Overlap in cams is good for FI cars
 

I stole this from Jeff Evans at www.Boosted-hybrid.com I thought would be pretty good info for this site since lots of people here are running log manifolds;


"Its been a trend lately to just "run gsr/itr cams without cam gear adjustment". Most cars that i tune do not have cam gears, so adjustment is not possible in most tuning sessions. After experimenting with cam gears on the last few cars i have been pleasantly shocked picking up 30-50whp, and 30-40 ft-lbs of torque at the same boost level with just cam gear adjustments. Gains were able to be made all over the place. Basically free horsepower. Here is an example of a car that i tuned on Sat with some cam gear adjustments:

http://www.evans-tuning.com/dynos/ca...djustments.jpg

The intake was +4, exhaust was -2. This is opposite of what people recommend for tuning turbo cars. I am dialing in overlap, not out. The other engines have been anywhere from +3 to +5 on the intake, and -1 to -3 on the exhaust. My thoughts on the matter are that with log manifolds (this car was using one), scavenging goes to crap. Increasing the breathing of the engine opens up the engine with the higher exhaust pressure from running the log manifold, and allows for less exhaust gas contamination to occur. I noticed that the plugs burnt much better after the cam gear adjustment, as well as less ignition advance needed to be run (and not because of retarding the ignition timing since the intake cam was advanced)"

Re:Overlap in cams is good for FI cars
 

how the ---- does that work? where does the timing belt attach? I call bs.

Re:Overlap in cams is good for FI cars
 

i hope you are joking....

hard to catch sarcasm over the net though

Re:Overlap in cams is good for FI cars
 

That's something new that I'll have to learn about with this B series is different cam timing for two cams. Oh, the possibilities, for ---- ups, and for good stuff. :-\

Re:Overlap in cams is good for FI cars
 

they mean to say adjustable cam gears. or they gave the trunk money a new job description, now he in the hood monkey and spins the cams.

oh wait thats gm. i guess he pushes the chromoly pushrods up and down

Re:Overlap in cams is good for FI cars
 

are you drunk tonight?


and yes, I was being sarcastic, I know the timing belt just hooks up the the vtack cylenoyed, duh.

Re:Overlap in cams is good for FI cars
 

yea a lil. im tryni to get stoned so i can fall asleep tonighit.

Re:Overlap in cams is good for FI cars
 

This is confusing because more overlap causes less cylinder pressure. the reason people raise compression, use nitrous, or turbos is to increase cylinder pressure. now he says he uses more over lap and less timing. So thnk about having a car that has too much boost or compression, and you're detonating, you would use more overlap or less timing to fix the problem. How would more overlap (less cylinder pressure) and less timing cause more power. That doesn't make much sense unless the initial tune was so bad that the spark was blowing out. I don't know but taht doesn't make much sense to me.Maybe hes got it backward and hes actually getting less over lap.

Re:Overlap in cams is good for FI cars
 

Did you read that the increased overlap might be preventing contamination from the reversion effects of the log manifolds?




p.s. Can the drunks go ---- up G.D and not the tech threads, thanks.

Re:Overlap in cams is good for FI cars
 

a good guy i know at a machine shop, said for me to change the cam in my 302 to something with less overlap... because i guess the HO roller cams have a lot.

but i bet he is thinkin about a really free flowing good system looking for max power potential

Re:Overlap in cams is good for FI cars
 

Yeah I did read that but it's kind of counter intuitive, I don't know for sure that it's not true but why would haveing both valves open at the same time cause less exhaust to go back into the chamber.

And I woudn't have been able to spew all that out If I were drunk. But sorry bout the spelling.

Re:Overlap in cams is good for FI cars
 

lol, I wasn't talking about you being drunk ;)

Re:Overlap in cams is good for FI cars
 

cool

Re:Overlap in cams is good for FI cars
 

See, ---- like this is just better with pictures and diagrams. Where the piston is, what postition the valves are in. You know, english. It's just hard to draw pictures in your head with numbers like -4 and +2.



I guess it's just all new to me and I haven't read about how "normal" people even tune these things.

Re:Overlap in cams is good for FI cars
 

It's pretty damn self explanatory. That's the cam gears adjustment. Most cam gears will have little number or tooths to count.
It's not that hard to understand really.

Re:Overlap in cams is good for FI cars
 

thats degrees people ::)

Re:Overlap in cams is good for FI cars
 

different engines will like different type of overlap or less of. It depends on the set up, which differs from one another. I tuned cars on the dyno that like increasing the over lap and some that don't. The only way to know if you are tunning your camgears right is to do it on the dyno. 98ctr

Re:Overlap in cams is good for FI cars
 

The theory behind this is having the exhaust gases exit through the exhaust valves while opening the intake valves. The flow of the exhaust gases going out the exhaust valves are supposed to help draw air in through the intake valves at the same time.

Stan

Re:Overlap in cams is good for FI cars
 

when working with FI if you open up the intake valve the compressed air is going to come in and push the exhaust gases that are hanging around in the exhaust ports out, you'll lose a little bit of boost, but if you time it just right you could find the balance between forcing exhaust out and letting boost out.

Re:Overlap in cams is good for FI cars
 

Hmm..never thought of it that way, that sounds like a reason explaination too. :)

Stan

Re:Overlap in cams is good for FI cars
 

its a chain effect.. if you have the exhaust open a lil while the intake opens, as stated above, you might loose a lil boost, but, the exhaust being PUSHED out by the intake spins the turbo faster...

Re:Overlap in cams is good for FI cars
 

thats probly why since my cam gear gear skipped a tooth my car has been spooling up way faster 8)

Re:Overlap in cams is good for FI cars
 

I see some really confused people making statements that simply can't be true in every case.

The best post was (I won't bother quoting) the one about how every engine will be different. No, I'm not being sacrastic, as this is true.

Overlap does NOT, in itself, cause lower pressure. Think about it...if you have a turbo that can fill your cylinders with the set psi of boost way before the intake valve shuts, then why not leave the exhaust open and let the turbo run free....then shut the exhaust valve and get the cylinder filled just before the intake valve shuts.

In other words, if you can pump the approriate MASS of air into the cylinder within the last 10 degrees of crank rotation, why try and get it in there on the first 10?

I don't think you'll be getting much exhaust reversion with a log manifold. Simple physics says that air moves from high pressure areas to low pressure areas. When going from a small port to a runner that's significantly lower, pressure drops, as does velocity.

The implication is that these log style runners, although cheap and easy to build, they are very large. This slows velocity. Having smaller runners would increase spooling, and likely never limit you on the top end. Take a look at some of the Supra headers....many use runners that are smaller than most guys use on honda engines.

Re:Overlap in cams is good for FI cars
 

because if you're just letting it blow through you could over spool your turbo

but the fact is a turbo isn't exactly a free flowing device, it blocks up the exhaust and uses exhaust energy to spin, it's not just a hollow device that lets exhaust pass as it pleases.

Re:Overlap in cams is good for FI cars
 

:-[

Re:Overlap in cams is good for FI cars
 

Isn't that what your wastegate is for? Isn't the purpose of a blow off valve to keep your turbo spooling?
===========================
Yes, and by NOT trying to cram boost in during the entire intake duration, and having overlap, you are allowing air to go through the engine and continue to spool the turbo with the intake valve open.

So if we say that your turbo can put air into the cylinder in 20% of the time that the valve is open, you'd rather cram it in during the first 20% of the time and then have your turbo fighting to keep that pressure....thus slowing the turbo? I think I'd rather have it freespinning and then jam the air in during the last 20% of the time the intake valve is open. Think about it a bit...this is likely what this guy achieved by putting in more overlap.

Re:Overlap in cams is good for FI cars
 

I'd sooner listen to engloid than stop drinking.

if that makes sense.

Re:Overlap in cams is good for FI cars
 

Kyle put the crack pipe down

Re:Overlap in cams is good for FI cars
 

holy crap kyle, you said that?

the duration that the exhaust valves would be open at the same time as the intake will most definitely not over spool a turbo... totally different from running with a charge pipe disconnected.

redemption will follow only with you ridiculing someone, me not excluded.

Re:Overlap in cams is good for FI cars
 

Sometimes you gotta wonder if people really have a grasp on the concept behind a 4 stroke engine to begin with.

Re:Overlap in cams is good for FI cars
 

Engloid is the master of stuff. And he knows ---- too. Ya crazy ass all knowing welding muthafucka

Re:Overlap in cams is good for FI cars
 

Haha!! Yeah, you guys just let me know anytime you wanna know about "stuff"...cause I know all about it. ;D

Re:Overlap in cams is good for FI cars
 

Well, im not any master on this kind of thing, so i can just look at it and give my idea or perspective of things.

Im not sure how much it has to do with overlap as it has to do with the timeing of the exhaust valve.

Since its a turbo motor, and the exhuast gasses velocity are what cause the turbo to spool, than buy opening the exhuast valve eirlyer or later you change the amount of work the engine has to do to get the turbo moveing.

In a natrually asperated motor, the cam opens the valve right at the pressure drop, somewhere halfway down the combustion stroke, depending on your motors r/s ratio and other things. The valve opens slightly into the pressure front that moves the piston. This causes exhuast gasses to rush out the exhuast, and as the intake opens since the gas is flowing (and objets in motion tend to stay in motion) the piston pumps gasses out the exhuast and pulls them from the intake valve.

So this lets me belive that if you open the exhuast valve later than you have all these hot gasses that are pent up in the cylinder that dont do any work, because there not quite enough velocity to move a piston/rods/crank/drivetrane/your car/, but yet still have the heat and mass required to spool the turbo.

Im not sure how you went about tuneing the thing so its hard sayin rather is overlap that created the power, or the timeing of the exhuast or intake valves that did it. Next time you are doing this, maybe you should try to adjust the exhuast only. See what it does, than try the intake.

Thats how ive allwasy done it on my motorcycles. Anyways just my $.02



Sean

Re:Overlap in cams is good for FI cars
 

Damn wish i had a B-series ----

Re:Overlap in cams is good for FI cars
 

This is making a generalized statement. I bet in the cars dynoed the intake pressure (boost) to turbine inlet pressure ratio was higher then 1. This would cause scavenging as in NA. If he was running a smaller more restrictive turbo like many stock FI cars, I bet the overlap would cause a reduction in power because the turbine pressure would be higher then the intake manifold pressure.

Cool observation though. I think a set of cam gears would be beneficial in either case.

Re:Overlap in cams is good for FI cars
 

yeah thats what i was thinking, the turbine inlet pressure is usually many times more than the boost pressure... I guess it depends on your setup

Re:Overlap in cams is good for FI cars
 

sure, overlap spools the turbo better, but without knowing the rpm of your turbine and the rpm abilities of your turbo you can't tell me I'm wrong saying that it is possible to oversping your turbo and cause problems.

if we're not working with diesel engines then the air going into the combustion chamber is mixed with fuel, so if you're blowing a bunch of that air out with too much overlap you're afr readings will be much richer than they really are inside the cc, so inside the cc you're likely to lean out and blow ---- up.

I'm not trying to say overlap is shitty, since we have a dyno graph at the top proving it can be beneficial, but you can't say that you need all kinds of overlap and the more the better. the point I'm trying to say is don't go overboard with it, find the point when the benefits are highest and the draw backs are minimal, that's the point where the most power will be made.

Re:Overlap in cams is good for FI cars
 

edit: found a better quote to describe how a wideband functions

"The wideband air/fuel ratio sensor combines an oxygen-sensing "Nernst" cell from the narrow band sensor with an "oxygen pump" to create a device that gives a wide range response to various air/fuel ratios. The Nernst cell senses exhaust gas oxygen in the same as a conventional narrow band O2 sensor. If there is a difference in oxygen levels across the ZrO2 sensor element, current flows from one side to the other and produces a voltage."
and again, we're not talking horribly long durations. even at idle, and I'm going to be lazy here and not calculate at all (hopefully engloid has a formula that he can whip out), 5° of overlap out of 360° is really not going to throw an impressive amount of fuel out of the engine and into the turbo. on top of allowing the compressor side of the turbo to spin without as much backpressure, and of the added velocity hitting the exhaust wheel, think about the cooling effects the timing will have on the cylinder itself. less chance for hot spots, less chance for preignition.

Re:Overlap in cams is good for FI cars
 

I'm not seeing where you're going with that. what effects will the timing have on the cylinder? what timing are you reffering to?

Re:Overlap in cams is good for FI cars
 

sorry, I got side tracked in thought.

the cam timing changes, dialing in the overlap as opposed to zeroing it out, will allow the cylinders to completely evacuate exhaust gas. having both intake and exhaust valves open at the same time will allow the fresh (charged) air to give a bit (not much, but still something) of cooling to the cylinder.