Mechanical Terminology Questions

[Oldman]

Whilst I’m ok with mechanical stuff, I’m no mechanic….
There are a number of commonly referred to terms that I would really like to understand clearly.
I’ll post others as I come across them but for now, could someone please explain:

1. Deck Height and its importance.
2. Compression ratio and its importance. 
3. End Float and its importance.

Many thanks…..

[KIWI]

Whilst I’m ok with mechanical stuff, I’m no mechanic….
There are a number of commonly referred to terms that I would really like to understand clearly.
I’ll post others as I come across them but for now, could someone please explain:

1. Deck Height and its importance.
2. Compression ratio and its importance. 
3. End Float and its importance.

Many thanks…..

Pretty good explanation of end float here:
End float explanation from TheLateBay
PR

[Barry]

Mark

Lots of good info here

https://www.aircooled.net/VW-Tech-Articl...ex/4449-2/

Grab a beer and read one story each night

Anyway…..

Endfloat
When the revs it moves laterally (back and forth) in the case. 
Over time it wears the case. 
The “thrust” bearing sets the majority of movement and the 3 washers under the flywheel the rest. 
It needs to be set at ~ .004”
Any more than say .008” and work is needed. 
If it is too loose the crank starts pounding the case causing real problems

---

Deck height
This is the space at the top of the stroke between the top of the piston and the top of the cylinder. 
It should be ~ 1.0mm - 1.5mm
This area is added to the head chamber volume. 
This is usually around 50cc - 55cc

Outside of this deck height and you effect the piston “squish”
Whole other conversation 

Compression Ratio
This volume above over the volume at the bottom of the stoke is the “static” compression ratio

There are compression ratio calculators where you put in
Head cc
Deck height 
Crank stroke

And it does it all for you.

[Oldman]

Thanks Kiwi and Barry 
I’ll follow the advice of grabbing a beer……….and…….
(I’ll go back and check………. )

[Barry]

Now it’s all a bit of a mix of
Politics 
Religion 
Black Magic

Hemi heads, semi-hemi, CR, cams - this is what separates the good and bad engine builders

Someone who “slaps” an engine together and some one who “builds” a motor. 

PS
I “my opinion” CR should be 8.0 - 8.5 or even a tad more. 

Gene Berg has the “Blue Book” and is worth a read, but I don’t agree with his CR 7.0 beliefs

[Oldman]

So what determines an “ideal” compression ratio and how does that affect performance ?
(I remember Rick saying that he wanted to increase the CR on one of his builds….)
Or does that differ with builders opinions….?

(I haven’t read through those links yet……)

[KIWI]

So what determines an “ideal” compression ratio and how does that affect performance ?
Or does that differ with builders opinions….?

(I haven’t read through those links yet……)

Some would say that the "ideal" compression ratio is that set by the factory as it is the best compromise between power, thermal efficiency, fuel economy and reliability.
PR

[Barry]

The higher the CR the bigger the potential for “BANG”

Now, depending what goes “BANG” it can be good…….. or bad

[rstucke]

compression ratios have little to do with original specifications if modifying your engine
If you have a standard cam stay close to original specs
Any modified cam should list a ball park figure for cr
Bigger valve overlap usually means higher cr because of losses
smaller overlap means less cr. I've just finished a 1200 build with a STD cam and set the cr at 6.9:1 (original spec 6.5:1)
Runs nice and smooth. 
If you don't stick to these ideas overheating and detonation will plague your day.

As for deck height
Your cr needs to be as above but the distance between the piston and the flat part of your combustion chamber (squish area) is critical.
Enough gap so your piston doesn't touch the head when everything is Hot and parts (conrod, piston, crank) have expanded.
Close enough so air/fuel is not trapped in that area which will cause detonation (knock) when things are at running temps
1mm to 1.5mm but I favour 0.8 to 1.2mm (1.5 is too much)
Rule of thumb. The better squish, the better turbulence, the better combustion.
So if your cr is right but your deck height is too big, reduce your combustion chamber (flycut your heads)
if too small, open your combustion chambers (best around your inlet valves)

By the way good luck finding someone to do that ( who knows what they're doing, specially on the central coast)

Another by the way, I really hate the way the text box jumps around when typing in and hitting the space key.

[Barry]

Mark

Just getting back to end float.............

It is easier to to do on a type 1 motor.
A bit hard grabbing onto the fan pulley of a Type 4

But, with fan belt off, grab the pulley and push and shove. In/out. 
The movement should be .004 thou of an inch.............a 1/10th of a mm

If you can see it move it is too much