deck height and compression ratio advice for the home builder

sand_man · 12-28-2005, 08:09 AM

One thing that has me sort of nervous is making sure that my deck height and compression ratio are all okay for my new engine (1987 930). Keep in mind that I don't have a lot of the expensive measurement tools that the top name builders and machinists posses. I have Wayne's book and a full set of factory manuals. My specs are as follows:

-New 98MM Nikasil cylinders with new 98MM 7.5:1 JE pistons and new rings - 3.4L
-ARP rod bolts
-ARP head studs
-Performance valve job with race springs, Ty retainers, and twin plug mod
-SC cams
-rebuilt rods

As I assemble this thing, how can I be certain that I'm not making any serious increases/changes to my compression ratio. Is this something that I should or can measure? Or for the home builder, is this just a shot in the dark...assemble the parts with love and care and pray for the best...hhhmmm, that's probably not it! HA!

FWIW, there were only 42,000 miles when I opted for the rebuild, just too many oil leaks to speak of and a failing fuel injector lead me to believe that I had a broken head stud or burned valve (it's a long story). So my point is that my heads, rods, crank, case, etc. were all in good shape...nothing actually broke internally.

Thanks In Advance,
Jeff

davidppp · 12-28-2005, 01:17 PM

Hello, Jeff.

I would not worry in the least about the CR itself..that will look after itself provided you check the deck height and valve to piston clearance as set out in the book..

At the sort of CR you are talking about, there is plenty latitude.

It gets a lot different when you are building a 2L racer with big cams and 11.5:1 CR...

Kind regards
David

Jeff Alton · 12-28-2005, 02:05 PM

Really? I would check the CR. What if the pistons are actually 8.5:1? JE has sent out a fair number of 9.5 pistons that were actually 10.5 pistons. There are a few good threads here on how to easily measure the CR with the need for any expensive or trick tools. Sounds like a nice motor you are putting together.

Cheers

sand_man · 12-28-2005, 02:38 PM

Thanks for the advice guys. I really think it will be a nice engine. I've already got a new K27-7200 turbo, a B&B intercooler, B&B headers, and I plan to have my fuel head modified. Planning on running with a .8 spring in a Tial waste gate just to keep things somewhat "mild".

It's these important details that involve measuring and machining that really worry me. The bit about the JE piston mix-ups has me a bit worried too. I'm placing my order through Henry Schmidt so hopefully he'll be sure it's right.

Anyone else have opinions on CR and deck height?

911pcars · 12-28-2005, 02:58 PM

I think we can agree that if there's any weak area of a 911, it's the cylinder head seal. However, this has been pretty much solved by using more stable cylinder head fasteners and making sure all parts are within spec.

This includes making sure the cylinder head stack is the same - the combined height of components between the crankshaft axis and the cam housing. Since there are three cylinder stacks to each bank, each cylinder stack should be the same as the others.

I would start by measuring deck height on the "shortest" cylinder. I know. All cylinders should be the same. More on that later, but from this one measurement, you can make adjustments by using a thicker base gasket and/or by trimming the cylinder height or cylinder head gasket surface. After measuring and giving you a reference point, you can now do the following:

Starting at the crankcase, the cylinder spigot gasket surfaces should be the same distance from the crank axis. If these surfaces are then machined, the prior deck height measurement will be affected.

Each cylinder in the bank should be the same (measured from gasket surface-to gasket surface). Measure the shortest, then trim all to the same dimension.

Each cylinder head should be the same height (head gasket surface-to-cam housing). If the stack heights are uneven, then the clamping force on each cylinder head/cylinder will be different.

Inside the cylinders:
Each crankshaft throw should be the same.
Each connecting rod should be the same length (big end-to-small end)
Each piston should have the same pin height (piston pin-to-piston crown)

Each will affect deck height. It depends on what machining is performed and when deck height is measured. Not every rebuild will involve the machining mentioned.

These are the lumps one accepts having separate components rather than cylinders cast together in one block or a single cylinder head for each bank.

Sherwood

sand_man · 12-29-2005, 06:50 AM

So beyond a Starret straight edge, decent Vernier calipers, and a decent dial gauge...is there anything else that would be needed? I'm wondering if once I get my bottom end assembled, I shouldn't just take the whole shootin' match over to my local machine shop (who is familar with Porsche engines) and have them check this stuff for me? You guys need to understand, I CAN NOT AFFORD TO MAKE A MISTAKE, HERE!

JeremyD · JeremyD's Garage

I checked my CR three different times - yes the JE's made me a little nervous.

first one was traditional - cylinder first, then head - fill up with a burette (found on ebay for $35.00). Did that twice. 3rd way was with the head on and fill to spark plug hole and measure. Search as there are a few threads on this.

The deck height is really easy to measure - just get a set of calipers (Harbor freight has cen tel calipers for $16)

All else fails - and if I was to do it again - I take to a machine shop and have them measure them (Cylinders, pistons, rods). In my engine the cylinders were 1-3 thousands out of spec, (and had some taper) and the flame rings were too deep causing the head to rest on on one of the fins and not on the sealing ring.

Jeff Alton · 12-29-2005, 07:05 AM

I think taking it to a knowledgable machine shop is a great idea if you have any reservations about getting it 100% right yourself. The key is "knowledgable" in finding a machine shop. Makes sure they guarantee their work, they may even let you watch and learn while they do the work.

Cheers

JeremyD · JeremyD's Garage

Some of the stuff I took for granted - cylinders from a known source, just been bored and replated, etc - and if you don't do it all the time (precisely measure stuff) it is not easy to correctly do.

Porsche engines have tight clearances - it's like assembling a swiss watch.

BTW - I was talking to JB Racing the other day. They told me that they do not put the stock JE 98mm rings in any of the engines they build. If I was building a turbo motor - I would definitely look at their iron sleeved cylinders in lieu of bored and replated ones...

sand_man · 07:43 AM

Quote:

Originally posted by JeremyD
Some of the stuff I took for granted - cylinders from a known source, just been bored and replated, etc - and if you don't do it all the time (precisely measure stuff) it is not easy to correctly do.

Porsche engines have tight clearances - it's like assembling a swiss watch.

BTW - I was talking to JB Racing the other day. They told me that they do not put the stock JE 98mm rings in any of the engines they build. If I was building a turbo motor - I would definitely look at their iron sleeved cylinders in lieu of bored and replated ones...

I was ready to purchase a set of the 3.2L fully finned cylinders bored and replated to 3.4L specs from EBS, with JE pistons, but I was really worried about the replating thing. Then Henry Schmidt came along with NEW 98MM Nikasil cylinders and 98MM JE pistons for a fantastic price. As for rings, I'll be using new Goetz (spelling?) rings not JE.

JohnJL · 12-29-2005, 11:19 AM

I would check the CR...all you need is a syringe, calipers, some windex and a cd (trim it around the edge to match the head and cylinder interior diameter.

I ordered 9.5JEs and they measured up almost 11:1! It was a 'happy problem" because I was interested in going twin-plug anyway, but I would be sure to check yours.

Everything else sounds great!