I am currently rebuilding the top end of the 4.6 V8 from my P38a. The tappets were in bad shape, probably still original after 325k km.
Now that everything is apart, I would like to make the most of the rebuild. I have a Piper Torquemax camshaft and will be shortening the trumpets as well.

One thing I'm not sure about is port matching of heads and manifolds. I have read BBC's long but excellent thread on properly building a V8. This has convinced me it is worth the effort, and some experimenting with an old head has proven it's not too difficult.
However, before crossing the point of no return, I's like some feedback. My plan was to enlarge the heads (after marking with the gaskets on) with a Dremel, as deep in the ports as possible. How smooth does the finish need to be? And is it worth the risk going a long way into the heads and especially the intake manifold. I'm a bit worried about the mounting of the injectors.

Any tips would be greatly appreciated!
Pictures will offcourse be provided.

Greetz,

Filip

I'm interested too, since at some point my heads need offing to have the valve stems done.

Porting is an art form and its not just wahaaay - loads of Ali removed ....



However some basics rules 1st

Make sure the heads your about to work on are good ones
Do they need facing valve stem guides etc etc - no point shoving hours into a poor head
And partly type - you want efi heads (better inlet ports and are known for better /closer casting sizes between cylinders)
Pressure testing by an engineering co would be a sound £s invetment
Best to have the heads also chemically cleaned pre work so as to remove all carbon deposits


There are a number of levels of porting:

Cleaning - ie Removing flash casting and marks from manfactuer

Blending - Matching the exit from the head to the exhaust etc

Modifying - Making the "Holes" bigger

There is much written about "Finish" years ago people polished ports - this now has been proven to cause fuel to stick like condensation to the mirroe surface - so a clean machined finish is more than good enough - aviod polishing inlet tracts



Basics on all 3 above :

Cleaning

Strip head - give to a engineering shop, have them check the face of the head and guides if OK then have cleaned, then its fit for work

Blending

Trumpet to inlet is normally fine the holes are often I think line bored, its the finsih inside where you'll see casting marks fins and lumps of ali,
These are - Valve seats and cylinder head backinto the ports - cleaning these off and go back around 15mm this is often enough and then removal of any deeper obvious flashing you can get to. Do NOT increase the size of the valve seat - if you touch the valve seat with the die grinder you'll be replacing it

Further in from the ports the castings are normally fairly well shaped on leter efi heads and porting won't get you too much benifit without other mods too.


Porting

This is the art form.

There is little point in making HUGE exhaust port exits without the entire tract being blended, this istricky skillfull stuff and needs to have balance between each cylinder, my exits for example are 44mmx27mm !. inlets 35mmx24mm..(I fink - from memory) but the machining goes right throught the inlet up upto and includes 48mm trumpets, this is hours of work and needs real skill to get right, Nice properly Ported heads (there some rubbish kicking about) can cost ££££££s as they are Labour intensive. If you just port these sizes matching to joins its of little benefit as the tracts remain factory sizes...It will improve gas flow but not much over just a good solid clean up and match port faces to each other.

Reducing the trumpet hieight will move usable power higher up the rpm range

• I would suggest therefore you limit the work to :
  
• Ensuring decent heads inlet manifold and trumpet base
• Checked and chemically cleaned
• Clean up all flashing and casting faults you can see / get to
• Blend the in and out of the ports of inlet and exhaust / exhaust manifolds that join each other back 15mm each side
• Not to reduce the trumpets height unless you wnat to rev the engine to get to the powerband / power

Nige

Thanks a lot for the info Nige!

The heads are from a 1995 4.6 GEMS and seem to be in good order. They will be tested, skimmed and have the valves cleaned/grinded.

Blending seems to be the way to go, there is an obvious overlap, both on the intake side (manifold and heads) and on the exhaust collector (head seems to be quite well matched to the gaskets). So I will take the leap and get to work with the Dremel.

As for the trumpets, I was told (by someone with considerable V8 experience) shortening them would make the engine more responsive. I definetely do not want to shift the power band upwards! Luckily I have not started work on the trumpets yet, so I have some time to sort it out.

I think I'll stay away from the valve seats as well, just to be safe.

Greetz,

Filip

Couple of additional points

Go and buy a set of GENUINE EXhaust and Compsoite Valley Gaskets etc. I say this becuase if you blend the ports in and out to match the gasket it would be good to know a new gasket in the future will be a perfect fit, pattern stuff often has holes slighlty off where they should be which would sort of be daft !

The "More responsive" bit re trumpet shortening is true, it helps make it rev, but shortening the trumpets does also move the power up the rev range, to get it back down again the trumpets also then have to be wider. Shortening trumpets was a trick used to get revy engines with hairy cams so do beware that doing this will lose you bottom end power. However, you have a 4.6 so the "Real Life noticable effect" would be less than doing the same to say a 3.5 as the cubic capacity can smooth out the effect a little. Frankly I would suggest you don't, unless you can find a big trumpet unit, in which case you'll then need the inlet ported to and through the heads to match and make the gains

GEMS heads are fine. Again I would suggest you strip them, ask for them to be inspected and cleaned then do the work on them, hand them back to the engineering shop for the work to then be done, this way they will end up being cleaned again so all the swarf will be ensured to be gone ?

Nige

Thx again.
I have a set of genuine gaskets, I was thinking exactly the same thing you're recommending.

I guess I'll leave the trumpets be then, too bad I already took them out... Moving the power band up would be contra-productive with a cam optimised for low-end, or would the 2 compensate and do give a net improvement?

Greetz,

Filip

Your guess as good as mine

Nige

Better to keep things simple then, plenty of work to be done anyway.

It is generally accepted that port matching and ‘flow enhancement’ without increasing the valve size will give much better low end torque. To answer your other question………… it’s the shape that increases that power not the shine. Many tests have been done on both machine finished heads and polished heads ………. the difference is about 1% ……… so looking at it another way…….. 99% of the improvement is in the shape.

As I have said before……… it is really easy to get it wrong, however, getting it wrong only ends up with not producing the results as expected, or even slightly degrading the standard power / torque, although this is unusual.

With the RV8 there is plenty to be gained from port matching the manifold to head and just generally cleaning up the inlet tract, especially the area below the vale seat (blend seat to head) and the throat (again, blend the flow path and tidy up). With any inlet port the high flow zone from the port face the throat bend is about 80% - 90% efficient, the medium flow area around the throat is about 70% efficient, and the low flow area directly below and around the valve is only about 50% efficient…………… so in the perfect world the greatest gains are to be made around that area of the valve. However, this assumes an in interrupted flow path, which is where port matching comes into play to achieve the uninterrupted path. Unless you want to enhance the top end performance, the exhaust ports are adequate on these heads……….. more can be gained from using the correct headers.

If you start playing with the combustion chambers then there are other factors to consider……… the new shape needs to be balanced across all cylinders. Also any metal removed from this area will affect the CR, so that will need to be corrected by skimming the head, decking the block, or fitting different pistons ……. or a combination of all three.

If you increase the valve sizes then consideration has to be given to ‘valve shrouding’. The bigger you make the valve then the closer it is to the bore wall which has a detrimental effect on gas flow. This is why big valve heads do not show the gains that were expected ………… these heads perform better in the higher rev range……… the same is true of large overlap cams.


HTH




Ian

Thx for your input Ian!



Just what I wanted to hear!



I have been working on the heads the past 2 evenings, and came to the same conclusion regarding the exhaust ports. They were quite well matched to the (new) gaskets, the exhaust manifolds on the other hand definitely need work, as there is an opening between the tubes and the flanges, with the tubes being considerably smaller then the gaskets. I plan to widen the tubes up to the weld on the flanges, so there is less interruption for the flow.



I'll stay away from the combustion area and valves (apart from cleaning and having them grinded).
Not really after top end performance or maximum hp, but while the engine is apart, it seemed like a waste not to do a bit of work to increase efficiency and low end torque.

As an update, I have finished the heads, they're now in the shop being skimmed and cleaned and ready to be picked up this evening. Port matching was easier then I thought.
Work on the manifolds doesn't go as well, it will take a good few hours before I'm happy with the result.
I'll try and post pictures soon.

Greetz,

Filip

rebuild pictures

More to follow.