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Old 10-30-2022, 07:02 PM   #1
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Engine swap help

Putting the finishing touches on a 2.5 to 2.7 engine swap. Here are the important facts:
1) I am using the stock 2.5L wiring harness. to do this I had to change the coolant temp sensor and use the 2.5 exhaust so that the o2's would be in the correct place (may change to an aftermarket after I work out the bugs.

2)It will be a 5 speed as that is what the car (99 Boxster) came with and it makes for no pedal work.

3)I am using the throttle body and the intake crossover tube from the 2.5 as the front crossover tube (I've already cut the proper size reducer boots to make it fit) and the back crossover tube from the 2.7 with the butterfly valve removed and holes plugged. This is because I needed to retain the cable driven throttle body.

4)Stock DME 5.2.2

5)Using the 2.5L fuel rail as the 2.7 was returnless and I didn't want to even think of the nightmare changing the fuel system would have been.

6)Using the 2.7L injectors because I have been told they will work.

Questions:

Vac Ports: I am having trouble figuring out the vac lines. I understand that the 2 main vac sources are ports in 2 of the 4 Intake boots. Which boots have the ports? I mixed mine up during disassembly. I believe they should both be on the front crossover (throttle body) as I think the changing airflow on the rear crossover with the butterfly valve would impact.

Fuel Rail: when bolting the fuel rail from the 2.5 to the 2.7 intake, the injectors don't line up. if you put the injectors in then you cannot bolt down the fuel rail. off by about 10mm. What have people done in the past to overcome this? I'm thinking of grinding off the mounting brackets and re-welding them in the proper location where all will line up.

What have people done for the lower mounting of the throttle body as the tube is farther back than on the 2.5? I assume I will have to fabricate something.

See pic below -
Would assume that all of the vac lines going to the Vac Unit K would all be eliminated and H would plug into the 3rd port of G.

Where does E install? clip into the intake slot?




Feel free to ask questions as to why I am doing things this way. Mostly the answer will be that I want this to go together as inexpensively as possibly then put money into working out the bugs. I already think that once I know everything runs, I will reinstall the butterfly valve in the rear crossover. the an exhaust upgrade. car needs some body work too and am attacking one thing at a time.

Note: I know I am calling the front crossover the "rear crossover" and the rear crossover "the front crossover". just used to the throttle body being in the front of the engine of the stuff I work on usually. apologies in advance.

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Old 11-27-2022, 12:17 PM   #2
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This is in reply to the message...
Yes, you no longer need to run vacuum to the flapper valve, so just use a 3 way coming from the vacuum source (H into G) as you said.

For the fuel rail, the simplest way to resolve it is to get a return type rail from a 2000 or 2001 model since your car already has all the return plumbing. There's probably some on eBay or I can sell you one or trade mine for yours (you pay shipping).

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Old 11-27-2022, 01:17 PM   #3
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You may find some useful information from Truegearhead's posts in mid 2019 when he did the same swap.
http://986forum.com/forums/performance-technical-chat/75535-2-7-swap-2-5-blown-motor.html
I did the reverse a few year ago and put a 2.5L engine into a 2001 base. I don't plan on buying any more 986s at the moment, but I think I've bought 18 of them now and have plenty of parts available from a couple 2001 2.7L engines I scrapped.
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Old 11-27-2022, 06:20 PM   #4
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Quote:
Originally Posted by 78F350 View Post
This is in reply to the message...
Yes, you no longer need to run vacuum to the flapper valve, so just use a 3 way coming from the vacuum source (H into G) as you said.

For the fuel rail, the simplest way to resolve it is to get a return type rail from a 2000 or 2001 model since your car already has all the return plumbing. There's probably some on eBay or I can sell you one or trade mine for yours (you pay shipping).


I’ll trade. PM me an address and I will send you mine off of the 2.5 and pay the return shipping on yours. Do you know if the crossover hose can be removed?
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Old 11-28-2022, 12:52 PM   #5
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This evening I'll get a look at the fuel rails and pick a good one to send. I'll message tonight some time.
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Old 11-28-2022, 05:33 PM   #6
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Here's what I plan to send:


That should be enough to replace the fuel rail on the 2002 engine and connect to the hoses in a 1999.

Sending a PM with address info...
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Old 11-28-2022, 05:53 PM   #7
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Regarding the cross-over tube with the flapper valve, if you are running the car on your stock DME from the 2.5L, you can either leave the valve closed or force it to stay open after removing the vacuum hose that controls it. I think that closed is the better option. Here's a post I did back in 2015 that discusses the function of that valve:

Quote:
Originally Posted by 78F350 View Post
Try looking up 'resonance flapper valve' for more info.
I think that any air going in or out of that hole is negligible unless you have excessive wear.

Quoted from: http://press.porsche.com/archive/products/press_kits/press_kits_2001/PDF/Boxster_in_Depth.pdf

"The Boxster inherits its twin-resonance air induction system from the 911
Carrera. The system acts as a “resonance supercharger,” allowing the engine
to draw from higher velocity airflow at certain engine speeds. A crossover pipe
connects the individual air collector/resonance chambers for each cylinder bank.
A flap in the pipe remains closed from idle to about 3,100 rpm. When it opens,
each cylinder bank can draw from airflow “excited” by the resonance created by
alternating induction between all six cylinders. In essence, “dual resonance”
creates two induction paths for each cylinder. Below 3,000 rpm, the cylinders
draw air from a “short” path. From 3,000 rpm to about 5,100 rpm – when the
resonance flap opens – the cylinders draw from a long intake path, which
boosts torque. Above 5,100 rpm, the flap again closes to allow the cylinders
to draw intake air from a shorter intake path to boost horsepower at higher
engine speeds."

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