Types of 7K Heads

[zorartic]

Hello folks!

I’m a 20 year old getting a 7K motor rebuilt to go into my TE71 fastback, the mechanical work will be done by the shop but I live in a 3rd world country where there’s isnt much technical theoretical knowledge but stuff just works haha

Now the head that came with my 7K (on the right) was said to be weak and had been faced so went out and sourced another one which was difficult, but the shop found one which is out of another engine and much better (on the left). However this new head seems to lack this depression cavity cutout around the valves as observed on the one on the right.

My questions are:

1. What does this cavity do, are there different types of 7K heads with and without this cavity?

2. What effect will there be if I pair dished pistons with each of these heads, the old weak valve cavity one vs the new flat head?

 

Cheers :)

[Banjo]

Welcome Aboard !  My guess is that the compression ratio, will increase, as you compress the greater amount of induced air/fuel mixture, into a smaller volumetric area, as a result, of the flat replacement head, with smaller head chamber volume.

You can readily find this new compression ratio out, if You refer to the 7K manual; or mathematically work it out by dividing the calculated swept volume, plus head volume; divided by the volume of the head, when the piston, is at it's highest point.

Quote

The compression ratio (CR) of a piston engine is the ratio of the cylinder's volume at its largest to its smallest, and is calculated by dividing the total swept volume by the total compressed volume: 

 

• Swept volume: The total volume of the cylinder, clearance volume, piston, gasket, and chamber
• Compressed volume: The total volume of the clearance volume, gasket, piston, and chamber

To calculate the CR, you need to measure the following: 

 

• Cylinder bore diameter
• Crankshaft stroke length
• Head gasket bore diameter
• Head gasket compressed thickness
• Combustion chamber volume
• Piston dome volume
• Piston deck clearance volume

You can also measure the CR by filling the cylinder with water when the piston is at the bottom of its stroke, then measuring how much water is needed to fill the cylinder when the piston is at the top of its stroke. The ratio of the two volumes is the CR. 

Time to get your pipette out !

Cheers Banjo

Edited January 12 by Banjo

[altezzaclub]

Banjo is right, the flat head will increase the compression ratio over the sunken head.

Here's how I measured mine before skimming it to a certain compression ratio. Use petrol or diesel or kero, not water, as water has too high a  surface tension.  Seal the plastic sheet on with a little grease and fill it up with a pippette or burette so you get an accurate measure. It will be 35-40ml I expect.

https://www.rollaclub.com/board/topic/42407-the-girls-ke70/

Just add up the combustion chamber volume you measured, add 7cc for the head gasket, and whatever the cylinder volume on a 7K is, about 445cc. So your compression ratio will be the total volume at bottom dead centre divided by the volume at TDC, so (35+7+445)/(35+7), or 487/42, or 11.6 to one if your head volume was 35cc.

After you've measured a combustion chamber in each head you will be able to compare the expected compression ratios.

What octane fuel are you running it on? Higher compressions, say over 10:1, will need higher octane fuels.

 

[zorartic]

Thank you for your timely responses

I will surely calculate the compression ratios although i believe the shop has finalized on the flat head being the best option.

Currently on a stock 4K I'm running 92 octane most of the time, but once the 7K is built ill run 97

[zorartic]

Also!

This is probably a stupid idea but as I mentioned, due to where I live nobody knows how to regrind a camshaft for higher lift and duration, the machine shops know how to work but have never heard of this process and none of them speak English.

Now sending a cam to be reground to another country is stupid expensive taking shipping and the exchange rate into account, so I wanted to know how do I explain to a machine shop the specifications of having a cam reground if they’ve never specifically done one?

I can show them videos but how do we know how much material to take off the base circle to reach the desired lift and duration? and how do we ensure that the all lobes are ground evenly by the exact same amount?

 

Cheers

[altezzaclub]

Those reasons you asked about are exactly why you get it done at a cam grinding specialist. They have shelves full of 'blanks' for each cam grind they do, giant cam lobe shapes that go on one end of the cam grinding machine and the machine copies them onto your cam.  I suppose these days its all done in CAD.

So you need to know the exact shape of some well-respected cam grinder's lobe, how much they took off the base circle and how rapidly they curved the cheeks, and did they cut a wave into it so it accelerate the valve, then slows it, then accelerates it again..  The shape is crucial to not hammer the valve as it hits it to open it, or not to let it slap onto the seat and damage it, and a few other factors like valve springs and lobe separation. There are differences in 'advertised duration' and '50thou duration'.

I had a Crow cut in my 4K, picked for mid-range and altitude, but there were these options too-

The 7K bore to stroke ratio means it won't rev happily, so don't pick a cam going to 7000-8000. The longer stroke means piston speeds get very high and the rings don't like it.

Make sure the cam grinder refaces all your cam followers too, they will have a wear pattern that suits your cam as it is and you want them fresh for the new cam.

I'd do it through some country that has plenty of cam shops and an old Corolla fan base, European or Australian/NZ or Japan. I assume you're in Western Asia, India/Pakistan/Sri lanka way??