MicroSquirt.com read only forum (Jan.15,2018)

Are you sure those are outdated heads on the Rover ?
6 grand , especially with larger displacement , should be
just short of imposible with the original design heads .
Never mind , I did a quick Google search and got some pictures .
Your heads probably flow OK , they are definately not the old design ,
225 hp stock is not bad .
Your only worries
(aside from head gasket/cylinder liner probs.)
are with the complete lack of a quench area in the combustion chamber ,
these heads were designed with reducing Hydrocarbon Emissions in mind
they tried to achieve this by eliminating any areas where the mixture
could hide and/or not be burned completely so they eliminated the
quench area , it was a big mistake , some American manufacturers
did the same thing but finally got wise and put the quench area back in .
Open combustion chambers are extremely dependant on having the
ignition advance just right .
But on a positive note , the completely open combustion chambers will
Detonate if you simply look at them the wrong way , this makes it easy to
find the maximum advance the engine will tolerate , which is especially
important for power and mileage with this combustion chamber design .

The 4-2-1 headers are called Tri-Y over here , they are the best way to
go on the street , are the 2 collectors also run into a Y ? (or an X pipe)
this can be worth a noticeable power increase and make the exhaust
quieter .

I was hoping the Jag was apart for a rebuild , there are several things that
can be done to improve the combustion chamber and make them
"fast burn" but it usually involves block machining , too bad .
P.S. I hope they put some kind of quench area in the Jags heads .

What is a quench area Jim?
 
Is this something I can have engineered when I do the gasflowing?
 
My Jag is what is known as a "Pre-HE" with what we call "flat heads" ie no profile whatsoever.  The piston tops are also flat...  The "HE" have flat piston tops and a recessed exhaust valve which produces a swirl an improves light throttle fuel consumption.
 
Kind regards
Philip
MS-2/V3/2.35/MT 2.25/Innovate LC-1/LSU4.2: Land Rover 4.6 V8i std, 9.35CR, 4-2-1 exhaust headers, IAC, VR(6 BTDC), TP100, std coil




Posted by email.

A "quench" or "squish" area is any area where a flat area on the
piston crown comes into very close proximity to a matching flat area
of the cylinder head .

This area , in order to have the desired effect , must be VERY tight when
the piston is at TDC , one rule of thumb used goes like this :

When remachining your engine block you want to "deck" the block
(I think you may call it skimming ??) this is when you put the block in a
holding fixture which uses an expandable steel pipe that runs through all
of the Main Bearing bores and then machining the head gasket surfaces to
insure that they are square and equidistant from the main bearing bores .
Before performing this operation the pistons and connecting rods and
crankshaft are carefully measured so that the amount of material to be
removed from the head gasket surface can be calculated .
The end result is that the piston crowns will be perfectly flush with the
head gasket surface at TDC .
This results in the ONLY clearance between the piston and cylinder head
being the compressed thickness of the head gasket .
This is usually about .040 inches or 1mm , this is just approaching the
ideal clearance , ideal would be about half of that measurement but would
result in too much risk of the piston contacting the head at high rpm .
Head gasket manufacturers sometimes provide this compressed thickness
measurement right on the package , if they don't their website or a phone
call will get you the specs. .
Another note of interest , sometimes "Steel Shim" head gaskets are
availible for your engine , the compressed thickness of these gaskets are
about half that of "composition" style gaskets , this may allow you to
reduce your quench clearance without machine work .

Here is a very good article I found with a quick Google search ,
it generally understates the advantages that I have seen .

http://chevyhiperformance.com/techarticles/94138/

And while it is referencing a Chevy V-8 , the principles apply to all engines .

Here's another one :
http://kb-silvolite.com/article.php?action=read&A_id=39
Their website has been having trouble try again later .

Do you know of any pictures you could give a reference to of your
particular cylinder head design and piston design ?