28 replies to this topic

[fenz]

I have a Yella Terra low comp head on my bench at the moment with chambers measuring 55cc.

Now from what I can gather 55cc is pretty standard for a low comp.(most probably got surfaced at YT and that's it)

 

I am looking at increasing the compression ratio and was wondering if there is a calculation (or rule of thumb) that you can do to work out volume of the chamber versus how much material to remove.

Due to the odd shape of the chamber I would imagine that calculating volume would be a bit difficult but I am sure this has been done a million times before and some one should be able to say yeah .040 thou off will reduce chamber to 50cc etc.

 

The motor is still in the car and flat pistons are .040 thou down the bores from TDC so not going in that direction but don't want to rip a heap out of the head and then render it no good for other blocks that may have been zero decked.

 

From what I can calculate current comp is around 8.5:1 and looking to get as close to 10:1 as possible.

 

Cheers 

[fenz]

Yep I get where your coming from.

Will have to do some calculations with different chamber cc and see where I end up.

 

Certainly don't want to machine a big slab off the head and then render it useless for future use.

Even though its only ST3 big valves etc. its done stuff all work since YT did their thing with it 

 

In an ideal world you would pull the motor and deck the block to zero height but aint got the time for that at the moment and motor is low kms.

Might have to build a big block 202 instead haha.. 

[fenz]

186 plus .060"

[fenz]

From my calculations(GSG Network.com) if I can reduce head chamber back to 50cc I should get   9.14:1.

using this.

 

Bore=3.685

Stroke=3

Head gas.dia=3.779x.040 thick

Chamber=50cc

Flat tops .040 down bores

 

Will have a play on weekend with 50cc in the chamber and see if I can get a measurement down to the fluid.

With head set up nice and level and using a depth mic. I will see what I get

[fenz]

Sounds like .020 might be in the ball park then.

Hope to do some measuring today to get some better figures.

[I'm a Red Motor fiend]

How much overlap has your cam got? Do you want to try and get to 10:1 just because? or are you trying to match compression to cam?

[fenz]

Cam specs unknown and chasing as much compression as possible with out destroying the head I have.

Car is to be run on 98.

Havent had a chance to measure yet. 

[LC-GTR-1969]

I've just had a low compression (large chamber) YT head machined . It measured 51cc had .010" removed and now measures 48cc . Going on 186 + 060" calculates to 9.5 : 1 .

 

Do you still have the witness marks showing? Just curious how far the l/c heads can go before they hit the witness marks... maybe 45cc?

[I'm a Red Motor fiend]

My low comp head has just a feint outline of the witness mark but the deck and witness mark are definitely now on the one same plane. 44cc chambers, a smidge over 100 thou off it.

[LC-GTR-1969]

Next question is how far can we push it past the witness marks before the chambers end up in the coolant passages lol?

[SA EH]

I recently bought a low comp head off Orangelj here for my EH, see for yourself. That middle thumbprint is flat with the deck, don't know any details though, I just bolted it on!

[I'm a Red Motor fiend]

Yep, thats just how mine looks.
Im not too keen to shave mine anymore tho. How far is in interesting question tho. I guess you could always sacrifice a junk head and drill through the deck to determine just how much is there.

Edited by I'm a Red Motor fiend, 02 September 2016 - 05:02 PM.

[fenz]

Yep that's had a bit pulled off it,

Mine measures around .250" down to this mark.

 

Tried filling the chamber with 50cc and measuring down to the fluid level with a piece of ground bar across the chamber as a reference.

But the fluid I am using (10w oil) forms a meniscus up to the ground bar making it hard to get a consistant reading.

 

Will keep having a play and try to come up with a figure that I think I will be happy with.

[rodomo]

use brake fluid

[grumpy xu1]

Hi guys, without sounding dumb, may i ask what you guys call the thumb print & witness marks, thanks Gary.

[grumpy xu1]

Good idea johno, lmao with borrowing the iron jug from the mrs long term. Gary.

[biga064]

Just go to the local chemist and buy a couple of the biggest syringes they have !! they have the measurement on the side so easy !!!

[fenz]

Old Jonno I measured the volume of the chamber with a piece of 12mm Perspex over chamber etc

.

But what I am trying to calculate by filling the combustion chamber with 50ml and then measuring down to the fluid is how much I need to remove from the head to achieve a desired cc/compression ratio.

 

I thought there would be a rule of thumb amongst holden six engine builders that could go "  40 thou off will give you 48cc chamber  "

I guess most people go for small chamber head for performance use but I am just trying to improve what I already have.

 

I will try using a thinner liquid and see if this stops the meniscus effect.

If I machine it myself I can check volume while its still set up and take another lick off it if needed I guess.

[LC-GTR-1969]

Old Jonno I measured the volume of the chamber with a piece of 12mm Perspex over chamber etc

.

I thought there would be a rule of thumb amongst holden six engine builders that could go "  40 thou off will give you 48cc chamber  "

 

I will try using a thinner liquid and see if this stops the meniscus effect.

 

No real rule of thumb that you would consider accurate... the measurement is progressive, meaning that the more you take off, the faster the CC becomes smaller. A virgin head with 20" removed will have less reduction in CC volume than a head that has 20" removed, after 60" has previously been taken off. The reduction in CC is not linear.