Richard,

When I talked to Troy at EBS he cautioned me by letting me know that the J&E pistons were manufactured to achieve the specified CR based upon everything else being stock. That means that the J&E product was as intended for a stock head 9.5:1

Your spread sheet that shows that with stock heads you would have had 9.5:1 CR. But because you milled that .005 off of your heads you ended up with a higher CR.

Did you find that the actual Dome Volume was different then J&E claimed?

Is it possible that you might have more than .005 milled off your heads?

My pistons are advertised as having a dome volume of 35.2, I will measure the actual displacement and reply later.

As for the math, I double checked and came up with the same thing the calculator did, the calculator was much faster. SmileWavy

Shane

CR check of assembled engine
 

Snbush and Richard - there is a way to verify compression ratios of engines after they are put together. It is a device called the Whistler. You put a cylinder at TDC compression, pull its plug, and blow air in. The audio frequency of this air movement will vary with the size of the cavity. Since you know the displacement, and have now measured the compressed size, you can calculate the CR. In fact, the instrument does this for you.

Kind of pricey piece of equipment. PCA Club Racing uses this to check CR on motors which are supposed to be stock. Some shops have their own.

I admit to not measuring my piston dome volume or my Cylinder head volume directly. But combined I measured them at 59CC which equals 9.94:1 CR
It's possible the heads were machined before I had them machined. I just have paperwork saying they removed .005". And though .005" removed is almost one whole CC, I have .25mm (.010") more of deck height than what JE advertises at, which gains two CC.

So I expected a net increase of one CC from the JE "stock" numbers, which should have dropped my 9.5:1 to 9.35:1 But instead I measured 9.9:1 even with the increased deck height. I can't account for the missing 3.5 to 4 CCs except as Piston dome shape. There's plenty of valve to Piston clearance (3mm) and plenty of piston crown to Cylinder head clearance (2.4mm) which implies to me that the Cylinder head has not been machined down excessively.

Conclusion, The JE Piston dome shape yields more than 9.5:1. That or I can't measure volume of CCs accurately.

Richard

I see a great deal of discussion about measuring everything after you buy your pistons. Strangely enough, it is and has been our practice to dimension the engine prior to ordering the pistons.

By giving your piston manufacturer the dimensions, they can machine the pistons to your requirements. This still requires verification but generally your results are much closer to your expectations.

Remember that "shelf" pistons are built using numbers based on a mythical "standard" formula.

Most retailers of JE pistons sell "shelf" pistons for both economic and convenience concerns.

And I think it's that "Economic" part that drives the home rebuilder as much as anything else.
I just think it's interesting that there is a general opinion that JE "shelf" pistons yield higher than advertised CR at least for our 911's. I would think as a manufacturer, I would choose to err on the low side of CR, rather than on the high side. Lower means less power, and therefore maybe a opinion on the street of not being as good. BUT Lower means fewer detonation issues, and fewer warranty claims.

I wonder if anyone has done a poll to collect actual data rather than just opinion on "shelf" pistons. maybe I will..

Richard

I would think that it is easier the lower the compression of a given piston than to raise it. I.E.: shave the piston, shim the barrels, off set a wrist pin
With that in mind, by manufacturing a piston on the high side of the compression expectation you will yield a piston that is easier to adjust (lower compression) making it more compatible to the needs of more builders.

Once again an issue of manufacturing compromises for the sake of cost reduction in mass production.

Ah! You've probably nailed it Henry. After all who does JE care more about as a customer? Me? no, I bought one set of pistons ever. Henry? Yes, create your "Shelf" pistons in mind that you can mod them to Henry's specific needs, rather than create new ones. After all Henry and his brethren engine builders are likely the life blood business for JE. Sound business practice to cater to them, not me.

Richard

That's not exactly how it works. The pistons are made from the same forging (shelf and custom) but they don't modify shelf pistons to make custom pistons.
When we order a custom piston, JE or any piston manufacturer pulls a forging from their inventory and starts the custom order from scratch. It takes longer for us to get them but by ordering a custom piston we can configure the piston (ring size, piston clearance, dome shape) with any specifics we deem necessary for the project we're building.

There are benefits to "shelf" pistons. First they are cheaper and there is no lag time. They are all the same so if you need a replacement you can order one. With custom pistons you have to order four.

Thanks Walt. SmileWavy

Katech Engine Development - Corporate Services

Is this close enough?
 

Today I finished up cleaning all the old sealant of the heads and cam towers and weighed the pistons and pins, these things were all dead on exact.

http://forums.pelicanparts.com/uploa...1268293151.jpg

The pins were each 110 grams and the pistons were each 470 grams.

http://forums.pelicanparts.com/uploa...1268293172.jpg

Post # 1978 Six Old Alusil Soldiers
 

Since this is the year my car was built and this is my 1978th post on this forum I am using it to pay tribute to these six solid performers who stuck in there well past any original expectation.

Each one of these units stood tall and were able to respond without hesitation. Although they are being replaced it is in no way due to lack of performance, or poor character, on the contrary these parts are still able to perform with valor.

Anywho, should I try to sell these? SmileWavy

Or just put em on a shelf?

Shane









http://forums.pelicanparts.com/uploa...1268294932.jpg

My experience with JE pistons is that going with a custom solution is imperative if going non-stock. For high lift cams, they will cut the pockets deeper for you but won't discount the volume change in the "spec" CR. You really need someone like Henry managing the process to ensure you get what you want. You really need to have your heads complete and your cams chosen BEFORE ordering pistons on custom applications if you expect to get the CR you are requesting. This way volume nuances from valve pockets, twin plugs, etc. can be accomodated.

I purchased 9.5:1 pistons, cut to clear .490" lift and after measuring the volumes of the heads with valves in place, the actual CR ended up at 8.9:1 @ 1mm deck height. Admittedly, I added a second plug, then relieved the piston edges, cylinder head plug holes and polished the chambers, which decreased the volume a bit, but between this and my dished '69S valves, CR was over a half point less than advertised.

This didn't end up being an issue for me as my goals changed, but rest assured I'd have been quite disappointed if I was still building the motor I had planned.

Plan, measure, measure, measure, measure again, then order pistons. Well worth the time.

I believe there is error in measurement of the volume the way you have done it.

Assuming you have the correct head cc's of 90cc - ~.9cc for flycut (according to your spreadsheet) and your're running a .050" deck with the JE pn#274638 which has a dome volume of 35.2cc, your CR is 9.37:1.

JE solid models the pistons and I've never seen a piston, especially a shelf item like what you have, be off from the advertised volume.

It is likely your heads actually have smaller chambers than you think - don't rely on pure calculations for flycut or assume your heads were actually 90cc, or your method of measure is causing you grief.

Hi.

Just to hi-jack slightly.

If anyone is looking for a good start on the way to a 3.2ss build, I have some 98mm Mahle pistons (9.8/1) and 3.2 Mahle cylinders for sale.

Following this thread with interest.
Db

Charles,

The spreadsheet is from Richhawks build on another thread. Your point is well taken and I have found the J&E pistons all the exact weights.

My heads have not been machined and (should) be (fingers crossed) 90cc's. Hope is my method for determining this. I will CC them tonight.

Shane

Getting there
 

Did a dry build and solder deck height test, yesterday got the P&C's on. Wow the J&E pic clips are a total pain in the thumbs:

http://forums.pelicanparts.com/uploa...1268549245.jpg

Today I got the heads and cam towers on, I am ready for cam timing, doing it on Monday.

I also discovered that the springs in the motor had a spring height of 38.7 so they were not stock springs.

Looking awesome Shane! Please keep the updates coming!

Getting those wrist pin clips in was probably the worst part of my rebuild. Someone really should market a tool for this.

Stromski Racing has a tool for it. No affiliation.
Stomski Racing

Thanks for the link. That is worth every painful penny.