An essay i wrote on Rsbb many years ago....


Cam Timing is not rocket science but its different for every engine.
You need to see peak lift at the point your piston is travelling at the fastest speed. This obviously varies from engine to engine so go and get a pen and paper and do some drawings...

Retarding from here will generally move peak horsepowqer higher and advancing move peak horsepower lower. Overlap will improve spool up but drop peak power.

Heres a basic rundown of the 4 critical timing events:

VALVE TIMING EVENTS IN ORDER OF IMPORTANCE:

Exhaust valve opening:
The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power stroke and it will make for good emissions, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down.

You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.

Exhaust valve closing:
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees ATDC, you will have a short valve overlap period. Less overlap makes less emissions, but it does not help power at the higher engine speeds. Closing the exhaust valve later, in the vicinity of 40 degrees ATDC, will mean a longer valve overlap period and a lot more intake charge dilution that will translate ultimately into poor low-speed operation.

Some compromise must clearly be made to determine just how much overlap one needs to use. Many factors such as idle quality, low speed throttle response, fuel economy, port size, and combustion chamber design must be considered in making this choice.

Intake valve opening:
A somewhat more important timing event is the intake valve opening.
Early opening allows for a greater valve overlap period and adds to poor response at low engine speeds. Now, for the high performance enthusiast, low engine speed could mean 3000 RPM, but I would not consider such an engine as appropriate for normal road use!

If you are not concerned about emissions, then early intake valve opening will help the power output of the engine. That is, earlier valve opening will have the valve open further when the piston reaches maximum velocity and that, in turn, will increase volumetric efficiency.

Intake valve closing:
Now, the last timing event is the most important, and the most critical to engine performance - The closing of teh intake valve. This event governs both the engine's RPM range and its effective compression ratio. If the intake valve closes early, say about 50 degrees ABDC, then it limits how much air/fuel mixture can enter the cylinder. Such an early closing will provide very nice low speed engine operation, but at the same time it limits the ultimate power output as well as RPM.
Another problem with early intake valve closing that most people do not consider is that if you have a high compression engine, say 10:1 or higher, you will have more pumping loss trying to compress the mixture. This might even lead to head gasket and / or piston failure!

These observations suggest that if you close the intake valve later the cylinder will have more time to take in more air/fuel and the RPM will move up. That seems simple enough, doesn't it? The later the intake valve closes the higher the RPM and therefore the more power, MAYBE?

In reality if the intake valve closes past 75 degrees ABDC, you could lose most of your low-speed torque and if your static compression ratio is only 8:1 like std cosworth, the engine will not be able to reach its horsepower potential. This should give you a better understanding of why the intake valve closing is the most important timing event.

I hope you have found that interesting folks...

 

nice clear write up mate

 

 

i dont think many people will realise that both the inlet and exhaust valve is open at 0 degrees

 

 

sorry to sound divvy,BUT could you possibly explain what overlap actually is?

 

or is it explained in gareths little diagram there?

 

overlap is a time period when both the inlet and exhaust valve are open(exhaust closing, inlet opening)

overlap happends between the exhaust and inlet stroke

induction. compression, power, exhaust (overlap) induction, compression, power, exhaust (overlap) induction, compression, power, exhaust (overlap) induction, compression, power, exhaust (overlap) induction, compression, power, exhaust

get me

 

got ya!cheers!
thats not explained in suck squeeze bang blow in my scouts book!lol!

 

Nice writeup Stu

And cool little diagram there garethT

 

Thanks. Just tidied it up and added some headings to make it a little clearer.

 

who would have though sticks with lumps on could be so complicated

 

 

what does BBDC, ATDC and ABDC mean? thanks

 

BBDC - before bottom dead centre
ATDC - After top dead centre
ABDC - After bottom dead centre

i think


so Stu, is there a point where you can reduce boost threshold without killing off maximum power too much? although sometimes i'd prefer a bit more low down punch to top end pull

 

sounds about right bud-weis i thought it was something along those lines but couldnt remeber!

 

You forgot BTDC - Before top dead centre


Do wild cams work as well on turbo engines?

 

he never asked otherwise i would have

 

totally class post here and tbh prob one of the best ive read this year, such a small post yet contains sooo much infomation

DEFFO need this in the tech section as a keeper

 

Q) my engine has a lot of overlap 24v BOA cosworth engine,once I put a free flow zorst it uses more fuel.

why ?

A simple test for all who know engines

 

yopu acidently welded the zorst direct to the fuel feed and so its just pissing out

 

lol

well it does not realy us more fuel, but more throttle is used at lower RPM so more fuel is used.

Clue

the orginal engine was desgned to be used with CATS

 

Gas flow is higher in the exhaust system as you've reduced backpressure.

Therefore more unburnt mixture is pumped through during overlap

Neil.

 

animal rights protests and so you have to drive quicker to avoid the time delays

 

bang on neil

bal you cooont

but you making me piss

 

Dear Stu'll fix it .................................................. .................................................. ............................ My mates got a "cam of unknown quantity",its a piper FR 391 it was destined for his S1 (i can hardly keep a straight face here stu........ )
it won't idle below 1500 RPM have you any ideas mate.Also can you type your answer in Spanish so he can read it as well.FPMSL

yours cheeky Dooer.

PS can you also fix it for me to dance with bucks fizz.

 

Stu,

Started trying to make some animations showing the piston and valves close up showing there movements relative to each other as i thought it might help those still struggling to full yunderstand it, especially with respect to overlap and the scavenging effect.

Gonna take to long though

Could always knock up some diagrams and/or schematics if they are to be of any help.

Just shout

 

Stu,

Can you either go one step further on this topic or maybe start a new with reference to Cams in Turbo engines???

I can only imagine that as Boost is above Atmospheric pressure the effects of the cam profiles with regard to cylinder filling etc will be different to that of an N/A engine.......

Or do they have the exact same effect as N/A cams do on N/A engines despite the induction being forced.....

 

good write up Stu

 

Thanks Stu another interesting read.

Could you still explain about the benifits (or not) of putting different cams in the YB

Steve.

 

come on then stu,

not mention'd lift, and how it changes bhp and torque @ different rpm points.

 

and the correct way to time the cams too, ie what degrees and whats involved

i have till tuesday

 

Found this one !



Loads more here

http://www.keveney.com/Engines.html

 

found this one two which is pretty cool

shows the cam movement, valves opening/closing, induction stroke/exhaust stroke all in one slowish movement

http://auto.howstuffworks.com/question685.htm

 

Now I know all engines are different and even identical engines could be different but...........
If you could put the same spec cam in lots of different 16v 20L turbo engines (cossie, gtir,scooby, evo etc) would if give similar results if run with the same boost? Or to phrase it another way, could a spec that works well in one engine work well in a totally different engine?

 

it works completely differently on each engine, every component that is different will affect how a cam works

turbo, inlet/exhaust manifolds , valve sizes , compression ratio, etc etc will affect the choice of cam.

you couldn't fit a cossie cam to another engine, even if it physically fits and expect any gains due to those points mentioned, this is all in my opinion though

 

Stu would you care to make comment on the following:

Data c/o Piper cams.

Cosworth Turbo Fast Road Cam
Power Band 2000-6200rpm
Duration inlet=264deg exhaust=264deg
Valve lift inlet=0.340" exhaust=0.340"
Timing 20/64 64/20
Full lift inlet=112deg ATDC exhaust=112deg BTDC
Lift@TDC with clearance inlet=0.034" exhaust=0.034"

N/A Cosworth Fast Road Cam
Power Band 2000-6500rpm
Duration inlet=268deg exhaust=268deg
Valve lift inlet=0.395" exhaust=0.367"
Timing 26/62 62/26
Full lift inlet=108deg ATDC exhaust=108deg BTDC
Lift@TDC with clearance inlet=0.067" exhaust=0.065"

To summarize:
N/A cam duartion is 4deg advanced compared to the Turbo Cam
Valve lift is Equal on Turbo cam and unequal on N/A cam
Valve lift is much higher on N/A cam than that of the Turbo cam especially on inlet
Turbo cam inlet opens and closes later than N/A cam
Turbo cam exhaust open and closes later than N/A cam
Full Lift is 4deg later on Turbo cam than that of N/A cam
Valve lift at TDC with clearance is almost twice as much on N/A cam than that of Turbo cam.

If i get a chance i'll plot them on a cam timing chart so as to give a visual representation of how they compare.

 

Thae main thing on N/A's and turbo cars,


is to make the power the N/A's relie's on the cam's alot more to make the power,

Wots piper's race cam's turbo-n/a look like against each other?

 

Thats exactly my question though.....how and why?

To be honest, I haven't looked at the race profiles since i thought a reference to Fast road spec was more in keeping with this discussion......

Race cams can and hopefully will be discussed once the basis for the differences has been established between the N/A and Turbo cams.......

 

Here's the charts: