High Compression RS engine

[Luke Hudson]

Hi Chaps,

After long and careful deliberation I've decided I'm going to build myself a high compression 2.7RS motor.

I'm going to leave my 2.4E engine in the car and build the new engine over the next year, I think I'll enjoy the learning curve of building the engine, and can still drive the Porsche in the interim.

Now for the big question, what engine should I use as a core? basically I want to get the case that requires the least modification as i'm trying to keep costs down. I reallise i will most likely need new P&C's no matter what engine I start with, but is there a case to avoid? Also do any "stock" engines have suitable heads for hihg compresion pistons? (bear in mind I'm in the UK so I'm looking at european spec engines)

I know this question's probably been asked before, and yes I've read Wayne;s book on engine rebuilding, but your opinions would be appreciated.

Cheers

[911SCfanatic]

I believe the 76/77 Euro case has all of the casting updates, oil bypass mod, and spigot size you need.

[Grady Clay]

Luke,

What is your planned use? Is your 911E a coupe or Targa? Do you anticipate suspension changes?

How high compression are you willing to go? In other words, what octane do you have available?

Are you willing to twin plug it? That can be done where it isn’t obvious. This allows slightly higher CR or lower octane and is great for use with the MFI.

A 2.7RS has MFI so you will need 911S MFI heads, S throttle bodies, S intake stacks, S cams, RS pump, piping, nozzles, etc. An original MFI green fan shroud would be nice to be a “sleeper.” Starting with a 2.7RS engine would be easiest but expensive. Starting with a 2.4S is less expensive and not difficult but still a lot of work. The other alternative is to collect individual pieces or sets of parts. This is time consuming and possibly more expensive.

Whatever you decide, you want to do it absolutely correctly. No half a$$ deal.

If you are willing to deal with the high octane fuel issue, use 92 mm 2.8RSR pistons and cylinders at 10.3 to 11.5:1 compression ratio. Time consuming to build initially but spectacular performance.

Another issue you probably should consider addressing is a front oil cooler. Adding an engine fan with 1.82:1 ratio is easy.

Sounds like a fun project.

Best,
Grady

[jgparker]

Also, the mid-76-77 911S (CIS) is a good core to start with. 7R case, all the mods, good size ports, larger oil pump, but you'll be starting from scratch on the induction system and exhaust. You might get lucky and find a good set of Nikasil cylinders on it too. They are cheep and easy to find, but it will need all the case work.

Good luck,

JP

[Bobboloo]

My preference when starting with a 2.7CIS motor is a 74' or a late 77' motor.

The reason being is because the 74' motors didn't have thermal reactors, so your cylinders didn't get "baked" causing excessive heat. The 75' thru early 77' cases tend to need the most work to get them "square" again due to warpage warpage.

Late 77' cases had dilivar studs on top and bottom rows and this seems to have help counteract the warpage problem incurred from the excessive heat. I own a late 77' motor that low and behold doesn't leak oil or have any pulled studs. Fortunently the dilivar studs haven't broken... yet.

A late 73'S motor would be a great starting point. They have 7R cases. The ports are already the correct size. The induction is bolt on ready. All you really need is the correct P+C's, enlarged spigots for the P+C's and a MFI pump recal. Basically a 2.4S and a 2.7RS are the same motor. They just differ in displacement.

Another good starting point in my opinion would be a 2.4T MFI motor. In alot of ways this is a better starting point than a lot of the 2.7CIS motors. The 2.4T cases don't usually need all the work that the 2.7CIS motors need because they don't don't seem to have the bad warpage problems. I just rebuilt a 2.4T and the case checked out great. No align boring or resurfacing was needed. I just decked the case and installed case savers even though no studs showed any signs of pulling. The heads on a 2.4T will already have the bore for the MFI injectors and you can use the T induction system. You'll have to buy the correct stacks but you can bore the TB's and recal the pump. The exhaust is also correct.

[Zeke]

I like what you say Bobby. I have a 2.4 T on the stand waiting for disassembly.

[obrut]

The big question is what is your intended use? If racing, then a lot of your choices will be distated by the rules of your class. If street, then its only $$'s.

I've just done the following - 2.7 capacity, 7R case, 10.5:1 AL Arias pistons, GE80 cams, massaged RS ports, MFI, drilled for twin plug (but running single due to rule change), early heat exchangers - and have just over 200 rwhp and a very fun engine to drive/race in a sub 1000kg car.

[Luke Hudson]

Thanks for the input guys.

In answer to some of your questions:
The car is a coupe, I replaced all the suspension last winter (alloy traling arms, all bushings replaced, brakes upgraded, new front struts), I realise if I get a heavier engine I'll need to beef up the rear torsion bars but everything else should be fine in that respect.
The car will be primarily for road use with some track days but i don't envisage entering a racing series, so it doesn't have to be "class" legal.
We have 98 octane petrol here in the UK, so i can go pretty high on the compression ratios, and I'm happy to twin plug (but it will be money dependant)
With regard to induction, yes, I'd love to keep it MFI but the cost may be prohibitive ( this of course depends on whether the donor engine has MFI), so PMO's are looking like the favourite option at the moment this is something I will look into closer to the time.
Exhaust is sorted, I have stainless SSI's and a stainless single out muffler on the E which I'll swap.

I have plenty of time on this project, so I'm happy to wait for the right donor engine to turn up. I really want to minimise the machine work and get the case that needs the least amount of work.

So if I understand the above, my options are a 2.4S or T with 7R case and MFI (this strikes me as an expensive donor engine to buy, especially as the S is a damn fine engine as it is) or a 76/77 2.7 long block and find the induction seperately.

Grady,

You mention 2.8RSR pistons, I assume to run these I'd need to severely modify the heads to fit these pistons? I suppose with all the effort I'm going to i could make this a 2.8.......

[Jim Richards]

Finding a good 2.4S donor engine at an affordable price will be a challenge, Luke. I'm finding it easier to find some of the 2.4S parts (cams, heads) and I'm hoping to use a 2.7 (7R) donor case, assuming it checks out with the machine shop. 2.4S MFI parts are pricey, too. Good luck on your project. Aubergine 911E's rule!

[safe]

The euro cars never had any thermal reactors, am I right?
So they shouldn't have the big problems the US cars have.

[911SCfanatic]

Magnus is correct. All the Euro cases should be fine and since Luke is in London he's all set. The 76/77 cases will eliminate some of the additional machining required.

[911SCfanatic]

just over 200 rwhp

So what is that at the crank, Ryan?

[Grady Clay]

Luke,

You said:

“Grady,
You mention 2.8RSR pistons, I assume to run these I'd need to severely modify the heads to fit these pistons? I suppose with all the effort I'm going to i could make this a 2.8.......”

I assume you have Bruce Anderson’s book, 2nd Ed. Chapter five and particularly Pages 138-139 give some explanation. If you can tolerate (with race fuel) 11.5:1 and higher it is relatively easy with GE80 or 935 cams. The issue with RSR pistons is to get the CR lower and back in the realm of fuel available at the pump. With 98 octane, twin plugs, and proper cooling, I think 10.5 to 10.8 is pushing the envelope with S cams. Now the issue becomes how to maintain a proper combustion chamber, particularly the piston-to-head clearance.

There is not a lot more effort and machining to use 92 mm RSR than 90 mm RS, it is just more critical. In neither case are the heads “severely” modified. The difference between 8.5:1 and 8.7:1 may make some more power but the difference between 10.7:1 and 10.9:1 may make the difference between the engine living or not on pump fuel.

I would only go this high using twin plug electronic ignition with data acquisition and knock sensing feedback.

In a relatively light 911 or 914-6 this 2.8S has neck snapping performance with MFI.

Best,
Grady

[jgparker]

This may be obvious, be another option to get a great HC engine is to bore out a used-up set of 90mm cylinders (2.7 Nikasil or Alusil) to 92mm and have them reprocessed to Nikasail. Then buy a set of J&E pistons at any compression ratio you want (9.5 or 10.5:1 are on-the-shelf). The bonus is that it's about half the cost of a set Mahle 2.8 RSR P/C, and will work with your stock 2.7 heads better than the RSRs. Just one option.

Good luck,

JP

[obrut]

Bill - Who knows?

If you use 15% losses you get about 235 fwhp. The dyno guys say times by 1.32 which gives you porsche 265 fwhp.

I do know that my old webered low comp 2.7 made about 150rwhp on the same dyno - so I'm pretty happy.

[snowman]

My 912, 1725cc measured 145rwhp at 7800rpm and that was the first run, before tuning. SO THERE

[obrut]

what's your problem snowman? - i was only answering bill's question

[Luke Hudson]

Thanks for all your help guys, I'll let you know how I progress!

Luke

[911SCfanatic]

Thanks for the reference point, Ryan.

[snowman]

No problem just hp challanged type of guy.