I have a question about the head being an integral part of the combustion chamber and how it should all fit together. This applies to a race engine.

Let's start with the block. If I take a stock d16a6 and bore it to an ITA legal .040" (1mm) from 75mm to 76mm bore, what should I do with the head? Most of the d16a6's I've seen in ITA that go .020 or .040 over don't touch the head. However, I've read that people recommend opening the head (where it sits around the cylinder) to the same 76mm so there is no "lip" that hangs over inside the bore.

The theory is sound but not for reason most may think. D16a6 pistons do not protrude from the deck at TDC so there shouldn't be a worry about the pistons hitting the gasket or the "lip" from the 75mm head (it's actually under 75mm). However, with the aluminum lip of a 75mm head I theorize this could cause a hot spot which would lead to warping or would lead to detonation which would lead to warping and then the head gasket would cease to function.

So my question... For anyone who is a profession engine builder or has a motor that's been bored and build professionally, is it recommended to make sure the part of the head that's sitting over the bore be same bore so there is no overlap?




Here's a photo illustration.

 

doesn't have to be the same bore, ie anything on a B20, but it is a good idea to smooth out the lip into a curve.

 

better picture:

 

I agree with the theory. However, ITA is limited prep and opening the combustion chamber on the head is a grey area. You can do a valve job and open intake and exhaust ports, but only 1" into the head from manifold surfaces. They say that if it is not in the rules you cant do it.
I would also think it would lower your compression and one would have to gain that back by decking surfaces and/or headgasket choice. I dont think that anyone would ever know or if it would really help on a D16 ITA motor. And seiing as how in its OEM state the head chamber is smaller then the bore tells me its not a issue. And unless honda curved the egde I dont see a real gain for a ITA application. Now if this was a fully built turbo or N/A Bseries etc, It may prove to be more important and needed but not for ITA.
My ITA A6 is .040 over and my head chamber was not touched. My DD is High comp LSVTEC thats 82mm (stock is 81) and the head chamber is stock. Never had problems with detonation or head gaskets.
BTW: I am not a "professional" engine builder. But I build all my own motors and tranny's.


EDIT: GSR heads also use a "square" chamber design which I believe overlaps also. As apposed to D16 and B16 "round" chamber designs.


Nick

 

I'm very familiar with ITA rules.

You are right with the "gray" area because it doesn't say block only nor does it exclude the head. Technically the head is part of the engine and depending on engine design (especially engines that have pistons that protrude past the deck) the chamber in the head is part of the cylinder.

Of course I intend on keeping it legal, without question, but I need to stop rebuilding my engines twice a year. I have an on going problem and I'm looking at everything that could possibly cause it.

 

Boring the engine .040" over will raise the compression, not lower it. The next part of your sentence makes me think you know this but I figured I'd throw that out there...

 

I was reffering to removing material from the cylinder head combustion area after the cylinder was bored that would lower compression. (You just made the combustion area bigger with nothing to fill the void)

Twice a year huh? What kinda of ECU/engine control do you run? The tune makes a huge difference. Is it Tuned on the dyno? If so whats the AFR at WOT and partial?

Nick

 

Nick97hatch was not writing about boring the cylinder he was writing about opening up the head portion of the chamber. And he is correct, your compression numbers will be lowered as your chamber volume will be larger.

 

Twice in 2009 due to broken ring lands from poor ecu/dyno tuning combined with whatever problem I'm having.

Once at the beginning of the season because the rear main seal wasn't install properly (builder) and it blew in the first race weekend.

And now, the gasket gave up the ghost. At the beginning of this season with the new motor I decided to go *STOCK* and used the stock ecu and stock FPR. No tuning, nothing fancy. I think the gasket went because of.....

1: overlap of head into bore of cylinder creating a "hot spot" that leads to warping
2: Fuel problem at high RPMs that causes a lean condition that leads warping
3: Improper installation of head/gasket/ARP studs
4: and finally.... *Unknown*

I like the 4th option the least.

I'm going to take the head off this week to inspect it all and when it get it all back together I will take it to the dyno with an Adj FPR and make sure my AFR's are between 12.8 and 13.3...