Hey guys i was wondering if when i do VT stage 1 cams if i will have to use 91 octane fuel? I would prefer to stay with 89 Octane right now with the high gas prices.

 

That will depend on your new static and running compression ratios.Static compression will likely be lower.Where running compression,in higher RPM ranges,will be higher.The best way to find out is to call the makers tech line,they know more about their products than anyone else.Does your engine "ping"under load with 87 octane now?If not,go back to using the lowest octane rating that doesn't "ping".You don't gain any power or MPG's by using higher than recomended octane.

 

Compression will not change with cams. As long as you use the same heads/pistons, you will have the same compression. My compression is well over 10-1 and I run 91 octane with no problems. I run VT Stage 1's also.


Adam

 

So i could use 89 octane and not have to pay for premium gas!!??

 

You could actually run 87 without trouble. Thats what our owners manual calls for and Ive heard of issues running 93 unless you have the timing bumped.

 

Should i not run 89 and just run 87 octane!?!?

 

I would stay at 89

Adam

 

unless your tuned there is no reason to run higher octane.

unless of course your blown or sprayed then you would want to. but with those cams you should be fine with 87 like most folks are. if you do ping then bump it up to 89 but 87 should do the trick.

 

I have an SCT 4 bank chip. I have a tune for daily driving set for 87 and a tune for the track set for 93 octane.

 

Mid grade fuel is nothing more than a little bit of super "watered" down with allot of regular. If your car runs ok and does not detonate with 87, by all means, run it. If you have a chip or tune and the tuner suggests high octane fuel, use 91, 93 or 94 whatever is the premium grade in your area. 89 is nothing but a waste of money. There is not one single benifit to running higher octane fuel if the engine that is consuming it does not require it to run stoichly. Contrary to popular beliefe, high octane fuel does not have any more detergants, cleaning properties or performance enhancing addatives.
That said, cams will not in any way dictate the octane fuel required in your engine. However, the tune that is required to make those cams work properly may. Ask your tuner what he recommends.

 

Yes,compression does change with cams.Thats one way they increase power.The more air you can get through the engine the more power you make.This air does not simply go straight through the engine and out the exhaust.Static(not running)compression doesn't change.Running compression does.This depends mostly on valve overlap.More valve overlap means higher power but at higher RPM's.This can even lower compression at lower RPM's.

 

How will valve overlap(both valves open) increase compression? Camshafts create power by allowing more air into the cylinder and more air out the exhaust valve(ie efficiency). I would like some more input on this compression increase theory.


Adam

 

Air has mass.So once it starts moving it wants to stay moving.Valve overlap allows the exhaust to help pull air into the cylinder.When the exhaust valve closes,the air,due to its mass and forward motion,will continue to move into the cylinder through the open intake valve.The air will continue to enter the cylinder even after the piston starts to move upward in the cylinder.Somewhere in the upward stroke the intake valve closes,trapping the fuel-air mix inside.This is what effects dynamic compression,it is also the reason that more radical cam designs don't idle well.The fuel air mix is being pushed back out into the intake at low RPM's and effects the fuel air charge going to the other cylinders.It also changes the amount of air coming into the cylinder depending on the speed of the engine even with the throttle body wide open.This can be seen on dyno sheets as the power increases as the engine speeds up.When you change only cams and do back to back dyno pulls you can see the effect of valve timing and overlap.

 

What you are saying is partially true, but unless you have a forced induction system pushing the air into the cylinder, the cylinder can "consume/accept" no more air than the vacuum that the motor has created. IE...a NA motor does not have the ability to go beyond a certain amount of efficiency. That is why NA cars are rated up to 100% efficient/load and PA cars can be well beyond that. Air does not have the ability to create a forced induction situation for itself. The only thing you can do for a NA car is make it more efficient....meaning allowing the air more unrestricted flow into and out of the engine. Theoretically you are saying that a 281ci motor can ingest more than 35.125 ci of air (281/8) because of cam overlap, which without some sort of forced induction simply cannot happen. In reality, the higher the engine rpm the LESS efficient a motor is because it simply cannot get enough air in the cylinders as fast as the valves are opening and closing. The time frame becomes so small that the engine loses power quickly....look at any dyno sheet. I can tell you that a 4.6 with cams and exhaust is most efficient at rpms around 4400. Which is not where peak torque OR power is made.


Adam

 

Newton third law says that an object in motion tends to stay in motion,that applies to everything that has weight,or,mass.This includes air.When air is accelerated by the piston it will continue to move forward untill acted upon by an outside force,the valve.Air will continue to fill the cylinder untill it's forward energy is exhausted as long as the intake valve is open and will even overcome a small amount or pressure in the cylinder.The scavengeing effect of the exhaust helps to get the air moving toward the cylinder during valve overlap.When the scavenge effect is spent the exhaust valve closes and the moving air will continue to flow into the cylinder even as the piston starts up on the compression stroke.Then,the intake valve will close,trapping the air that was forced into the engine by the law of inertia.The easiest way to see this is to use a garden hose.Turn it on and then place your thumb over the end suddenly.For a short time the pressure against your thumb will increase untill the water loses it inetia,or "momentum".Then it will be at whatever the line pressure is.Air will also obey the part where it says an object at rest will stay at rest.That is why the faster you spin an engine the less efficient it is.Air can only move so fast.And as we all know,the faster that air moves the lower it's pressure.

 

I think we will have to agree to disagree on this one.

Adam

 

You guys are too funny. I hate to break it to you Chopper but cam's don't change compression at all in any way, no way.

Compression is nothing more than a messurement of pressure. A given amount of space can hold a certain amount of air. It dosen't matter how fast you put it in there or how efficiently it moves in and out. It's only going to fit so much and the pressure created by that specific amount of space filled with that specific amount of air is called compression. Valve overlap, centerline, durration, lift and lobe sepperation will not change that amount of space or the amount of air that can be compressed in that space in any way. Bigger, better, aftermarket cams make more power over smaller, weaker factory cams becasue they allow an engine (which is essentially nothing more than a giant air pump) to flow the same amount of air as before, faster and more efficiently. The compression is what it is in exact reletivity to the size of the combustion chamber and the volume of air forced into the chamber when that chamber is compressed....Done.
The piston is only at TDC for fraction of a second. It dosen't make any difference how long the cams keep the valves closed, once the stroke is complete, the piston goes back down. Even if the valves close before the piston get's there, it's simply going to stop allowing air to enter the chamber in turn leaving that same amount of air as before. Your confusion is understandable, but all the same, it's inccorect.

 

The compression you are refering to is static compression.The numeric relationship between the volume of the combustion chamber and the stroke of the piston.Cranking,or dynamic compression IS directly effected by the cam design.If you don't believe me go to [url]www.camhelp.com.This is Comp Cams home page,go to their tech page and read it for yourself.

 

Static compression is the only type of compression that will affect the rate of fuel burn in turn causing the need for a fuel change WHICH IS WHAT THIS THREAD IS SUPPOSED TO BE ABOUT. :icon_joke

 

The last point in the cycle is the intake closing. This occurs slightly after Bottom Dead Center, and the quicker it closes, the more cylinder pressure the engine will develop. You have to be very careful, however, to make sure that you hold the valve open long enough to properly fill the chamber, but close it soon enough to yield maxi mum cylinder pressure. This is a very tricky point in the cycle of the camshaft.(The previous paragraph was copied from the Comp Cams cam timing tutorial)Cylinder pressure has everthing to do with whether a certain octane rating will detonate or not with a certain cam design.