Long duration cams and why they work (By panic mechanic)

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Long duration cams and why they work (By panic mechanic)

Post by Fran »

Re: How does it work. Long duration cams
Oooh ooh and look at this beauty - glad I saved this one. Hard to work out at times. This was a post I made on the board many moons ago about CR and long cams.. The youngsters can learn a few things here too.

The relationship between long duration cams and compression ratio is as follows. A cam's duration is measured in degrees of the crankshaft travel since top dead centre(TDC) of where the cam starts to open the valve and to the point where the cam closes the valve. The agreed standard is to measure those degrees after the valve have achieved 1mm of lift away from the seat and when the valve is 1mm before closing. So if you have a 236(standard G-grind) it stands to reason that the valve will close 236 - 180 degrees after tdc(assuming the cam lets the intake start to open on tdc-most don't- it starts to open about 24 or so before tdc but for ease of calc lets leave that) - so 56 degrees after the crank turned at the bottom or of the pistons's travel up the sleeve does the intake valve close. Both valves being closed now it means the compression stroke. Now take a 288 - only after 108 degrees of crank movement after bottom dead centre(ABDC) does the intake valve close - that only leaves 72 degrees of compression stroke as opposed to 124 degrees for the standard cam. In effect this will lower your effective compression ratio. Why does it work then?. The speed of the air rushing in the port of the open intake will at some point be higher than the effect of the rising piston wanting to push the air out of the open intake valve. At this point you feel the effect of the cam working as it lets more air into the chamber. This effect depends on things like intake runner design - length etc. That is why short intake runners and long duration cams can make things worse than better - a short intake runner is not condusive to high air speeds - neither is a too big intake runner (for that reason why you might lose power going to 50mm tb's instead of 45's that made power well). So to counter this effect of a lowered compression ratio some guys will raise the compression ratio to the reasonable limit of standard fuel (around 11.5:1) because this will recover some of the bottom end that the cam loses. On the other side of the coin - a higher compression ratio means more fuel/air mixture being present in the chamber which aids power and make for a stronger motor at top end as well. The problem with that is that you now have to watch the timing advance to stay out of ping with the motor on wide open throttle and high rpm. Here is where programmable systems comes into it's own - you can advance the timing low down to aid acceleration and pull it back slightly at high RPM wide open throttle to avoid pinging.

Just looked up at this old post from panicmechanic;

One thing I cannot grasp: 'a higher compression ratio means more fuel/air mixture being present in the chamber which aids power and make for a stronger motor at top end as well'

My understanding is that you are compressing the same column of air/fuel mixture, although to a higher pressure, so not sure how it means that more fuel/air mixture is present in the combustion chamber.
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Re: Long duration cams and why they work (By panic mechanic)

Post by Ady »

I'm sure the answer is in this section:
The speed of the air rushing in the port of the open intake will at some point be higher than the effect of the rising piston wanting to push the air out of the open intake valve. At this point you feel the effect of the cam working as it lets more air into the chamber.
The way I understand, by making logical assumptions:
The velocity of the incoming air allows more air to remain in the chamber due to the higher pressure than stroke. Since there is more air, the amount of mixture being compressed in the given space is more overall. That is how the compression is increased.

That's my guess, I know close to nothing about these things, correct me if wrong. :lol:
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Re: Long duration cams and why they work (By panic mechanic)

Post by Fran »

We will have to see more replies! :shock:
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Re: Long duration cams and why they work (By panic mechanic)

Post by Solo786 »

First you need to understand CR

lets use 10: 1, this means 10 units of air/fuel mix compressed into 1 unit of space (ie combustion chamber). So when the piston travels from bdc to tdc, its taking 10 units volume and pushing into 1 unit of combustion chamber space.

By skimming the head (a mod done to increase CR) you are making the chamber smaller.. there by increasing the volume that is being squeezed into 1 space. If the volume increases, you are allowing for more air/fuel mix to be drawn into the cylinder.
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Re: Long duration cams and why they work (By panic mechanic)

Post by Torker »

Higher c/r = denser mixture, denser mixtures burns more efficiently and completely, hence more power.
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Re: Long duration cams and why they work (By panic mechanic)

Post by ACiD_Omega »

Fran wrote:One thing I cannot grasp: 'a higher compression ratio means more fuel/air mixture being present in the chamber which aids power and make for a stronger motor at top end as well'

My understanding is that you are compressing the same column of air/fuel mixture, although to a higher pressure, so not sure how it means that more fuel/air mixture is present in the combustion chamber.
There is static CR that is calculated from the volume of the cylinder and head, that is what you are reffering to, but when you change cams, you alter the dynamic CR, dynamic CR is affected by many things like load, throttle opening, charge density and cam duration and phasing (or cam timing) and it's dependant on where in the rev range you are.
so longer cams have lower dynamic CR in the lower rev range, but higher dynamic CR in the higher revs
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Re: Long duration cams and why they work (By panic mechanic)

Post by panic-mechanic »

Correct acid omega. The longer duration cam effectively kills the static CR becuase it closes after bdc. So you need tomraise the staric cr to get the dynamic cr high enough again to be what the static cr would have been.
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Re: Long duration cams and why they work (By panic mechanic)

Post by AlexTDi »

Thanks for this great post :hurray:
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Re: Long duration cams and why they work (By panic mechanic)

Post by Khanman001 »

:thumbup: :troll:
always good to learn!! Great post!!

:hijack: Can anyone please tell me if a 288 Estas cam & vernier will work on a 2E sub+ABA crossflow head? Stock internals
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Re: Long duration cams and why they work (By panic mechanic)

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Re: Long duration cams and why they work (By panic mechanic)

Post by VWicked »

:thumbup: :hurray:
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Re: Long duration cams and why they work (By panic mechanic)

Post by Fran »

Why did everyone only reply two weeks later? Aggenee mense

solo786: 'By skimming the head (a mod done to increase CR) you are making the chamber smaller.. there by increasing the volume that is being squeezed into 1 space. If the volume increases, you are allowing for more air/fuel mix to be drawn into the cylinder'

Can someone verify this?
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Re: Long duration cams and why they work (By panic mechanic)

Post by Ady »

I thought it is the smaller volume of mixture but compressed into much a smaller (relatively speaking) space, therefore raising the compression?

Example:
Before skim - 1000cc (900+100) into 100cc combustion chamber = 10:1 CR
After (10cc) skim - 990cc (900+90) into 90cc chamber = 11.1 CR

That's what I conclude from a Google search.
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Re: Long duration cams and why they work (By panic mechanic)

Post by smoothscott »

Also, they way I understand it, is that longer duration cam/s aid in valve overlap and increase 'scavenging' i.e. increase the vacuum created by the exhaust stroke which 'helps' draw more intake velocity...in essence 'supercharging' the intake stroke velocity which helps draw more volume of air per stoke in turn increasing volumetric efficiency which is directly translated to more power...
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Re: Long duration cams and why they work (By panic mechanic)

Post by Fran »

The compression ratio theory looks correct. The way I see it is that the more you compress the mixture, the higher the subsequent temperature and that probably means a bigger explosion.

Yes, performance cams are a way of super-charging the chamber :cool:
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Re: Long duration cams and why they work (By panic mechanic)

Post by keval »

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Re: Long duration cams and why they work (By panic mechanic)

Post by smoothscott »

panic-mechanic wrote:The longer duration cam effectively kills the static CR becuase it closes after bdc. So you need tomraise the staric cr to get the dynamic cr high enough again to be what the static cr would have been.
This makes perfect sense, but at what point does scavenging overtake the problem of the 'low' static CR?...if it does at any point.

I'm finding it hard to wrap my head around this concept coz, for example, on something like on a Vtec motor, it effectively changes the valve timing at the crossover point to a longer duration cam lobe (with a little more lift) but this change in valve timing does add more power...BUT...then again you mention that longer duration cam/s kill static CR which kills power...

Does it have something to do with the dynamic CR that the longer duration cam has it's advantages or is it just at higher RPM that the vacuum created from scavenging overtakes the problem created with the 'low' static CR?
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Re: Long duration cams and why they work (By panic mechanic)

Post by Solo786 »

Ady wrote:I thought it is the smaller volume of mixture but compressed into much a smaller (relatively speaking) space, therefore raising the compression?

Example:
Before skim - 1000cc (900+100) into 100cc combustion chamber = 10:1 CR
After (10cc) skim - 990cc (900+90) into 90cc chamber = 11.1 CR

That's what I conclude from a Google search.
This is correct, my explanation of there being "more" air/fuel mix present was poor, but this puts it into perspective. The mixture ends up being more dense

Swept volume + Clearance volume / clearance volume = CR
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Re: Long duration cams and why they work (By panic mechanic)

Post by missioner »

all theory aside vtecs/VVTi/VANOS (etc) cant be compared to SOHC motors. Long duration cams do effectively improve the breathing of the motor regardless of whether its single or twin cam motor. This is the thing that grates me the most is when guys think a cam alone will resolve their search for more power. A camshaft (or a pair of) must be part of a bigger overall picture. Lots of power can be made with other methods before a performance cam is needed.

now back to variable timing valve trains, these systems are ways of compromising between a mild smooth idling, good torque curve at low rpms camshaft and a wild screamer at high rpms making massive kW.

Any performance mods performed on an engine should have this as the focus, besides all engines are simply large air pumps.
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Re: Long duration cams and why they work (By panic mechanic)

Post by OLDSKOOL »

Thanks Panic
Learnt alot from this tread!
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Re: Long duration cams and why they work (By panic mechanic)

Post by Deranged_9n3 »

Might be in the wrong section here, but I have read that on 'high performance' diesel motors they tend to LOWER the CR, that way they can add more boost and fuel

Makes sence to me but I am a noob. Told my dad this and he said that he cannot understand why they do this. For instance a pd130 motor has a CR of 19.5:1, people then make use of VW racing pistons tht lower the CR to 17:1 by having bigger bowels ( if that is the correct word ). Some people even stack mutliple head gasketts to lower the CR, a cheap way to do it.

Is there someone on this forum who can explain this to me ? Just an interesting read I had from a UK TDI forum earlier this week
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Re: Long duration cams and why they work (By panic mechanic)

Post by Fran »

They lower the compression ratio in order for the chamber to accommodate a larger volume of air and fuel to be compressed. It is pretty much compulsory, otherwise you will harm or break the internals.

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Re: Long duration cams and why they work (By panic mechanic)

Post by Abnormal »

That does not sound exactly right but I get the point.

You need to lower the compressio ratio in order to boost more. Same as you would a na petrol engine that you are going to turbocharge.
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Re: Long duration cams and why they work (By panic mechanic)

Post by WillemT »

Interesting thread to read up.

The way I see it is as follows.
For more power you need more diesel and air and you still don't want to break something.
For that you need to lower static cr on a diesel by increasing the chamber size by means of different/deeper pistons.

In effect you'll now have the same chamber size as a bigger diesel engine and can accomodate more air and fuel mixture at safe compression ratios. Due to the smaller engine you will not be able to get the same static cr that you had or that the bigger diesel has.

Since there's a turbo on the static cr doesn't matter that much as you can adjust the dynamic cr with the turbo boost to get it where you want.

So then in other words the relation between the static and dynamic cr will determine what boost you need to get from the static to the dynamic. And that's why you need to carefully control you boost levels to not let the dynamic cr get to high...

Just my understanding, might be a bit off... :wink:
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Re: Long duration cams and why they work (By panic mechanic)

Post by 1hotTT »

High my 5c lol

Just for thoughts for the technical peeps:

Say 2 engines:
1) 90mm Bore x 60mm Stroke = 1526cc
2) 90mm Bore x 90mm Stroke = 2289cc

Cylinder head exactly the same with same valve sizes etc.

The relationship to valve opening is vastly different as we have a much longer travel with a longer stroke hence time to open valves can be greater?

Just a opinion :type:
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