924Board.org

[indi9xx]

As a Porsche specialist based on the south coast of the UK, we have performed performance modifications to most Porsche models for the last 15 years.

For all this time I have had a burning desire to find out why the 924 is such an underperformer and does not respond to the normal kinds of modification.

After almost 10 years of study, and recent contact with one of the design engineers who developed the 924 project, we have discovered what the factory designed into the engine to limit performance, and what is needed to develop more power from this engine.

We have a couple of test cars in at the moment, one will be retaining the CIS injection system (although modified) and the other is an all out "money no object" car with omex engine management, DIS, custom manifolds, cam and most importantly both cars will be running our new heads.

Now the reason for my post, would there be much interest in these heads, along with our modified injection components once dyno and flow bench figures of the final items are ready?

Are there other performance products for the 924 that you guys would like to see? (such as lightened flywheels, stroker kits etc)

All the best

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[Lizard]

what type of cost would we be looking at for a new head?
_________________
3 928s,

[-nick]

and how do the flow numbers compare to just a port&polish on a stock head?

-nick

[indi9xx]

Now thats the big question!

In an ideal world, being a business, we would like to get as much money as we could.

But also being a business run by enthusiasts (albeit Factory motorsport trained enthusiasts!) we know what its like, for example, not many people will go out and spend $1000 making a $2000 car as fast as a $3000 car. We have to be realistic.

So... its going to be a balancing act, between us having to ensure that we do not go out of business building heads at a loss, and pricing the heads so that 924 owners can afford them. So the cost of the heads, as an initial estimate, in pounds sterling, would be somewhere around £500+ shipping and core exchange.

This includes the head being 100% recon, without cam, completely reprofiled intake ports (long and short side), ported exhaust ports. 3 angle valve seats, de-shrouded valve faces and options on bigger intake valves.

Everyone bangs on about the 924 turbo head, well initial figures show that our head out flows the 931 head, with much increased port velocity, all with the head still in development.

We also hope to be able to supply short fully blueprinted engines, with compression ratios of either 9.5:1 or higher, tubular exhaust manifolds, adjustable fuel pressure regulators with vacuum boost, larger throttle bodies with matched ported intake manifolds and more.

Anyway, at the moment, we are dipping our toe in the water to see what the response is... even to find out if this is what people want.

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[indi9xx]

Nick,

A port and polish head will not compare with our head at all... anyone can make a head flow increase on a 924 head, but that increase rarely produces an improvement on torque and BHP.

One of the features of the 924 head is that the intake port shape was bodged on purpose by the design engineers to ensure that the 924, a better handling car, did not go whipping 911 owners on the track... cast your mind back to 1975/6, and the 911 available at the time, for much more money than the 924. Porsche wanted to make sure that the cheap model didnt beat the expensive one on the track.. They also didnt want aftermarket companies making bolt on parts to make the 924 faster than a 911... For example, did you know that the 924 was originally to have the head seen much later on the 931? We didnt believe it until we were given a photo of one of the first prototype engines dated 1972, many years before the 931 was even thought of!
We found out this by talking to a design engineer who worked on this engine during the development of the car since 1972. Changing this port shape involves some serious work. Not only this, the port shape is seriously important, the 924 shape is almost the oposite of whats needed, the port shape we use is the result of borrowed time and favours owed by a couple of engineers working in F1 and indicar engine design, along with destroying tens of 924 heads during testing on the flow bench.

Anyway, in short, the new head out performs either the 931 head, the 924 head, modified or not.

Now we just need to find out if this has just been an interesting diversion and triumph over the factory efforts to restrict this motor, or if we can turn this into a product!

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[Lizard]

£500 is a decent price for a head with the figures you are talking about, however what would really make it sell woud be a dyno chart on a completely stock 924 with just that head to see what type of gains would be yielded,

do you have any ideas as to what type of gains would be seen with just the head, and maybe a header, also what is the possibility of having the ports mating made similar to the 944 so that the 944 headers would be able to be bolted on as there are alot more sources for them and they are a better design imo, not sure if the spacing would be possible,

also would a custom intake manifold be needed to be able to get it to work, or a stock one with slight modification to the ports,

will the spark plug remain in the current position or would it be moved to the top of the head like the 931,
_________________
3 928s,

[Chrenan]

Jon

It is clear you guys have put a fair amount of thought into this, and it is great to see a business so enthusiastic about the 924.

I have to echo Lizard's comments. It would be awesome to see a dyno of a stock engine vs a stock engine with your tweaked head as the only performance improvement. If it is really as simple as buying the head from you and bolting it on, you could get quite a few orders, including one from me! Would it be that simple to get performance increases? Or would it be more involved than just swapping to your revised head?

What about the North American cars with different pistons and compression, would this be a factor for or against buying your head?

CB
_________________
1987 951 - M193 Version for Japan

[Lizard]

also would it turn the engine into an interferance engine?
_________________
3 928s,

[indi9xx]

Lizard,

Dyno print outs will be available as soon as the project cars are complete.

The first of the two cars to recieve this head is 100% standard, 80,000 mile 1984 924. This car is just receiving this head, a larger throttle body with the intake matched to the throttle body. Which on their own, produce very little if any gain. We are doing this car as a good example of the benefits of this head in a real world situation.

The second car, is just an experiment to see just how much power the 924 engine can produce, if money were almost no object. Not a real world example, but just a project to see just what could be achieved.

No custom intake is needed for this head, the standard one fits just fine, the port entry is almost untouched.

Getting the 944 headers to fit, is probably beyond the scope of what can be achieved without becoming a massive project in itself.

We are however planning on making a tuned tubular manifold and exhaust system, which will be compatible with the standard parts (the join from the down pipe to the system will be the same type in the same place as standard)

The spark plugs will be staying in the same place. We did investigate moving the plugs, partly because we wanted to see about a twin spark system, but found that during the simulation of the engine we found the only benefit to be beyond 7000 RPM... so that idea got shelved.

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[indi9xx]

CB,

This head will work on low compression US cars as well as the Euro spec engines.

We have consentrated on Port velocity more than chasing peak CFM figures.

A high PV makes torque, which at low rpm is what you need, which in turn, if maintained at higher rpm makes BHP as a side effect of the relationship between torque and BHP.

As it turned out, concentrating on PV, and creating a well designed venturi port, CFM is improved. The heads are still being worked on to perfect them as we experiment with valve cuts, but already the PV and CFM figures beat the hell out of any flow figures we have seen for the standard, 931 and modified heads we have seen.

We have a 924D head here, and we were amazed how poor it really was, it seems that even with that head, the politics at Porsche ensured the head didnt release all of the available power, as to have the "wrong" model getting all the lime light in racing.

North american cars with the lower compression will still benefit from these heads. except we would really advise you get a set of Euro pistons and get the full benefit!

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[CMXXXI]

Any similar projects in your shop for those of us with a 931?
_________________
'79 Eurospec 931

[indi9xx]

No Lizard, this head will not make the 924 become an interferance engine.

Bring up the compression by skimming the block, putting in a high life cam etc, could make the engine an interferance engine, just the same with this head, as it would be with the standard head.

Jon Mitchell
Independent Porsche Specialists
Bournemouth, UK
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[indi9xx]

CMXXXI

We have found with the 931 head, that it also suffers (to much less extent) from low port velocity. Which really does no favours for off boost performance.

Projects we would like to attack for the 931 would be...

1. CGT alternative intercooler, which is a little inefficient and hard to find, but is a better location than a 951 item.

2. Alternative to fitting a 944T (951) cooler in the badge pannel with a more efficient design. the standard 951 cooler is also a little behind the times.

3. Modified breather system. Even the S2 system is ineficient.

4. Boost controll system to manage boost versus detonation detection. (we have a russian electronics guy we call the "nutty proffessor" working on this)

5. Launch control / anti lag (we have already fitted this to a customers car as a one off, but want to produce some production version if there is interest)

6. Custom exhaust manifold to mount the turbo on the other side of the engine, replacing the KKK with a garret model and the injection with an omex system. (this is a bit far fetched, but we would love to see how far the 931 could really go, if the piston crowns were tougher!)
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK

[Lizard]

so essentially you are not redesigning the head but rather giving it a much more advanced port and polish on a proper flow bench with tests being done on the different valve grinds.

there was talk along time ago here on the board talking about relocating the intake port, basically swapping the position of the intake port and the injector so the injectors would be under the intake manifold and the intake would be dropping air straight down into the cyl, any chances of seeing if this is a possibilty, maybe mounting 4 individual throttle bodies on top of head sitting them perpendicular to the ground,

you are also stating that you are doing work to the exhaust ports as well is this correct?

also what are the chances of a kit to convert the 924 to EFI and a price to be able to do that?

what are the current PV and CFM stats that you are looking at achieving, compaired to the stock #'s unfortunatly I dont have the factory #'s to compair to myself.

also the early US 924 came with smaller valves/ports would those be considered acceptable trade in/core as well?
_________________
3 928s,

[indi9xx]

Lizard,

With the 924 head, the intake port, in particular, is probably the poorest port since the side valve engines used in the early days of production cars.

Lots of people and companies port an polish these heads, with various levels of success.

We took the angle that for this head to be viable, the valve position (so standard and non standard cams will work) and inlet manifold (header) position (so people didnt have to buy a custom manifold) would have to remain in the standard location.

These specifications, were used to completely re-design the intake and to a lesser extent the exhaust port. Once the design was settled, we made the first prototype heads and flow benched them, made some perfections and find ourselves where we are now.

Modification of the heads is quite far removed from the levels of port and polish, partly because if anything, slightly more material is added than is actually taken away, as with traditional port+polish methods.

Rough CFM figures at the moment are about...

52 CFM at .100 inch valve lift for the standard head intake and 69 CFM at the same lift for our head

146 CFM at .400 inch valve lift for the standard head intake and 212 CFM at the same lift on our head.

Port velocities are hard to measure as a "standard unit" such as meters per second on our equipment (or nearly any superflow), but we can measure this on a relative basis, which showed that our head produced an increase in PV of between 25% at .100 inch and 15% at .400 inch lifts over the standard head.

To put our PV figures in perspective, with nearly every ported head you can find to test, you will find that PV actually drops on the modified head, which increases BHP at the expense of torque. PV, is especially important for torque production. This is the reason why most cylinder head modifiers concentrate on CFM figures, as getting good PV is much more difficult (anyone can grind out material, but adding it at the right place is much more tricky!) but also PV figures are much more difficult to put into units of measurement or even measure in the same way from one machine to another, the best you can often do, is test various ports (standard versus modified) on the same machine and make a comparison.

Also most port volumes increase following a port and polish, where as our head actually decreased the port volume by about 2cc from 103cc on the intake to 101cc.

I hope all this makes sense, its not often I get a chance to talk about my work, so can be prone to babbling and not making my self clear.

In short, this is pretty far from a P+P, where PV would be down, CFM up and port volume up (not all good!), on our head the PV is up, the CFM is up and the volume is down. (all good!)

I also advise against a full polish, which I find gives good cfm improvements in many cases, but on the car tend to cause fuel drop out and poor atomisation of the fuel, especially at certain rpm's... we tend to favour a matt finish, which promotes a little port wall turbulance, which in turn stops drop out and increases fuel atomisation. Also at some PV (such as certain stages of valve lift), the effect actually increases CFM due to the turbulant area on the port wall, forming a kind of boundary layer, which reduces drag of the air over the port walls, I think it may be along the same lines as the aerodynamic theory behind golf ball dimples assisting stability... In fact I often thought about either dimpling or adding rifling "turn" (like a gun barrel) to a port to see the effects.

anyway, babble over for now )
_________________
Jon Mitchell
Independent Porsche Specialist
Bournemouth, UK