Engine Blueprinting

Now we've covered some principles, how do we use them? Let's consider mass produced engines and blueprinting for starters... Here we are making 100,000 units a year with a specification for an output.  We know we have production tolerances and that castings vary, camshafts vary, machining varies.  The boys in the labs have done the work and shown that under every possible convolution of production tolerances we can meet the specified performance of say 165BHP at 5000rpm and 200lb/ft torque at 2500rpm.  In practice a certain percentage of the engines will actually be allowed to under perform by a small amount and a number will over perform, by say 5%. Now if they were all supposed to be the same, why would they vary?  It turns out that the quality of the cylinder head castings varies quite significantly because they're sand cast and the internal core shifts around significantly between lots, causing visible difference in the shape and finish of the ports and combustion chambers (and externally too).  It turns out that some shapes work much better than others, especially where the machining and casting meet around the back of the valve and valve seat and also where the head and manifold ports meet.  The good ones produce more torque everywhere, particularly at high rpm.  These areas can usually be corrected, but it’s not economic to do so on a production line.

Note also that there are two basic head designs around, the early type, usually fitted to engines with 8.13:1 and 8.25:1 CR with dual springs on small valves, short reach spark plugs, possibly with steel rockers rather than the later alloy ones.  These heads can give good results but generally they need much more work and it's better to start with one of the later head designs.  

Through the late 80's up to 1998ish the head is essentially the same, with some variation in valve size.  All these heads will take the latest largest valves with a spot of machining on the seat.  After this the outer row of head bolts was dropped as they have always been superfluous, and the head faces were milled to accommodate the thicker composite head gasket.  As a consequence, the chamber volume went from 36 to 28cc to allow for the volume in the space between the top and bottom of the thicker gasket.  The definition of the ports is improved so they flow better although the shape and size is basically the same and a bit of extra headwork will result in the same performance from all.  The valve guides also have a different stem seal so the tops of the guides are machined accordingly.  It is possible to mix and match all parts for any of these heads except the early ones because the early spring seats wont accommodate later springs.  Gas flowing and preparation of heads is a topic in itself and it's better if you go and read some of the books about it.  It's not especially difficult but it is time consuming and boring and it's easy to trash one or more heads in the process so it might be worthwhile to pay someone to do it.

CC'ing the heads 

It's also worth CC'ing the heads (measuring and correcting the combustion chamber size and shape) to equalise the combustion chamber volumes as far as possible.  0.25cc deviation is realistically about as far as you could go and 0.5cc deviation is an easily achievable and sensible target, resulting in a variance of about 0.08CR, i.e. a spread of say 9.31:1 - 9.39:1, which is good enough for most purposes. The pistons should also be CC’ed when correctly fitted in the block.  CC'ing is not difficult but requires a wordy explanation.  I've covered it in another FAQ and it's usually covered in books about gasflowing and engine building.  

Cam offset

It also appears that due to tolerances, the camshaft can be offset either way from it's design value by a degree or five and t

his affects where the maximum BHP and torque appear.  If the cam is offset by -4 degrees we get more low end torque and less power but +4 degrees gives us less torque and more power.  It’s usual to dial in a camshaft with 1-2 degrees of advance on a new chain and gears to allow for chain stretch so allow for this.  In practice the vast majority of standard timing gears and cams are quite accurate, however it is possible to buy vernier gears and stronger chains to make sure the cam is set exactly as you want it and some cams such as the 218/MC1 require a certain offset.  Again there are several production camshafts around, but the only ones worth bothering with are the latest 3.5 and 3.9 profiles if you stay with the stock camshafts. 

These can produce outstanding results on a properly prepared engine, however there are better camshafts around, depending on the engine requirements.

Lift

Finally it seems that again due to tolerances, some engines have slightly more or less valve lift and this also affects the power and torque. The ones with a little more lift seem to have better torque all the way across the midrange and top end with just a little less at low rpm.  Shimming and preload are covered in a later section so I won't repeat it here.

Putting together a combination

Armed with this information we should be able to build an engine with production parts that outperforms the vast majority of standard production engines, possibly by a significant amount.   And we only just got started...

Bottom end

Before we move on to what else we can do lets have a think about the other bits.  If things vary that much in the cylinder heads and cams, what about the crankshaft, connecting rods and pistons?  What effect will it have?  We have rods of different length and weight, pistons with varying heights and weights and a crankshaft with varying offsets and out of balance flyweights.  This will result in extra vibration and stress on components and may have small effects on compression ratio.  It may also be that the position of the end gaps in the piston rings can affect the amount of gas that blows past the piston as well.  We want a quiet, smooth and strong engine that will last for a long time so lets optimise here too, especially if we decide to get even more torque from our engine and use it at higher rpm. Lets get the crankshaft offsets checked and corrected if necessary.  It might need machining to correct it but we have other bearings available if we need to remove a few thousands of an inch off the crankshaft journals.

Rods & Pistons

We'll get the rods and pistons matched and checked for weight and corrected if necessary.   On competition engines we should get the crown heights checked and machined so all the pistons come the same distance up the bores as well.  We'll also get the crank balanced as far as possible and make sure the journals and oil ways in it are radiused correctly.  When fitting the rings to the pistons, we should check and if necessary correct the ring end gaps by putting the ring in the bore and measuring the gap.  The rings should then go on the piston with the end gaps positioned according to the notes supplied with the rings.

The Block

What about the block?  Many blocks leak slightly here and there so they should be pressure tested.  On highly stressed engines or ultra reliable engines it’s usual to replace the original liners with shrunk in ones which will mot leak around the joints to the block. The standard ones are cast in and are the source of leaks from between the supporting casting and the top or bottom of the liner.  The new liners are shrunk in after any holes or cracks in the supporting alloy walls are welded up.  They are stronger and will not leak. This does apply mainly to the bigger engines with bores bigger than 89mm but it's not uncommon on 3.5 litre blocks either. Since the machined height of the decks (top faces) vary, they should be corrected if necessary otherwise the CR will vary as well.  

Bores must be honed correctly.  The blocks should be scrubbed and jet washed with the core plugs removed to remove all traces of casting sand in the waterways.  Check for small pieces of flash from the casting process hanging around in the bottom of the valley, fragments may drop off and find their way into somewhere they shouldn't.  On older blocks and higher output engines, it's well worth replacing the main bearing and head bolts with high tensile studs made by ARP, these can be torqued higher and fit better, reducing the tendency for the main bearing caps to fret at high rpm.  On the oldest 3.5 blocks I'd strongly recommend this since the bottom ends are known to be weak.  Post 1985ish blocks have extra webbing internally and are considerably stronger and the blocks from around 1997 have cross-bolted bearing caps so there are 4 bolts holding the cap in position.  You can tell these from the 5 bolts on each side along the base of the block.  Even so, I would recommend it on all performance engines. So now we have a smooth, quiet and very strong bottom end that shouldn't fall apart if the engine gets buzzed accidentally or used enthusiastically on a regular basis. We've also got a top end which is as good as or better than the best production items.  We'll assume for now that it makes exactly 170BHP at 5000rpm and 205lbft at 2500rpm, running on a pair of SU’s and a standard exhaust system.