Thoughts on 911/20 ST Type Engine

[cameron.arnott]

Hi guys

I'd really appreciate your collective wisdom on a 2.3 ST type engine I'm thinking about building. The engine will be used for fun road use, tarmac rallies and occasional track work. 7500rpm redline and reasonable torque from lower revs.


• 7R case, shuffle pinned, oil bypass mod, turbo pump, case savers, resurface & line bore
• 66mm ‘T’ non counterweighted crank (to spin up faster) Magnafluxed + micro polished
• 2.0 ‘S’ rods – balanced & resized
• 10.5:1 JE pistons with Mahle 2.2 barrels bored to 85mm
• DC 40 Cams or similar
• Twin plug distributor
• Quality head studs and rod bolts
• 2.4 ‘S’ heads with standard ‘S’ porting. Polishing, racing valve springs & retainers for safety.
• Webers with 36mm main venturis,140 main jets, 160 air jets, 60 idles, F3 emulsions, tall secondary venturis, double phenolic spacers to give extra manifold height. Port matched manifolds
• Triple outlet rally muffler

Questions

• Any thoughts on the ‘T’ crank vs ‘S’ crank in the way of preparation of the crank, bearings or case? Also torque characteristics vs ‘S’ crank?
• Is it worth porting the heads a smidge to say 37mm intake leaving exhaust at 35mm
• What have I forgotten or what would you do in the way of preparation not mentioned here?

Thanks in advance!

Cheers, Cam

[bumble]

Cameron,

That sounds like a neat engine. I'm soon to take delivery of a 2.0L engine that's been in the works for a while. I spoke to several people about these issues before I finalised the spec. Some guys who run these little engines swear by the nc cranks and spin them to >9K (albeit, they don't last long between rebuilds)so longevity shouldn't be a problem for you - actually, I think you might be a bit conservative at 7500rpm - 7800rpm should be achievable with good reliability. Apparently, they spin up real quick but vibrate a little bit more.

Finding a good 92mm 7R case might not be easy.

I have many of the parts to build this engine.

Cheers,

[cameron.arnott]

Cheers Bumble

Any stuff you want to get rid of?

I should add that I'd like to achieve around the 200HP mark if possible.

Also retaining SSI's

Cheers, Cam

[bumble]

Hi Cam

I have quite a few bits that may help. But I think there are a few issues you need to consider before you commit to any particular configuration - particularly with regard to Tarmac Rallies. These are probably better discussed off-line. If you want, call me on 0410 559 395.

Cheers,

Mark

[cameron.arnott]

Mark,

Will try to give you a buzz tomorrow.

No other input here guys? HP figures achiveable with this spec?

Cheers, Cam

[Walko]

CAm,

I think the factory acheived 22ohp with the 2.3St motors so you should get you 200hp quite easy.

Michael

[Grady Clay]

Cam,

I have raced sorta similar engines starting in the late ‘70s. I’m just about to rebuild the three identical engines and build a fourth. Remember, these are race only and totally unsuitable for the street but may be instructive for your purpose.

7R cases (all ‘73.5 CIS 911T originally) as you describe with additionally the case and cylinder spigots modified for better crankcase ‘breathing’.

66 mm counterweight crankshafts (it turns 8300 every shift with occasions to 8800 and some unintentional 9000).

2.0 ‘normal’ rods. X-rayed, Magnafluxed, Zyglowed, polished and shot peened. I started with about 150 rods and ended up with six really good sets and a lot of spares.

91 mm Mahle Nikasil P&Cs at about 11.5:1 CR. The 2.4 cases opened to fit the larger spigots.

Factory 935 ‘sprint’ cams.

Single plug (rules).

Dilavar cylinder studs and standard rod nuts & bolts (inspected as the rods).

2.2T heads with the ports enlarged appropriately. 935 lash cap adjustable rockers, Ti retainers, stock valves, stock valve springs set to stock (street) height.

46IDA, 42 mm venture, 906 intake manifolds, two 911T phenolic intake insulators.

Fairly standard 3-into-1 headers with ‘cookie cutter’ megaphones.


Some other ‘features’ are:
No alternator, just a shaft in the empty housing. It runs off just the battery.
935 Ti jackshaft.
Turbo oil pump.
Aluminum 935 chain sprockets
Both center oiling cams and spray bar.
908 air presser sensor to detect broken fan belt.
771 oil light switch set to 40 psi.
906 215 mm flywheel.
‘69S alu/steel pressure plate.
‘69S ‘thin’ disc.

And lots of TLC. ‘Maintenance’ rebuilds every 25 ‘at speed’ hours (add up lap times). That consisted of rod nuts & bolts, rod bearings, top ring, head gasket and touch-up the valves. I never found a problem. Of course each had the deck slightly reduced until the deck height/squish kept the edge of the piston clean but never contact, even with excursions in the 9K range.

The light weight of the rockers and retainers allowed proper valve control without high spring pressure taking power, adding heat and wear.

These are in the 250 hp range from 2576 cc.
In an SCCA GT2 914-6 I could take the GT1 track records.

I hope this is instructive.

Best,
Grady

What is this worry about tarmac rallye? I thought there were only two coppers in all of NT sorta like Wyoming.

[JCF]

Grady, why the "normal "rods instead of the nitrated S type ?

[Grady Clay]

JCF,

Because I only had a few sets of S rods and lots of 'normal' rods to choose the best from.

Best,
Grady

[JCF]

So then are the S rods a bit of overkill for , say a normal converted to 2.2 S running at standard red line ?

[cameron.arnott]

Hi Grady

Thanks for your input.

Yeah, not too many cops up here. Not too many people at all really Unfortunately we have recently had a speed limit of 130kph introduced. Gone are the good old days of no speed limits out on the open road.....I have to go interstate to do the rallying unfortunately

I have purchased a 73.5 7R small spigot case today which is a start, and I also have the crank & rods. Some serious saving to be done for a while now

I was interested to see that you were using the stock valve springs with those sorts of revs A light valve train is obviously a good thing!

Cheers, Cam

[obrut]

cam

are you aware that the twin plug will put you into C3? (i.e. MS)

[cameron.arnott]

Ryan,

Bugger! Hadn't thought of that, although it probably won't kill the idea

I'm still a little undecided as to whether I should go a little more agressive with the cam.

Hopefully I can get you to take me for a lap around the block in the 906 engined beast so I can get and idea of the driveability of a stroppy cammed engine.

Also hoping the ST that Dave Belford built will be at the Phillip Island Historics to sneak a peak at!

Cheers, Cam

[Grady Clay]

Ryan, Cam,

The class issue depends on what you want to do, how ’competitive’ you want to be and the seriousness of your effort.

My racing this car was to be reasonably competitive in a spec. controlled class (first CP and then GT2) with a not open ended budget. The three engines allowed me to have one in the car, one ready to go in the van and one apart at the shop. It is so much effort and expense to go to an out of town event that I always wanted to be prepared to successfully finish the weekend. (Only once did I have to use the spare.)

Over the years I have competed in much ‘higher’ classes than my whatever car was prepared for and had great fun. For example I entered my street ’68 911 in the fastest race car class at a Parade because it had (at the time) an Empi 2.5 and race tires. (I still took 3rd overall.) The point is to have fun. If the twin plug lets you use higher CR and/or tolerate less octane, so much the better. You have just as much fun regardless of which plastic trophy you win.

On the other hand there are situations where winning is the goal. That is where you pick a class you like and build a class-specific winning car. Every little detail is just within the rules. That is what I did with the 914-6 when I raced it. Those valves were just 0.01 mm below specified maximum diameter – if measured carefully. That plastic trophy represents a lot of effort; expense, technical, driving and racing. For example I have three sets of race wheels and would rotate new Goodyear race tires on to get the absolute best performance each race.

Even then common sense must prevail. At a National event I pulled off the pole grid position and back to the paddock when a thunderstorm approached. Sure enough less than half the field finished and many were severely damaged. I don’t think that is much fun.


I gave the 914-6 to my son 10 years ago at age 13 but he couldn’t drive it until he graduates from college. That happens this May. We are building the car with more ‘flexibility’ and not to one set of rules. It will comply in some class in SCCA Club Racing, PCA Club Racing, NASA, Nostalgia, RM Vintage Racing, POC and more. It may not be absolutely competitive in some situations but we can race somewhere almost any weekend.

The engines are becoming 92x70.4 = 2808 cc, twin plug (electronic), as high CR as I can get, GT3 oil pumps, lower max rpm – say 8000 (with a rev limiter), 906/910 cams, additional water vapor cooling (the ‘Rubbermaid Solution’), a new cage and seat, lighten it up somewhat (to about 1700#) and more. The transmissions are getting pumps, filters and coolers. The goal is to extend the service life of the components (at the expense of some performance). He will just have to drive faster.


In any event, evaluate what you want (and can) do with this project. For some the fun is in the planning and building a project with a timely end result secondary. Others do anything to make it to the next event. I am of the mindset to do things ‘right’.

To add to your discussion, the 906 cam (and like) are very, very ‘aggressive’ for the street and will only work reasonably in a very light (<2000#) car. Even then it will use up clutches getting started from a stop. Those cams “almost don’t run at all” below 4500 rpm and are designed for power in the 6500-8300 rpm range.


There are the sayings: “If you don’t have time to do it right the first time, how are you going to find the time to do it over again?” "It is far better to be a big fish in a small pond than in deep water with the sharks." And the classic: “Do you know how to make a small fortune in racing? Start with a really big one.”

Best,
Grady

[kenikh]

Just ran across this great thread. Adding my 2 cents on my ST "replica" motor...
Building it now, 2.3L ST replica motor for my '69S:

Fully detailed 3R case (shuffle pins, squirters, light mooning, every oiling mod known to man, etc)
85mm JE pistons @ 9.5:1 CR
'69S heads - in line to be ported and extrudehoned by Dick Evelrude
Twin plugs (still debating which system to use)
DC44 cams (custom grind; .490 lift at both lobes; 102 deg lobe centers)
Titanium valve retainers
RARE 47mm 4-bearing cam carriers
Nickie cylinders
Rebuilt 2 liter 'S' rods w/ ARP bolts
Cross drilled crank
Fully balanced internals
Fresh 930 oil pump, with full race chamfer on the internals
001 MFI pump
38mm (bored out) magnesium MFI stacks and TBs
Lightened '69S flywheel

Planned to be good for between 210 and 220 HP (crank) at 7800 RPM. First dyno run should be sometime this summer. Should be fun; the car is already running a fully built 901 gearbox w/ AFKQX short gears.

Original motor will be treated to a concours rebuild after this motor is done.

[aws]

Grady: I was surprised that you are using the 2.0 rods instead of the 2.2. The 2.0 rod is light but not near as robust as the 2.2 rod. Over the years I have always heard the 2.0 rod was not up to the task, but you seemed to have proved that notion wrong. The other surprise was stock springs set to stock height. Again over the years the stock springs have worked well for me all the way to 9000 (on a regular basis) but I always have shimmed them 1mm tighter than street spec. The last surprise was the dilavar studs, I have always had good luck with steel. (although with a aluminum case). My opinion has always been if the engine is kept in normal operating temperature range you shouldn't have stud problems.

aws

[cameron.arnott]

Cheers KenikH

Are you still going 36/36 on your ports? (yes I have been reseraching your posts )

Now pretty certain I'm going to use a CW crossdrilled crank and I'm leaning towards your cam specs.

Whats the ETA on your engine?

Cheers, Cam

[kenikh]

Originally Posted by cameron.arnott

Cheers KenikH

Are you still going 36/36 on your ports? (yes I have been reseraching your posts )

Now pretty certain I'm going to use a CW crossdrilled crank and I'm leaning towards your cam specs.

Whats the ETA on your engine?

Cheers, Cam

I have decided to go 38/38 on Steve Weiner's advice (Steve, the heads are coming, I promise ), unless the porter insists otherwise. Those cams with their 'S' like overlap and with 2.3L of displacement on the motor, should overcome any low speed intake velocity issues. The smallest this motor will be is 37/37, guaranteed.

ETA is end of summer. I've been wrong on ETA before, though.

[cameron.arnott]

Time for an update....

The build is getting close just getting together the last few bits. Here's the updated spec list so far:

• 7R 2.4 case, shuffle pinned, oil bypass mod, 996 turbo pump, case savers, resurface & line bore
• 66mm ‘S' counterweighted crank, magnafluxed + micro polished, cross drilled
• Pauter rods – balanced
• 10.5:1 JE pistons with Nickies barrels of 85.5mm as per alowable specs for historics here in Oz
• DC 44 Cams on 102 lobe centres (Thanks KH!)
• JB Racing Twin plug distributor
• Supertec head studs
• Rennsport Systems ported and massaged heads, racing valve springs & Ti retainers for safety.
Australian made 'Cairns' Muffler - light weight and ads enough mid range torque to flatten out S cams 'on cam' feel in a 2.4 - 2 in 2 out
• Webers - currently with 34mm main venturis,140 main jets, 160 air jets, 60 idles, F3 emulsions, tall secondary venturis, single phenolic spacers, PMO tall manifolds - port matched.

What sort of specs to you guys think for the Webers? Do you think any scope to go a little bigger venturis on a 2.3 liter engine revving to 8K? Or is 34mm enough to get good power AND driveability?

I'm also leaning towards running indivdual sock style air filters on each trumpet for light weight and a vintage look. Any downside to these?

[bumble]

Hi Cameron,

Good to see you're making progress with your engine

Your spec is very similar to mine except you have 300cc more capacity and I run 11.5:1 C/R. Several questions/comments:

What headers are you going to use - specs?

What ignition will you run? I have a single MSD 6AL driving two high output coils (the brand of which I don't have to hand but can find out if you want) which works really well.

Whether twin plugs will bump you to C3 is inter-alia, a function of build volume of the model you want to race. In my case, I could prove that sufficient 914/6 GTs and kits were produced to satisfy volume requirements for C2 so they had to allow it (although I think sometimes their decisions are a bit arbitrary and you have to argue the case )

'Cairns' muffler - what's this, who makes it and what info do you have that compares it with any alternatives? How light is light? IMHO headers/muffler combination will be a major determinant of how well your engine performs, all other things being equal. We spent days on the engine dyno testing various combinations with sometimes conflicting and confusing results. I'm not done with this issue

Cams - interesting input from Kenikh... I wonder whether these would be good for 2L? I have cams that are in retrospect too aggressive. Great power but undriveable below 6000rpm - REALLY undriveable... Any comments from knowledgeable folks on this issue would be welcome. What makes the DC44s so special?

Carbs - your specs look pretty close to me. I would test both 34 and 36 venturis - you might find you run out of puff above 8000rpm with 34s. With the components you have you should be able to run reliably to 8500rpm I currently have 46mm PMOs but will revert to 40mm Webers when the engine goes back in.

The only problems with sock style air filters is dust filtration and whether rain can get through your engine grill into the trumpets - for that reason I run rain-shields and don't worry about looking cool

Cheers,

Mark