924Board.org

[Euro924S2]

[Raceboy]

Turbo is not making power out of nowhere, it also consumes power, though not as much as supercharger.

Regarding belt thing, I have had two belt failures with brand new belts and I confirmed that all the ther hardware (pulleys, tensioners etc) were straight, nothing turned too hard etc.

If I would settle up to 300hp, I wouldn't bother with anything other than stock NA engine.
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'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[Euro924S2]

2 new belts failed????? I would be buying my stuff from another supplier in future!!! That's aweful!! I had a belt tensioner seize on my 924 and drove it for a steady 10 miles home with it screaming and pooring out smoke as it burned over the tensioner. When I removed it, it was 40% thinner but still never broke!!!


I take your point that you can make 300hp from the NA, but is it the best way???

Would euro NA pistons and a skimmed turbo head (to give 9:1 CR) be non-interference still?

Other than the stronger ring lands, is the NA piston better in any other way?

Still can't help think that the better flowing head trumps the other NA benefits.

I'm only asking sooo many questions since my 'ultimate engine' is only half assembled (£1k rebuild to date) so its not too late to use a hybrid arrangment if it proved advantageous.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[Euro924S2]

[Raceboy]

"Best" is very wrong word describing extracting power from our engines, be it 924 or 931. Because both of them are ancient and take whatever bit more modern engine, it outperforms it by quite a margin.
But I would consider "best" approach when you can get a certain power level with good torque curve that lasts for more than one track day.

My point is that if you don't want more than 300hp, why go for much-much more expensive option? Especially if the engine is meant for street, not track-only car.

I agree that turbo head is better, especially the plug placement and also better flow but it is expensive and hard to come by.

BTW, my 931 Gulf engine has 931 head and VW flat tops and it is also non-interference. The belts that failed were Contitechs, so not a no-name brand.

[WEASEL149]

[Raceboy]

[leadfoot]

Well known or not as in my case, in all configs na and turbo head - clutched/no clutch i never had a problem with off idle droop or lag, my boost was always on tap, up to 6500rpms where i set my limiter.
I dont know how you tuned your load cells or idle control solenoid but my advice would be to look closer into this if your having off idle issues?
I was using a pwm solenoid from a 4 ltr engine and on fast settings so it may have been able to react alot quicker, its pretty obvious if your afterburning fuel though as it tends to surge coming off throttle, as your aware its always finding the balance in the tune depending on your resolution and functions available.
With my cam profile and piston choice my last engine using the turbo head was non interference
Custom sc projects vary beast to beast, there are so many variables that can be adjusted,
Maybe rc can chime in on how his blower is going as it's off a 3ltr engine and mine was only off a 2ltr, i suspect the latter m series blowers are maybe a tad more efficient
Youve done well, not taking anything away from what you've accomplished, just did things a little differently as there wernt many peeps doing sc cars when i tackled my projects
Cheers Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Lizard]

Peep,

Contitech belts are shyt. They stretch.

Only use gates for the timing belt.
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3 928s,

[leadfoot]

Using stockna pistons and turbo head you would probably need to deck it almost completely of any chamber to get 9.0, think about it stock euro is 9.3 and has a flat head!!!
A flat top piston sit around 1mm lower in the bore and has the chamber in the head as well creating around 8.7 - 8.9
Raceboy can tell you how much punishment they can take...
But for a sc setup i would also look at cam profile more with less overlap or why not design a twin cam or multivalve head?
300ponies will be around 20psi+ or just spend the extra dosh just lightening the car
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Euro924S2]

Thanks Stu.

Genuinely interested (and pleased for your sake) that you have no idle droop. I've never suffered with it either on my 924 but I installed twin throttles from the outset, because of the torid time I had with my brothers car previously (miata) - that said with the standalone ECU (again adaptronic like mine) it is a million times better - almost OEM like. I have Miata throttles on my 924 with the PWM controlled idle valve and it works perfect. I also think it's difficult to comment without driving 2 different setups and taking a preference - people have very difference standards and expectations. My 924 now starts and runs in all conditions more like my new BMW than like with the old CIS. As you rightly say - we can only be guided from our own experience or that of others, and with soooooo many of the miata (mx5) guys having trouble with droop on FMIC supercharged cars I rightly or wrongly presumed that would be the case with all. Maybe it's a feature of that engine - as you say they are all very different beasts. I guess the main thing is to be happy with whatever works for the individual - mine never stalls or droops so works for me.

As for the turbo v's Na head, (twin cam is pointless exercise on this engine in my opinion - just drop in a 3.0L 968 engine if thats your budget) I'm still glad I went with the better flowing head, although they're not too hard to find in the uk still.

Raceboy - you really don't need to to tell me these arn't the 'BEST' engine in the world - it was said in the context of; which combination of OEM 924 variant parts are likely to give best outcome - ie most reliable, best breathing etc - but anyway - each to their own!
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[WEASEL149]

I think if I build another engine it will use the standard NA 924 engine as a basis. As far as I can remember the only difference in bottom-ends apart from pistons is the fact that 931 engines were supposedly hand-built by Porsche, but if you're rebuilding anyway I suppose it's irrelevant.

I would still keep the 931 head though.

I agree that the cost of a twin-cam head for one of these engines would be prohibitive and pointless when you can go for the 2.5l and upwards engines.
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1979 UK 932

[Euro924S2]

Stu, if your keeping the turbo head and using a stock (Ie not stroker) bottom end then the only other bit not turbo are the pistons. Crank and block are identical - in fact mine are off a NA on my rebuild as my turbo ones were tooooo worn!!

I still haven't been convinced that given the choice, NA pistons are the way to go, sure they may be cheaper But with all the extra work getting the CR correct I'm not sure there's any point - UNLESS you can't get a turbo motor easily but in the UK this shouldnt be an issue for the mean-time. As time goes on and more are scrapped / rescued this may change.

For me, the take home message from Raceboy is if you've got a good NA engine and want more power then you can safely crack on and boost it without worrying. Which for many members on here will save them £££££.
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UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp

[leadfoot]

In my first itereation I use N/A everything and a sc, cranking at 3psi
then changed crank pulley and cranked it up to 8psi...

changed the head to turbo and also used 88mm vw flatops, psi using same drive setup dropped back to 3psi... I also redid the manifold at this stage to a dual plenum with diffuser...

changed clutched drive to fixed and wasd running 10psi in the end before running out of injector flow...

So yeah you could keep the N/A head and keep throwing boost at it,
I would also consider the dynamic compression ...
One thing to note is that after sticking on the turbo head I could run much more advance timing.
It was always the impression of my tuner that the more advance you run the more power you can make...
It's kind of annoying that we all seem to choose different ecu's and that the maps are not really transferable,
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Euro924S2]

Apologies Stu, was really directed at Weasel but put ur name at top by mistake. Very useful to hear the process you've been through though. Completely agree it's a pain timing figures arnt transferrable, but when I looked at Raceboys initial table his top end figures are significantly lower than mine.

I'm convinced my manifold is holding me back so I'm designing a velocity stack version - then once it's fitted I plan to dyno car again at the same place with nothing else changed to get an accurate measure of how the current one is flowing. I've read much, so will be changing much too. Plus it'll be all aluminium.

I'll keep u posted. John.
_________________
UK spec '83 N/A with 931 motor with Eaton MP62 'charger @ 15psi. EFI - 565cc inj. Standalone Adaptronic ECU. 951 FMIC. Ally rad. Twin throttle. Recirc valve. Custom manifolds and CAI. 232bhp