HP Question '73 vs '77

FranksPlace2 · 10-15-2003, 12:41 PM

The 1973 911S was 2.4 L, 8.5:1 CR and developed 190HP.

My 1977 911S is 2.7L and develops 165 HP. It is said this is due to the exhaust system.

If I put a 1973 exhaust system on my '77, will I gain 25 HP? What about an SC exhaust system without a catylst?

Thanks

Frank

Randy Webb · 10-15-2003, 12:56 PM

you'll be lacking a cam with a good power profile and an induction system with good ar flow. SSIs will help a lot tho.

emcon5 · emcon5's Garage

Nope. There are big differences between the 73 911S and a 77 911S engines. The piston shape, induction system and cam profile, Port size in the heads, in addition to the exhaust are all different.

The 73 had Mechanical Fuel injection, high lift, high duration cams and pistons with deep valve pockets in the pistons to accomidate the cam and larger intake and exhaust ports.

A 77 has Bosch Jetronic (CIS) fuel injection, which can only tolerate a fairly mild cam, and a piston dome designed to reduce emissions.

Tom

bigchillcar · bigchillcar's Garage

Lose the thermal reactors (pollution control stuff) and you should pick up 10 hp...the '74 911S has the same engine minus the reactors and is rated at 175 hp. What's been said about the CIS fuel injection is true: not much if anything can be done to extract more power from it...it's advantage is driveability and reliability over being 'modifiable' for performance like the MFI or carbs.
Ryan

Early_S_Man · 10-15-2003, 01:08 PM

Frank,

Your car still has the Thermal Reactors and smog pump, right? Even less, down to 160 hp, I believe! You would gain nothing by going to an SC exhaust system!

The '74 version of your engine without reactors, and with the good '67 thru '74 heat exchangers with tuned, equal-length headers ... made 175 hp, so that is the best you could ever expect.

The '73 was the last of the line of the true 911S models with high-revving 'S' cams and MFI injection. Your pistons, cams, and injection limit the power of your engine, in addition to the exhaust ... if it had the appropriate parts ... it would be a 210 hp 911/83 RS 2.7 engine, with the following power and torque curves:

FranksPlace2 · 10-16-2003, 05:43 AM

I bought the car in Mar 02 and I had a (unplanned) complete stock rebuild of the engine by local guru Randy Greff. I have new Mahle P&C, serts, 11 blade fan, tensioners, valve job and SC exhaust with Cat bypass. ( I can here Wayne now, "This is a hobby, not an investment. This a hobby...")

Warren, No TR no Smog pump

I understand the lack of cam and, perhaps air induction. But the 73 1/3 T went from MFI to CIS with no reported change in HP. So CIS can't be that bad. Plus I have 2.7L vs 2.4. And 8.5 CR for both That has to be worth something.

Warren, is the curve above measured at the engine shaft or at the wheels?

Frank

Tim Walsh · Tim Walsh's Garage

Also keep in mind that the MFI T camshaft is very mild compared to the MFI S camshaft. The MFI T pistons look almost like turbo pistons, no dome and almost no relief. The S pistons on the other hand half lots of both. Your CIS pistons don't have any relief and have a swirl in them to reduce emissions. To make the sort of HP that a 2.4S is making you would need a much more wild cam than the CIS S is and different pistons. ALSO because the CIS doesn't like the fuel reversion that the MFI S camshaft makes at idle you would need carbs or MFI.

The CIS was made almost exclusively for emissions and that's the biggest reason you won't be able to make the same HP as the 2.4S.

Early_S_Man · 10-16-2003, 08:05 AM

Frank,

The above curve is at the crank, on a dyno test stand ... from the factory service manual. It is the engine that propelled the '73 RS 2.7 into history as the best, most genuine, most visceral Porsche ever available to the driving public! Also, the basis for a few racing wins over the past 30+ years ... THE high point in 911 history, by most accounts!

RallyJon · 10-16-2003, 08:19 AM

Quote:

at the crank, on a dyno test stand

A very well-respected 911 tuner recently told me that the Porsche factory tested engines without the fan or oil pump connected--that the test stand had it's own oil and cooling air supply.

If true, that would take a bit of the shine off the high revving HP numbers the factory put out. I bet the fan is costing at least 10 HP at redline.

Tim Walsh · Tim Walsh's Garage

Jon,
But also remember than those #'s are the minimum that porsche would accept before putting the engine in a car. Frequently they put out a few more ponies than that.

Early_S_Man · 10-16-2003, 08:49 AM

Jon,

That is PURE BUNK about the oil pump and fan!!! Your source is highly misinformed, or opinionated, about the test facilities in Stuttgart!

Of course ... you do have to supply air to an enclosed dyno room, and dry sump engines DO require a hookup to a dry sump tank full of oil, right??? A quick look in Volume I of the factory service manuals shows an engine being tested ... just prior to being installed in a car! It would be impossible to rig up a special engine without an oil pump purposes of generating 'fake' numbers for PR purposes ... much easier to just 'pull' numbers out of the air -- like Detroit did in the '60s for purposes of deception!

It is quite true that power output numbers from the factory are conservative, and represent the worst output you would ever see in a production example! The 3.0 engines supplied to Roger Penske for the batch of IROC cars dyno'd at anywhere from 315 -325 hp ... much to the chagrin of the highly-competitive racer egos involved!

jluetjen · jluetjen's Garage

If anything, it was the Detroit automakers who were doing stuff like that because the SAE Gross numbers that they used to quote didn't include a whole lot stuff like alternators that real world cars use. Porsche has always quoted the HP as DIN which has a very rigid process (this is Germany remember

) for taking the measurements. If my memory is correct, DIN required testing the engine as installed. It would be very doubtful that Porsche could get away with mis-quoting the HP and avoid running into regulatory problems with German government.

RallyJon · 10-16-2003, 09:57 AM

Thinking about it, I guess it would be pretty hard to test a normal production 911 engine without the oil pump. But leaving the fan/alternator off does make intuitive sense in a production environment where a sub-par engine has to be pulled apart immediately.

Curious if he's wrong about this--it's not the sort of thing one would just "make up".

FranksPlace2 · 10-17-2003, 03:48 PM

I guess I am an early S wannabe but settled for a mid-year when what I could afford wasn't what I wanted.

I still have enough zip for my needs.

Frank

Jim Richards · 10-17-2003, 04:55 PM

Quote:

It is the engine that propelled the '73 RS 2.7 into history as the best, most genuine, most visceral Porsche ever available to the driving public! Also, the basis for a few racing wins over the past 30+ years ... THE high point in 911 history, by most accounts!

I love it when you talk like that, Warren.