compression

Post by **PrecisionBoost** » Mon Dec 12, 2005 12:47 am

As for dishing the pistons.... my opinion is that it would be easier to simply order new forged pistons in the correct compression.

It would seem that machining pistons is fairly expensive ( I checked into it for my JE pistons )

I do like the con rod thing..... but it also requires the disassembly of the engine.... and rods aren't cheap.

For your case..... where you want to use the stock rods/pistons..... you either have to limit your boost or go with a thicker gasket.

Cometic makes custom gaskets in various thicknesses..... I was talking to Tom about it a couple days ago..... he listed off a half dozen different gasket thicknesses that are specificly for lowering compression on Naturally aspirated cars so that you can turbo them.

Audacity Racing · Post by **Audacity Racing** » Mon Dec 12, 2005 12:59 am

you do have to be careful if you try and shorten the con rods to lower compression... there area couple things that can go wrong that just plain suck...

if the piston sits too low inside the cylinder while at bdc, there can be a lot of blow-by gas which leads to oil break-down and seal failure.

i would think this would also lead to hp loss because the volume of the combustion chamber would be so large (granted turbo fixes that)

if you did this to an existing engine... you would need to have it re-bored and sleeved to do this because the rings would have worn a "sleeve groove" in the cylinder wall... which would severely increase blow-by

the kicker of that whole thing is that the con-rods are a significant amount of weight. just lowering the deck of a piston wouldn't make for a lot of material loss... especially since the material is just displaced to teh backside to support the new lower deck... the issue lies in the significant loss of balance from the crank...

i would stick with piston changed instead of con rod changes

Post by **PrecisionBoost** » Mon Dec 12, 2005 2:26 am

I've put a little thought into the whole compression calculation thing.... the missing link is the fact that our displacement really isn't 0.5L... it's 0.5L plus the volume of the combustion chamber in the head

If you calculate the displacement via our 86mm X 86mm (bore/stroke )

You get...

8.6cm X 3.14 X ( 8.6cm /2 ) ^2 = 499.54 Cubic Centimeters

Since 1000CC= 1L our bore/stroke works out to be 0.49954 L

Multiply by 4 cylinders and you get 1.998L

So.... I didn't take this into account.

I'm going to guess that our standard head gasket is 0.040" ( 1mm )

I could figure out the exact calculation now..... but I don't really see a point to it.... the forumula I used is fairly close and the rayhall tuning site will give you the exact compression if you plug in the numbers.

Audacity Racing · Post by **Audacity Racing** » Mon Dec 12, 2005 2:31 am

volume of combustion chamber doesn't matter though... just combustion chamber plus piston deck volume.

the reason i say that is compression can easily be figured based on material added or removed from the original design.

www.racingdownloads.com

find the forumlas and look at it (excel file). it has a head milling data calculator to determine material taht needs to be removed (or if you get a negative number... added)

you'd have to use the stock gasket thickness and then use some paper to add and subract for your new gasket

tango · Post by **tango** » Mon Dec 12, 2005 2:33 am

You can compensate for the timing by using a slightly longer belt.

Post by **PrecisionBoost** » Mon Dec 12, 2005 2:34 am

Now.... I haven't had time to do actual calculations but I'm guessing that if our stock gasket is 0.040" (1mm) and we double that to 0.080" (2mm) the compression should drop from 9.6:1 down to something in the range of 8.6:1 to 8.9:1 (just a rough guess )

So... as far as connecting rods.... the rod only has to be a millimeter shorter to change the compression ratio by a significant margin.

One bad thing about shorter connecting rods and thicker gaskets is that you loose the "quench" area around the outsides of the piston.

Quench area is important.... it allows a slight heat transfer to cool down the piston.... the piston and cylinder head never really touch... but they get close enough for the cylinder head to absorb some heat from the piston.

A hot piston will cause detonation.

This area is also known to give a "squish" effect where it pushes the air/fuel on the outsides towards the center.

So.... replacing or machining the pistons is the correct way to do a lower compression engine..... the con-rods and thicker gaskets are somewhat of a "hack"...... they work.... but they have some disadvantages.

Audacity Racing · Post by **Audacity Racing** » Mon Dec 12, 2005 2:49 am

based on teh calcs on tha website...

the combustion chamber of the engine in question is 51.8cc's

to drop the compression to 8.6:1, you would need to add .053 (more than another gasket alone) onto it... i guess an mls would work though

as for doubling the gaskets, you'd get 8.76:1 cr

Post by **PrecisionBoost** » Mon Dec 12, 2005 2:53 am

tango wrote:You can compensate for the timing by using a slightly longer belt.

Hmmm.... that's an interesting idea.... I don't know if that would work or not.

Even with a longer belt.... you'd still have the same distance between the crank pulley and cam shaft pulley.... the notches are evenly spaced.... so even if your belt was slightly longer..... I don't think it would change that distance.

I dunno... I have to give that some thought... I'm pretty sure the only result is that the timing belt tensioner would go back to it's original postition.... you would have to change the spacing of the notches in the belt to make a difference.... then they wouldn't line up with the grooves in the pulley.

debo890 · Post by **debo890** » Mon Dec 12, 2005 3:14 pm

info: thr rods on the let are a few mm shorter than the nubi's. effectivly droping the compression. if you mill the head when you rebuild, you get back some of the squich area.

Post by **PrecisionBoost** » Tue Dec 13, 2005 6:17 am

That's wierd that you found the Nubira rods shorter than the LET rods.... I'm going to have to go back and check mine.... I thought the Nubria T20SED, Sunbird C20GET and Opel C20LET rods were all identical in length.