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Engine mods

NONE OF THE DIAGRAMS ARE TO SCALE!!! I just quickly sketched them for you to show how I did my engine.

Heads

Let's start with the heads. First off, you can modify the inlet and exhaust ports as per diagrams.

Some words about porting. You can make the exhaust port as big as you want within reason, as long as you make sure that the flow of gas is smooth, and that it matches with the exhaust manifold - the outlet of the head can actually be smaller than the entrance to the manifold, and this can actually help lower speed running, and bring the engine onto the cam a little earlier in the rev range. Polishing the port and the back of the exhaust valve give some small economy and power gains.
The inlet port is THE most critical part of the engine if you are going to get power. Don't think that just hacking out loads of metal will make it bigger and therefore flow better, as it doesn't work like that. Also, it is a misconception that polishing the intake will make every thing better. Sorry, you actually make things worse doing that. What you want is the smallest diameter port that lets the air through at a good flow rate. The way to do this is to remove only the metal that stops the airflow flowing easily into the cylinder. Obviously, any lumps or bits of metal that stick into the port are bad, and therefore best removed, and concentrate on the port roof where the flow is fastest. The valve guide boss is crudely cast and can be improved, but don't shorten it.
The most critical area of the port is the area just before the valve throat, where the diameter of the port is at its smallest. Alfa boxers are terrible in this area, as the valve seats are machined after insertion, leaving a horrible sharp edge and steps in the port wall. This is where you want to make everything smooth and curvy, with all the different diameters blending into each other.
In both cases, a good three-angle valve seat is also important, and make sure you don't damage the valve seats or thin them too much.
The inlet port is much better left roughish in finish as this helps promote swirl and evaporation of the fuel, which makes low speed running smoother.

Valves

The valves can be machined using the principles of the diagrams.

Photos of the actual valves are there too.

The white coating on the valve faces is PYSZ (partially yttria stabilised zirconia) ceramic coating which I managed to get done for free as I work in the aircraft engine industry. It's a thermal barrier coating, whose benefits to the engine are largely confined to my head and the pub 'who's got the best engine' conversations. If it makes any difference, I can't notice it! The little hole in the centre of the valve head is so that when it was machined the valve didn't bend.

The 'anti reversion groove' is a way of preventing the inlet valve from behaving like the exhaust valve - it effectively makes it more difficult for the exhaust gas to flow past the valve into the inlet tract. This has the following benefits:

it aids the scavenging of the cylinder

prevents contamination of the inlet charge with exhaust gas

strengthens the exhaust pulse signal

This helps bring the engine on to the cam a little earlier, and can make the low speed power a little better, and has no disadvantages if properly done.
The inlet valve is better if it is shaped like a penny on a stick, instead of a tulip shape like standard, but the exhaust valve can be left as it is. In each case, a three angle back cut to the head improves things, but make sure that the valve seats are the right size, and that there are no sharp edges on the back of the head. Thinning the stem of the inlet on the part that sits in the port helps too.
Make sure that the inlet manifold is matched to the inlet port, and that none of the gaskets or spacers create 'into wind' steps that obstruct flow. The standard matching of these bits are normally terrible, and are dead easy to improve.

Carbs

Did you mention that you have 45mm carbs? They are too big for a 1.5, the maximum you need is a 40mm. 45's will just make the engine run like a pig at idle and low speed, and the engine never pulls through enough air to take advantage of the 45's extra flow capacity. 36 IDF's are probably the best if you are not going to put in rally cams or big valve heads. They also would give you a lot more low and mid range power, which is more important on the road.
One area that is absolutely crap as standard is the little ram pipe casting that sits on top of each carburettor. The finish is really rough, so get your emery cloth out and polish it to make it look like a proper ram pipe. It also makes it look good, and it seems to make the engine sound more aggressive too.
If you do want to remove the pistons and do the block skim, 1,0 mm can normally come off the block faces without any danger, but obviously check that you won't run into piston to valve contact. Any good shop should be able to help you determine that. Note that for any aftermarket cams to work, they need a compression hike, otherwise you are wasting your time.

Conrods

The connecting rods can be lightened and balanced as per my diagram, and I've included some pictures of my rods after I reworked them.

I unfortunately didn't record how much the weight was before lightening, but after lightening the rod complete with bolts all weighed less than 664 grams. By lightening the pistons and selectively fitting the rods, bolts, gudgeon pins and pistons, I was able to make all four assemblies weigh 1139 grams plus or minus 0.5 gram. Polishing the rod ends and beams also makes them less likely to break. Note that lightening and polishing does not give you more power! It only gives you a smoother, more responsive and reliable engine. If you do this, don't forget to replace the big end and main bearing shells. Whatever you do, don't mix up the big ends!

Flywheel

The flywheel can be modified as per my diagrams.

, this will take over a pound from memory. Again, no extra power will be available from this just a better response. Don't take too much off though, otherwise pulling away from a standstill can cause the engine to bog down. Remember to get the flywheel balanced afterwards too, preferably complete with the crankshaft and clutch assembly.

General Tunning Info

For some general info regarding tuning, how about considering the following:-
later 1.7 engines were fitted with bigger valves as standard, and the heads bolt straight onto the 1.5 block with no mods. In addition, all the mods to the valves and heads would apply to these heads too. Note : as pointed out to me by Richard Jones, you have to use the heads complete with valves and cams and carriers due to different sizes internally, but the assembly will bolt straight to a 1.5 block.
I have in the past 'rebuilt' a 1.5 engine using a second hand 1.5 and a 1.3 block. Confused? I rebored the 1.3 block to take the used 1.5 pistons, fitted new rings and got a 'new' 1.5 short engine without the price of expensive new pistons. You could get a tired 1.7 and bore out your 1.5 block to accept the 1.7 pistons etc.
Try and get hold of the 105bhp camshafts from the 1.5, or the cams from the 1.7 (unknown spec). Of course, make sure of which type of cam you have - single or twin lobe.
When your engine is on the floor, replace the core plugs. I didn't, and guess who had leaking core plugs 3,000 miles later?
Check the oil pump - if it is knackered, you will stuff your modified engine really quickly.
If you get your block rebored or skimmed, get it cleaned properly to make sure that all the honing grit and swarf is out, otherwise you could have big problems.
I put on a pair of K&N filters - they make the engine sound better, and increase the power by 2 or 3 hp.
My manifold is a standard TI Bainbridge one, they are just as good as any other and a lot cheaper that the Ansa super trick jobs.
The system I'm using is Ansa, it's bloody expensive, but it does work, giving 4 hp or so more. There are probably other aftermarket systems available that are just as good though, and one thing that I hate about the Ansa system is the awful 80mm chromed tailpipe - this looks like it's been stuck on at Flash Harry's Go Faster Emporium, it really is tacky.
The cheapest power you'll ever get is to rolling road tune the car. Not only that, but when your spitting, recalcitrant, temperamental, gas-guzzling, evil minded car comes back purring like a pussycat, sweet and easy to drive, then stomps all over everyone and screams off into the distance, you'll be a truly happy man. A good tuner will make all of those mods you do really shine.

Well, I've rambled on a bit, so I'll let you go away and see what you are going to do. If you get stuck, or have any questions, just send me an e-mail and I'll try and help.
Good luck
Les