Pistons, Turbos, and Milling Machines

[code109]

hi all

some may be aware of my car and the recent purchase of a turbo. after some research ive realised I need to lower the compression ratio of the engine / pistons if I want to run any worth while boost...

 not too much obviously as I don't want to go bending con rods!

ive read that you could just use a thicker head gasket, or even (I wont be doing this) use two normal gaskets?! but those options seem a bit "heath robinson"...

the problem is I don't have500+ pounds to blow (no turbo pun intended ) on forged pistons, but I am a cnc machinist so I considered making some pistons from solid ali billets...?

then I wondered if I could just dish the standard piston? so does anyone know how much material there is left to play with on standard pistons, or better still does anyone have any experience in something similar, either machining solid billet pistons, or machining the standard ones?

any help would be appreciated

thanks

craig

[93fxdl]

Carving pistons from billet sounds like a fun idea, but don't know how they would hold up also which of the millions of grades of aluminum to use. The reason for forged components is grain flow within the item, which gives the strength.
Why not gas flow the head? Tidying up the combustion chambers till your volume is large enough,
Ttfn Glenn

[code109]

So shave some material out of the combustion chamber? Sounds like a plan!

[93fxdl]

If you want to blow an older style motor, there's lots of things you can do to help it stay together.
Older tolerances were a lot bigger than now. Options available, match all the pistons weights to the lightest. Lighten, weight match, polish, and shot peen your con rods. Crankshaft, lighten and balance. Upgrade oil pump, possibly fit cooler and/or enlarge sump.
Another add on can be water/ methanol injection to reduce knocking, polishing all sharp edges off combustion chamber helps aswell
Ttfn Glenn

[tim124]

As said, water / meth' injection will keep the engine safe, you can run very high compression ratios..... but it has to work every time, needs to be fool proof or piston melting time!
leave the engine stock, just freshen it up....

[3point141]

May I ask what the current static & dynamic CR is? Also, what kind of boost number are you wanting to run?
Without knowing what your standard pistons or rods are like (rods are quite likely to bend before melting/detonating a piston CROWN due to hugely increased torque.

Forced induction can be done with moderate boost levels on many engines without much hassle, although you might be limited to short periods of high engine load before your engine goes...

Don't forget piston rings of a N/A motor are of a different design & also gapped differently to a F/I motor. long periods on-boost could easily help bind a ring & destroy your engine... At least get the top-ring gapped right if you are keeping the standard pistons.

If you can safely manage intake air temperatures (after turbo) you might be ok. fitting an intercooler is certainly the best way of doing so, but, don't underestimate the amount of heat that an incorrectly matched turbo (one that's working too hard) can put into your intake charge. Get this right & you might not need water injection (meth not ever really needed)

Whatever you do, don't put in a thick head gasket! This will almost certainly decrease your static CR, but will ruin the squish band of your combustion chamber. Having a good squish area is essential in preventing detonation. (will help charge mixing at the top of the compression stroke, promote mixing of lean & rich areas of the charge & therefore & help prevent detonation.)

A clean combustion chamber always helps prevent detonation. Get in there and clean your head & piston CROWN.

Static compression ratio really doesn't mean anything.. Dynamic compression ratio is what you really need to know. You could always look at adjusting your cam phasing, perhaps retarding your intake cam (on a twin-cam setup) thus decreasing the dynamic CR?

Some head work might obtain you a few tenth's of a point CR, but if you do, mind you leave the squish land alone!

Good luck with the build!

[3point141]

Oh yeah... decrease the water temperature a few degrees. Running a cooler engine greatly helps decrease detonation. Although FI engines place a huge demand on engine cooling, so you need a really top-condition coolant setup (oil included) to guarantee this. also, heat-soak is a top engine killer.

[froggy]

Yep I run an 82 deg stat which keeps things under control . Running 92 c I could see Cyl head temps of 115c but keeping it at a lower temp I see max 95c after a long pull .

[v8ian]

Got my Engineer head on here, Pistons as far as I am aware are not round, they as slightly ovoid and some taper too, rings are round,
To make pistons, there is some serious and complicated measuring and programing to get them right, material is going to be a tricky one too, There is a reason that forged pistons are generally preferred for performance options,

Would a decompression plate be an option,??

[mdh]

^ what he said!

Decompression plates work.

www.ferriday.co.uk/decompression-plates/

[code109]

thanks for all the help guys! really is helping me to start understanding it all more

would decompression plates not affect the "squish" area though?

and its quite a large turbo for a 1.8 so it shouldn't be overworked... I think ha. and I will google what water/meth injection is lol

is an oil catch can needed too really?

and 3point141 how are they gapped differently? would they be available to buy to fit standard pistons?

mdh thanks for the link really interesting and helpful

[mdh]

Piston rings are gapped so that when they expand they get the best seal. In a turbo motor they run hotter, so expand more, so the initial gap needs to be bigger.

Squish worked by promoting turbulence in the combustion chamber which made the burn faster. I don't think it's as necessary on an injected and turbod engine. Forcing the air in under pressure should be sufficient.

There's a really good turbo thread here - www.16vminiclub.com/forums/showthread.php?t=3208

Basically the guy dropped a scrapyard 1400 k series into his mini then turbod the hell out of it, but did it all himself, total DIY.

He got some ridiculous power. It's well worth a read!

So what engine are you boosting?

[code109]

only a 1.8 mx5 lump... (cliché I know lol)

I was hoping to run at least 12psi, but ive been told the usual, safe maximum load for a standard mx5 engine with no mods is about 6psi? but I want something that will have a small amount of lag then go crazy

i'll give the link you gave me a read though, cheers

[mdh]

Nothing wrong with a turbo mx5! ?

We need pics though ?

[code109]

its all on my build thread for the Hillman

[mdh]

Good work dude!

There's some good info on turboing mx5s here - www.miataturbo.net/diy-turbo-discussion-14/diy-faq-all-your-answers-one-big-post-4288/

[sparkyt]

6 psi isn't a lot
....

That's the max boost for a n-a engine

have you taken the pistons out at all to have a look

You may have room to just pocket the center a few millimetres

[mdh]

Mx5 lumps were designed to be turbod, as they were fitted to the 323 turbo.

They're pretty solid up to 250hp if you get big injectors and a proper ecu.

That should be enough to be scary in an old Hilman :-)

[froggy]

A mate has a megaasquirted turbo mx5 with a t25 turbo and runs a bar with no problems

[code109]

I see.... :/

Well now I'm confused! Haha I've seen someone else on this website take an mx5 engine to 12 psi and blow his engine with a con rod coming out the side of the block! Lol

I was planning on just milling a pocket out of the original pistons but if I could get away with just using a decompression plate that'd be epicically easier

As for the fuel system I've "uprated" the whole lot, well basically started from srcath anyway as the hillman wasn't FI believe it or not ha so it has LP and HP fuel pumps, adjustable rising rate regulator, swirl pot, stainless braided lines and a custom solid billet fuel rail I made at work with RX8 injectors