924Board.org

[Kenodog]

Hey guys I am just putting together a list of stuff for a stroked motor. Going to be using the stock (but rebuilt) CIS system for now and was wondering if it's worth,or needed, a big valve kit for the head ? This is an area I really don't know much about and there seems to be varying opinions on the matter where it concerns a turbo charged car.

Any FACT based opinions are appreciated.

Thanks,
Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[fiat22turbo]

I believe the consensus was that larger valves weren't as helpful as having the ports reworked to improve flow as the larger valves were shrouded by the combustion chamber negating much of their gains.

The fix would be deshroud the valves by leveraging the larger bore provided by the over-bored engine block, but I don't know if you could fully deshroud the valves.

Looking over the data and the discussion in the head flow test thread and some of the newer discussions it strikes me that the gains in the Eurorace head are likely due to improved port work and not to the larger valves.

I'm looking forward to reading this thread as it develops, hopefully some good data and discussion results.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ideola]

[ideola]

The above-referenced thread, in case you haven't seen it.
http://www.924board.org/viewtopic.php?t=39442

Notably:

[ideola]

And in case you get bored with the above, here is a much longer discussion:
http://www.924board.org/viewtopic.php?t=20953
_________________
erstwhile owner of just about every 924 variant ever made

[MikeJinCO]

On the thread I started, I am looking at the 931 head from a NA aspect where shrouding can have a much greater effect than a turbocharged system. For me the shrouding effect throughout the valve cycle due to the cylinder wall clearance negates much of the advantage of the larger valve. I would think that the shrouding would be a moot point with 7 or 8 PSI of boost versus the vacuum generated during the NA cycle. Knowing nothing about the turbocharged systems I would think that being able to run a slightly higher boost would provide much more than the valve sizes and shrouding. Look at the modern diesels from Ford/Cummins/GMC, the valves are pretty small but that can run upwards of 25psi of boost. They run400HP,750 ft-lbs of torque with potential to double that. The gains from reduced valve train weight issue is something else completely.

Reading David Vizard's porting book most work is referencing valves at an angle to the cylinder where once the valve clears the head it angles into free space--ie an inclined valve SBC V8. On our motors the valves run parallel to the cylinder making the shrouding issue on NA motors much more significant in my opinion(he never mentions it).

Dan's approach of covering all the bases is probably the best for engine performance, unfortunately many of us have to pick and choose what we can do. If I could get lighter weight, smaller stem valves of the same size I'd be very interested as that would allow smaller/softer valve springs which combined with solid lifters would save horsepower required for driving the cam, but I don't know if those parts are available. Many US high performance V8's have gone from double springs to progressive wound singles with significant gains at a not extravagant cost.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[Kenodog]

In building my last motor,which I dyno'd at 210hp at the flywheel @12psi, I realized that it was just about out of grunt without upping the boost to a level that wouldn't see the engine last long. This time I want to stroke it and get the 88-90mm pistons so it has much more torque. Combined with 12-14psi the motor will last and also put out the torque that will leave me happy and not regretting just going half arsed in the build.

I have had a stroked N/A and the torque it had was unbeleivable compared to a stock car. Sideways off the line was no problem at all and it just felt so much more ....alive.

Right now I am just slowly buying what I need to do the build, trying to cover all of the bases to ensure that the motor will be as good as it can be but also last at least a few years without grenading. Next stop is sending the crank down to Castillo's in Cali...


Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[gegge]

Check chapter 4: Heads - Standard and modified in David Vizards book "how to MODIFY your MINI" and figure 14-15,8.

David suggests that chamber work is extremely important in shrouded-valve heads and illustrate this by fitting a 1.156" inlet instead of a 1.093" in a standard 850 head. Flow decrease by 10% Fitting a larger 1.467" instead of STD 1.312"into a modified 1300 head resulted in less flow at high lifts. The MINIs are bore shrouded in the same way as 924.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[Kenodog]

Thanks Gegge that is the kind of info I was looking for. Sadly there are not many people around here that would know enough to do a good job unshrouding the valves. Anybody have a good guy that could also flow test and show some results ?


Leigh
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140