Need advice: 3.6 or 3.8?

[plexiform]

I have been in the process of restoring my 75 911S for the past several years and am finally at the point where I have to decide what to do with the engine.

I'm choosing between:
1. Freshly rebuilt 964 3.6 engine to stock spec
2. Take the freshly rebuilt 964 3.6 engine and upgrade to Motec engine management and add a custom intake,
3. Take option #2 and upgrade to billet camshafts, Pistons, High rate valve springs.
4. Change out the cylinders for Mahle motorsport 3.8L setup plus all the other mods mentioned above.

If my goal is fun trouble free low maintenance spirited road driving once in a while without plans for track time what would be the way to go? Is the decision simply how much i'm willing to spend or is there a benefit to keeping it simple and staying stock despite the known performance increases with options #2 and #3?

Appreciate the advice.

[Bill Verburg]

Originally Posted by plexiform

I have been in the process of restoring my 75 911S for the past several years and am finally at the point where I have to decide what to do with the engine.

I'm choosing between:
1. Freshly rebuilt 964 3.6 engine to stock spec
2. Take the freshly rebuilt 964 3.6 engine and upgrade to Motec engine management and add a custom intake,
3. Take option #2 and upgrade to billet camshafts, Pistons, High rate valve springs.
4. Change out the cylinders for Mahle motorsport 3.8L setup plus all the other mods mentioned above.

If my goal is fun trouble free low maintenance spirited road driving once in a while without plans for track time what would be the way to go? Is the decision simply how much i'm willing to spend or is there a benefit to keeping it simple and staying stock despite the known performance increases with options #2 and #3?

Appreciate the advice.

Depends on the rest of the chassis/trans setup and $ that you want to spend

For sporting street use option 1 of a stock 964 in the stock '75 chassis is a ball,
the trans will be happy, both in gearing( It'll be a little short for really comfortable highway cruising) and reliability, You can use ss cams for a little more sporting character
assuming stock body work you can fit 4x 7x15 or 16 wheels, the 3.6 is a bit much for the tires but that can be part of the fun
you'll likely want to upgrade t-bars to maybe 20/26 and then evaluate for further upgrades
i'd look into an effective lsd before going beyond this, 40/40 min 40/60 prefered w/ a loose preload

going to option 2
adds a lot of expense and additional torque/hp. This stresses all of the components from trans to suspension and wheel/tires, I'd want 8 & 9.5 x15,16 or 17 w/ 225 & 255 tires & beef the trans as much as possible. The wheels and tires will mean at least RS body work

option 3 & 4 besides adding exponential expense further stress the rest of the car, I found in my '76 that the 915 even a late beefied version isn't really happy w/ 3.8 +mild cam motor, the issue is the torque transmitted more so in the lower gears

[plexiform]

[QUOTE=Bill Verburg;10336304]Depends on the rest of the chassis/trans setup and $ that you want to spend


Thanks, yes I should have opened with more details. The majority of this project has already been done over the course of the past 5 years. I am at the point of engine now so looking for specific advice about that.

Here is what I've already done:

Body: Early 930 turbo look, All steel excpet fiberglass tail and carbon fiber hood.

Brakes: 996 Brembos all around with Rebel Racing hardware to make it work.

Suspension: Bilstein coilover all around with Rebel Racing parts and Eibach springs, torsion bars removed, triangulated front strut bar by Rebel Racing, Rebel Racing 935 rear spring plates, Rebel Racing front and rear bushings

Trans: 915 5speed

And then other cosmetic stuff including full leather/suede interior and guages redone by North Hollywood Speedometer.

Its been a long standing project and I'm finally nearing the end. It currently has a 3.0L SC motor in it that works great but I want more power so I purchased a freshly rebuilt 964 3.6L motor and am now edging closer to the slippery slope of how much more to do or just install it and go.

Wheels are Etoile 16X8/10 with 225 in front and 255 in rear. I will likely leave the wheels and tires package upgrade until the very end. But I am leaning towards Fikse Fuchs RSR 17x9/11 and would like to go as wide as possible with the tires to finish off the project.

[Catorce]

If it's a freshly rebuilt 964 motor, then you have to weigh the options of simply installing the motor and driving, or totally rebuilding the motor as a 3.8, which will cost money and also change lots of the stuff that is already on the 964 motor (crank, cams, heads, valves, etc).

All things being equal, if I was starting from parts, I would go to a 3.8 just because. IMHO there is never a good reason to go with a motor with less displacement and less power.

[Bill Verburg]

Originally Posted by Catorce

..
All things being equal, if I was starting from parts, I would go to a 3.8 just because. IMHO there is never a good reason to go with a motor with less displacement and less power.

I can think of 1, the 915.

like I said, been there, done that. 3.8 + 915 isn't the happiest combination. Yes it works for a while at least, but the torque in the lower gears distorts the shaft alignment and the stock gearing is really a bit to low.

I've had stock '93 964 a stock 993RS and a lightly breathed on 993 in my '76 w/ 915/44 and 915/67. By far the happiest combination is breathed on 993 + 915/67.

ive laos used 7 & 9 205/55 & 245/45 x16, 8 & 9.5 x17 w/ 235/45 & 275/40 and 225/45 & 255/40. The 16s were not quite up to where I wanted grip to be w/ any of the engines, the 235/275 was ok for track days but there was too much energy cost for stop light drags, best compromise is the 8 & 9.5 x17 w 225/45 & 255/40 x17 w/ the 993 & 915/67

If going to 9 & 11 my only suggestion is to keep the wheels and tires short and light, something like P7 Corsa 235/45 & 285/40 x15 on 9 & 10.5(11 is a tad too wide for this tire), do some research It's hard to get decent sizing for 9 & 11 unless you go to 18s

[clutch-monkey]

i'm thinking of going 3.8 without changing compression ratio, not sure what i need. 964 3.6.
if the 915 fails i'll go sti box.

[plexiform]

What can be done to the 915 trans to make it work and last long with a 3.8? My style of driving is not going to push performance to the limit hardly ever.

[Catorce]

Originally Posted by plexiform

What can be done to the 915 trans to make it work and last long with a 3.8? My style of driving is not going to push performance to the limit hardly ever.

Billet side plates, better lubrication and a transmission cooler are the usual additions but this is usually for track cars.

As long as you are committed to not doing hard launches and generally taking it easy on the trans then you should be ok.

Also as far as 3.8 goes, there are lots of degrees of this. Singer builds a 3.8 that makes ~380 HP. Taking a normal 964 and throwing on bigger jugs might only net you ~310HP so there is a wide range.

[Trackrash]

A stock 964 with headers and a good muffler with a chip to match should make you happy. You will be surprised at how good one of those will work in a light weight car.

[shbop]

I'd go with the 964, and have some fun. Save the 3.8 for a future project, along with a sustainable trans.

[tmaull]

Can you say more about the STI transmission? I assume out of a scoob? I haven't heard of this approach - a quick google didn't tell me anything.

[clutch-monkey]

Originally Posted by Trackrash

A stock 964 with headers and a good muffler with a chip to match should make you happy. You will be surprised at how good one of those will work in a light weight car.

agreed for a street car. my track/street car has a 3.6 at about 320hp which is up for rebuild.
strongly considering 3.8 since the cost is almost the same.
my issue is i want to leave the compression ratio as is, don't want to put more strain on components, just want that bump in torque/hp.

Originally Posted by tmaull

Can you say more about the STI transmission? I assume out of a scoob? I haven't heard of this approach - a quick google didn't tell me anything.

it's been done a few times on this forum.
mate has done it on his car. it is a functional solution not an elegant one (linkage comes out at the 12 oclock position).
i'd have to put in some fab work to make it look nice.
but it is stronger and cheaper than trying to keep a 915 alive.
that said, my 915's seem to survive really well, so it's not something i am jumping into at the moment.

[Bill Verburg]

Originally Posted by Catorce

Billet side plates, better lubrication and a transmission cooler are the usual additions but this is usually for track cars.

As long as you are committed to not doing hard launches and generally taking it easy on the trans then you should be ok.

Also as far as 3.8 goes, there are lots of degrees of this. Singer builds a 3.8 that makes ~380 HP. Taking a normal 964 and throwing on bigger jugs might only net you ~310HP so there is a wide range.

As far as the trans that is correct, if you had a late al. case then a girdle for the main and lay shafts would be desired, but the mg cases come w/ one stock

Here are some typical rear wheel torque figures all were done on the same dyno



to get an air cooled 3.8 to match a 996GT3 you would need to spend ~$50k for the motor, it would probably be cheaper top t/p in a GT3 motor

[Bill Verburg]

Here's a comparison of the result of a stock 3.9 993 n-ver into a '78 SC, the difference is startling

[rkeele]

Bill is correct, I have been running a mag. case 915 with custom billet side cover and GT racing short gears in my 3.8 race car for some years trouble-free. understand this is a low mileage race car not a streetcar that's driven daily, that and I am using a Wave track which keeps the heat down compared to the clutch type LTD also a Wevo shifter and coupler is VERY IMPORTANT, It can be done, street 911 drivers tend to shift to fast from 1st to 2nd and that's where the complaints come in,

[Black 993]

I'd do a stock-ish rebuild but add cams, exhaust, and live remap. Great motor and cheap (as these things go).

[Mixed76]

Seems a little crazy to not put the 964 motor in as-is and see how you like it. Doesn't cost much to install and remove, relative to the costs to build a different motor.

I would do that just to have a driving car so the suspension bits and miscellaneous could all get properly sorted. Then come back for power if it needs it.

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[Trackrash]

Agreed with above. You will need some sort of custom muffler/ exhaust anyway. That and a chip update will really wake up a 964.

[dannobee]

I agree with Gordon, put it in a drive it.

If you start hotrodding a 3.8L, the 915 will NOT be happy in a street car, regardless of what other people say. It WILL break unless you baby it in lower gears. And if you're afraid to push it, what's the point in having all that power?

Yes, I know that 915's have a cult like following, but when the G50 came out, some of us at dealerships saw it as a godsend.

[Carbster09]

I am doing Option 2 now! I started out by getting a 7R case for my stock 71T, looked at machining costs and just bought an SC motor to help, then it was going to be a 3.5 ... low and behold TRE said they had a 3.8 built for someone that no longer needed it! What a boost. Then sourced a good 915 and had it built to take the power. Its a slippery slope and it ain't cheap. I would tune the 964, maybe add some cams and have fun! Easy!