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Old 12-29-2008, 11:38 AM
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Does a tunnel ram raise or lower Compression??

well since the internet apparently sucks and has no good information, i was wondering:
On a stock engine, and no the brand is not important at all, dont care wheter or not it is hemi, chevy, or ford, high output dont matter either, when you install a tunnel ram intake does it raise or lower or maintain the present compression ratio?
as well as would it require that you get any special applications?
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Old 12-29-2008, 11:46 AM
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the intake doesn't change the compression ratio.
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Old 12-29-2008, 12:10 PM
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Old 12-29-2008, 12:15 PM
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While it doesn't change the compression ratio a well tuned one can raise the cylinder pressures. The static compression ratio stays the same but you can get higher running compression. I suggest reading a lot of books about it- mostly to prevent another endless squabble over it.
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Old 12-29-2008, 12:26 PM
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You could check compression with no intake on the motor and it would not change the compression ratio of the motor. Now if you meant to say "blower" or "supercharger" that is a totally different story altogether.

Compression is more of a calculation than anything, you take the volume of air at its most uncompressed (Bottom dead center) and measure it. Then you take the volume or air at its most compressed (top dead center) and measure it and do some fancy calculations and you come up with the compression ratio. This is also done with calculators that are available on the internet.

When you do the calculations based on a motor that has only done 1 revolution it is called "static compression". However when you factor in the camshaft duration and lift with the motor running then it changes because the intake valve might be open for a short time during the compression stroke of the motor. When you calculate this it is called "Dynamic compression ratio" and usually this number will drop with a more aggressive camshaft.

So if you have a motor with a static compression ratio of 10:1 you could have a dynamic compression ratio of 8.5:1 with a mild cam and 6:1 With a really aggressive cam.

But as you can see there is nothing before or after the combustion chamber that changes the compression ratio of the motor. Unless you are talking forced induction like turbos or superchargers.

It can be confusing because it says "tunnel RAM" but just like any other intake the air and fuel is sucked in by the piston traveling down and once it has traveled to bottom dead center no other air or fuel is getting into the motor.
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Old 12-29-2008, 12:28 PM
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Quote:
Originally Posted by XNTRCI-T
I know, I know, ....but if you think about it...it is kind of a good question.
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Old 12-29-2008, 12:33 PM
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Quote:
Originally Posted by Double_v23
Unless you are talking forced induction like turbos or superchargers.

It can be confusing because it says "tunnel RAM" but just like any other intake the air and fuel is sucked in by the piston traveling down and once it has traveled to bottom dead center no other air or fuel is getting into the motor.

That's not ENTIRELY true...

you can have a "ram" effect created if your intake, overlap, and exhaust are tuned correctly... See exhibit A- NASCAR, or B- Formula 1.

It is USUALLY NOT PRESENT on the street... but it can happen.

It DOES NOT AFFECT COMPRESSION RATIOS but it does affect running cylinder pressures.
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Old 12-29-2008, 12:38 PM
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I agree, if it is tuned just right and you are traveling over 100 MPH than you can have a ram effect. But just for that split second of dwell every other Revolution at BDC.
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Old 12-29-2008, 12:49 PM
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Quote:
Originally Posted by Double_v23
I agree, if it is tuned just right and you are traveling over 100 MPH than you can have a ram effect. But just for that split second of dwell every other Revolution at BDC.

speed has nothing to do with it. you can have it on a stationary engine.
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Old 12-29-2008, 01:01 PM
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Static compression ratio wouldn't change
Dynamic compression ratio wouldn't change
Actual cylinder pressure might. If anything the tunnel ram would shift the peak cylinder pressures to a higher RPM. If the rest of the engine is optimized for that change, then its a good thing, otherwise its going to cost torque and not really make any more HP.

How the intake mass flows through the intake can affect how much actual mass makes it into the cylinder.

But if the OP is asking if the intake manifold affects compression ratio... it doesn't.
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Old 12-29-2008, 01:04 PM
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Quote:
Originally Posted by d&c repairs it all
well since the internet apparently sucks and has no good information, i was wondering:
On a stock engine, and no the brand is not important at all, dont care wheter or not it is hemi, chevy, or ford, high output dont matter either, when you install a tunnel ram intake does it raise or lower or maintain the present compression ratio?
as well as would it require that you get any special applications?
There are two compression ratios in an engine, the first is called Static Compression Ratio (SCR) the second is Dynamic Compression Ratio (DCR).

Static compression ratio is the ratio most often referred to when we talk compression ratios, it is the mathematical relationship between the measurements of all the volumes that make up a cylinders total displacement divided by the volumes above the piston when its at top dead center. There can be positive, neutral, or negative volumes in the equation these are usually the result of the piston crown shape. A positive volume would dish that adds volume to the total, neutral would be a flat top that neither adds or subtracts volume, a negative volume would be a domed or pop-up piston where the projection above the crown height would subtract from the total volume.

Dynamic compression ratio takes into account the affect that breathing dynamics has on modifying the absolute pressure density in the cylinder of an operating engine. This also modifies the cylinder pressure into something different than the calculated static compression ratio would indicate the compression pressure to be. This is sensitive to RPM, cam timing, and intake thru exhaust port efficiency. The absolute pressure density may be lower or higher than the static compression ratio, however, in most cases it is lower. It takes an extraordinary engine (i.e. full up competition engine or a supercharged engine) to cause the absolute cylinder pressure to exceed the static compression ratio. That would only happen when cylinder filling exceeded 100% volumetric efficiency, which is most uncommon.

Generally dynamic compression is at its lowest when engine speed is insufficient to maximize flow into and out of the cylinder.

Things that reduce dynamic compression

- slow port flow due to excessively large ports, valves, too much carburation, too little or too much exhaust system for the RPM range the engine is operating at.

- excessive cam timing (duration), too much lift, too much overlap which is a kissing cousin to close Lobe Separation Angle, too late closing of the intake valve for the engine's operating range.


Things that increase dynamic compression ratio

- Mostly having all components sized and timed correctly for the engines operating RPM range.


So far as a tunnel ram intake is concerned, it or anyother such component can only affect the dynamic compression ratio. Its effect will always be to lower the DCR at lower thru mid RPM ranges and will add to the DCR at high RPMs assuming the head ports, valves, carburation, and exhaust can also meet the flow demands. Things that do not change the in-cylinder volume relationships have no effect on the SCR.

Most DCR's, regardless of the engine's SCR, tend to be around 8 or 9 to 1. This changes with RPM in proportion to the effects of reverse pumping, overlap leakage out the exhaust side, and velocity of the incoming and outgoing gasses. The reason most competition engines run SCRs around 13-14 to 1 is that the loss of DCR below the torque peak is so great that the engine will hardly run in the RPM range below that point because the mixture density is so low. Extraordinary compression ratio is used to improve the existing density to overcome this problem. It is to some extent self balancing on the top end when the port flows are maxed out and the density again begins to fall off. For a street driven engine these extreme compression ratios pose a problem in that the cams are usually more efficient in the lower RPM ranges which leads to excessive absolute pressures at lower RPMs which in-turn leads to detonation sensitivity.

Whew, this is the kind of discussion that Phd degrees are made of, there is much literature both general and technical on the subject. When you understand this, you're well on the way to being a competitive race engine builder.

Bogie
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Old 12-29-2008, 01:07 PM
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Quote:
Originally Posted by Double_v23

It can be confusing because it says "tunnel RAM" but just like any other intake the air and fuel is sucked in by the piston traveling down and once it has traveled to bottom dead center no other air or fuel is getting into the motor.
Well, actually the air/fuel mix is not sucked in, but filled by atmospheric pressure.

Try sometime to suck the beer out of a bottle. If the bottle is not vented to the atmosphere, you can have the greatest suction around & you are still not gonna drain the bottle.

For more push than atmospheric pressure, going to a supercharger or blower is gonna do the job. 5lbs of boost is on top of the 14lbs pressure at sea level, no?
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Old 12-29-2008, 01:09 PM
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Quote:
Originally Posted by curtis73
Static compression ratio wouldn't change
Dynamic compression ratio wouldn't change
Actual cylinder pressure might. If anything the tunnel ram would shift the peak cylinder pressures to a higher RPM. If the rest of the engine is optimized for that change, then its a good thing, otherwise its going to cost torque and not really make any more HP.

How the intake mass flows through the intake can affect how much actual mass makes it into the cylinder.

But if the OP is asking if the intake manifold affects compression ratio... it doesn't.

That's why I stated it in bold, but the increase in cylinder pressure won't necessarily occur at a higher RPM- it can occur at a lower RPM if designed for it. A well tuned tunnel ram is perhaps the best carburated manifold you can have, for any application. (watch how many people get upset at that one!- just don't forget we're talking about manifold tuning, not carb tuning) Tuning it for some applications can be next to impossible, but they do work well.

IR setups are a very contested replacement- it all depends on how you want to go about things. When tuned properly they both beat any other single or dual plane manifold.

I know a lot of people are going to say "well you can't run a Edelbrock box stock intake and 2 1050 dominators on a street driven 327," and you're right- but that's not the ONLY tunnel ram. A properly designed and built custom one can be effective at almost any RPM desired.

Not practical for most of us (due to costs), but it is possible.
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Old 12-29-2008, 01:14 PM
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As has been pointed out...and there were many other things, that aren't so related, discussed...it doesn't alter CR. The Tunnel Ram, while many of my friends at one time ran them on the street (gas was 20 cents a gallon), require a different gear and cam...at least...to get the benefit. You mention a stock engine, maybe for discussion, but if you are anticipating putting one on a stock engine I would advise against it.
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Old 12-29-2008, 01:23 PM
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You guys are thinking way too small. most off the shelf tunnel rams should be reserved for taller gears and higher revving engines, but its not a necessity for all tunnel rams.

Look at the LT-1 intake, its a tunnel ram with fuel injection and it works quite well in a heavy caprice wagon with shallow gears.

If you can design one to take advantage of pulse tuning and prevent air/fuel seperation they can be made for any application. it is expensive and would only be done on a stock engine for a fun project just to show it works, but it is possible.

I wish somone running a thin/long runner tunnel ram would chime in. The closest factory example I can think of is MOPAR, its not a tunnel ram either but it shows what can happen with runner tuning.
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