2.5L Twin Plug Rebuild Options

[CaryPhotography]

Hey Guys,

I'm new to the Pelican Parts forum and after seeing all sorts of great information I decided to join in to get solid information, bounce some ideas around, and get some help.


Down to business. The project I'm starting to work on is a 1970 911T which already has a fiberglass wide body kit, various performance upgrades, rear tires are a 285/40 15", and more importantly a 2.5L twin plug 13:1 compression motor with weber 40's. I also have the Bosch mechanical fuel injection system for the motor. Some 20 years ago, the motor was built by Wayne Baker of "Personalized Auto Haus" in San Diego. The motor has been seating for over 15 years either in a garage or in a storage unit and when I checked the other day, I could turn the fly wheel over by hand and oil (not water) poured out from one of the ports as I leaned the engine over. From what I'm told, that's half the battle.

The motor was pulled from the car because both work needed to be completed and the motor needed to be rebuilt. From what I'm told, this is a race motor that was detuned for street use, meaning rebuilds are generally done due to hours of use and not miles of use. The motor was functioning when it was pulled, just needed a rebuild to freshen it up.

Unfortunately, my budget is tight, VERY tight which seems dumb because tight budgets and Porsches do not go hand in hand. As much as i want to rebuild this engine back to it's original glory, my check book simply can't hand that kind of stress. The big question is: Is there a way to freshen up this motor without doing a full rebuild? Could it be as simple as replacing main bearings, seals, and pistons rings in order to get this beast running again? I don't want to but I may be forced to, put the Bosch mechanical fuel injection system up for sale in order to afford a rebuild.

The other idea I've had, sell the motor with the fuel injection system and purchase a stock 3.0L so I can get the project going sooner but I really do not want to lose these twin plug heads.

This may seem like a stupid question but... Has anyone driven a twin plug engine and how would it compare to a stock 3.0L?

[Flieger]

The MFI and 2.5 will have a character that cannot be duplicated with the 3.0. That is a very special engine you have there and it would be a shame to part with it. I would just save up to do things right. But that is just my opinion.

Nice project!

(If it is really 13:1 compression you will be running race gas even with twin plugs. If you want to run on 91 E10 you will need to tone it down to 10.5:1 or so. The cams make a difference as well. A racy cam like a 906 (or even a mere S cam) will allow you to use lower octane at the revs you would normally see on the street. This is due to the overlap taking away some of the compression at lower revs.)

[CaryPhotography]

That's what I keep getting told, this is a very special engine. The MFI was not used for sometime because it needed to re-calibrated and in order to save time and cost, the dual weber 40's were installed. When the car was being raced it was running a mixture of AV gas and 91 octane. As the final mixture, I'm not sure what that is. What about tuning the engine to E85?

Any idea on what a rebuild would cost?

[Flieger]

E85 would require new fuel lines and would not be good for the MFI due to its corrosive properties, but it would have the octane rating. Perhaps water/methanol injection like on a turbo would be enough? But you would really be going through that stuff. Wouldn't want to run out.

[CaryPhotography]

I'm already planning on replacing the stock fuel tank and fuel lines because the car has been seating for so long. The goal is a 15 gallon capacity.

What do you think about just rebuilding the Weber 40's instead of running the MFI?

[Flieger]

Well, I am an MFI person. You will find that most users of MFI are fanatical about it and think there is no substitute. MFI has throttle response and a character that carbs can't match, and you can't match a properly set up MFI system's performance with EFI unless you go to modern, individual throttle body, sequential injection. Most of the problems people report with MFI are due to worn (due to miles/age) components that are no longer in spec. You can read up about "Check, Measure, Adjust" though. And there are places like Eurometrix and Pacific Fuel Injection, or Mark Jung here on the board, who rebuild the components back to spec. My throttle bodies are worn and the idle is high but it is plenty fun above 4000 rpm once it comes on cam.

That said, you can still get most of the way there with carbs.

Given that your car is a 1970 911T, it did not come originally with MFI as an S would have, so selling the system would not be such a big deal as if it were original to the car.

I have never heard of running MFI with alcohol, but I think the Indy engine, later used in the 936 and 956 on gasoline, may have used MFI with methanol.

If you do convert to E85 with MFI you will really be facing an uphill battle since you will not really find any information to help you from others who have already done what you are trying. Conversely, if you want to refurbish the MFI there is a wealth of information here to do so, and you will find many people willing and able to help.

I suggest you also join the Early 911S Registry and check out their forum. There are lots of experts about the early cars there, and MFI. Don't let the name fool you, they allow all early 911s- T,E, and S to join. I am a member and my car had been updated before I bought it to the short hood body style. And my engine is not original either. It is a 2.7 RS spec (with MFI ). Your twin plug, high compression, short stroke is something I would like to have. At least the twin plug and slightly higher compression. The short stroke would be more in keeping with the 2.2 that would have originally been in my car.

[CaryPhotography]

From what I gather, the MFI's are quite magical. When I get the information back about the engine later this week I'll make a decision which way I'll go for the rebuild. I'd like to run the MFI so I can extract every bit of potential out of the engine.

[CaryPhotography]

Does Mark Jung have a different screen name or is it just "Mark Jung"?

[Flieger]

"356RS"

Steve Weiner ("Steve@Rennsport" I think) at Rennsport Systems and Henry Schmidt ("Henry Schmidt" or "Henry_Schmidt" I believe) at Supertech also can refurbish and tune MFI. They are regular posters here.

[CaryPhotography]

Hey Guys, It's been a while since I posted anything about the project. Life has slowed down the rebuild process. Thankfully, I have started working on the project again.

After some research and discussion I've come to the conclusion that rebuilding this engine to it's former glory is not going to suite the plans I have for the car. Ideally I'd like to build a street legal track car that can withstand city driving. Running a high compression race engine is not exactly the greatest of ideas in those types of setting. With that said, I've given serious thought to getting a 3.0L single plug engine or a 993 3.6L conversion setup. Any thoughts on that?

As far as the engine parts go, I split the case, removed the crank, and pulled everything apart. Internally, everything looks great.

For a while, the big question was, are the heads still usable. To answer that question I spoke with the team over at TLG Service (amazing shop) and had them inspect the heads. After inspection we found out that the heads are still useable and from there we made the decision to get the heads rebuilt. The goal now is to sell the complete MFI system paired with the twin plug heads. Any suggestions on that approach?

[Walt Fricke]

Why not sell the whole engine? Someone like Ron Mistak down in San Diego, who has a Wayne Baker 914 race car already, might want it whole - even if it is a box of parts.

A stock 3.0 would be a pretty easy conversion, and not cost an arm and a leg. Car won't be super fast on the track, but not a slug. Lots of guys seem to like installing 3.6s, usually from 964s. I suspect the 993 motors command a premium over the 964s. But the 964s, stock, in a lighter early chassis haul the mail. Like everything, it is largely a question of cost.

The twin plugs don't, by themselves, make the engine more powerful or otherwise better. What they do allow is running higher compression with the same quality of gasoline, typically pump premium. That's the only reason anyone would want these heads, really - to build a race motor. And these heads can't be used on an engine larger than 2.9 liters, on the cases through the 2.7s. The 3.0s used a wider head stud spacing.

[CaryPhotography]

Would anyone happen to have a good idea on street value for an original Bosch HBF MFI system with twin plugs heads? (picture of recently rebuilt heads coming soon)

[CaryPhotography]

The heads are Alive!!

Over the weekend I picked up the twin plug heads from TLG and did some cleaning on the High Butterfly MFI system. Can someone help me determine the system based off the numbers found on the throttle body?

[356RS]

The high butterfly casting number 911.110.107.5R indicates a 2.8 - 3.0 RSR's. Both the 2.8 & 3.0 had 43mm to 43.5mm ports with a 50mm top opening. The ST high butterfly had the casting number 911.110.107.2R and had 41mm to 41.5mm ports also with a 50mm top opening.
What a difference the heads are now after seeing the first photos of them. They look great after a good cleaning and valve job. Casting numbers 911.104.307.0R with the 1819h marking on the heads indicate there early 72S heads.

[CaryPhotography]

Any takers on the complete system?

[CaryPhotography]

System is being put up for sale. PM for questions and more pictures to come.

[Boydyrs]

You have PM sir?

[CaryPhotography]

I have responded to your PM

[CaryPhotography]

Here is the ebay posting.

[CaryPhotography]

All right, it's been a while since I've posted anything about the project and that's mainly been to a busy work schedule and being 100 miles away from the chassis. With that said, the past few months there has been some momentum on the project. As some of you may or may not know but I decided to not go down the road of rebuilding the 2.5L engine. After numerous conversations with various sources and defining what the goal for the project this is what is being done.

Engine: 3.2L from an 85' 911 w/ Rebel S Racing semi hard engine mounts
Transmission: swap out the 901 for a 915 and Rebel S Racing Blue Transmission mounts
Suspension: Rebel S Racing bushing kit Front and Rear plus RSR front shock bushing set, Elephant spring plates, and a fresh set of Koni's.
Wheels & Tires: 17" cup wheels with Hoosier R6's

Still working on getting seats, painting the interior, adding a roll cage, and getting the chassis ready for the engine install.








Decided to make a stand to hold the chassis in order to help move the chassis around the garage with ease while everything was removed.