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Posted: Tue Sep 27, 2005 11:46 pm 
Getting Side Ways
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Age: 35

Posts: 516

Joined: 20th Dec 2004

Gallery: 4 images

Ride: EL Manual

Location: S.E. Melbourne
VIC, Australia

Sounds like good info. Could you explain more about valve overlap and dynamic/static compression? I am having trouble picturing exactly how it helps cylinder filling when you have large overlap cams - squeases more air out into intake and exhaust??

Is it possible to get a good stable idle and smooth running with a big cam assuming you use aftermarket management? How big can you go before it becomes a problem. Would one of those crow cams be ok. I am aiming for a strong but very drivable street motor.
I have a manual EL.



Manual EL Gli.
Mods: Crow Stg3, Chiptorque Dynotune. Lukey Exhaust. DBA slotted rotors, EBC Pads. Whiteline Low Susp. EL GT Swaybars, ROH Envy 17 x235 wheels. Vehicle Datalogger Display V2.0,

Posted: Wed Sep 28, 2005 2:41 am 
Getting Side Ways

Age: 36

Posts: 533

Joined: 12th Jan 2005

Location: Melbourne
VIC, Australia

Ok. Static and dynamic compression. Here's the crux of it.

Static compression is the compression ratio that's quoted of an engine. If a cylinder has 500cc capacity and it's capacity gets reduced to 50cc on TDC, then you have 10.0:1 CR.

Dynamic compression ratio comes into play when the engine's running. If you have a big cam with lots of overlap, at low speeds the intake charge gets expelled through the intake port on the compression stroke since the valve stays open longer into the compression stroke.

So, if the cylinder has a capacity of 500cc, and it sucks in 500cc but then expells 100cc through the intake port on compression, you've now compressed 400cc to 50cc, so your compression ratio is only 8.0:1. By increasing the static compression to say 12:1, you're also increasing the dynamic compression to 9.6:1.

When the engine speeds up and you get into the cam's power band, the gas speed entering the engine is much higher, so while the piston is trying to push air out of the engine on the beginning of compression, the inertia of the gas entering the engine is pushing the gas in. This is why you have to keep gas speeds as high as you can.

Now, as for tuning idle with big cams. The problem with big cams is that the air/fuel charge gets diluted with exhaust gases that get sucked back into the engine on the induction stroke as the exhaust and intake ports are open simultaneously for a short period of time. That period of time is the overlap funnily enough.

Because there is exhaust gases mixed in with the fresh air and fuel, the charge in the cylinder takes more time to burn. Advancing the spark will help, but you also need to run richer mixtures. Running richer mixtures reduced the burn time of the charge in the cylinder, hence why you reduce spark advance with an increase in manifold pressure and increase spark advance with very low manifold pressures.

This is why you need a programable ECU with a big cam. The standard Ford ECU is trying to maintain 14.7:1 ARF @ idle and with a big cam, you may need to run 13:1 or even as rich as 12:1. Increasing the idle speed will give you a better idle and let you run AFRs closer to 14.7:1.

I really have no idea on how big you can go in a particular engine before it gets un-streetable. I have a sound understanding of the thoery of how everything works, but I can't give you any recomendations.

However, with manuals you can afford to get a fair bit more top end and still keep the engine well mannered. Getting a smooth idle is easier as the engine doesn't have to drag the converter round at idle. A high stall will overcome this, but you're still stuck with higher gearing in an auto, and with the higher gearing, you need that low down torque to pull the engine into the power band. Since manual ratios are shorter, you can sacrifice more low end torque and move it up the power band.
Posted: Wed Sep 28, 2005 4:20 am 
Oompa Loompa

Posts: 49

Joined: 25th Aug 2005

Location: brisvagus
QLD, Australia

thats some of the best info ive read in a while
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