. For instance... if i loose the bolts on my fidnazas I have to remember that the black part of the gear is irrelevant basically and is tied in constant to the belt and basic timing alignment than it is to the relationship between the cam timing and compared to the cranks timing on each particular cycle. A lot of it is figuring out if I'm advancing or retarding the cam in what direction i turn it.. but this is how i use a mental picture to do it properly


A few things to keep in mind...

1.) the Cams and crank go clockwise if you're standing on the driver side looking at the face of your cams gears on our engines (there's a pic in one of the links about a Honda engine and the arrows are back wards to what our engines do)

2.) Depending on your cam gear's marks and how old the gears are (some old ones were not so DSM specific) the +/- or R/A markings might not be labeled correctly so pay attention to the dowel pin for reference in it's relation to the outer "ring" of the gear

3.) Turbo cars and N/a car will react a tad differently to the same adjustments and sometimes need the opposite (depending on the grind but I'm not getting that deep in) just remember that "in general" an N/a car will gain top end from more overlap because of scavenging with a properly built exhaust system... A turbo car will not gain as much top end because turbo manifold back pressure is keeping us from being able to scavenge, so we have to decrease our overlap to shut the door and have the most time of just the intake valve being open so we can force as much fresh air in as possible for top end pulling power.

Basically an N/A car depends on scavenging for top end, out cars use it for mid range off vacuum areas and depend on the turbo for "top end" power (there's arguments in each favor but these are based on OEM style cams and "slightly hotter" but meant for their application (some N/A cars when turbo'd pick up power like crazy because of their OEM style NA cam

Now down to da' bid-ness.........


First of all i think as the combustion chamber as a "room" and in it are the piston, intake and exhaust valves.. they come and go

For me it helps to start analyzing the cycle by using the exhaust stroke at it's ending (or the exhaust valve leaving the room) as "step 1" in the even that is about to take place that I'm adjusting (the overlap mainly)

on my gears i just think of the shiny billet legs of the gear as "more lobes" as a visual reference and it will be moving from roughly 5 o'clock (from where i start to paying attention) to where it's roughly going to 10 o'clock in a clockwise direction... and the intake valve is starting at say 2 when the exhaust is at 10 O'Clock (and this is probably not physically correct, just for the mental picture of it pushing down the valve stems that are pointing up )

Now picture the event you're changing to be starting with the piston at the top of the exhaust stoke and the exh. valve on it's way to closing ( say 7 o'clock) so the intake stroke can start opening (coming in the room ) and the cam lobe is at 3:30

the 'overlap" is when the intake valve is going to open (come in the room ) and start letting in air before the exhaust valve has fully closed from letting exhaust out (this is the "overlap - It is easiest for me to think of it as the intake valve coming through the door before the exhaust valve has left the room)

Now the earlier "in the room" the intake valve comes the longer it will be open while the exhaust valve is still is open and should be scavenging (and theoretically help pull in more air/fuel) this is done by advancing the intake valve or retarding the exhaust valve This is increasing overlap, or tightening "lobe separation angle" or LSA This is for more high RPM power at the cost of low end and mid provability (DD'ing duties mostly are lost)

Now the more the exhaust valve is retarded the later it's "in the room" and the more advanced the intake cam the "sooner it comes in the room" and the longer they are there together (overlapped)

Now decreasing overlap or widening LSA will give you the opposite (more bottom end and torque at a lower RPM at the cost of peak RPM HP) but sadly the effect in the direction of "bottom end power" for daily driving is far less "bang for the buck" (or degree in this case) as compared to increasing overlap or tightening LSA

I've attached a couple links that i use and have as printouts in case I'm questioning my thinking just to be safe (copied and pasted them into notepad and printed them)

now that's playing with overlap and LSA...there's a whole other thing taking place and that's the relations ship of the complete event (and not factoring in what the overlap is doing for power)

And that is when exactly the transition from exhaust to intake (decided by what valve is open ) is happening in relation to where the crank is in it's reference to TDC.. I hope this helps you understand if not become less afraid to play with the cam gears and use the advice in the links to make your decisions on what to do and how you do it. The exact procedure is outlined in the dyno tuning link and tells you the correct ways to set both portions of cam tuning on the dyno or street.


What these links should help you understand are the differences in the 2 ways you are changing relationships of the 3 components (both cams and the crank) and why although the same amount of lobe separation is present whether you have the exhaust retarded 4* and the intake at 0* or the exhaust at retarded 2* with the intake advanced 2* (same overlap or LSA but at a different time in the cranks cycle)

This has some basic cam info with DSM reference in them
4G63 Camshaft Degreeing

This explains LSA and overlap as thought about in where the lobes are to each other (This is based around a Honda engine and the arrows in this diagram of a cam are back wards from our engines as theirs rotate the opposite direction)
Cam gear tuning basics - Team Integra

And this one explains the process of tuning in the proper order (but as the last link this one is on a Honda again) but it's still an in-line 4 with 2 cams
Cam gear tuning basics - Team Integra


In Summary... more overlap also brings on these effects besides the difference in turbo and non turbo apps from mid-range to top end... Some of these can give you some good advice on a DSM to preventing knock, or if you have non what you may stand to pick up.. and even how to calm some knock happy cams and use your timing form the ignition accordingly

example,... you cot some tap happy 272's and are limited to about 12* at say 5500 RPM and are getting some knock still at say 25psi assuming safe AFR's

1. Moves Peak Torque to Higher RPM

2. Increases Maximum Torque

3. Narrows Power band

4. Builds Higher Cylinder Pressure (higher pressures induce knock)

5. Increase Chance of Engine Knock (knock for no other reason)

6. Increase Cranking Compression and Effective Compression (hard starts and higher effective compression creates heat which creates knock)

7. Idle Vacuum is Reduced and Idle Quality Suffers ( some like em lopey - like I do as you will there)

8. Open Valve-Overlap Increases (duh!)

9. Closed Valve-Overlap Increases (this can let late burning gases if not make power still provide a burn out the exhaust valve giving high EGT's but helping in turbo spool - particularly if the unburnt fuel is from overly rich tuning and not poor timing)

10. Decreases Piston-to-Valve Clearance ( we all know what this can do when it gets too small and things get hot)

Due to these rules you can try to get rid of some mid RPM knock and timing limits by advancing the exhaust cam, reducing overlap and helping top end power or retarding the intake to do the same... and if this allows you to run say 20 - 21* timing in that range you may choose to further dial in your new found power from the greater timing... do things in 2* increments. and there's a couple ways to get some of the same goals with different side effects

check this
(on a turbo car with turbo style grinds)
advancing the exhaust cam = less overlap = more top end...going 2* per adjust you may stop at 4 to be safe and see how that is

or you may leave exhaust alone and retard the intake cam, again by 2*'s and up to 4* again and see how that is

Or you can try and may end up at exhaust advanced 2* nd intake retarded by 2*

all 3 ways you have decreased overlap by 4*, but changed it's relation to the crank, the more retarded in relation to the crank the whole even the more top end, the more advanced the more bottom and mid so actually you'd end up better off taking the cams in unison either 2* each towards each other or away from each other depending on what you want with over lap depending on if you're widening to allow a more aggressive tune and if you want more top or mid-range with fast spool... after starting to put this together tonight i now know why no one has touched on this before as it can be a HUGE can of worms with variables and what out-weighs what in terms of your goals, and ultimately your engine combo from CR,lift, duration, and turbo choice will all play a role in what is best for your setup

in spirit of that I'm going to starts a poll to see how people have done theirs and what their goals were and if there were any noticed pro's or con's

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Old 07-06-2011, 01:18 AM Show Printable Version Show Printable Version   Email this Post to a Friend Email this Post      #2 (permalink)
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Update Suited to turbo cars specifically


Ok, in the areas of LSA (lobe seperation angle) and "Overlap" (when the intake cam opens before the exhaust has fuly closed)

Earlier i stated that tightening LSA or increasing overlap would gain top end at the sacrifice of bottom end and that Widening LSA or decreasing overlap, would make more torque/bottom and cost you top end

Well this IS true, for naturally aspirated cars, but a theory i've had for years is now proven on paper so i feel i can back up my own thoughts with paper if needed.

ON a turbo car "our top end" is something differenty than an N/A's car engines "top end" since they are non forced induction they rely heavily on as much scavenging and ways of using airflow pricipals (think Bernoulli) to gain HP

Well on turbo cars where there's boost and exhaust turbines and housings, we can't freely flow our hot gasses fast enough to use scavenging once we are thoroughly in boost because the back pressures in the exhaust are fighting to keep the stuff in the combustion chamber from getting out, let alone letting it out fast enough for it to scavenge new air in. In my own logs and now in the dyno thread about cams, it's been proven that a turbo car will pull better with MORE LSA or LESS OVERLAP. with less overlap the exhaust has it's best time to escape and then the door is shut so that as little of it as possible can work it's way back into the fresh charge coming in and hence not spoiling it alowing the most fresh backed up (boost pressure) air and fuel in for the coolest cleanest charge

Now, some have argued this is not the case in the past and after much of my own testing i can see why. it's perception of "top end" for some people "top end" is 4k RPM through 7k RPM, for others its' 5500 to 8500 RPM

Well, more overlap will make the car "hit" harder around boost coming in (and come into boost sooner) with more overlap becyse before we're making boost we ARE using scavenging (just not once we're really into it and hauling a$$) so this added pull from 4k to 6500 or so is what many will call their "top end" But i found with 264/272 combo that it did give me more in the turbo engines "down low" but i couldn't barely rev past 7200 RPM..

Then When i changed to less overlap, i lost even MORE bottom end than with the "scavenging try" (but i'm talking the areas before 4k RPM all together) but low compression ratio and other turbo specific parts of the entire combo make this method of tuning cams almost useless at stock or near stock engine build levels. BUT, ONCE YOU START REVVING TO 8K RPM +... you'll find that the less overlap you have the further and harder the car pulls (as well as gaining vacum at idle and running much smoother at low RPM)

The same car taht wouldn't run past 7200, now with 4* advanced exh cam and 4* retarded intake cam was spooling a little slower but once it was on it was like a locomotive, she was pullin and nothin was stoppin' her.. The car now rockets past the entire 8k RPM range and buries the stoock tack to the 9K RPM limiter i have if i don't shift early in 1st and second gears when they start to "freewheel" the LSD and both front tires the revs are insanely high. I'm shifting at right around 8k in 1-3rd gears, and have no need to get near 8k in fourth as long as i'm DSM brakes LOL But before with the cams both overlaped, i could spin the tires and even with no traction the engine hovered around 7200-7500 RPM untill the car caught traction or caught up, now it will rev to the moon if i let it

So that's why i'm adding this

IF you want midrange 4k - 7 -7.5k play with overlap a little

IF you want top end 7500RPM + when play with getting rid of overlap (also, decreasing overlap moves the exh valve further from the piston as it comes up and the intake not start moving untill the piston is further on the down stroke, but you MUST remember that they will do teh opposite on the intake stroke so don't get too insane, and if you do post results :)
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