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compression vs flow 

 

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 Post subject: compression vs flow
Posted: Wed Oct 26, 2011 10:17 pm 
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whats better compression or flow? and why
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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 10:51 am 
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Ride: EF XR6 Wagon, AU2 XR6 VCT

Power: 148 rwkw

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Not sure what you're asking, but the two are not mutually exclusive - you can and should improve both. The best street engines are the ones that keep the mods in balance, so they complement each other.

Flow is normally easier and cheaper to improve, and you can do it in stages. And anything you do to help the engine breath usually improves fuel economy too - if you can resist using the extra power. :D It is also normally a significant performance bottleneck on a factory engine. The Falcon sixes aren't too bad from the factory (I'm assuming you have a six), but can still net decent improvements from straightforward upgrades:
- XR8 or XR6T snorkel, or F6 or SS Inductions if you want to go really big
- low restriction panel filter (K&N, Simota, BMG, ITB, Amsoil, S&B, etc) or pod filter in the airbox
- replace airbox exit with 3" plastic pipe
- larger BA induction tube
- headers
- 2.5" cat-back sports exhaust
- high-flow cat, if you have the budget
And so far you haven't even opened the engine! The engine will flow better at mid to high revs if you get your cam reground a step or two hotter, but fuel economy will probably suffer.

Compression increase basically gives you more mechanical leverage from each power stroke, but is harder and more expensive to achieve. You need to remove the cylinder head and have it milled, or install high-compression pistons. Pre-AU's can get more compression by using the thinner AU steel head gasket. Higher compression also tends to help fuel economy a little. But the performance improvement from an extra .5 compression probably won't be as noticeable as putting the same money into flow improvement.

If you are going to seriously mod your engine and run a lumpy cam, you might go for head porting and a multi-angle valve grind. You would mill the head for compression at the same time, as the head's already off the engine.

That's my take on it, FWIW.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 10:56 am 
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im sure if you fitted a cam that improves low end torque it would be better on the fuel as its getting the car moving a little quicker off the line

 

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xr8 snorkel,typhoon lower snorkel,territory intake pipe,20% underdrive kit,pacemaker extractors,high flow cat,xforce exhaust,t56 conversion hd turbo clutch/ flywheel,m86 3.45 lsd,custom tune187.1rwkw as an auto

Last edited by bry40l on Thu Oct 27, 2011 5:42 pm, edited 1 time in total.

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 4:32 pm 
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yea i was just wondering the Au i have is a strait gas set up( after market) impco 300a 1/20 but it uses a ea head...... just wondering if i sould recondision the ea head beacaus it has a smaller combustion chamber or ues a Au head because it flows better....
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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 6:05 pm 
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compression increases torque output, its beneficial up to a point as at radical compression ratios the motor begins to fight itself.. it takes power to compress the air/fuel mixture as the piston is rising.

in your case go for the better flowing head.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 6:52 pm 
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Ride: EF XR6 Wagon, AU2 XR6 VCT

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Gas has much higher octane but less energy (power) than petrol. If you will never use petrol, you should go for high compression - even 11:1 or more would be OK - to gain back some of the power you lose by using gas.

I didn't think that the EA head flows much worse than the AU; and running aftermarket gas, you probably won't be pushing the limits of the head anyway. The small chambers don't affect total flow, in themselves, but they do increase compression. I would bet that any gain in flow going to an AU head would be more than lost by the lower compression, and you would make less power. Happy to be corrected if I'm wrong though.

The other thing about gas is that it needs a completely different ignition advance curve to maximise the power. I'm sure TI Performance could whip you up a chip to get the best out of your combo.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 7:00 pm 
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so i wil take the Au head, any real benifits from shavin the head to up the compression? cause its on gas it can have a fairly high compression
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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 7:04 pm 
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come to think about it.. i dont think changing to an AU head will make any notable difference.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:28 pm 
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On a bench the EA head out flows the AU head.
But out right flow doesn't even come close to telling the whole story.

The simple answer is get a AU series 2 or 3 head. Have it reco'd. While it's there, get hem to take the valve guides out and shorten them and put a tapper on the bit that protrudes into the port.
30 degree back cut on the valves.
Then have it shaved 30thou. (0.75mm).

Doing that you will have more compression, and a higher flowing head with higher port speed.


The long answer is a thread all of it own.
the start of that thread is here. ford-4l-and-6-cylinder-f1/possible-head-porting-docco-coming-soon-t86312.html

Notice the the age of the first post and that it's 10 pages long and isn't even half done yet and you get the idea of how completely not simple your question is to answer.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:31 pm 
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ok back to the ea head lol, i think its running the Au cam and valvetrain, so would that be less lift at the valves then the standed ea cam it was runing?, if so might mean i might be able to shave the head a bit...... how much would you guys go?
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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:34 pm 
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Why would it be less lift?

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:50 pm 
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just thought that the ea head with the Au valvetrain has a smaller ratio lift 1.8:1 where the ea head with ea valvetrain has a lift ratio of 2:1.... dont know if thats right but thats why i asked
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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:55 pm 
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OK.....EA head, cam & std valves and valve springs ( std cam timing too).....on a std ED 4.0 bottom end.
The head did not have the crap shaved out of it, but it was ported and polished, and the swirl ridges are intact. Has 4480 Pacies, homemade CAT and exhaust system. It also has an ICE 7amp ignition system too.
Also has a Moristech piggyback ECU with a std ED GLi ECU.....also has a 5sp EA box and 3.27 LSD.
I don't know what the CR is.....can't be much more than 8.8:1.
This is video footage at a dyno run ......204.5hp....or 150kw....at the treads... http://youtu.be/jJTh9F3Vgg0
I'm happy with what it has with whats been done to achieve that.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 8:59 pm 
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backyard_fun wrote:
just thought that the ea head with the Au valvetrain has a smaller ratio lift 1.8:1 where the ea head with ea valvetrain has a lift ratio of 2:1.... dont know if thats right but thats why i asked



the AU cam is designed to use the 1.8:1 rockers. It's not the same just with different rocker ratio.
The type of head it running on doesn't change anything.

Have a think for a second. The AU head flows less, yet the AU makes more power more torque and uses less fuel. Over ten years of factory development improved over the original EA stuff.

 

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 Post subject: Re: compression vs flow
Posted: Thu Oct 27, 2011 9:03 pm 
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yea tickford_6 your right, will the Au bottom end handle the higher compression alright? dont want to do this then have it drop a rod or something
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