924Board.org

[Peter_in_AU]

I've been working through the possibility of stroking the 924 motor by using a set of rods from the Mitsubishi 4G63 motor. The first mention of using 4G63 rods I heard was from sandgroper from Western Australia in this thread http://www.924board.org/viewtopic.php?t=5495 and of course I totally missed the implications.

If you haven't heard of it, the Mitsubishi 4G63 is a 2 litre motor found in millions of Mitsubishis from Evos and VR4s down to Lancers and other throw-aways as well as Hyundais, Dodges, Kias and probably a lot of others.

The conrods in the 4G63 are 150mm long (c to c) and have a big-end bore of 48mm. The rods from the early motor (up to 1995 I think) known as the G1 motor have a wrist pin diameter of 21mm. The later G2 rods have a 22mm wrist pin diameter. The G1 rods are stronger than the G2 rods but unless you're building a 400hp motor the difference is not an issue. In case you don't remember, the 924 rod is 144mm long and has a big-end bore of 51.6mm. According to my rather rough measurements a 924 NA Euro piston has a compression height of 41mm and sits 4mm down the bore at TDC.

Now, if you offset grind a 924 crank down to the 4G63 rod size by taking metal off the bottom of the throw you end up with an extra 3.6mm of stroke. This would give the 924 a stroke of 88.0mm. If you could find a set of 87.0mm pistons that work then you end up with 2093cc or an increase of 109cc. The thing to do is to find a set of pistons that give a useable compression ratio and that you can hopefully buy on ebay for less than $100.

I've worked through the Mahle-ACL piston catalogue and come up with some possibilities. The Australian piston catalogue is here it's about 15meg.

A web page showing initial results is here and the Excel spreadsheet it's based on is here.

I used Dyno2000 to calculate the compression ratios so I'd really like someone else to check them to make sure I didn't stuff up somewhere. The numbers were calculated with an 87.0mm bore and an NA head (using the 6cc volume that's been quoted here).

Assuming I didn't make too much of a mess of it there's a couple of good possibilities; for an NA the Mitsubishi 4G64 2153cc or the Toyota 3SGE pistons give around 10:1, if you want to supercharge or turbo an NA then the GM/Holden Vectra 2.2/Daewoo 2.2, Ford/Mazda "FE" 2.0 Telstar/626 or Mazda B2200 pistons at 8.3:1 might be a good choice.

A 2.1 litre 924 with 10:1 compression.

Can someone check my numbers?
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[ESC944]

Peter,

I know their are a few companies that make stroker motor kits, but I understood they are based around the crank, up to 2.4 liters.

I am curious if you could use a stroker crank, some vw set of forged pistons with the 4g63 rods.

The post on the VW pistons that sat so far down in the combustion chamber, wouldnt this be the best mix???

Even if we just used the 4g63 rods.. and those VW pistons.... that would be interesting.

I dont remember the exact details on those VW pistons.. hemm and use the VW stainless larger valves in the 924 HEAD, like the 44/36... or some such....

Anyway I hope someone can comment on this... would love to try something like this, but really want a lot more displacement, but any increase would be good....

I assume the limit is still 2.4 liters.... hemm perhaps the same companies offer the stroker kit could modify it for the rods.

Well just my thoughts.... hemm of course.. their is alwasys skirts and sleaves and oversized pistons...

I dont know the history of the race motors... but can anyone comment on the maximum bore and stroke that could be achieved and still have a solid motor that can handle some serious boost?

[Peter_in_AU]

The two main problems with a stroker "kit" is the cost and the availability. All the price estimates/guesses I've read seem to be around US$2,500 and from what I can work out even if you wanted one you can't have one because they don't actually exist.

I don't know what the absolute limit of the 924 bores is. Given that the largest standard oversize is 87.5mm and the very narrow bore spacing of the block any more than 89mm probably wouldn't hold together. Assuming you could get an 89mm bore then to get to 2400cc you'd need a stroke of 96.5mm which is 10.1mm over standard. You'd need to get the crank welded up and offset-ground a long way to get 10mm even if you used rods with a smaller big-end bore you're still going to be welding, grinding and heat-treating that crank.

The reason I'm looking at 2.1 litre is that the pistons are within the bore service limit and are standard shelf items, the rods are shelf items and the crank doesn't need to be welded so shouldn't need to be heat-treated. Besides, how many guys on this board are prepared to spend $2,500 on stroking their motor. Given the number of stroked 924s we've got I'd say that number is very small.

What I'm after here is bang-for-bucks. If you're like Joe and just spun a bearing then you're going to have to get the crank reground anyway so why not add an extra couple of hundred for some new pistons and rods and get it stroked at the same time.

Just had a look on ebay, there's a set of 4G63 1G and 2G rods and pistons (4 of each) for $44 for the lot and a set of new 4G64 pistons with a buy-it-now of $124 which seems a bit high as there's two sets of new 4G64 pistons on ebay Australia for $59 a set.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[ESC944]

I checked on the price for the stroker kit it was under 2000.00 and comes in 2.2 up to 2.4 flavors.

I agree it seems like your solution is a lot easier... now if we could find slightly larger pistons that would work and be suitable for boost... add the deck braces, seal it up with some serious head bolts...

But as long as we are looking at this... hemmm.. PISTON Squirters and oversized pistons, longer rods...

Coatings from Swain:
Dome (TBC) and Skirt (PM) $ 32.00/ea.
Dome (TBC) and Skirt (PC-9) $ 38.00/ea.*
Dome (GC) and Skirt (PM) $ 48.00/ea.
Dome (GC) and Skirt (PC-9) $ 52.00/ea.

Dome Only (TBC) $ 24.50/ea.
Dome Only (GC) $ 42.50/ea.
Skirt Only (PM) $ 14.00/ea.
Skirt Only (PC-9) $ 18.00/ea.
Heavy skirt build up (PC-9 .002-.004") $ 24.00/ea.

Build a stroked high boost capable beast.

I am just wondering still about all the different VW pistons that even forged are cheap....

Forged Cima Piston & Liner Set 88mm X 69mm Stroke
Price: $231.00

Mahle Piston & Cylinder Set 87mm x 69mm Stroke
Price: $139.95

I guess I am just thinking that if you go slightly oversized on the piston and combine that with the longer rod... you get just a little more displacement and for any block with issues that require an overbore isnt this reasonable? That is boring and using the 88 instead of the 87? I have never bored out a 924 block... but I am thinking about what can be done...

[ESC944]

So thinking about this from my experience and knowledge with other types of motors, some things to consider when building a stroker:

Building a stroker with longer rods and increased stroke... you have to consider the angle of the rod, this has a lot of impact on the motor in more ways than one...

The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection (this is not the angle of maximum thrust, which occurs when the rod’ beam axis is at 90° to the crank throw or journal, typically between 70-76° ATDC; however, the math is easy to do).

Angles beyond 17° (where the rod axis is 90° to the crank throw at 73° ATDC) promote excessive wear at the piston major thrust surface, and piston breakage could be the result.

So one thing to do is calculate the new rod angle. High rod angles will require quality rods, I k ow the 4g63 motor having run a GST eclipse for some time and a lot of boost. It has strong stock internals.

But you will have to use quality (ARP, etc.) fasteners. Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.

Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
(or)
Sine of Rod Angle = .5 ÷ R/S

To make your own calculations using the Microsoft Calculator (every Win95/98/00/ME has it):
Double-click the “Calculator” icon to open it
Click “View”, then “Scientific”
Input the result from the formula above
In the left margin of Calculator, look for the check-box that says “Inv” - check it
Make sure the box marked “Degrees” (not Radians) is checked
Click on “sin”
The rod angle in degrees will show in the window.

Connecting Rod vs. Stroke Analysis
The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:

Ratio “n” = Rod Length ÷ Stroke
The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length.

The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.

With increasing the stroke and rod length should be very interesting With the limits of the Head.

Rod Ratio vs. Intake Efficiency
An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results.

Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed.

“n” = 1.45 - 1.75 more compatible with:
Large intake port volume vs. motor size
Single-plane or 360° intake manifoldsS
Large carburetor vs. engine size
Moderate engine speed (pick-up, RV, towing)
Tall axle ratio

“n” = 1.75 - 2.1 more compatible with:
Small intake port volume vs. motor size
Dual-plane 180° intake manifolds
Small carburetor vs. engine size
High engine speed (peak power more important)
Short axle ratio

Hemmm... lots of possibilites... if you are going to do this and I hope you do... it should prove to be one nice motor.

What I am wondering is what would be the maximum we can we can do with the stock crank... well in my case, my 931 crank and finding a rod that would work to give me max stroke and bore for max displacement on the cheap.

Hey didnt the VW eurovan use 150mm rods?

[ESC944]

I am just wondering if we can get 2.2, then upgrade the turbo on the 931 or add a supercharger for the NA... Some standard head work, rework the intake to use a larger plenum with velocity stacks or similar plenum to runner mod.

2.2 with out a stroker crank would be fantastic. Even with just the 88mm pistons, and the 4g63 rods... should be getting closer to that 2.2 that I want.

88x88 =2141.78 correct?
931 1981-82 US 84.4 x 86.5 =1984

So you would pick up 157 right?

[john h]

Looking at Peter's catalogue on the Pistons, if you go to 88mm bore you could use either a Toyota 2H - or the Isuzu KBD28 (motor type C223)Piston however you will need to confirm the compression height for the 931 piston. Both of these are diesel motors so the piston's should be designed for higher comrpessions.

I tried measuring the compression height for the 931 but the piston's I found sitting in the box of engine bits seemed to give a compression height of 53mm so I'n not sure if they're the right ones - I think they are as one of my motors destroyed a piston and a rod and the box only had three piston's in it.
_________________
Remember a Porsche is not just for Christmas,

if you take it to pieces slowly it can provide anguish all year long!

[Peter_in_AU]

as always, the devil is in the details.

Yes you could probably take the 924 bore to 88mm, it would appear that all you'd need are 88mm pistons of the correct compression height and pin diameter and bowl volume. Just get hold of the catalogue of a major piston manufacturer and plug all their pistons into some software until you find the right one or you can calculate the values you need and get JE to build you a set.

However, when you measure it the standard head gasket bore is 88mm so when you compress it it will stick into the combustion chamber. Bugger! OK, lets get a custom gasket laser cut. Don't have a CAD .dxf drawing of the head gasket, no worries, the shop will run one up for you for $150. How many do you want cut? One? OK, there's a $75 setup charge for the laser cutter and a minimum $200 cutting charge. Hmmm that's a very complex cut, it might be a bit more, let me just work that out for you. Of course your mileage may vary but that's just for a single gasket.

Then of course if anything stuffs up and you need to clean up the bores you can't because the linings are too thin to be bored any further so you throw the block away and go hunting for another one.

Now if the rent isn't due and the you don't need to pay for the kids' class trip to Antarctica then all this is just annoying.

At the end of the day/month/year you've picked up an extra 10% capacity which if the god of normally aspired engines existed would give you an extra 12hp. Honestly, you'd probably pick up the equivalent to 12hp by just throwing all the junk you carry around out of the car.

The point of my original proposal was to get a $200 crank grind, throw in a set of $50 rods and a set of $60 pistons and pick up an extra 109cc virtually for free 'cause I was getting the crank ground and needed new pistons anyway.

I wish they'd actually published the true story on that "Heavy Breather" 2.4 924 because I'll bet that after investing $10,000 in his 924 the guy either 1. blew his motor and sold the car for $1000 or 2. bought shares in the company that did the work and retired rich or 3. got blown into the weeds by the first Euro 931 he came across.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[ESC944]

Peter I waited all night to see your post and when I did I ended up LMAO.....

Yea I see your point about not going that big. 2.1 will do. I always like the idea of more.... if we could grind the crank more....... ah well, the cheap approach with durability is the best.

Hahahaha

[ESC944]

Anyone want to comment on increasing the stroke? Peter? just a little more to get that 2.2 without severe angle? How about it?

I guess I am still thinking about it... if we could get anohter mm or 2 ...

?????????

[Peter_in_AU]

Some people are never satisfied

The rod/stroke ratio of the standard 924 is 144 / 84.4 = 1.706. The ratio of a 2.1 stroker using 4G63 rods would be 150 / 88 = 1.705 and as the 4G63 rods are 6mm longer, with a 3.6mm increase in stroke the piston wall and rod loading would probably be about the same as the standard 924.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[Lizard]

1 thing that wasnt mentioned is that for the 2.4L stroker kit you have to grind the block and oil pan for clearance,

in regards to the idea of different rods, I think it would be easier to look at the TDI diesel crank, do a board search
_________________
3 928s,

[ESC944]

Yea never satisfied

Like I said... if you are doing it cheap... I am wondering if their isnt a way using your idea, to get a little more and still have a stable engine.

I have seen 150, 152 and 156mm rods... depending...but thats not the big issue, the big issue is rod journal size... for that extra stroke...

Ok so I am thinking that if engines like the Race version Oldsmobile Aurora Indy Racing League (IRL) V8, can go down to a 46mm rod journal and still produce insane power... well could we go that far down on the Crank, with out sacraficing reliability?


So anyone have an idea if the 924/931 crank can take a hit and go down to 46mm and still be stable in a high boost motor?

Forged rods and pistons... arp bolts, etc....

AS FOR THE TDI crank, I thought that nighmare required a completly different flywheel and all that machine work?>

I assume sticking with Peter's idea, its a simple matter of some VW forged pistons, 87mm variety... or something similar, but those sit 5mm lower... use 150mm rods or some combination, but do it with 46mm rod journal size.... that would add another 2 mm to the stroke, would it not?

I think the 2.1 is great... amd darn cheap. IF we could get another 2mm on the stroke....

I have followed the other posts about the VW pistons... the only thing I see is getting a low compression setup together for boost. say 8.0:1 anything under 9.0 should be good.

Peter any comments? Can I follow your footsteps here and do the 2.1 witht he VW pistons?

What about grabbing another 2 mm os stroke....? Sticking with 87mm bore of course.

[ESC944]

HERE IS WHAT I GO BUY WITH REGARD TO crank modification:

Journal Size and Bearing Speed

It’s a complex subject. Simply stated, there is a trade-off among features of strength, durability, weight, heat, and friction.

Small journals reduce friction horsepower losses present, an opportunity to run smaller journal and lighter weight rods. They accelerate more quickly but also offer less strength, higher unit loadings, and increased oil temperature (at the bearing surface interfaces). Typically, the highest unit loading occurs at or just past TDC (depending upon combustion conditions), and the greatest rate of change in surface friction appears at or near maximum piston velocity (in the range of 60-75 degrees past TDC during the power stroke). Generally, engines required to operate over a wide range of rpm seem to benefit more from smaller connecting rod journals than those in a narrow span of engine speed. In making decisions about journal size, the larger the diameter, the greater the bearing speed (relative motion between journal surface and bearing surface during any interval of rpm). As journal diameter is decreased, unit loading (psi) increases with bearing surface area, oil temperature can rise (cooling ability decreases) while the rotational weight reduction aids crankshaft acceleration. Consider these and related effects before making a change.

Connecting Rods

Again, buy the best parts that fit into your budget. Higher engine speeds require the best parts. Even budget builds should use premium rod bolts. Rod lengths and their affects on the engine cycle and performance will be discussed in a future article.

Wristpins

Wristpin problems are common in high-rpm, high-horsepower engines. The loads placed on the pin tend to cause two primary types of distortion: pure bending and hoop bending. Pure bending, as the name implies, tries to bend the pin along its length. Hoop bending tries to distort the pin radially in the pin bores of the piston.

[ESC944]

SO last question I didnt see anything posted about ROD width, getting a connecting rod who's big end width matches the 924 Crankshaft rod journal, along with bearings.... Did you already have this covered.

I an not sure but I understood the following:
Engine Series
Pin dia
B E bore
Length
B E width
Weight
Part Number

924
.945”
2.032”
5.670”
1.098”
670g

4G63 early
.827”
1.890”
5.906”
1.116”
610g


4G63 late,
.866”
1.890”
5.906”
1.032”
590g

Just want to understand how you are covering the width issue? The early 4g63 is wider, the later is narrower.

I am very serious about doing a stroker, not that peter has my attention...

It will be the first thing I do after completing my other project (turbo and supercharger ktis)

So I want to start gathering parts and pulling the crank to be modified....