Head combustion area

[campbellju]

I've been thinking about heads again now my 87.5mm bottom end is finished.

A standard bore is 86mm which means the shape of the head will also be 86mm

My HG is 88mm and if I find a clean standard shaped head there will be a 0.75mm lip between the head bore and the cylinder bore?

This sounds like a "det" target and it probably explains why even on a properly mapped car with 87mm pistons you can get nibbling away in this area?


....................................
Head 86mm ___________¦ <--- 1mm of det target?
HG 88mm ___________
Bore87.5mm __________
..................................¦


Is my logic correct and should the edge be chamfered to reduce the issue?

Jim

 

[red reading]

Yes and yes, I dummy build my engines for my cars and match the head to the bore after being able to mark it properly via bolting the head to the block and scribing the head around the bore area.

 

[Makaveli_se7en]

As above, not only risk det but also loss of potential power through incomplete burn caused by poor squish

 

[campbellju]

Thank you both. Good to know my logic was correct, bad to know I'll need to sort it out. I'll speak to the engine builder next week.

Cheers,
Jim

 

[fubar andy]

Would that be the case for 86mm head on a 86.5mm bore, det wise?

 

[MarkTurbo]

When I had my combustion chambers cnc machined I had to send the head gasket & the dowells with it so it could all be matched up accurately.

 

[red reading]

Would that be the case for 86mm head on a 86.5mm bore, det wise?

Worth checking if you are building an engine, mine is 86.5mm bore and was surprised at how much I had to remove from the combustion chamber edge.

 

[campbellju]

I know the engine builder will answer but to check do you just grind the edge of the head to a chamfer or do you mill back the edge to the new size?

 

[red reading]

If your good you can grind and shape it back to the chamber bowl shape, then cc the head and match the volumes and finally skim the head to bring the overall volume down if needed to match o.e.

 

[campbellju]

Thanks Danny, sounds like it's something I'll need to speak to the engine builder about.

 

[fubar andy]

If your good you can grind and shape it back to the chamber bowl shape, then cc the head and match the volumes and finally skim the head to bring the overall volume down if needed to match o.e.

That's interesting, so if I just chamfer the edges and bolted the head to the block, would that still be ok or is it 100% necessary to cc the head?

 

[The Doc]

Better to cc the head, and make sure that you leave absolutely now ruff spots or jagged edges...if you are concerned or not sure give it to someone who can do it.

 

[fubar andy]

Better to cc the head, and make sure that you leave absolutely now ruff spots or jagged edges...if you are concerned or not sure give it to someone who can do it.

It's at my engine builders now; but I'm curious to whether I really need to do this or whether its just an extreme precaution that really is not going to be that detrimental to my build/engine in the long run?

Not only that, I'm buying my house right now, so I'm not rolling in cash like Scrooge McDuck, just not trying to go over board with this refresh :lol:

 

[The Doc]

An engineer should be able to do it easily and relatively cheaply as they have experience , it is worth doing as it will massively reduce the chance of det from hot spots in the combustion chamber.

 

[campbellju]

I was looking at the combustion chambers over the weekend on my old head that I wasn't going to re-use but now wonder if they might be okay?!? I'm a bit confused.

The chamber itself seems to be made of a different material of around 1mm and it's this area that has some minor det attack.

Is this a trick of the eye created by the previous HG which was 87mm on an 86mm chamber? It sounds dumb but is the head is 100% cast aluminium or is there a coating applied to the combustion chamber area that needs to be left intact?

As my new build will use 87.5mm bore and an 88mm HG then I have quite a bit of space with which to tidy the edge of the head's combustion chamber. A small skim to refurbish the head face/chamfer and it would be good to use again.

 

[The Doc]

It is all just a casting, there is no special coatings on the combustion chamber. Many have welded the heads up and re machined them in the past after detonation and even valves snapping off there heads and what you would think would render the head as scrap can be repaired.

if you measure it all out and find that the head is usable after the chamber work and a skim use it.

 

[campbellju]

Thanks Doc. I was looking at it again last night and I became more convinced this was the case. I ran out of time today so I can put the head in the car and take it to the builder tomorrow.

Cheers,
Jim

 

[Fusion Ed]

I have never had a problem with any det from small lips around the edge of a head. And I've mapped enough of these to say if they were det limited like that. Not a bad idea to neaten these things up though.

 

[The Doc]

I have never had a problem with any det from small lips around the edge of a head. And I've mapped enough of these to say if they were det limited like that. Not a bad idea to neaten these things up though.

It it does happen Ed, have seen my own and a few others and even an nme engine have this issue. It does not start because of it that comes from else where but once the area is damaged that is where it carries on the worst, this will also help reduce the hot spot that has caused the top of the deck to drop from excess heat in that area.

 

[campbellju]

I don't think they are det limited in a tuning sense, more heat soak causing very minor det that over 20,000 miles eats away at the edge that may then lead to other problems such as sealing etc.

Another head I'm looking at has similar issues in the chamber from an 87mm bore after 15,000 miles at >400hp. I'm not a metallurgist so calling it det might be wrong.