Machining head for more compression TB42

[351ciofgrunt]

Has anyone had this done?

How much can you raise the compression? How much can you machine off before piston to valve clearance becomes an issue?

I'd like to get up around 10 to 1

[351ciofgrunt]

Bump

[create]

I have a TB45 head that I have taken a fair bit off. I have the compression at about 11:1. You could do the same to a TB42 head also. You can only run straight gas after doing this tho.

[Brett S]

From a bad memory i took off 2mm to get it up to 9:1.
piston was basicall level with block so just head gasket thickness for squish area.
dodn't even bother checking for valve clearance the cams barely have a bump on em..

[create]

Standard engine is 8.5:1 removing 2mm would have got you heaps more than 9:1

[Brett S]

i think from std mine measured about 8.3:1 allowing for head gasket thickness when using burette in combustion chamber.
i'll try and remember to dig up the figures..

[Fozdick]

when i did mine , i took off about 60thou (1.5mm)

this gave me a CR of 9.5 with a .5mm over bore on the pistons

unsure of how much had been taken off of head before i got it

this was done by ccing the comp chambers

as mentioned .have to run premium afterwards and didn't really help with power much at all.

[jessie928]

Has anyone had this done?

How much can you raise the compression? How much can you machine off before piston to valve clearance becomes an issue?

I'd like to get up around 10 to 1

performance is great when you do this on LPG, economy is also alot better.
if you go over 10:1 you have to always run 98 octane petrol , it stil may ping on hot days , gas loves it, as create has, go 11:1 at least.
advance the timing accordingly and it will be even better.

piston/valve clearance is not an issue with the standard cam.

Jes

[351ciofgrunt]

Has anyone had this done?

How much can you raise the compression? How much can you machine off before piston to valve clearance becomes an issue?

I'd like to get up around 10 to 1

performance is great when you do this on LPG, economy is also alot better.
if you go over 10:1 you have to always run 98 octane petrol , it stil may ping on hot days , gas loves it, as create has, go 11:1 at least.
advance the timing accordingly and it will be even better.

piston/valve clearance is not an issue with the standard cam.

Jes

cheers mate

Yeah it's on LPG, 11 to 1 sounds good!

[Brett S]

ok dug up me measurements..

Standard:
Piston flush with block
Head gasket 1.2mm thick @96mm = 8.7cc
Measured combustion chamber = 87.8cc using burette and perspex
Swept volume = (3.14*9.6^2/4)*9.6 = 695cc

therefore compression ratio = (695+(87.8+8.7))/(87.8+8.7) = 8.2:1

now because the actuall combustion chamber is not a full bowl it has a large squish area the actual cross sectional area is only 58cm^2 This i drew on autocad to get the area.

So for every 1mm machined off head you decrease combustion volume by 5.8cc.

So say target is 9.5:1
9.5=(694.5 + x + 8.7)/(x + 8.7)
82.65 + 9.5x = 703.2 + x
8.5x = 620.55
x = 73cc target combustion chamber volume

therefore change in combustion chamber volume = 87.8 - 73 = 14.8cc change

Now with combustion chamber area being 5.8cc/mm
Amount to machine is 14.8/5.8 = 2.55mm to machine (a fair bit)

this assuming new head gasket same thickness. (and 4.2L)

also 9:1 required 1.7mm

hope this is of some help to others

[PGS 4WD]

Get a hardness check first, take a lot of material off and you will have gasket failures as the head will deform between head bolts if already soft.

Joel

[jessie928]

Get a hardness check first, take a lot of material off and you will have gasket failures as the head will deform between head bolts if already soft.

Joel

X2 to that advice. its also a good idea to pull out the hex plugs under the rocker gear and weld up the holes, the expansion rate differences crack the head here aswell.

JEs