924Board.org

[Racing]

Thanx.
I´ve recieved your mails alright,just that when i on occasions have tried to get onto the site i´ve failed outta some reason.
Beats me,but anyways..here i am.

Cheers!

[-nick]

Hey Racing,
While we've got your attention- could you give me an idea what you had to do to the head to remove enough material to lower the compression ratio? Did you just smooth out the sharp edges around the combustion chamber? Or did you take even more material off than that?

I'm calculating numbers way over 8.5:1 using these pistons with an otherwise stock head & headgasket. I''m going to keep the stock headgasket (I've had bad luck with keeping a copper gasket sealed around the water passages in cold temps). The stock gasket is slightly thicker than the copper gasket you're using, so I'll need to take a little extra off the head to compensate. I'm not planning on hitting anything much (if any) over 1bar, so I think the stock gasket and studs should hold the compression in well enough.

You wouldn't also know what your compression psi is with your current configuration? (I assume you're using the stock cam, so I can compare apples to apples). Did you cc your heads before and after? Have the numbers handy?!

Here's the plan for this beast- I'm just going to start off with these flat-top pistons in an otherwise stock motor (the intake & head are gasket matched, that's it)- well, I'll have to remove some material from the head to get the CR down (maybe somewhere just above 8:1) and I'll polish the head combustion chamber and have them cc'd to make sure they're even. That's just what will be necessary to run these pistons. I'll be running the stock US 7psi too. I'm going to try and get it on a dyno at that point. I'm also going to get the crank trigger wheel setup while I've got the engine opened up. I'm looking at having EFI, dizzyless ignition, and a wideband o2 setup by the time spring rolls around (all programmable via megasquirt). Wait, why stop there?! I'm also planning on sitting an IC in the space where the airbox & CIS pieces used to be in front of the engine and duct air from the front vents into it (I'm picking up a lower profile and better performing radiator fan while I'm at it). I'm going to hack off the plenum from the intake and have a new barrel welded on to the runners with the throttle body opening on the front end so that I'll have a straight shot from the intercooler outlet to the throttle and avoid bending IC pipes all over the place. Then we'll re-dyno and play with the boost to see what things look like.

gotta get to work...!
nick

[Racing]

Nick,have in mind that i´m by no means some sort of guru on the subject.
Just sought and found what i thought would be a feasable alternative,and struck home doin so.

First of all i´ve got about the same climate you do.
When using a copper head gasket there´s a couple of thing that HAVE to be adhered to.
First is to have it properly tempered-anealed.If not there´s no way in gods zoo that you´ll get the softness needed to get it to be pliable enough to seal.
Second is the use of high temp silicone in small amounts around the water outlets.As far as around the cylinders the carbon residues will take care of that if there indeed is a problem-which there shouldn´t be if the gasket´s been properly anealed in the first place.The copper will "grow" under the heat of combustion anyways so...

Head then.
What i di was basically to "lower" the entire roof a small margin.
Now this is a while ago,so please don´t take this to the bank,but as far as memory serves me the stock CC clocked in at 21cc.I ended up at 28.No sinking of the valves or anything like that.
While i was at it i installed new guides,seals and cut new seats.
With "my" height head gasket that made me end up at 7,8 i believe,but then again...have in mind that i was shootin for VASTLY higher boost levels than you are.

Enter EFI(or more to the point the poss of altering ign timing really)
When building high end turbo engines these days builders really just drop timing and increase boost.Of course a lot depending on the stature of the CC space and how tolerant it is to detonation.
My findings was that these engines would cope with 22-24 deg at full throttle at around 1 bar-of course a lot depending on the compressor and intake temp.When reaching for the starts boost wise it needed to be dropped accordingly-and the engine will tell.
A good tell tale is the WB O2 sensor as when doing so the AF number will start to fluctuate,and if equipped with a pyro you see the same beavhaviour and the temps will raise.

Timing really needs to be load dependant,as this after all is a boosted engine.

Nope.
Don´t have any numbers as far as cranking psi,then again i ran a reground cam anyways-and that SOB let me rev into the 7200 region..

[-nick]

Thanks for the notes & numbers. Your head cc numbers sound about right. It shouldn't be too tough to find ~10 cc's to take off of the head chamber. It may take that just to smooth down the chamber in the head for cylinder #4 after the detonation hammering (I'll try to get a pic up of this).

FYI- I found the VW forged pistons on ebay:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33623&item=7934622350&rd=1&ssPageName=WDVW
They have 30 or so sets of them with a buy it now for $135 (~$25 to ship). I picked up one set, I'm thinking about grabbing a back-up at this price...

I believe I can set the timing up to retard as a function of boost. I'll have to look into this though.

Re: the copper headgasket- I don't know why I wasn't able to get mine to seal, I used the spray headgasket dressing per the instructions, it was a new part, and the mating surfaces were true- I was still using the stock headbolts which the gasket maker told me might have been the problem. The first time the temps dropped I started the car and saw coolant leaking around the rear of the engine where the head&block mate. After it warmed up it was fine, but I didn't want to have to deal with the mess. For my goals, the stock gasket should be just fine.

Thanks
-nick

[Racing]

Slugs seems right Nick.

When it comes to gaskets...
I hear ya,and frankly speaking the amount of abuse the gasket will take is a matter of cylinder pressure and extracted power(heat)

When it comes to CU gaskets one has to be extremely anal about the decks being absolutely and completely true.
Next up is that the surfaces can acutally be TO smooth.

[Peter_in_AU]

hey -nick, thanks for the heads-up on the pistons on ebay. I got a set and even with the $75 shipping to Australia they're still a good price.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you