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Combustion Chamber Issues


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#1 samboson

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Posted 06 May 2019 - 02:15 PM

Hi, wonder if I could ask some advice.

 

I got a head with 33 and 29mm valves that has had some porting and flowing. I have cleaned it up and been preparing it but just measured the chambers and they vary from 18.3cc to 19.7. Basically it looks like the inner 2 are 19.5 and the outer 2 are 18.5cc when allowing for minor measuring inaccuracy. 

 

To me this seems like just a bad job been done but is there any reason you would make the Siamese chambers lower compression on purpose? It looks like they have worked the chambers out at the edges but not in the middle between the valves etc as this looks more course like the original casting. And the chamber depth is deeper for the middle 2 chambers from deck to between the valves. It’s like the original casting varied in depth and they didn’t take that into account when working the chambers out. 

 

I guess if I ground it down on the outer 2 then I can even them up but is 1cc a lot to ground out? I have the tools but not done it before. 

 

Any advice would be great. Thanks guys.

 

 

 



#2 DeadSquare

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Posted 06 May 2019 - 04:08 PM

What size engine is it going on ?



#3 samboson

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Posted 06 May 2019 - 05:27 PM

1275



#4 Fastorq

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Posted 06 May 2019 - 06:32 PM

How much has the head been skimmed?

#5 samboson

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Posted 06 May 2019 - 07:22 PM

I don’t know it’s as it was when I bought it. I haven’t done any machining.

How do these questions relate to my questions? Have I missed something?

Cheers

#6 unburntfuelinthemorning

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Posted 06 May 2019 - 08:59 PM

It's good you measured the combustion chambers, most people don't. 

 

It isn't normal to have such a difference between them.  David Vizard in his book states that the chambers should be within 0.2cc for a race engine and 0.4cc for the road.

 

Trying to even them up seems to be the best decision.



#7 samboson

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Posted 07 May 2019 - 08:46 AM

thanks for the reply, yeah i thought it would be outside of tolerance. in the Dave Vizard books there is no mention of grinding out the floor of the chamber though only the edges. I think i will go steady and see if i can take a bit away to even them up.



#8 unburntfuelinthemorning

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Posted 07 May 2019 - 10:26 AM

Grinding out the floors of the chambers seems an odd thing to do but as long as it hasn't weakened anything shouldn't be too major a problem.



#9 Turbo Phil

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Posted 07 May 2019 - 11:22 AM

The chamber floors should be ground flat then radiused into the walls. They should be ground to an even depth when the head is modified, not only to equalise chamber volumes but to ensure even valve heights.
1.5cc variation is size is to large. 0.5cc for a basic road motor will be fine.

Phil.

#10 carbon

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Posted 07 May 2019 - 06:39 PM

OP, I would also check the centre exhaust valves for any signs of recession.

 

If all of the valves are from a matched set the distance from the surface of the head to the face of the exhaust valve should be identical. If the exhaust valve is recessed by 1mm this will effectively increase the chamber volume by 0.7cc 



#11 samboson

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Posted 07 May 2019 - 06:49 PM

Thanks for the replies guys this is all useful stuff. I will check if the valves are recessed this could be the case but would only be about maybe .2 of a mm. if this is the case does this mean that really it needs new seats? I don’t want to spend anything on it really as it’s just to run while I build my proper engine so I will check and grind out a bit to try and even the chambers up.

Is 1cc a lot to grind out? Not done it before.

#12 carbon

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Posted 07 May 2019 - 07:01 PM

1cc is pretty simple even with electric drill and cheap stones. Just takes a bit of patience and quite a lot of measuring while work in progress to ge the volumes balanced. As Phil says, getting all chambers to within 0.5cc is fine for a road engine.

 

Would also suggest if you go down this route to get some scrap valves and put these in to protect the valve seats when grinding. If they are a little bit smaller than the valves you have fitted this will be even better.

 

And some duct tape on the cylinder head face for same reason, easy to slip and mark the head face. Critical area to protect well is between cylinders 2 and 3, which is where you often see head gasket failures.



#13 samboson

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Posted 07 May 2019 - 07:05 PM

Thanks carbon I’ll bear all that in mind.

Thanks for the help really appreciate it.

#14 samboson

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Posted 09 May 2019 - 07:45 PM

So before i went carving into the head i thought i would do some more measuring just incase, and as Carbon said above i meaasured the exhust valve depths and inlet depths for that matter.

 

what i found is i measured the chambers wrong or read it wrong for one of them so this time the volumes were 18.6, 18.7, 19.7, 18.6 so all very close apart from one of the central chambers, the one closest to the thermostat. measuring the Exhaust valve depth in this one it is over 1mm deeper than the rest so as carbon said this would pretty much make up the 1cc difference in readings.

 

looking at the seat it doesnt look pitted or damaged it looks like it would when cut i think but its just a lot further into the head. all the seats look the same with the valves out and the other valve depth measurements were all very close to each other, max 0.2mm difference.

 

so my question now is what do i do with this one chamber being bigger, seems a bit mad to grind the other 3 out? or is this the way to go? i guess the only other option would be to install a whole new seat, anyone know the cost of doing that and a skim?

 

looking at the compression ratio i think this would give roughly 10.8:1 in three of the cylinder and 10.5:1 in the other, although i do need to check the deck height to check this. is that a big issue?

 

cheers



#15 mini13

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Posted 10 May 2019 - 09:44 AM

new seat should do it, if its recessed then its not unleaded, so may as well do all 4.

 

TurboPhil would be my choice for getting it done, but i think hes pretty busy currently, slark race enineerig is near you, also Rob walker is good.






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