GTR Forum banner

1 - 20 of 31 Posts

·
Registered
Joined
·
175 Posts
Discussion Starter #1
A friend just got excited with the dremmel and was wondering if it's a good idea to 'smooth' out those ugly flat surfaces on the cyl head of the RB26

Then he sent me a page from the JUN website and I was gobsmacked:eek:

Would anyone here do this on a lightly modded skyline? (not talking 1000bhp cars with standalones etc)
 

·
Banned
Joined
·
2,097 Posts
I think this has been discussed before, and the general consensus was that for a road car in order to retain drivability and reliability it should be kept.
 

·
Banned
Joined
·
7,219 Posts
Reduce det mainly i think as it will lower the compression ratio to a bit more pump fuel friendly level rather than 8.6:1.

The squish areas match with the ones on the pistons, thats how squish areas work, so best not change the ones in the combustion chamber in that case.

If your pistons are a different shape to standard then you can change them to suit (sharp edges create hot spots which encourage det), tho getting rid totally is not always a perfect plan as the ones on the combustion chambers also help direct the fuel/air charge into the centre, where the sparkplug is, though the effect of that is debatable.

Also, look at heads on engines that have died from det, the det marks are always on the squish areas.

Im not sure how much removing the squish areas from the combustion chamber lowers the compression on a GTR actually, interesting to find out.
 

·
Registered
Joined
·
175 Posts
Discussion Starter #4
SteveN said:
Reduce det mainly i think as it will lower the compression ratio to a bit more pump fuel friendly level rather than 8.6:1.
It will reduce the static CR of course --- but at the expense of the mixture quality, especially at lower revs
If your pistons are a different shape to standard then you can change them to suit (sharp edges create hot spots which encourage det), tho getting rid totally is not always a perfect plan as the ones on the combustion chambers also help direct the fuel/air charge into the centre, where the sparkplug is, though the effect of that is debatable.
He's getting JE pistons and their shape is different around the squish (at least from pictures I've seen)
Theory is nice, but nothing beats condensed experience in a particular engine.
I was just wondering if others have done it on the RB26 and if so, how it affected det 'sensitivity'
Also, look at heads on engines that have died from det, the det marks are always on the squish areas.
Yeah, but they are (by design) the areas further away from the plug, so that would be expected, wouldn't it?
Im not sure how much removing the squish areas from the combustion chamber lowers the compression on a GTR actually, interesting to find out.
Burette before/after would show the difference alright.

I was quite surpised to find the 'mod' top of the list on a website like that one.
I thought maybe I'm missing something?!
 

·
Skyline Section Manager
Joined
·
11,872 Posts
I've got no squish areas on my head. The the only think that seems different is the MPG.
I don't get any knock really, but I do run a little more advance than with the STD head setup, which is good.
Driveability is ok, no real change there.
 

·
Banned
Joined
·
7,219 Posts
R32 Combat said:
I've got no squish areas on my head. The the only think that seems different is the MPG.
I don't get any knock really, but I do run a little more advance than with the STD head setup, which is good.
Driveability is ok, no real change there.
Was gonna reply myself that the drivability shouldnt change to an extent youd really notice.

More advance due to lower compression id guess.

Be interested to see what kind of ign timing people run actually.
 

·
Registered
Joined
·
1,568 Posts
Off boost and driveabiltiy will be effected. Boost threshold will also be higher. Instead of removing the squish, just put a small radius or chamfer on it to break the square corner, this is all that is needed to remove potential hotspots on a mildly tuned engine.
 

·
Registered
Joined
·
175 Posts
Discussion Starter #8
Most interesting guys, thanks.

More advance would be expected of course, since the combustion chamber is now larger (bits further away from the plugs are now inside the chamber, and it takes so much longer for the flamefront to get there)
 

·
Skyline Section Manager
Joined
·
11,872 Posts
I run 29degs @ 7000 @ 1.5bar. I dunno how much more advanced over STD timing that is, but it's gotta be a few degs.
 

·
Registered
Joined
·
1,568 Posts
SteveN said:
Surely the people who consider doing this are doing it to engines a lot more than mildly tuned tho!

Im sure ive heard of an OSG RB30 with it all removed...
EH?????? you've lost me??????? (mind you thats not hard!)

I mean that even in a mildly tuned engine you will get benefits from just breaking the square edge and removing potential hotspots. And in my opinion this is all thats necessary right upto 900bhp, because the chamber on the RB26 works very nicely.
 

·
Registered
Joined
·
175 Posts
Discussion Starter #13
R32 Combat said:
I run 29degs @ 7000 @ 1.5bar. I dunno how much more advanced over STD timing that is, but it's gotta be a few degs.
The speed of burn depends on AFR and charge density (mainly)
So it's not hard to calculate the extra ignition degrees needed to accomodate the extended combustion chamber edges.

Trouble is that without squish the mixture won't be as homogenous as it was meant to be (so it will be more prone to detonation). That's the easy bit. The hard bit is to quantify the effect.

I'm not familiar enough with the RB, that's why I posted here.
Has it got offset intake ports (for swirl) like the supra?
 

·
Registered
Joined
·
568 Posts
JohnA said:
The speed of burn depends on AFR and charge density (mainly)
So it's not hard to calculate the extra ignition degrees needed to accomodate the extended combustion chamber edges.
That's a bold statement. Care to give an example?
 

·
Banned
Joined
·
7,219 Posts


Head modifications are well documented. What Jamie has done is right in line with the Nismo GT500 circuit race engines. This info is even published. The port shape does not change. Smooth the bowl and short-side radius, knife edge the splitter, push the splitter back about 1 cm into a "V" in higher rpm engines. Exhaust side gets the "lump" removed and the bowl and short-side radius cleaned up. Knife edging the splitter in the exhaust port will give very small returns. Both ports can be manifold matched and the valve guide boss' gets smoothed into a "V". Remove the intake squish pad and chamfer the exhaust. These are the basic head mods that every tuner in Japan does.

The pads in the combustion chamber are not quench pads designed for inert effects, like an OHV American V8. They are squish areas to promote chamber turbulance and a faster burn at lower engine speeds. Mid level engines have the intake side removed and the exhaust side chamfered. Full race engines have both pads removed (the Pulsar GTI-R SR20 chamber is like this from the factory). Removing the pad on the intake side has two effects, besides lowering compression, it unshrouds the intake valves for a substantial power increase and eliminates pocket detonation. Without this mod, you will burn the pistons on the intake side when getting near 600hp. You will not hear the detonation, but it's there. A knock meter won't pick it up in a 700 hp engine because mechanical noise is substantial.
 

·
Registered
Joined
·
568 Posts
JohnA said:
What exactly is a bold statement?
It's all documented mate, nothing extraordinary.
So given all the other variables, what will the difference in ignition timing be exactly? In the days of the internet, being documented is not the same thing as being proved.

Paul
 
1 - 20 of 31 Posts
Top