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engine_stuff_explained

The aim of this document is to give yourself the reader an overall understanding of what is required to sumount to a race engine, lightness, power and overall “easy power tweaks”

CRANKSHAFT

Effects of reduction rotational mass in a similar way to the wheel in the toy cars you used to rev up and release and let it zoom off. The crankshaft builds ds up rotational force with speed and momentum effectively storing the energy and helping the bike resist changes in engine speed - good for cruising at a steady speed but bad when you need a fast engine response.

Drawbacks – it takes effort to get the crankshaft rotating and stops the engines revs increasing or slowing down quickly. A lighter crankshaft takes strain off the engine and allows the engine to rev more freely, as a bonus as there is less weight the engine is able to release more power. You’ll notice a race-tuned engine increases and decreases revs a lot more quickly than a standard engine. The big downside to a lighter crankshaft is that engine momentum or inertial spin is reduced – most noticeably on a hill. The lighter the crankshaft the faster engine revs will rise and fall but you will lose momentum on a hill more quickly.

Whereas the momentum in the engine is maintained with a heavy crankshaft the momentum is reduced and the hill has a much more direct effect on the engine output. Best used in a race situation where the track is flat with a demand for fast engine speed changes and the engine has been tuned to output power matching the crankshaft capacity (high revving). The rider will often dance around on the gears and braking taking advantage of the greater responsiveness from the engine.

It must not be forgotten that included in this rotational formula is the weight of alternator and associated rotor weight, hence their removal from racebikes, also don’t forget the use of billet clutch baskets and such for reduced weight and increased strength. Basically anything that has to be spun-up takes power and reduces engine responsiveness. Other factors that can affect power-loss are windage (air displaced by the crank webs rotating) and collecting oil and adding to frictional drag, This is the main reason dry sumping is adopted, knife edging a crank can also help with these frictional losses and any works carried out will require the crank to be re-balanced, although this should always be done as bad harmonics can be induced into the crank if not balanced properly. I would only entrust this too a specialist, I have done several with very expensive equipment and obtained excellent results, but don’t forget I have been trained to do this.

CONRODS

These are designed to withhold the forces of stretch and compression, stretch when the piston travels upto TDC and momentarily stops to then travel back down the bore where at BDC the momentary change is to compress the rod and resist bending. Standard rods are designed to withstand such forces within a certain safety criteria and once you start tuning these can be severly compromised. This is why many serious engines use rods such as Carillo, Cosworth, and others for racing apps, the other area often overlooked is the parts that hold them together “rod bolts” ARP are one such manufacturer of aftermarket bolts, to cope with the higher stress’s and strains placed on them. If you have a limited budget you can opt for shotpeened rods, these are stronger than standard but at a lower cost to “H” beam rods, although not upto the full monty they are within the range of most peoples budgets and can be a cost effective route depending on application. The balancing of the Conrods is quite easy and requires great care and time, one of the easiest ways to balance will be included later for your perusal. Do not forget when mixing and matching rods from different models that journal centers vary and journal sizes, and may have bigger boss’s around the caps and small ends, these may well interfere with other parts (crankcase mouth, piston crown etc)

PISTONS

Now this is an area that is a whole science in itself, when dealing with race motors, basically buy the most expensive you can afford, they are your insurance. A lot depends on the comp ratio you wish to run and time between strip-downs, Wiseco are probably the popular choice for cost and spare parts procurement (you can buy individual pistons/ rings and Gudgeon pins) . Extreme care has to be taken with these as piston/valve clearances have to be measured along with skirt clearances to crank webs, squish band clearance etc

Rules Of Thumb

  • Allow .004” per inch of bore for ring end gaps
  • Allow a minimum of .020” for squish clearance
  • Balance (match weights) them with wrist pin/gudgeon in to within 1gramme (Cosworth spec) material removed from wrist pin boss’s or underside of skirt/crown, extreme care here as you may weaken the piston, various methods can be employed but you generally find they are very close.
  • Pistons utilizing “C” clips to locate the wrist pins, should have all sharp edges removed from their ends, I have seen clips rotate in the piston and the sharp edges have cut their way out of the piston!!

Work out your required piston/valve clearance at required lift and timing and ensure adequate clearance (.020” min), if it tighter than this get your pistons machined for valve pocket depth.

CAMSHAFTS

I will not cover at great length this subject as it is very complex, it is very difficult within the scope of this brief document to cover all aspects of cams, in brief there is:-

Understanding cam specs and their affects will help you select the best cam for your specific engine. The four important camshaft specs to understand are duration, centerline, separation and lift.

DURATION
Duration refers to how long a valve is opened in relation to crankshaft rotation. This open valve time period is expressed in degrees of crankshaft rotation. So, a cam specification of 220 degrees duration simply means the cam holds the valve open for 22 0 degrees of crankshaft rotation.

As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the piston's stroke. However, at lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder.

CENTERLINE
The cam's centerline specification is used to tie the valve timing to the crankshaft's rotation. This spec is expressed as the number of degrees the crankshaft must rotate from top dead center until the cam has rotated to the peak (or centerline) of the lobe.

The centerline spec and the duration spec can be used to calculate when the valves open and close in relation to the crankshaft's rotation. When the valves open (or close) relative to the crankshaft's rotation is known as valve events or valve timing. Some cam manufacturers will provide valve event information and others only provide duration and centerline information. If your cam manufacturer doesn't provide valve event information, give them a call and request it!. Understanding the effects of valve events or valve timing is the real secret to understanding engine performance. For the engine to run at its peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed

SEPARATION
Separation refers to the spacing between the intake lobe and exhaust lobe on the cam shaft. This spacing (or separation) is expressed in degrees on the cam, not on the crankshaft. So, a 108 lobe separation means the intake and exhaust lobes are 108 degrees apart from each other on the cam shaft.

This spec by itself really doesn't mean anything. If you hear someone else is using a cam with 108 separation, don't think that you should use cams that only have 108 separation!

Separation, just like centerline, is another way to tie the duration to the crankshaft rotation and end up with valve events. This spec is a little more complicated though, because it is in cam shaft degrees and the crankshaft rotates two degrees for each one degree of cam rotation. Also, if the cam has been installed either advanced or retarded, the valve events will be different. This is where the Cam Shop software can make your decisions easier because the software does all of the calculations for you and you don't have to guess at these confusing relationships.

LIFT
The final cam spec to understand is lift. While duration refers to how long the valve is opened, cam lift is used to determine how wide the valve is opened.

If the valves are not opened wide enough, they will cause a restriction for the air trying to enter or exit the cylinder. However, opening the valve past a certain point will not increase the flow to (or from) the cylinder. A good way to demonstrate this is with the garden hose in your back yard. When you first start to turn the water on, the flow increases but after a turn or so, opening the valve more has no effect on how fast the water comes out of the hose.

It is necessary to understand a large amount of science to understand how the flow is related to how wide the valve is opened and how this affects the engine's power.

CYLINDER HEAD

The cylinder head is the powerhouse of any engine, the shape and length of inlets determine how much air will flow and what power it will develop, coupled with the exhaust ports. The main areas to be addressed are:- Compressed volume area – all cylinders with valves installed and plug in should have the same volume, this can be rectified by material removal of the smallest chamber to bring it into the same region/volume as the others. Port volume from casting face to back of valves, this is also matched in all race motors to achieve volumetric balance, as is the exhaust ports. Multi valve seat angles also offer advantages with better airflow over the valve head for varying amounts of lift. The shape of these ports will determine torque and power characteristics when coupled to the right choice of cams. Get yourself a decent head-worker for your port work, as this will make or break the expenditure on your engine, Roger Upperton has excelled in this area.

VALVES & SPRINGS

This is another area that can rob power, contrary to belief I have built race engines with slightly weaker springs than the full on race items. The reason being that a strong spring has to be compressed and thus drains your power, extreme care has to be taken and religious use of a shift light to prevent damage from valve bounce. Again if you are lucky enough to get a set of ultra light valves, you don’t need strong springs because their inertia has reduced. This is the main reason for running lightweight valves – less energy required to move them off the seat!. The other area which is worth doing is the installed height of the valve springs, namely the springs when installed and subjected to full lift should be .020 from coil bound, this is achieved by placing shims under the springs to gain the required distance, if you remember the clearance at full lift to piston it was set to .020, subtract this from the coilbound distance and will have a zero clearance but no contact – insurance. From some figures we obtained at Cosworth the power required to drive the camshaft/valve gear on one bank of a F1 V8 was 25hp, that is a total of 50hp drained just to run the camshafts, on a 600+hp motor, that’s 9%. Imagine any increase in that order to any race engine and your talking serious gains, that is why Air-springs dominate for valve operation now!!

I hope you have enjoyed reading this brief overview of what must be considered when modifying an engine and the due considerations that would have been done by the original manufacturers, and the reasoning behind some of their decisions. I have used some referenced some material from publications and included them, as they explain in easier words than I can express.


- Ozzyfzr

engine_stuff_explained.txt · Last modified: 2009/08/08 22:46 by 173.168.75.179