Haynes and the factory turbo Workshop Manual state the turbo has two compression rings. Maybe some did. Mine, with Mahle pistons, had the usual 2 compression 1 oil. Haynes also lists turbo compression at 7.5:1 across the board. The factory Workshop Manual specifies 8.0:1 from the 80 model.
924 and 931 pistons are cast, (except the GT piston which is forged.) The 8.5:1 piston used used on the 177HP engine has the same size, so it is possible to fit a GT piston. The cast piston, PN 931 103 048 01, is manufactured by Mahle and the weight is 530 gram. The forged one, PN 931 103 042 01, is made by Kolbenschmidt and the weight is 471 gram.
EBS sells all sorts of 924 pistons including, OEM, GT type
Kolbenschmidt 8.5:1 forged, 471 gm, $595.00 w/ rings, wrist pins and circlips, Mahle cast, std 7.5:1, $395.00 w/ rings, wrist pins and circlips, and finally, J.E. in custom CR's $625.00 W/ rings, wrist pins and circlips. If you buy from the manufacturer you pay more money. In this case it is more economical to go through a large retailer.
All engines use the same forged crankshaft. (924 & 931)
The 924S got an additional 10 hp for 1988.
a good place to put a picture of a naked block. Note the non-stock headers and the short
More more pictures of naked blocks, and actually a sequential series of a rebuild of the 924NA motor, check out PUB Racing's Engine Rebuild Page.
General Description and pictures of Australian and Euro Spec cars at Darrins
A tuning trick for the 924 is to fit a 931cylinder head gasket. It is thinner and will give around half a point higher compression.
The part number is 931 104 337 03 - Ole Kammersgaard
The most frequently asked question is, "How do I get more power ...". Uusually implied is the unstated thought, "What is the single bolt-on piece which is effective or cost effective?" With this engine the simple and short answer is Nothing! There. That was easy and saved a bunch of money. If you look in the references section there are brief reviews of several articles in several different sources all demonstrating that the engine does not respond particularly well to bolt-ons. Additionally, Eric Lynch has compiled a very interesting library of 924 Power Tricks. This includes scans of many older magazine articles from various attempts to hot-rod the 924. Jon McCullough (AKA AppleBit) has also compiled his Tricks'n'Mods on his website: http://126.96.36.199/porsche/p924.html. The following is from a message to the Porsche List by Jim Pasha. Jim knows these cars better than anyone and explains it concisely.
The mods that make the most difference in a 924 NA engine are displacement and ignition curve for US cars. Most other mods just add to reliability, that is, port clean up, multi-angle valve grind, Headers ,intake mods, etc.
The 933, the DP road race 924, used the 931 head as it has a slight combustion chamber in the head , but requires some very strange looking pistons to get any sort of compression. The stock intake and exhoust from the NA engine bolt right on, though a 4-into-2-into1 collector header works best. Join #1 and #4 together with #2 and #3 then those two into a single collector. I can give the lengths that work to anyone interested. Done correectly there is an 8HP gain.
If you are rebuilding, check the camshaft lobes for lift and visible damage as a slightly worn lobe will cause a bad idle. One thing worth considering, Ottinger made a 2.3 Liter kit for the 924 many years ago. This would bring a Euro engine to 145Hp. Compression is 9.5:1 so HC should not be adversely affected and it will run on most US fuel as well. (ed note: Jim Pasha mentions a 2.3L kit which may or may not still be available; PAECO in Birmingham AL makes a 2.2 stroker kit for the 924. Furthermore, there are reported to have been one or two 924's with an Audi 5-cylinder installed, at 2.2 liters...)
Balancing makes a big difference in smoothness to the NA engine. Have the cam bearings checked for wear and refinished if necessary. Worn cam bearings, which are actually the head and cap can cause oil pressure readings to fall off. Always install a new cam oiling tube and connector. The connectors are plastic and will leak or break, keepping oil from reaching the the cam lobes and bleeding off oil pressure. I generally use a new oil pump, though they are expensive. The pump gears will wear on the plates, causing pressure bleed off. A new pressure bypass valve should also be considered.
For the injection, If you have a rough idle, check the o-ring seals for being hard and leaking air past. Squirt a littl water around each injector and see if the engine runs rough. If it does, then it's sucking air. The injector sits in a phenolic housing that is both an insulator and mount. These are threaded and can leak around the threads. Any air leak will cause a rough idle.
The injection is pretty fool-proof. Oil residue on hte air sensor plate causes idle flutter and roughness. Simply remove the boot and wipe both the top and bottom of the plate to get the crud off. You will notice a difference on an engine that was running rough unless it has other damage.
The biggest cause of rough running or idle is a worn camshaft. Any scratch on the cam follower will eat against the cam and lower the lift of the particular lobe. This is an endemic problem with the engine type, both in the Porsche application and the Audi/VW application.
A broken right side engine mount spring will also cause some vibration at idle. Battery acid is flushed from the battery compartment right over the mount and will corrode the spring. Replacement is the cure. I have used left side mounts in that location, but be forwarned that heat from the exhaust will affect the lift of that mount.
Read the Porsche technical spec for any model that uses head studs, the 944
being one of the best. Use a light oil, as Dave has pointed out, when you
torque the nuts onto a head stud.
Bolts may be reused in any stock application providing the threads are in
good condition and the engagement point for the socket, inner or outer style
is not damaged. The 924 and 931 studs are not really stressed that hard in a
normal application. If you want to build a hand grenade motor with 20psi of
boost, then worry about studs.
The primary advantage of studs is the ability to use a finer ptich thread to
make for a more evenly distributed torque. You actually get s more accurate
torque with the finer threads that you would wioth the coarser threads
usually employed with bolts.
Finer threads also reduce the amount of twist, the use of oil stress relieves
the torque and twist on the stud.
Studs do work better when bottomed, however, the more thread engagement you
have, the beter the pull is. Just because a stud is bottomed, it may not be
any better than one that is not. For heads, engagement of twice the diameter
of the stud is usually adequate. In all applications, the block threads must
be chased with a bottom tap to ensure there are no galled threads and that
all the crud is out of the bore. This especially true for the 944 engines.
If the total compression (dynamic) will be over 10.5:1, use studs. Some
static calculations assume a complete closure of the valve at all times,
however, in the real world, ramp angles on the cam lobes reduce the total
compression by limiting the amount of time the valve is closed and therefore
lowering the dynamic compression. Many people that have 11.5:pistons don't
realize that the dynamic compression is much lower. A turbo reaces peaks in
compression only under boost. And not usually for long periods of time, even
on the sutobahn. A static compression (high) is around all the time and
causes much more stress. At the low rate of boost in the stock 931, studs
only are stressed thermally in most cases, and with the knowledge gained in
the last 20 years, most of those problems can be minimized or avoided.
- Jim Pasha
by Steen Andersen, 76 924, firstname.lastname@example.org
I keep reading that cutting up your air intake box, so only a small rim is left for holding the air filter, will give your more horse power a cheap way. SO! I did it, and it makes a wonderful sound, but it decided to put it to a test. And the main result is that it doesn't give any gains in HP. Therefore my advice is, Don't Cut Up Your AIR BOX for more HP, You will only gain more noise, and a more dirty air filter. The complete test is on my website under http://home13.inet.tele.dk/steand/924/Dyno.htm
I have made a test off the effect off cutting up the air box, among some dyno runs i made. The result is on my homepage on the address. http://home13.inet.tele.dk/steand/924 The sub page DYNO contains the tests.
My conclusion: It only have an effect on the sound.
The right engine mount is a coil spring inside rubber. The engine hangs on both. The springs are hidden inside rubber when installed. The way the mount is constucted it may not be obvious that the spring is broken. The rubber will continue to support the engine, but without correct dampening. The picture is of a right side mount from a 1982 924. The car had been through several tech inspections and the broken mount was not found until removed. Difficult to see, but the spring is broken inside the rubber. This is caused/accelerated by corrosion from the drain out of the battery tray. When this mount was replaced the engine was suspended only slightly higher than before, but was smoother. In other words, you could not tell it was broken until it was removed.
If you have an old 924 and the right mount looks origninal: it probably should be replaced.
You racers might be interested in this received from the UK:
My company specialise in the design and manufacture of competition engine and suspension mounting systems.
We were recently contacted by a team running a 924 in the UK Porsche 924 championship to design an up-rated engine mounting system. We produced a kit to up-grade the existing mount (an insert to fit into the mount metal pressing). This has run successfully in many events.
If you have any owners who race the 924 they may be interested in the kit.
If you want more information about us please look at our web site www.vibratechnics.freeserve.co.uk.
Haynes makes no mention of the vibration damper. Connected between a bracket behind the alternator and the crossmember. It is probably bad. It is rediculously expensive, (list price, finally found one for circa $65). I wonder if it is critical; the 931 does not have one?
924 exhaust side with spark plugs
See how flat the head is on the combustion side.
Blow this up to get a good look at the flat chamber.
German head gasket, the OEM ones (Victor Reinz is the OEM manufacturer), are the only ones to use. The American ones are a waste of money and incorrectly made for the 924 engine. They are intended for the AMC cars that used a variation of the engine in Gremlins and Hornets in the late 1970s.
(1) screw studs into the intake manifold or head (if removed);
(2) align gasket correctly over port openings;
(3) slip gasket over studs and tighten down with nuts;
(4) use a fine felt-tipped marker to trace an outline of the inside of the
gasket onto port;
(5) remove nuts and gasket and grind away up to and including the marker
(6) repeat steps 2-5 to verify port fully opened.
According to what I read you only need to go about 1-1.5" into the ports.
Make sure you blend the end of the port area you grinded smoothly into the
untouched port area.
Places that need work on the head are the casting marks that run along the
inside of the intake ports of the head (use you finger to feel around on the
inside of the port), the area next to the valve guides (sharp corners), and
the ridge where the valve seat and head come together.
All it takes is a little common sense and a lot of patience
-Peter A. Holiat
Lots of opinions. One of the big mods is the 'big valve' head. But reports are that mod tends to crack between the valve seats because there is too little material left. There is some question also as to whether the larger valves alone make any difference or are just along for the ride and other changes made do the real work. When replacing the valves you can find standard replacements (in stainless no less) cheaper and at least as good as OEM. So far I know: turbo exhaust - TRW 3085/1
Schrick in Germany had a few good grinds 15 years ago; but I do not know who is dealing for them here in the US. JPasha
Webcam is another cam grinder, favored by a lot of PCA Club Racers in the US
David Ewing, "I was chatting with Mr. Dimitri Elgin at Elgin Cams about the 931 camshaft profile. He related the camshaft used in the 924 and 931 are
identical. Porsche engineering relied on the boost to
increase the charge volume, no provisions to optimize the breathing of the head with the turbo.
The part number is located on the casting near the distributor gear."
Intake Duration@.050" Lift Exhaust Duration@.050" Lift
222 .470" 222 .465"
The throttle body (TB) upgrade is a well-known modification, and very attractive due to it's simplicity. Based on experience with various TB options available for the 924, the serious performance results are minimal; however, the upgrade can be performed very inexpensively, and performance often feels subjectively better to the driver. The swap may be even more useful on turbo cars, since they are more sensitive to intake restrictions than NA cars, but it is generally believed that the TB is not the main restriction in the turbo's intake system, perhaps unlike more recent import hot-rods.
Neuspeed and Weber both make (or have made) big-bore TB's that fit the 924, though availibility of these may be a little tricky. Market price seems to be around $200 USD new. Somewhat less expensively, the TB from a 5-cyl Audi or, though less desirable than the Audi, a 16V VW, can be used with similar results and ease of installation, with substantially reduced financial impact, since these are stock units readily available from a junkyard or wrecker's.
The stock 924 TB has bore sizes (diameters) of 36mm/46mm primary/secondary - as the throttle is opened more and more (loud pedal pressed down), first the primary, then the secondary opens up, providing a non-linear throttle response and better driveability at low throttle openings. The Audi and VW units retain this primary/secondary arrangement; more recent aftermarket TB's also have this feature, though some early models may feature only one port.
There are a couple of different Audi TB's available; the best way to identify them is to bring along a set of calipers to take measurements at the yard. Some TB's have primary bores the same size as the stock 924 TB, with only a larger secondary bore; others (such as some 5000 and 5000S) have a 39mm/53mm primary secondary. Regardless of size, they can typically be found in US junkyards for around $25USD.
To obtain the most from the modification, port the intake manifold to match the enlarged throttle body - the old gasket can be used as a guide for this. The bolt pattern on the TB is the same, but the throttle cam from the 924 TB will need to be swapped onto the donor TB - the nut on the top of the throttle shaft is removed, and the swap is easily made. Later 924's will also need to ensure that the throttle switches are properly aligned; the dual switch can be positioned with longer bolts, with nuts; the third reed switch on '81 onward cars may need to have a mounting point drilled and tapped. YMMV, depending on what car you use as a donor. Don't forget to make sure idle speed and mixture are adjusted after installation.
What follows are a couple of driver's impressions of the changes:
Driveability is slightly improved over stock, retaining the nice primary/secondary function, but spooling up a little faster. It doesn't feel that, in spite of part-throttle gains, that the bottom line power has been improved, but it'll take acceleration runs or dyno time to show that. - Vaughan Scott
Power gains are noticeable in the higher rpm’s. The lower rpm’s are unchanged. This modification works well on both stock and modified cars, turbo or non-turbo. - Daniel Varholy
Thanks to Dan Varholy for input on this section!
The Pressure Bypass Valve (which is not discussed in the Haynes manual) is easily accessed via a large (17mm) plug in the front of the oil pump, at the front of the engine. Unscrewing this plug will allow the valve piston and valve spring to be removed and inspected - but it is usually worth just replacing these two components outright, in addition to the copper crush washer on the plug, as they can usually be had from the dealer for less than USD$10. Don't get caught replacing the plug itself, however; it is many times more expensive (and does not typically need to be replaced anyway).
The standard filter is Fram PH2870A or Pennzoil PZ12 or equivalent. Cars with headers installed may have interference with the filter. Use a shorter filter: Fram PH3614 or Pennzoil PZ21 or equivalent. These are about 1 1/2 inches shorter. The only spec I can determine which is different is the bypass pressure. The shorter filters bypass at around 10 psi rather than 30psi. A review of the applications for both size filters would indicate that this is not an operational concern.
From: Dan email@example.com
...warming the car at idle takes longer, burns more fuel, lets more stuff settle in the oil pan, and makes more carbon build up on valves etc. than if you just drive the car easy for a few miles. I used to let my car warm up in the driveway on cold winter mornings until I went to mechanic school (I'm a truck mechanic). They make a BIG DEAL about this subject.
From: Charles Ditmore RbrSidDown@aol.com
The only purpose of the vacuum amplifier is to operate the EGR valve. The vacuum signal from the throttle body is not strong enough to open the EGR valve so a series of spring loaded diaphragms ports the high vacuum stored in the vacuum tank under the fender to the EGR valve.
From: Allen Z firstname.lastname@example.org
Date: 11 Jul 2000
the needle bearing in my crank went out and caused my clutch to fail so I went to the bearing supply shop and bought two high speed sealed ball bearings for twenty bucks. I have never had a problem with it scince.