Combustion chamber volume

[MerkXRTurbo]

Hey guys, I'm gathering info to have my custom pistons made and I'm going to have to have some data to know how much dish I will need. 1st of all, I'm shooting for 8.0:1 compression (I could live with it a few tenths lower even), and I really need to know what the combustion chamber volume of our engines is.

While we're at it, how about the compressed head gasket thickness? How about the deck to piston crown clearance (shoulda measured that before I disassembled the engine )?

 

[NovaSS]

SHOnut can get you forged units at 8.5-1 to 12-1 these are a tried and true design and come with rings and wrist pins. You could always have a little removed out of the dish to drop anphter 1/2 point but I would not worry about it. Guys are running 600 + WHP with these things out of the box.

http://www.shonutperformance.com/

 

[MerkXRTurbo]

Yes, I'm aware of those and I appreciate the info. There are a couple of reasons that I've decided not to go with those:

- I'm not a big fan of Wiseco pistons. Although I'm sure I'd never regret purchasing those and never have problems with them, I know that there are much higher quality forgings out there.

- I AM a fan of CP pistons, and a guy I know does group purchases on CP pistons to GREATLY lower the price for a set. He will take the measurements for me, make me a few recommendations, and send them away with requests from about 10 other customers at the same time.

In the end I get a stupid strong piston, I get EXACTLY the compression ratio I want, I'll probably even spread the rings out a bit for a thicker compression ring landing (just to insure maximum strength, but once again I'll have my guy give his opinion on that), and I have it all for about the price of the one's that SHOnut offers.

No disrespect meant to anybody here. I'll be doing plenty of non-piston related business with SHOnut.

 

[1993MTXSHO]

Yes, I'm aware of those and I appreciate the info. There are a couple of reasons that I've decided not to go with those:

- I'm not a big fan of Wiseco pistons. Although I'm sure I'd never regret purchasing those and never have problems with them, I know that there are much higher quality forgings out there.

- I AM a fan of CP pistons, and a guy I know does group purchases on CP pistons to GREATLY lower the price for a set. He will take the measurements for me, make me a few recommendations, and send them away with requests from about 10 other customers at the same time.

In the end I get a stupid strong piston, I get EXACTLY the compression ratio I want, I'll probably even spread the rings out a bit for a thicker compression ring landing (just to insure maximum strength, but once again I'll have my guy give his opinion on that), and I have it all for about the price of the one's that SHOnut offers.

No disrespect meant to anybody here. I'll be doing plenty of non-piston related business with SHOnut.

If you cant get stuff cheaper and better why buy them then? No disrespect should be taken by anyone including Josh. **** I have weisco pistons, but I called them direct and ordered a custom set and paid much less then Josh charges, you're paying for the ginny pig work he has to do as far as getting specs and figuring everything out, also there is no hassle of havign ot make phone calls. I decided making phone calls and getting specs was worth saving 1-200 bucks.

Here this might help you out a lot this has everything if not almost everything I used when I ordered mine. If you have any more stuff you need shoot me a pm as I don't check threads as much as I used to.

 

[MerkXRTurbo]

Awesome! Thank you! That's a start!

 

[NovaSS]

I was getting ready to post those same pages, nice to have factory manuals Should be the first thing you purchase if you plan on pulling a sho apart. Check ebay they turn up once in a while.

 

[zak]

Of course, compression and deck heights aren't listed . . . . here is a hint, run a search. zak

 

[MerkXRTurbo]

I've been searching like ****. The problem is is the fact that most topics that come up are so old that you can't access them, and in the field that shows what section they're in it's blank. It's actually a PITA cause I can tell that there would likely be some good info in them.

 

[1993MTXSHO]

Stock compression is 9.8:1 if that's what you need.

Oh and if you mean how far can you deck the cylinder heads its .001-.003"

 

[93rev2sev]

Old school way of measuring the combustion chamber volume...

Drill a tiny hole in a piece of glass(plexi-glass works great) and use it to cover and seal the chamber with the head sitting on your bench...deck side up...valves closed. Then, fill the chamber with water from a measured syringe. Once full, look at how much is left in the syringe and subtract that from what you started with.
this will give you ACTUAL combustion chamber volume.

 

[MerkXRTurbo]

Good thought. I'll probably do that after unshrouding the valves and polishing the chambers.

 

[zak]

I've been searching like ****. The problem is is the fact that most topics that come up are so old that you can't access them, and in the field that shows what section they're in it's blank. It's actually a PITA cause I can tell that there would likely be some good info in them.

My apologies, I forgot that the Performance sections are all still scrambled. If I get a chance I will dig through some of my engine notes.

The pistons, though hypereutectic, are pretty stout, with careful tuning they can hold north of 500 hp. Consider destroking and using the stock pistons by using the 0.75 mm undersize rod bearings. IIRC, this should get you down around 9.5:1, fine for a reasonably sized turbo.

zak

 

[luigisho]

I thought the pistons were cast not h-e?

 

[zak]

"Hypereutectic" is a form of (aluminum) casting, stronger than conventionally cast aluminum. The slugs are not forged, stock.

Hypereutectic aluminum has higher thermal expansion than a forging, allowing closer piston/wall clearances and better cold start NVH and emissions/blowby reduction.

 

[MerkXRTurbo]

Yes, I know the factory pistons are quite stout for a casting, but I've built and modified a lot of turbocharged vehicles with great success, and I like 'em best when the compression is hanging right around 8:1. That's where it should be for a street car, IMO. For dedicated track use you can even go a bit lower than that for more top end power, sacrificing off boost performance.

I plan on running boost numbers in excess of 20 psi.

 

[Toolman]

My 9.5:1's have seen over 22psi before with no problems. What makes 8:1 more ideal in your opinion? Are you wanting to run 91 octane most of the time at those boost levels? I would think that with an 8:1 CR, your lag will be increased as well (what turbo are you planning on running?)

TT

 

[MerkXRTurbo]

Yes I am planning on running pump gas at that boost I have found that one can run pretty extreme internal cylinder pressures if the base compression is low.

Yes, lag is increased a teeny tiny bit, but it's really not too bad. If you look at the VAST majority of factory performance turbo cars (WRX and STi, EVO, SRT-4, DSM, Starion/Conquest, 3000GT/Stealth, Isuzu Impulse, Maserati Bi-Turbo, 300ZX, every turbo Porsche, Supra, etc.) you'll find that even they have compression ratios of 8.5:1 or less, and sometimes as low as 7.0:1. If you've ever driven any of these cars you'll know that the lag still has more to do with turbo selection than it does with compression.

I'm planning on using a pair of WRX turbos. Not only can I get good examples for pocket change, but I have dug through dozens of compressor maps and cannot find one that fits my projected needs better than these:

I'm projecting that I'll be consuming air at a rate of 20lb/min (~.14 m3/sec) PER cylinder bank. At 23psi (~2.5 presuure ratio) that will put me in the center island of the compressor map, at 76% efficiency.

 

[Toolman]

Ah, twins, yes. I am a single minded individual (if you catch my drift! haha).

On my next build, I plan on using a fairly large turbo, and no matter what CR I am at, it will be a tad high on lag. But it is a race car, and will see very little street time (and no pump gas), and so when it is running, it will always be in boost!

 

[Silvapain]

"Hypereutectic" is a form of (aluminum) casting, stronger than conventionally cast aluminum. The slugs are not forged, stock.

Hypereutectic aluminum has higher thermal expansion than a forging, allowing closer piston/wall clearances and better cold start NVH and emissions/blowby reduction.

Just to expand on this: Hypereutectic means that there is more than 12% by mass Silicon in the metal. Hypereutectic metals are stronger and more brittle than hypoeutectic metals.

 

[luigisho]

"Hypereutectic" is a form of (aluminum) casting, stronger than conventionally cast aluminum. The slugs are not forged, stock.

Hypereutectic aluminum has higher thermal expansion than a forging, allowing closer piston/wall clearances and better cold start NVH and emissions/blowby reduction.

Yeah I know what they are, I just didn't think the SHO pistons were h-e.