I have an idea to raise the detonation resistance of EA82t engines through some headwork.

 

I noticed that the combustion chambers of all EA82's looked remarkably similar in shape to that of an aircooled vw. ACVW's have really, really bad detonation resistance. Which is a big problem because they are aircooled and run hotter anyway.

 

There is a machine process available called "semi-hemi" cutting that does two things: It increases combustion chamber volume which lowers compression, which is NOT my point, and it also cuts away a lot of the wedge-shape of the combustion chamber, increasing the detonation resistance.

 

I'm going to attempt to post two pictures now, one of a stock vw head, and one (the only one I could find, and it's not real good) of a bunch of heads that have been semi-hemi cut.

 

http://www.jacklockamy.com/head_chambers_200x150.jpg

 

 

http://www.johnmaherracing.co.uk/images/imageheads155.jpg

 

 

Ok, maybe that will work. Theoretically, one could have this machine process done, and then make up the compression difference through decking or head milling, and still make more power with a turbo because of the increased detonation resistance.

 

Questions? Discussion? Verbal Abuse?

 

I might be willing to contribute to this experiment. It would have to be someone elses engine, because I don't own a EA82 powered-car. I do have a shortblock I may be willing to contribute, also. Of course I'm in VA, so that makes it more difficult.

I have the heads pulled off my EA82T right now and I am thinking about maybe trying this. Lowering the compresion on a turbocharged engine usualy causes an increase in power becase it allows you to run much more boost which makes up for the lower compresion. I don't understand how changeing the shape of the chamber would increase the resistance but I don't know that much about engines. I would like to hear others feedback before I consider this.

i might have a hole ea82T you can play with for me, lol.. got 5 or 6 of them layin around.. my ea81 hatch is going to become an ea82T hatch this summer.. and i'm pretty much local..

Basically, a wedge chamber is detonation prone because it takes too long for the combustion event to take place.

The longer it takes, the more time detonation has to occur. Also, it takes more total ignition timing to get the engine to make decent power. The quicker the burn=less total ignition timing=more detonation resistance.

 

That's one of the great things about the Chrysler Hemi. A hemispherical combustion chamber burns quick, therefore reducing the chances of detonation occuring, and incidentally needing less ignition timing. It becomes less timing sensitive, in a sense.

 

I'm pretty sure I could send a set of EA82 heads to a VW engine builder and have them perform a semi-hemi cut on them. I could CC the chamber on a stock head, do the cut, and then CC the chamber again to figure out how much to mill to get the compression back.

 

I NEED THREE MORE THINGS!!! Does anyone make adjustable cam gears for an EA82? The aircraft guys, maybe? If a significant amount is machined off of the heads, then the distance between the cam gears and the crank will change, messing up cam timing.

Also, i will need cam specs and a degree wheel so i can adjust the cam gears to get the cam timing back in spec. I could probably swing the degree wheel and maybe the cam gears if people are interested enough in this experiment.

If I may be so bold as to ask a couple of questions here..

 

First, the EA82 is a non-interferance engine. Given its propensity for eating timing belts, I would say thats a good thing.. If you start shaving the heads, theres a possibility of losing the non-interferance status, no?

 

Second. Instead of shaving, decking, milling, or otherwise changing the valves distance in proportion to the crank, would an easier solition be to use the 8.5:1 compression piston from a carb block?

 

What formula would you use to figure out what the compression loss would be from the 'semi-hemi' modification?

The carb block might be a good idea.

For a stock VW head, chamber volume is somewhere around 50-52 cc's and the hemi cut increases it to 54-55 cc's. I'm not sure if this is a big enough change to warrant the jump to the higher compression block, nor do i know how much would be milled off to make up the compression lost on the turbo block. That would be something I'd have to measure before and after (by cc'ing the combustion chambers.)

 

Anyone know the bore diameter of a stock ea82 offhand?

3.62" or 92mm

Ruckersville isn't too far away. Maybe if you have a pair of heads you aren't going to miss, i can perform diabolical experiments on them.

 

Calebz :

 

a=cylinder volume

b=deck volume

c=combustion chamber volume

 

a+b+c

-------- =compression ratio

b+c

 

So I would need the bore and stroke to get the swept (cylinder)

volume.

And I would need to measure the cylinder deck height in order to do the volume calculation on that.

Then i would have to cc the heads to get the combustion chamber volume, this is the one I'm concerned with. I don't think I would need to shave off a whole lot do do what I want.

I'm serious. I'll take care of the headwork if someone has a car they are willing to guinea pig. Someone send me a pair of heads.

i was going to suggest the 9.5:1 SPFI pistons. or get forged pistons sized how you like, even.

Jules and I gotta clean his shop out and strip an RX before we can get to anythig else at the moment..

 

but i'll toss a set of head your way if you want to play, heh we have a ton of them just sittin around.. (still on motors)

 

give us some time, i'll pull a set for ya at some point...

Roger that, I'm interested to see what comes of this.

Where is Richmond are you. I live here also and I have some EA82T engines around. I would be more than happy to donate them to the cause. One I rebuilt two years ago and then I thought it failed again, but I'm pretty sure it was just the turbo blowing steam out the tailpipe. Anyway I have two blocks and head assembly's you can use for this experiment if needed. email me and we'll see if they will fit for your purposes.

You have mail.

one of the biggest reasons why the semi hemi reduces detonation

besides the lower compression is the machining gets rid of some of the sharp edges /corners wich reduces hot spots that can contribute to detonation as well

 

a nice blending and polishing job in the combustion chamber would help achieve the same results but with out the expense of milling

and a few other things

It's NOT just the lower compression. It's the combustion chamber shape. I plan to mill the heads to regain the lost compression. I don't want to say this might work, and have it work because the compression was lowered, I want to see if it will work because the combustion chamber shape changed and the compression remained the same.

BTW I think I can do it all on the mill at work.

Can you say no overhead cottage industry?