I have an LLY piston. I want the top decked on the piston. With some very crude calculations IdahoCTD and I figured .056 should drop it 7.5 cc's. If loosing 3cc is a rough estimate for one point of compression loss, my hillbilly math shows that .056 off the deck should get me to 15:1.


Am I way off base here?

Here is the program that I use now. Guy made this up. Very easy to use.

A stock piston has 42cc's in the bowl. Stock heads have a volume of 4 cc's.

head gasket volume is found by thickness x 3.1416

Great info. This might become a sticky!

According to that calculator, the piston is at .0009 protrusion for deck to piston height. I have not compensated for the reduction in bowl volume from decking the piston. Cutting .030 off the deck of the piston should net a deck to piston depth of .021 below the deck. Input that into the calculator and you get a compression ratio of 14.97:1.

Figure in the CC gain in the bowl and .040 will get me close to my 15:1 goal without a head gasket thickness change.

I think you mean .009 protrusion Bobo.

Cutting the top 0.03" reduces the bowl volume ~1.2 cc

Stock gaskets vary. Thickest I think is 0.03

Plugging all that in gives you 16:1

You either need to cut an additional 0.020 off the top or open the bowl to get to 15:1.

Bob,

I actually worked the equation myself and I got the following:
.040 off the top at 15.8 to 1
.050 off the top at 15.47 to 1
.060 off the top at 15.1 to 1
.070 off the top at 14.7 to 1
.080 off the top at 14.49 to 1
.090 off the top at 14.1 to 1
.100 off the top at 13.9 to 1
This is using a 2.375" diameter bowl and a 4.00" piston.

After this we can figure out the amount that needs to be removed from the lip of the bowl with standard spray angle injectors to hit the same spot on the lip. Just not now. :)

If you want, I gave Trippin a profile for 51.5 cc bowls that doesn't take any off the top.

If you want, I gave Trippin a profile for 51.5 cc bowls that doesn't take any off the top.


A stock bowl is 42cc, so that is 9.5cc less (14:1??). Will that bring the compression below 15:1? I'd like to stay at 15:1 if at all possible. What does Mahle think about this method? Will it hurt the strength or make the walls thin from bowl to oil galley? Are you doing this to a stock LLY piston? How will the high heat that my stock pistons seen play into the strength of doing this? Should I buy new pistons?

Thank you for sharing this info. I appreciate it.

that only works though if you dont change cam.. Gotta make more P/V room with Cams

Or, get a cam that doesn't smack the piston. :D

So is there a performance "dis-advantage" to removing the "quench" area in diesel engine.

FYI, every .010 removed from the top of the piston seems to reduce the bowl volume by approximately .7cc.

Remember to include that in your calculations. :D

So is there a performance "dis-advantage" to removing the "quench" area in diesel engine.

Well again I am speaking out of context, but in the vw diesel world we have found definate disadvatages to removing or enlarging the quench area. This is one of the reasons that we opt to modify the bowl of the piston vs shave the top or, do the easy thing and stack headgaskets.

The things that have been found are cold starts are harder and idling is ruffer, with increased white smoke. At higher rpm's we see more smoke. We have had to add timing to compensate. Well the acutal hp differnce is within ~5hp there is a definate drivability difference, even with the added timing as compensation.

Our (well the most accepted amongst the vw diesel tuners) theory as to why the difference of squish or quench is that we need the turbulence created by the squish area to get that more complete burn. The natural or created swirl alone is not enough. The theory goes quite abit deeper yet then that, but my do to my lack of inteligence that is as deep as I understand.:o:

This really is were the details of building a high performance engine (of any sort) come into play. These little details are what seperates the guy making 700hp to the guy making 900hp, they run the same exact same componets, yet one motor makes 200hp more, or in another senario you have 2 motors that make 700hp one lives twice as long despite runing the same stuff.


This is starting to get off topic, but you guys have the fuel and the air, and now the control (thanks to EFI LIVE) to make 1000hp+, but the current growing pains, are the same as every other engine out there, you need to address the weak points, and the detials. I know for certian I will be scorned for this, but im a big boy I can take the heat. There is alot to be learned from other performance area's (which you can see showing through in a number of the members of this board, bringing in backgrounds of building bbc, or bbf ect... race motors, and head work, turbo GN guys with good knowledge of boost but one area to take a good look at is Import guys. As they like to run big chargers and turn alot of rpm's. Yes there are a ton of pimple faced kids that drive honda's with electric BOV noise machines that clutter the forums, but if you can get beyond that into the hardcore guys at the leading edge, they are no differnt from us they have a passion to make it faster, stronger and better. Now I am diesel through and through my truck is diesel, my car is diesel, heck even my lawnmower is diesel, but I still find a whole world of knowledge outside the diesel comunity that translates back into making our stuff better.

How much bigger is the bowl on the LBZ pistons? If a guy was using LBZ pistons and didn't want the bowl size to be smaller than the LB7's , how much could be taken of of the piston? What would the compression be? I want 14:1 compression on LBZ pistons.
I think maule said to clearence for valve relief then take any extra material out of the bowl to get to the desired compression ratio.

Enlarging the quench area via shaving the tops of the pistons increases the dead space in the chamber where fuel can't get to readily. The combustion pushes the bowl gas back into this pocket where it doesn't interact until the piston is low enough that the injectors spray over the tops of the piston.

So for each 0.010" shaved from the top, you get about 0.55 cc of additional chamber.

Please define the "quench" area. Would removing any of the lip on the top of the piston bowl help any?

"Quench" area is the area between the top of the piston and the head outside of the bowl area. Several things begin to happen when you shave the top off the piston and you need to be careful when doing it. Compression ratio is affected in several areas when cutting the top as you reduce the bowl volume and you reduce the volume between the piston top ring and piston top that you must figure into it. This are is being skipped over by most and it does matter. Once you cut too much then the injector patterns get screwed up as well, so just cutting the top or putting a thick head gasket isn't the answer here.

Can somebody please tell me how much I would have to take off the top of a Lb7 piston to get 14.5:1 or 14:1. I dont want to take any out of the Bowl.

Can somebody please tell me how much I would have to take off the top of a Lb7 piston to get 14.5:1 or 14:1. I dont want to take any out of the Bowl.

Depends on a lot of factors. However if you take .080 off the top of a LB7 piston and run a "C" gasket you will be pretty close to 14.5

See my previos post. The calculations were for a LLY but it's the same as the LB7.

Can somebody please tell me how much I would have to take off the top of a Lb7 piston to get 14.5:1 or 14:1. I dont want to take any out of the Bowl.

Taking some out of the bowl is a good idea. I've heard that the LBZ bowl design is different and better. Can an LLY/LB7 bowl be made to match? How important is the lip on the edge of the bowl?

Bob, your pistons are done. 15.14 as near as I can tell for your final compression. Blake and I spent lots of time on these calculations and had to fire a call to Trippin ( Thanks Guy ) to make sure our calculations/math was correct.

Motor is at the machine shop, I didnt get any pics of them before they went, but will when we get them back

Speaking of Guy, I owe him. He has helped a lot with my build on the sidelines and never asked for anything. Thanks Guy. Socal Diesel will get my business because of this! Thanks again!

Taking some out of the bowl is a good idea. I've heard that the LBZ bowl design is different and better. Can an LLY/LB7 bowl be made to match? How important is the lip on the edge of the bowl?

Anyone know the answer to Bob's question about the lip around the bowl?

anyone know the answer to bob's question about the lip around the bowl?
x2

x2

Yes.





















Oh you wanted me to elaborate... :p:

The lip is there to improve swirl (picture a donut of air that is constantly turning into its middle as well as rotating in a circle) The lip helps that rotation towards the center at low rpm's for more complete combustion for better idling charicteristics as well as reduced emissions. As Rpm's go up and cylinder pressures increase, this becomes less and less important.

If you are going to modify your pistons, just shaving the top is going to mess with the squish or quench, and this does far more harm to your idle and your cold starting ability then removing the lip does. However when the lip is remove you need to have a chamfer on the edge of the bowl or you will encounter hotspots, which opens a whole 'nother can of worms.

I'm sure there must be an engineer that can explain this whole thing in much better detail, and much more precisily then myself, hopefully one of them will chime in. :cool: