E-pistons in a 2.4L T engine - whassup?

Jim Richards · 10-02-2002, 08:20 AM

A '72T I'm interested in had it's engine rebuilt, supposedly with E pistons. Does this make sense? Is it a good thing? What are the implications? I'm clueless on this issue. Thanks!

T Bird · 10-02-2002, 09:02 AM

Jim
Do you know if they are 2.4E pistons, or 2.2E pistons? Makes a big difference with the compression ratio. When we rebuilt my 2.4T, we used a new set of 2.4S pistons for an 8.5/1 CR. This is significantly better than the 7.5/1 CR for 2.4 T pistons.

2.4E pistons are 8/1 CR, while 2.2E pistons are 9.1/1 CR. I can't remember now, but I seem to think that if you use 2.2 pistons in a 2.4, the actual CR is higher than what it would be in a 2.2. I could be wrong-- the memory isn't what it used to be!

Joe911 · 10-02-2002, 09:10 AM

E pistons would have a more aggressive dome (more compression) and deeper valve reliefs cut in the crown so a higher lift cam can be used without unwanted contact between the two (valve and piston). The 2.4 had reduced compression compared to the 2.0/2.2s and a slight up in compression would mean more efficiency (better burning of the fuel - assuming of course high enough grade of fuel is used).

Using different piston and cylinders at rebuild time is/was not uncommon on the early engines. So this alone should not put you off - the overall quality of the rebuild IS what's important. Just search the BBS and read lots of threads on this topic..... or buy the B/W engine book in the next week or whenever it is available.

Bobboloo · Bobboloo's Garage

Expanding on what T bird said. Using the 2.2E pistons that were 9.1:1 in the 2.2 motor should give you 9.6:1 in the 2.4 motor because of the longer stroke. But according to BA it usually comes out to be a little less. Either way they are preferable to the T pistons.

Bobby

Jim Richards · 10-02-2002, 10:53 AM

OK, so I need to know whether 2.2L or 2.4L E pistons were used. I guess I also need to know if the cams were changed and what is the compression ratio for the engine. Does this modification have any implications on the heads or any effect on the MFI system? Thanks!

T Bird · 10-02-2002, 01:54 PM

Jim
I don't believe there are ramifications for the heads. As noted, I went from 7.5 to 8.5 CR and retained the stock T heads. It works fine! However, I also switched to Webers from MFI at the same time. I have seen one other 2.4T with MFI that was rebuilt with 2.4E pistons. That set up seemed to work fine without any mods to the MFI pump.

If this car still has MFI, I would bet that the E pistons are of the 2.4 variety with 8/1 CR. I would think a significant increase in compression would alter the engine's fuel delivery requirements and thus neccesitate some adjustment to the MFI pump.

Good luck.

Jim Richards · 10-02-2002, 02:05 PM

Thanks John! I got a clarification from the seller that it was indeed 2.4L E pistons and cylinders that were used for the engine rebuild. The cams and MFI don't appear to have been changed.

I'm assuming that this setup will still run on regular gas and probably have a little more HP than the stock 911T. Does this sound right?

Early_S_Man · 10-02-2002, 02:48 PM

All 2.4 engines run on regular gas.

But, I'm afraid you won't have any additional power, because the USA 'T' MFI engine had the same 8.0:1 compression ratio as the 'E' engine! No net change with the new pistons ... just bigger valve reliefs! The 911E cylinders were Biral construction like the 'S' ... for additional heat dissipation capability. Are your cylinders cast iron, or do they have aluminum fins around the cast-iron liner? Or, maybe later replacement Nikasils???

T Bird · 10-03-2002, 07:13 AM

Warren
Are you sure about the CR for the 2.4T? As I have always understood it from the workshop manual, the spec book, the owners' manual, and road test info from around the time of the introduction of the 2.4 series, the 72/73 MFI "T" was equipped with 7.5/1 CR pistons and iron cylinders. The p/n for the 7.5/1 CR iron cylinder set for these cars is 911.103.942.01.

Further, as I have always understood it, the T got the higher 8/1 CR in 73.5 when the switch was made from MFI to CIS. The p/n for the 8/1 CR piston set for the CIS 2.4T cars is 911.103.940.01. The cylinders in this set are also of iron.

The Biral 8/1 CR piston/cylinder set used in the E cars for 72 and 73 show p/n 911.103.943.01. I believe the configuration of the piston dome is different from the CIS 8/1 CR piston.

John

Early_S_Man · 10-03-2002, 07:24 AM

Well, guys, my '73-'73 spec book shows compression of 8.0:1 on p. 12 for the '72 'TE' USA engine ... and the '73.5 'TK' data is on p. 22.

Oldporsche · 10-03-2002, 09:17 AM

The E piston was the only piston offered by Porsche in a oversize. Thus you could bore a set of cylinders slightly and use new pistons, saving you the cost of new cylinders.

This may be a clue as to why they are there.

Good luck,
DAvid Duffield

Jim Richards · 10-03-2002, 09:28 AM

In this case the seller's receipt indicates both the pistons and cylinders were replaced.

T Bird · 10-03-2002, 03:49 PM

Warren
Not to beat a dead horse--- but I hope this resolves the confusion on the CR for the 72-73 911T. Upon getting home tonight, I checked my spec book. On page 6 of the 72-73 spec book, the 1972 and 1973 type 911/51 engine, which is the USA spec MFI engine, is shown with a 7.5/1 CR. The 1973 911T with CIS (type 911/91) is shown with an 8/1 CR.

I also checked my Porsche workshop manual. Volume III, Section I, page 0.2-1/1 (USA only), shows the technical data for 1972 models. The CR listed for the USA 2.4T is 7.5/1. This also comports with the data in my 1972 911T owners manual, and the 1972 Porsche sales literature that I have.

There are other citations in both the spec book, the workshop manual as well as other less official sources that I have that confirm this information.

While somewhat unclear, the reference you cite on page 12 of the spec book to the USA 2.4T having an 8/1 CR must be a reference to the 1973 CIS model. Indeed, the data on page 12 covers 1972 as well as 1973 CIS models. Also, on page 22, the chart you cite identifies differences between the CIS model and the 1972 (MFI) model and reflects as a difference that the CIS model has a CR of 8/1.

John

Early_S_Man · 10-03-2002, 04:07 PM

Well, John, I just don't know ...

The Stoddard catalog jives with p.12 of the spec book.

It is another case of a discrepancy in factory publications not easily resolved! It would have been so easy for the factory to issue revised editons of the spec books, but I guess the issue of correct information being in the hands of the public ... just wasn't that important!

marc weintraub · marc weintraub's Garage

I'm with Warren on this one. Also, My 73"S" has 2.2"S" pistons in it, with this being the only change, my new compression ratio is somewhere around 9.1:1 essentially becoming a median between the two respective ratios. This has helped with mid range power and the car absolutely flies.

All The Best,

Marc

Oldporsche · 05:51 PM

Well I dunno, however you can take the cam chains off a '72 911T and turn the crank over without hitting anything. That I do know!

I also know that Marc is right. I have an acquaintance whom I believe races in the PCA events with a set of 2.2 pistons in a 2.4 for the very reasons he mentions.

Good luck,
David Duffield

T Bird · 10-04-2002, 04:54 AM

Guys
I don't have the current Stoddard catalog here at work so I can't check, but I do have the catalog from around 1993, with the sepia 73 S on the cover. The spec chart on page 103 shows the same data as shown on page 6 of the official 72-73 spec book, i.e., the CR for the 911/51 MFI is 7.5/1. Also, on page 50 of the catalog, Stoddard lists all the piston cylinder sets with factory part numbers and shows that for the 2.4 MFI T, the set (911.103.942.01) has a CR of 7.5/1.

As I noted in my earlier post, the information on page 12 of the spec book showing that the USA 2.4 T had a CR of 8/1 is a reference to the 73 CIS model. If not, then all of the other "official source" data in the spec book (i.e., pages 6 and 22) and the factory workshop manual are incorrect, as well as the data in the 2.4 T owner's manual and factory sales literature. I believe even the R&T road test of the 72 E indicated that the 72 2.4 T had a CR of 7.5/1. I think it is unlikely that all of these sources are incorrect, based solely on the ambiguous reference on page 12 of the spec book (which is explainable), but each is free to believe what they will.

Jim Richards · 10-04-2002, 05:01 AM

Will the E pistons & cylinders have any effect on performance in any part of the power band?

T Bird · 10-04-2002, 05:18 AM

Jim
Yes. The higher CR will yield more power and torque. The bump from 7.5/1 to 8/1 is probably good for 5 -7 HP. When I installed the 8.5/1 S pistons, I recon I got about a 10-15 HP bump, and I could feel it, especially from 2500 RPM up.

Wayne 962 · Wayne 962's Garage

I'm with T Bird on this one. I've been working on this table for the book all this week. In my table the MFI cars are 7.5 and the CIS are 8.0:1

In this 1973 book, there are a lot of confusing things. Like which plugs does the CIS engine use? I had to look at several different sources...

The book is not clear because the CIS motor is the 1973T, the MFI USA motor is the 911T-E, and the carb Euro motor is 911TV-E.

I believe that the 8.0:1 CR is for the CIS engines only...

-Wayne