[Walkinshaw]

Well, I thought id post something to get some conversation around the all-important aspect of Compression in relation to turbo/supercharged engines (in particular the EA/AU 6's)

We all know a high compression and high boost don?t really go hand in hand (except in extreme conditions.) For my proposed application something around 8 up to 8.3:1 would be the aim, to allow for a possible 20lbs boost. Now the easiest way to achieve the compression would be custom pistons, but we are not all cashed up that we can ring Arial and order in some custom forgies.

I?m thinking the way to do it is with Head gaskets, piston dishes, and head chambers.

Its common knowledge that

1) EA has the largest piston dish (11.3cc iirc)
2) The EA head with the swirl port removed has the largest volume
3) The Au gasket is the thinnest allowing for compression increase
4) The EF==> heads flow much better than <==ED heads

So to achieve that compression I'm assuming EA pistons and deshrouded EA head would be the way to go (as stock compression is 8.8:1 and deshrouding may result in 8.5:1)

To use this will loose the possible benefits in an EF/AU head and its inherent flow characteristics however the compression decrease by using the EA deshrouded head may surpass the better flow of the EF/AU head.

Or, the following may be attempted. Use an EA piston set however use a ported EF/AU head which may result in around 8.9:1 compression. I?m assuming new (say ACL race) pistons would have a decent crown thickness which could be machined down slightly to reduce the deck height of the piston/rod/crank assembly. This method if possible could yield 8.0:1 compression quiet easily (maybe 7.5 if used with EA head).

Id like to hear peoples views on this topic, from research, experimentation or theory.

Regards

Daniel

 

[ebxr8240]

Carefull with taking away quench will lower compression, BUT it can cause detonation.
It pays to cc the combustion chamber including piston deck height, valve reliefes and thickness of head gasget.. So check my Morrinsville College maths ?

4000 div by 6 = 666cc sweep area volume..
666 div by 10 = 66.6 cc chamber etc...
666 div by 9 = 74 cc chamber etc
666 div by 8.5 = 78 cc chamber etc

So thats the total area reqired @ top dead centre [tdc] of combustion chamber gasget,piston to deck height, valve reliefs etc.. O/K ????
Tickford heads have too high compression to start with imo. Best to use a lower comp head if possible....

 

[Soxx]

Don't forget it's a piece of piss to machine a bigger dish in the pistons, so in that case, I would still use AU pistons so you can also use the longer AU rods.

Rick.

 

[Walkinshaw]

Im not sure if the pin position was the only change on the Au pistons, or if the deck height was also reduced meaning less crown thickness to play with.

Will AU rods bolt up to EF crank. The EA pistons would still be more benificial as they have a larger cast in piston dish.

What is the diffrence in rod length from EF/L to AU, assuming the total deck height is the same.

 

[Soxx]

Deck height is the same.
As I said, the dish in the AU piston can easily be machined to be the same size as EA, or even bigger.

I'm not sure on exact rod lengths, but any extra length is a good thing.
If building a hi-po engine from scratch, you'd be crazy not to use the complete AU botom end anyway, the stud girdle, integral sump, all lead to greater rigidity at high HP.
It can be troublesome keeping a botom end together at high HP, but it is a snap to shave off a few points of C/R.

Rick.

 

[mongrelEB]

Sorry to crack in, I'm thinking of doing something similar with my old dead AU motor so am interested too

If the AU rods are longer, theoretically the AU crank would have a greater throw, yes? Resulting in slightly better torque.
My understanding of high-performance engines, which isn't good - yet - is that the best option would be to machine out the combustion chamber, allowing you to redesign it to control the flow of gases better.
Is this correct or am I just dribbling shit?

 

[Soxx]

mongrelEB -
No, the crankshaft throw is the same. The piston has a higher pin height to keep piston from poking out of the block.
Longer rods help with higher rpm's as the thrust angle isn't as severe, this particularly helps with undersquare designed engines.

Rick.

 

[EA Turbo Clone]

isnt the 4L class as an over square bore, due to the longer stroke. Doesn't the longer rod actually causes a change in power curve slightly, you pickup slightly but you will also loose slightly elsewhere in the rev range? EA pistons with EF/EL head cause a compression around 8.5 to 1, without the need for machining piston top, and the EA piston has a dish off 15.2cc.

Insadently you realise that 8 to 1 comp with 20lps of boost gives a static compression of around 18.8.

 

[DansEdgli]

How is it possible to find out your compression ratio? Can it be done in the car?

 

[Soxx]

EA Turbo Clone -
The 4L I6 is an undersquare design, that is, the stroke is greater than the bore.
Torque is improved because the rod has more leverage on the crank, think of it as using a longer spanner to gain more leverage when undoing a stubborn bolt.

Longer conrods give less thrust angle on the bores which leads to less friction, less chance of splitting bores and cracking pistons. An engine typically revs more freely with longer rods. They can also broaden the power band, but the effect would be subtle in a Ford I6.

DansEdgli -
No, you need to know chamber volume, piston volume, and head gasket thickness.

Rick.