Engine Internals, Comp Ratios & General Info

[Gerg_R31]

The following is intended to help some forum memebers with a better knowledge of the engine internals, after market parts, different materials, some machining processes, formulas and the "why is that done" this wont cover everything (based on the RB series but similar principals can be used on other motors)

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Under standing how it works

A internal combustion engine works on a 4 stroke or a 4 cycle rotation, intake (piston moves down air & fuel enter via Intake valve/s) compression (piston moves up compressing the cylinder) ignition (piston goes down as spark plug ignites the air fuel mixture) exhaust (hot exhaust waste is pushed out the exhaust valve)

Or in short, suck squeeze bang blow

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Choosing the right parts

When modifying any motor the first thing you need to ask yourself is what is the purpose of the car?

Why should you ask yourself this? Because all modifications will have benifits and losses. For example a factory car is designed for the lower rpm range and run out of steam in the higher revs, big turbos big cams will highly benifit in the higher revs but sacrificing power in the lower revs

The general thing is to choose where your most likely going to spend the most time in a rpm range for your purpose

(based on a twin cam rb30)

street car 1000rpm-3500rpm

street car / enthusiast track days 2000rpm - 6000 rpm

street car / competitive track days 3000rpm - 7000rpm

dedicated track car 3000rpm - 8000rpm +

Choosing all the parts that will work with in that rev range will give a more benificial result in that rev range

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Compression Ratios

Compression ratios have more involved then what is stated on a piston box.

i.e customer: "can you organise me some pistons with a comp ratio of 8.5:1?"

"what head cc's head gasket you got?"

custmer "oh no i dont want any head work done" and a few weeks later came back going off saying we ordered the wrong pistons, blah blah his mechanic found it was 9:1 blah blah" why was it higher? because he got a thinner head gasket,

With that example out of the way, the following should give a bit more information for helping with ordering the right parts and having a job done right the first time. There are 2 types of compression ratios. Static and Dynamic

Static Compression Ratio of an engine is the ratio of the cylinder volume compared to the combustion chamber volume where the swept volume of the cylinder, with the piston at BDC (Bottom Dead Centre) pushing via the crank stroke, into the closed combustion volume when the piston is at TDC (Top Dead Centre)

Or in simple terms, for example 8.5:1, a piston at its lowest point can sqeeze the volume of air and fuel mixtures above it 8.5 times to make it fit into the volume above the piston when it is at its highest point (formula below)

Static Compression Ratio Formula

(metric units)

Swept Volume

Bore x Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x stroke = Answer3

Answer3 ÷ 1000 = Swept Volume ( SV )

Compression Volume

Head CC

Piston dome/ dish volume

Deck height (piston below the deck)

Bore x Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x block deck to piston crown measurement = Answer3

Answer3 ÷ 1000 = deck height volume

Deck height (piston above the deck)

piston measurement above top ring land x piston measurement above top ring land = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x block deck to piston crown measurement = Answer3

Answer3 ÷ 1000 = deck height volume

Gasket

Gasket Bore x Gasket Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x Gasket thickness = Answer3

Answer3 ÷ 1000 = Gasket volume

The Maths

Head CC + gasket Volume + Piston dish Volume(- Piston dome volume ) + deck height (-if piston is above the deck)= CV

SV + CV + Answer

Answer ÷ CV = Compression ratio

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Dynamic Compression Ratio, this is a bit more complex and uses the position of the piston when the intake valve is closing ABDC (After Bottom Dead Center) rather than the crank stroke to determine the swept volume of that cylinder

For example a RB30DE motor with a static 10.5:1 and 10.8mm lift cams with 280° of duration (seat to seat) where the intake valve closes at 60° ABDC will rob 19% of the stroke making the rb30's 85mm stroke equivalent to a 68mm stroke and turning the 10.5:1 into a 8.7:1

The formula/ how to

Measure how far the piston is down the bore just after the intake valve has closed, this to TDC is your Dynamic Stroke

Repeat the same formula used to acheive the satic compression ratio but replace the engine stroke in the swept volume section with this measurement

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Componant Materials/ info

OEM (Original Engine Manufacturer) manufactured parts were quite often only desgined for the general basic driving and not expected to see past a certain rev or power range, or high stress like that of track days. Many OEM parts are still quite strong and can and will keep pushing past their expected limitations where others do not

Alot of parts manufactured these days have improved their quality and consistancy, which has eliminated most of the old school "blue printing" as it has already been acheived through these high quality standards and more accurate machining, tools and machines for a machinist to machine things with spot on accuracy. for example cam followers in the older motors once had to be individually machined to match the curvature of the varying cam lones, but with the standards of today, the cam followers will generally have a consistant 0.002" to a 0.004" radius and are able to be mixed around without causing any issues (unless it is a solid shim set up which it is best to keep in order unless you plan to re shim it, or re machine to correct the clearances)

Pistons

If your measure a piston from top to bottom at room temperature the measurements will vary a fair bit, but at the engines operating temperature adding thermal expansion piston material thicknesses, the piston will change and become exactly the same size from top to bottom. This has all been designed to do this intentionally by the manufacturers.

Cast pistons are cheap to make and can endure a long life under the standard conditions designed and built by the OEM

Forged pistons are desgined with a higher silicon content which gives more control over the thermal expansion rates of the forged steel molecules, which in turn provides a stronger material to with stand more pressure and to with stand, absorb and transfer more heat.

Connecting Rods

Connecting rods come in many different materials, shapes, rods bolts etc.

Material; Many of the OEM stuff is quite strong and can with stand a fair bit of high level stress, but consistant high RPM (6000+) will fatigue on the con rods material on a molecular level, where the rods will start to stretch after 6000rpm and go back to normal once below 6000rpm. Though after some time, the consistantly stretched rod will increase in length and form weak points where it can crack, split or snap. Below is how much the rods will stretch

factory cast 0.0012" @ 6000 rpm

steel 0.006" @ 6000 rpm

billet 0.002" @ 6000 rpm

alloy 0.020" @ 6000 rpm

and times how much the material stretches per 1500rpm after 6000rpm

Design; The rods design will help with strength from additional cylinder pressures and engine torque, factory I beam is the weakest, then H beam, and I beam being the strongest

Rod bolts these things are what is holding the rods onto the crank, the torquier the motor the better quality the bolts should be for extra security. between the bolt head and the thread is usually a pattern that is designed to press and hold itself into the rod which will minimise its movements. Then the higher torque settings secure the rods onto the crankshaft.

Bolts/ studs with torque settings

Anything where there is something that will increase the torque settings from the OEM settings will most likely oval any bores, or tunnels, always best to measure with a bore guage or an inside micrometer for roundness, correct with honing, boring, or torque plate honing

Heads Gaskets

head gaskets are to keep the combustion chamber sealed, and prevent oil and water mixing / entering the cylinder

Composite; this type of gasket is your every day head gasket. They usually dont vary in thicknesses. It needs a flat but not too smooth surface (or still shows signs of grinding marks) to seal properly. During temperatures above operating temperature heads will distort which can seperate the materials causing the head gasket to leak

Metal; Or Multi Layer Steel gasket, this needs a smooth finish on both the block and the head with about a 7ra finish (smoothness rating) these come in many different thicknesses which will help to keep the compression ratio where it needs to be between machining. These aid with a better seal and during temperatures above operating temperature the gasket will stay intact and not seperate like the composite

O-ring & Copper this is where the head and block is machined and an o ring is fitted to both to seal the cylinder. A copper head gaskets then seals around the o rings and the rest of the oil and water jackets

Valves & Guides

OEM steel valves are good for street aplications, and with the exhaust valve consistantly glowing red the wear factor is more expected. When the chamber conditions change it puts extra strain on the parts, more so the exhaust valve. Common failures that follow are exhaust guide bores becoming bigger, exhaust valve stems becoming smaller and the valve head starts to melt away, commonly know as a "burnt out valve" Stainless steel, nitrided (chemical process that hardenes materials) Inconel, Titanium valves are most common aftermarket valves with the metals more durable to harsher conditions. The after market materials also create an extra friction with cast guides so it is recommended to have bronze guide sleeves or bronze guides fitted to counter this

Valves Springs

Valve springs are there not only to help close the valves but to allow for sufficiant valve/ cam lift. to test a valve spring first you will need the installed height with the valve closed. At this point is where you geat your seat pressure, if your seat pressure is too low you will get valve float (where the valve will bounce off the seat instead of snapping shut) if its too high it will chew out the valve tip, cam followers, cam lobes.

For max spring lift, installed height minus spring height at coil bind minus 0.080" thermal expansion & spring vibration clearance will equal your max lift. If the cam says more valve lift then your springs have then its time to invest in new springs. Using springs that dont have sufficiant lift to match the valve lift will create valve bounce. If you have a rocker then times the cam lift by the rocker ratio to get your valve lift

Springs also come in various materials for better heat absoption tolerance and strength

Camshafts

Choosing the right camshaft is sometimes a bit of a mystery or a guessing game. The first thing you should ask yourself is the what is my car used for and pic a cam that is rated to give power with in that rev range. Going for the biggest lift with the biggest duration will be good for a dedicated drag or track car but not for a daily driver as it will rob you of all the low down power, this is called "over caming". Many cam grinders have tested each cam profile and have recorded when the cams work their best. You may find some cam grinders will have similar cam profiles to another, usually this is from someone desigining the cams testing them, then selling the master cam to their competition who may slightly alter the design to call it their own

Turbo's are more happier with smaller lift medium to long duration. if you have a bigger stroke, i.e rb30 with rb26 cams then subtract about 500-1000rpm from the cam grinders rev range

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Machining

to some the machined surfaces dont appear to be that "important" and a home job is all thats needed, but it plays a large role if its done right the first time so things dont go boom or burn smoke etc. If you find yourself in this situation it is best to measure with bore guages, inside micrometres, normal micrometres. Vernie calipers arent accurate enough for a precision measuring tool

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Bores

Cylinder bores have a pattern known as the cross hatch. which is a series of 30° and 60° angled scratchs. These are designed to hold oil for lubricating the rings to prevent friction. Friction creates heats, heat creates premature wear, wear creates smoke. Glazed bores is where the bores look glassy and have no cross hatch pattern. Using the bottle brush hones you attatch to your drill, should only ever be used after you have measured the bores and there is no issues, other then a faded cross hatch, such as taper (smaller - bigger end to end), barreling (smaller top and bottom, bigger middle) hour glass (bigger top and bottom, smaller middle) or ovality (its an oval shape)

Measure in 6 spots, top middle bottom front to back and side to side. For alloy bores or chrome moly piston rings or greater material a finer cross hatch is required, a plateu stone usually follows a hone to take any microscopic sharp edges off the cross hatch pattern

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Crankshafts

Measuring a crank should be done at to spots on each journal with a micrometer, north & south, east and west. This will not only determine if the crank is with in size or if its ovaled. If a crank has been ground, it doesn't hurt to recheck the size before assembly incase it is too small/ big or is ovalled. Also a commonly overlooked part is where the journal meets the counter weight webs should have a rounded edge, if its a sharp corner then that has not been ground right and the corner is where the crank will snap. Quite common for the gallery bungs to be drilled out to allow for a proper clean of all the oil galleries

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Balancing

Balancing is weight matching the rotational (rotating) and dynamic (up and down) componants. 1 gram of weight at 700rpm becomes 1kg of weight at 7000rpm, this extra weight will cause flex vibrations stress and can lead to early componant failure. Rods pistons rockers fall into the dynamic balancing section, to balance these you find the smallest weight and machine off in areas that wont weaken the componant to match the lightest. With cranks, flywheels and harmonic balancers they are all part of the rotating assembly (rotational),

When balancing a crank and or flywheel you remove weight by drilling, linishing so the front to rear is the same as well as the rotational and "throw" of the crank, where in some cases an over balance is introduced to create extra throw(throw is the rotational force that keeps the crank spinning it the direction it was ment to turn).

Most cranks are Internally balanced, meaning you can swap flywheels, pressure plates etc without having to re balance the whole lot

Externally Balanced cranks are balanced by roughly equal weights, on both the harmonic balancer and the flywheel. to change one item you have to change both and have it re balanced.

Sometimes mallory metal (a heavy dense metal) is needed to be machined to fit when you cant remove enough weight to counter the issue. When having to fit mallory metal it is best to put it in parralel to the journals as its less likely to come out then if its pressed or welded in via the drilled holes in the counter weights

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Blue Printing

Blue printing is making everything as identical as the next, i.e every rocker weighs the same, every piston and conrod weighs the same, every conrod is the same length, every clearance is the same, ever combuston chamber is the same. With everything running the same, there is an even balance of work being done in each cylinder componant, which creates less pre mature wear on the harder working parts cause there is none, and the benifit more efficancy and increase in power.

Balancing is pretty much all that is left with blue printing these days as all parts have a better quality, back in the old days when the quality was not available you would for example get a box of pistons that said 0.005" piston to bore clearance and the each bore was machine to suit each individual piston. technology has improved, machines improved and blue printing is almost fased out

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Porting

Porting jobs vary depending on the aplication, for example a N/A motor with a stroker kit need bigger ports to help the motor breath. too big and it can make it harder to breath as its removed the velocity created by cylinder and atmospheric pressures, but bigger ports may help with turbo'd cars or dedicated race cars. Many port at home but the risk of each port varying in the flowing characteristics may make 1 cylinder work slightly harder then the other.

The best thing to remeber with porting, is the manifold sides are wider for volume, higher for rev range. the bowl area is for atomising and creating a swirl pattern. You want the swirl pattern so the air flows horizontally out past the valve as the valve head blocks the vertical path. Single valve heads should have a snail shell like pattern around the back of the guide, or a ramp that starts at the front of the guide and drops at the rear of the guide, and twin cam heads only need the area where the port splits into 2 to have a "knife edge" on it and same in front of the guide

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Valve Seat Angles

Most alloy heads these days come out with the 3 angle valve seats, 30° crown 45° seat 70° throat (bowl/ port side)

valve lapping is possible to get the seats to seal but what it is actually doing is bedding the seat's 45° angle into the valve creating a indented area on the valve face, which carbon can build up on it over time and create a non sealing valve again

Flow testing has proven that adding a 58° between the seat and the throat increases the head flow on the intake seat. Radius' are a continuous round seat, these are best for the exhaust side with a minimum of a 1mm wide 45° valve seat angle. The radius is not good for the intake as it disrupts the atomising processes

After seat have been cut and the springs on a "leak down" test should be performed, with the manifold side facing up pour a liquid in, and if any has leaked through in a couple hours then it is not sealing 100%

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Reconditioning Cylinder head

The reconditioning process is maching the head so its just like new. The following is the process in how it should be done

Strip and inspect cylinder head, CC chamber

Acid bath head and parts

pressure test cylinder head - report to customer if no good

organise parts if needed

bead blast valves

face and machine valves

bead blast head

run a tap through all bolt holts inspecting for damaged threads, repair or helicoil if needed

Remove and replace guides if needed

Remove and replace vavle seat inserts if needed

cut valve seats making sure they seal 100%

adjust shim lash adjustments if it has solid lifters/ clean and inspect hydrualic lifters

machine head gasket face to within tolerance of original head cc's

de burr and final wash of head

assemble with new welsh plugs, oil gallery bungs and valve stem seals

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Reconditioning Block

Reconditioning of the bottom end

Strip and inspect measuring every bore, tunel, journal in the north south, east west fashion

acid wash everything

organise new parts if needed

linish crank - grind if not within specs

remove any press in bungs to help clean out and dirt and sludge that may get stuck behind

run tap through and inspect every bolt hole

bore and hone block, finers stones for allow bores and chrome moly rings

chamfer top of block

wash

dummy assemble and get deck height

work out compression ratio and see if it matches OEM specs

close and rods conrods if needed,

remove and replace pistons if needed

wash everything

asseble or send back to customer in "kit form"

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RB series specs

ARP RB Main Stud Torque settings 73-75ft lbs

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RB20

bore 78mm/3.071"

stroke 69.7mm/2.744"

main crank journal 2.1634" / 54.951mm to 2.1644" / 54.975mm

big end crank journal 1.7699" / 44.956mm to 1.7706" / 44.974mm

main tunnel 2.3089" / 58.645mm to 2.3094" / 58.658mm

con rod big end tunnel 1.8897" / 48.000mm to 1.8893" / 48.013mm

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RB25

bore 86.00mm/3.39"

stroke 71.70mm/2.82"

main crank journal 2.1634" / 54.951mm to 2.1644" / 54.975mm

big end crank journal 1.888" / 47.956mm to 1.8887" / 47.974mm

main tunnel 2.3089" / 58.645mm to 2.3094" / 58.658mm

con rod big end tunnel 2.0079" / 51.000mm to 2.0084" / 51.013mm

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RB26

bore 86.00mm/3.39"

stroke 73.70mm/2.90"

main crank journal 2.1634" / 54.951mm to 2.1644" / 54.975mm

big end crank journal 1.888" / 47.956mm to 1.8887" / 47.974mm

main tunnel 2.3089" / 58.645mm to 2.3094" / 58.658mm

con rod big end tunnel 2.0079" / 51.000mm to 2.0084" / 51.013mm

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RB30

bore 86.00mm/3.39"

stroke 85.00mm/3.35"

main crank journal 2.1634" / 54.951mm to 2.1644" / 54.975mm

big end crank journal 1.9670" / 49.961mm to 1.9675" / 49.974mm

main tunnel 2.3089" / 58.645mm to 2.3094" / 58.658mm

con rod big end tunnel 2.0866" / 53.000mm to 2.0870" / 53.011mm

(any advice given here is more based on a 'push in the right direction' if you dont fully understand parts of this. or a job, ask some one for help, re read info, google or leave it to the professionals)

[Duncan]

Excellent post....and it leads me to ask a question.

Is "closing and honing" a rod's big end or cam tunnel the same operation as "tunnel boring" a block? What exactly is involved?

And does closing and honing a rod's big end shorten your stroke? I've had interestng results when measuring capacity/stroke on some of my race motors

[zebra]

Excellent post....and it leads me to ask a question.

Is "closing and honing" a rod's big end or cam tunnel the same operation as "tunnel boring" a block? What exactly is involved?

And does closing and honing a rod's big end shorten your stroke? I've had interestng results when measuring capacity/stroke on some of my race motors

#1 yeah very similar but differnt machine/tool

#2 No, rod length makes no difference to capacity, grinding the crank journals can, if the machinist is a little out

[Gerg_R31]

Boring pretty much making a hole bigger using one or many single point carbide cutting tools on a round centralised bar, and if set up right can make any hole perfectly round. Boring can take out a large amount of material in one or two attempts

Honing is a process done on a round centralised bar with stones instead of a carbide tip. Honing stones come in various grades of grit and are used to take out smaller amounts of materials. Honing on its own is used for reclaiming the cross hatch in cylinder bores, getting the last couple thou out to get the cylinder bore on size, honing of the little ends after new bushes or converting to a floating rod, honing con rod big ends, valve guides, rockers, lifter bores etc

Closing and Honing Is usually required after spinning a big end bearing, wear and tear, getting new bolts with a higher torque setting creating the tunnel to be out of round, or mixing the caps up. The process of closing and honing is done by, a light machining of two joining faces, i.e a con rod and a con rod cap so they sit perfectly flat, toqued to specifications and then honed until round and with in OEM spec.

Line Hone a long bar with stones that go nearly the entire length of the bar to ensure that each cap bore is aligned with the next, sometimes a light hone can make the tunnel round again, some times it will need to be closed and honed if its unable to reclaim the roundness and alignment it is then needed to get lined / tunnel bored

Tunnel Boring is usually done for jobs where a light hone wont correct the tunnel. The process is done by milling an exact amount off the two joining faces, bolting them all back on, torquing to specs, then running the boring bar through the entire length to get it close to factory size, perfectly aligned and round. it is then followed by a light line hone to get it within OEM specs

honing or boring of any main or big end tunnel will most likely bring the piston slightly higher towards the deck, with closing and honing of rods and main caps, usually 0.001"-0.003 is removed, boring 0.005"-0.010" (the average hair on someones head is about 0.004" thick) but unless the crank has been ground offset the stroke is never affected

[AngryRB]

very useful post.

[Eps]

This needs to be pinned

[MRHD66]

awesome well written too

[G37Sam]

Very interesting read, I'm building an RB26/30 and this will sure come in handy

[gtsttrk]

+1 for the pinning. This has been invaluable as I look to strengthen my engine in the near future!

[B-rice]

hey great write up. Willing to use it to calculate my compression ratio if i use a metal head gasket. I dont understand what you mean by "block deck to piston crown measurement" Also "piston measurement above top ring land x piston measurement above top ring land"

[TeeDow]

Dude thanks for this man a lot of useful information on here to help me get started and what to look for when I get my shit together to do mine