step cut heads vs compression

[Chico]

Hi All

Im dropping the motor out at the weekend to do some work (leaky push rods flywheel seal etc ) , and was going to swap the heads over
for some new ones I have .

Currently im running a 1776 , with. compression set at 7.6 , I think( from memory deck height was 0.07" using barrel shims ), but looking at the new heads im not sure I allowed for the step cut, when I built it ,
I presume I should ?

New ones have a 1.57 mm step , When I set up before I measured gap from piston to top of cylinder , but didn't think about step , should I ?

thanks

Chico

 

[paul_q]

If you mean a step in the chamber you will measure that when cc the heads
You don't allow for it in the deck height.

 

[Chico]

Hi Thanks
Yes step in combustion chamber.

Just trying to learn and get my head around things, got various books and all say leave 1mm min to be safe (piston /barrel)
but don't mention the step.

So in theory if I had a set of heads without the step , would I allow 2.5mm top of piston to top of barrel?
Or if I had a set with a 2.5mm step would I not leave 1mm between top of piston and barrel ?
What should the measurement between chamber and top of piston be, to be safe .
or am just talking nonsense and should ignore the step completely .

Thanks Chico

 

[paul_q]

Measure the cc's in the head including the step

https://youtu.be/8QJKkCFFTEI

Use this calculator

http://cbperformance.com/v/enginecalc.html

Aim for a minimum of 1mm deck (piston to barrel top)

The step is part of the cc's not deck

 

[paul_q]

See what compression ratio you get.

If it needs lowering space barrels for more deck

If it needs to be higher you can get the step removed in the head or have the the barrels cut down (if there is enough room in the deck height )

 

[Chico]

Hi Thank you

Im comfortable cc heads , ( 57.5 ) well more than 57 less than 58 on all 4 , so close enough for my requirements .

which works out at 7.9 with 1mm deck , 7.6 with 1.5mm , all good im looking for 7.5 to less than 8. w100 cam (I have it loaded up with turf sand cement etc at times ).

I think im just confusing myself with deck height , as the cylinders sit on the step the piston "could" in theory be flush and still have 1.5mm before hitting
the head , so I was working out 1.57 step plus 1 to 1.5mm piston to cylinder so 2.5 to 3mm deck height which would drop me into 6's CR

Sorry probably not being very clear ,but you have helped massively as I was driving myself divvy

Chico

 

[atafonso]

If tou have a step, do not measure it's depth as you have to measure the head chamber volume in cc's and the step is part of it.
First step is to measure your actual deck height, or, the distance between the top of the piston to the top of the barrel. With that measurement and the head cc measurement, you will calculate your compression. Using a w100 you should be at around 8 to have some efficiency. Forget the stock compression of 7.3-7.5 at this time because your cam has different intake valve closing timing, which affects the dynamic compression(the important number).
For best efficiency, you should have a head with no step, and a deck height of between 1-1.5mm. That is called the "squish area", and it works best within that measurement.
If your compression is too low with the measurements you have, cut the step accordingly to get the desired compression.
Do not touch the barrels unless you have more than 1.5mm deck height which i am sure you don't with your parts.
Abel

 

[Chico]

Great thanks guys
Ill update when i've got it pulled apart

Chico