924Board.org

[ideola]

Picked up the crank today. Also received my pistons and rods. Bought a 12" digital caliper so I could accurately measure my deck height, and will be taking block in tomorrow for cleaning and prep work.

First for the pix:




This pic shows how much smaller the big end is on the Manley rod as compared to our stock rod:


Comparison of pistons. From left to right: OEM NA; OEM turbo; Diamond Euro-spec 931 replica; Wiseco off-the-shelf piston.


OK, now for the numbers. Bear in mind, this is specifically for using a 931 head on a NA application in order to achieve a D-Prod-like build. There are many additional configurations that can work depending on whether you want to use a 931 or NA head, and whether you intend to run forced induction or NA.

• My deck height measured 231.15mm. I am going to have my machinist remove .025" (.635mm) to bring the deck height down to 230.515.
  
• The pistons are 88mm bore (also available in 87mm and 87.5mm bores). I decided to go big or go home.
  
• The piston should end up about 1.3mm down from the deck. Not 100% ideal for quench, but this particular piston had the highest compression height I could find, and consequently the lowest quench available, which is why I opted for the Wiseco pistons even though they were $150-$200 more than some of the other alternatives.
  
• Wrist pins are 22mm floating. The rods are available in the correct width for either 21mm or 22mm pins. I chose the 22mm pin because of other piston selection criteria, most notably CR, compression height, and minimal quench.
  
• The total additional stroke on my crank ended up being 2.8mm over stock, bring total stroke to 87.2mm.
  
• Rod length = 150mm
  
• I am going to run a standard .051" Cometic gasket.
  
• Calculated CR comes out to 10.7:1
  
• Calculated displacement comes out to 2122cc

I am hoping to receive the rod bearings next week, and also that the block will be done early in the week. As soon as I have bearings and block, I will do a quick assembly on everything to verify that there is no rod-to-wall interference with the additional stroke. As you can see from the pix, the Manley rod is quite a bit smaller on the big end, so I do not anticipate any issues there.

The crank work ended up costing more than expected even though he again confirmed that nitriding was not necessary. Apparently, our cranks have what are called "rolled radius" journals, which is a hardening process where the journal meets the lobes on the crankshaft. It is impossible to tell if the radiuses have been rolled (resulting in signficant hardness and strength) or simply machined (less durable but easier to work with) until the machinist starts the grinding process. Well, our cranks have rolled radiuses. According to my machinist, no one else in this city will touch a rolled-radius crank for this kind of work. He claims that while working the rolled radius, the crank will bend slightly as each journal is worked, and requires re-straightening after each journal is completed. A tedious process. So no need to nitride because the crank is pretty beefy to begin with; but a little more than we had originally anticipated for labor.

Here are the dollar figures:

Crank work = $400
Rods = $350
Pistons = $600 (off-the-shelf with side skirt coating, no teflon tops)*
Total = $1350
* there are cheaper options available but with less desirable quench distance...

By comparison, here are the numbers for the weld-and-regrind approach:
Crank work = $650
Rods = $870 (custom Crowers)
Pistons = $880 (custom slugs with skirt coatings AND teflon tops)
Total = $2400

Frankly, I have not been able to find an ideal piston configuration, particularly as it relates to the quench distance. Using off-the-shelf pistons means you have to take what you can get for compression height, and NONE of the hundreds of configurations I've evaluated have sufficient compression height to bring the piston flush with the deck. However, for a high compression NA application, the quench area with this particular setup is still significantly better than stock (which is over 6mm), although it of course results in being an interference design. Based on the numbers, I don't see any way to achieve increase stroke AND increased CR while retaining a non-interference motor.

It's also worth noting that at $600 for the off-the-shelf pistons, it may be worthwhile to just spec out custom Diamond slugs for only $100 more in order to eliminate the valve pockets, while also achieving the precise quench distance and CR desired. Just another option.

I hope to report back next week with an update on the rotating assembly.
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erstwhile owner of just about every 924 variant ever made

[ideola]

Forgot to post weights:
Manley rod = 544g
Wiseco piston = 365g
Wiseco wrist pin = 112g

I don't seem to have a record of the weight of OEM slugs or wrist pins, but the tech specs booklet indicates the rods are between 815g and 927g, so up to a whopping ~1500g of weight savings is possible on the rods alone.
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erstwhile owner of just about every 924 variant ever made

[924RACR]

Conveniently I just weighed my stock (S2 '82 931) rods, as I just balanced them out. Without substantial weight removal from the big end, I got them all balanced at about 851g. Not too bad. That's for the Series 2; the Series 1 rod I swapped in (due to the original damage that necessitated the rebuild) was 4g heavier. Which meant more grinding to balance.

The Series 1 piston is, likewise, the same 4g or so lighter than the S2 piston. So the rotating mass would work out the same... though of course in my case I just reused the stock, still perfectly good S2 piston, so needed to balance the rod.

I was impressed by how well it was all balanced from the factory; definitely much better than I recall the 924 NA stuff (from the race motor build).
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[RC]

FYI its not too difficult to shave up to 50 gms off the big end cap alone. This set, balanced individually and end for end came in at an even 200gms on the caps.






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World`s quickest 924 2L slushbox

[BionicBalls]

Any updates on this thread?
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1980 924 NA
1982 931

[ideola]

The head was completed last week, and the dual DCOEs arrived this week. I'll be assembling the motor in September, and it'll go in the silver D-Prod-bodied car I just brought back from Florida. That car will get paint before the motor goes in, so it will all depend how long my body man takes on prep and spraying.
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erstwhile owner of just about every 924 variant ever made

[AFRICA DEE]

My stroker conversion... long winded with other stuff I am doing....

I started with a 931 totally original and painted in Guards Red, 93 000km’s. Some photo’s and development/progress work are on the forum.
Three years ago I reconditioned the engine with new standard bearings; rings, a fairly proper F/M intercooler and the boost turned up to 1 bar it was used as a track day toy. A bucket seat, smaller steering, half roll cage and a 4-point harness went in, with a local aftermarket coil over suspension. The front discs were upgraded to cross drilled items, better pads went in and Goodridge braided hoses. But with 3 head gasket failures, piston destroyed due to detonation and one turbo failure (1 bar Boost, maximum 98 octane, CIS and standard radiator etc with an average day temperature in summer of 30+degrees C in South Africa do not go well together …) So I have decided to convert it to a narrow body ‘light weight” normal aspirated RHD “street legal” track day car. This is what I have done and/or acquired to date:
RHD steering box went in with all the standard steering gear.
Brake booster has been deleted in favour of a 3 Pedal Box with separate master cylinders for clutch and back/front Willwood master cylinders.
Boxster 16 inch rims with 225 and 205 tires respectively
GTS headlight conversion, ex GT Racing
Interior has been totally stripped including dash and gauges except for door panels and roof lining.
Electric windows to go, side window glass to be replaced by Lexan , electric mirrors to go.
Aluminium fabricated dash with electronic DigiDash and Innovate MTX-L Wide band AFR system to go in.
Wiring harness is considerably lightened.
All interior sound deadening has been removed lots of no longer required brackets and spare wheel well and LHD battery box floor has been cut out and holes are covered in aluminium. Weight loss +20kg.
Total heater delete, only modified interior fan will be kept with directional outlets.
Bumpers lightened with some discreet holes cut in. Bumper fog light deletes with ducts for piping to brakes. Original head light areas and wheel arches has been modified to run cooling pipes
Fuel tank has been replaced by a 40l aluminium tank with a single h/p fuel pump and a small aluminium overflow tank.
Two OMP TRS bucket seats and 6 point harnesses
2.5 kg fire extinguisher
Light weight battery to be fitted, position still to be decided on.
Lightweight Toyota forklift alternator 1.2kg
BMC filter in ‘cold box’
Gotech Pro X Engine management system (ECU) (www.gotech.co.za)
Larger capacity aluminium radiator and twin fans controlled by ECU
Modified 931 intake manifold with VW VR6 throttle body
Fuel rail with Bosch 440cc injectors and head has been machined to lower injectors.
Big Valve Conversion with H/D Springs from Ideola. The 931 head has been gas flowed with reworked 280 degree stage 2 cam. ARP head studs to go in. Await “new” Cometic gasket.
Ideola lightweight lifters and lash caps bought and distributor gear cut off from rear of cam. Rear of head has now been blocked off.
Vernier cam wheel.
Due to heater delete could modify the rear water outlet on head to 32mm with 32mm pipe to water pump
Engine: Ideola stroker conversion bought with Wiseco pistons( 11:1 CR) and Manley Rods. I await the arrival from US of A….
Crank will be knife edged.
Baffled crank and scraper system to be made up.
931 Flywheel -2kg
New exhaust manifold to be manufactured, D-Prod like...will have to see about oil filter and oil cooler pipe modifications (Ex 931)
Development and work to continue…
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1980 924 N/A Ex-931 Track car: My cash guzzler
2004 996 4S: My Weekend plaything
2012 VW POLO GT: My daily commuter

[Khal]

How's the toofah handle in that snow?!
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'80 924 Turbo

[musicalannette]

So the tab thats on the sump?, stopping it from being removed easily?, is that for this type of "blueprinting" you should be doing if you want to go racing? Or do they say something at scutineering if your going 20mph faster than everyone esle down the straights and say its because you only have one door mirror? made me chuckle. looks a good hidden mod if too expensive fo me though.
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....

[CorsePerVita]

[itbracer85porky]

Hello all. Any updates on this motor build yet? I am very interested in this. I would like to know how successful it is/was, and how much to get one built. I would like to go this way with my 924 DP replica, as I found other uses for my spare "hot" engine.
I would gladly be a guinea pig for something like this, and let you all know how it works out. I have a few questions though:
1) what is an estimate on the HP of a 10:1 engine in this configuration?
2) how streetable would that be?
3) could a well tuned CIS handle an engine like this?

I just want to play around with the car a bunch more, and think this would keep my busy. Thanks in advance!
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1977 Porsche 924 DP Replica (in process)
1984 Porsche 944 Euro conversion
1990 Eagle Talon SCCA ITA
2010 Ford F150 Supercrew
2012 Nissan 370 sport pack

[the_mad_electrician]

I would be highly interested in this if I get another 2.0 car
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81 924 N/A

2004 Ranger "Edge"

2005 Mazda 6

[ideola]

[itbracer85porky]

Hello all. I just had a thought regarding using a 931 head on an NA motor such as this build.
Would it be possible to have the stud holes filled in/welded shut, and then re-tapped to accept the bolt pattern of the N/A headers? There would still need to be some match porting for sure, but would that work? Has it been tried?
It would require some aluminium welding skill (not hard at all), and some precise drilling tapping (again, no hard), but it could solve a lot of problems with putting a 931 head on an N/A motor...
thoughts?
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1977 Porsche 924 DP Replica (in process)
1984 Porsche 944 Euro conversion
1990 Eagle Talon SCCA ITA
2010 Ford F150 Supercrew
2012 Nissan 370 sport pack

[fiat22turbo]

I wonder if the NA head could be machined with the 931's combustion chamber (or something similar)? Would that be easier for use in a NA application? Just thinking out loud since the heads don't seem that much different other than the combustion chamber, the location of the ports and the spark plugs on the opposite side (hey maybe put a set on either side and run dual plugs!)
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)