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Join Date: Oct 2002
Location: South Carolina
Posts: 133
Less exspensive 2.8 upgrade idea......

DISCLAIMER:
This is just an idea at the moment there are a lot of holes but it seems like it MIGHT be possible at first glance. I WOULD LOVE TO HEAR YOUR THOUGHTS PRO OR CON. This isnt something I would do unless It was time to go ahead and rebuild the motor anyway.


I have been toying with the idea of changing my 2.5 liter engine to 2.8 without changing to the exspensive 3.0 crank. I have talked with a machine shop here and think we have found a way to do this for $1500.00 including new pistons and machine work. This of coarse does not include labor to remove or perform work on the engine. It seems we do have enough meat in the walls to do this work and we will be getting away from having to use expensive Porsche pistons as we are inserting a cast iron sleeve. We are also thinking of connecting the cylinders by welding in some support webbing (A little idea I am playing with.) Here are the specs I have worked up so far ------


Porsche 1986 951 Specs
Bore x Stroke: 3.94 x 3.11 in, 100.0 x 78.9mm
Displacement: 151 cu in (2479cc)
Compression ratio: 8.0:1
Turbocharger: KKK
Waste gate: Porsche Maximum Boost Pressure: 10.9 psi
Valve gear: Belt-driven single overhead cam, hydraulic lifters
Power (SAE net): 217 bhp @ 5800 rpm Torque (SAE net): 244 lb-ft @ 35

Forged Pistons available -
4.155inches = 105.537mm
Cost $250.00 for 4 race pistons (I have a source)

Machine block and insert cast iron sleeves for new 4.155 pistons.
Weld in support webbing between cylinders to keep them stable under high RPMs.
Machine piston end of the stock 951 rods to fit the new 4.155 pistons
Deck pistons to equal 8.5:1 compression ratio

New Specs
Bore x Stroke: 4.155 x 3.11 in, 105.537 x 78.9mm
Displacement: 169 cu inm (2764cc)
Compression ratio: 8.5:1
Waste gate: Porsche Maximum Boost Pressure: 10.9 psi
Valve gear: Belt-driven single overhead cam, hydraulic lifters
Power (SAE net): ??? bhp @ ??? rpm Torque (SAE net): ??? lb-ft @ 35

Change to engine
Number of Cylinders are = 4 to 1
Piston Diameter = 4.155 Inches
Engine Stroke = 3.110 Inches
Computation Results:
Computed Engine Displacement (CID) is 169
Computed Engine Displacement (Litres) is 2.8
Computed Engine Displacement (CC) is 2764.1

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Randy Wilson
Columbia South Carolina
"SPEED COSTS! How fast do you want to go???"

1986 Porsche 951
1986 Porsche 928 S

Last edited by Randy803; 12-08-2004 at 05:24 PM..
Old 12-08-2004, 12:38 PM
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This is a very cool idea. The thing that I am skeptical of is how you can get an engine sleeved and all of the other machine work done for $1500? That tops some of the highball figures I have seen just to have a block sleeved.

However I am very interested in the process seeing as how I have my motor out for a complete rebuild and a parts motor sitting in the corner just begging for something crazy.
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Eric Devansky
986s - Radio Flyer
20% of a 951 parts car remains
Old 12-08-2004, 01:22 PM
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The machine work pricing is a friend of mine so I need to see what he is going to charge the average joe to do this work. Honestly if I can find a nice big milling machine (I sold mine a year ago) I will start doing the modification myself if it works. I already have a large hydraulic press to insert the sleeves and a nice lathe to face the pistons. What I need to find is a cheap donor motor to experiment on before attempting to do this work for anyone! I am going to start looking for one. Does a 944 engine have the same bore and stroke as a 951? If so I can pick one up pretty cheap, that would be a fast 944 if the mod works out! I know that I could perform the work fairly cheap as boring isnt that big of a deal its one of the simpliest of machine operations and pressing in a sleeve is cake. Modifying the rods is where I am going to be a little challanged and figuring out how much metal to face off the piston to get the correct compression will be trail and error unless someone has a math formula to help me figure it out.
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Randy Wilson
Columbia South Carolina
"SPEED COSTS! How fast do you want to go???"

1986 Porsche 951
1986 Porsche 928 S

Last edited by Randy803; 12-08-2004 at 05:21 PM..
Old 12-08-2004, 05:17 PM
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The more I research this the more exspensive it becomes. It might end up being more practicle just to buy the $5000.00 kit with the 3.0L crank at this point and build a stroker. Doesnt mean I am not going to play if I can find a spare 951 engine.......
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Randy Wilson
Columbia South Carolina
"SPEED COSTS! How fast do you want to go???"

1986 Porsche 951
1986 Porsche 928 S
Old 12-09-2004, 03:58 AM
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I have the 2.8L big bore motor done. Believe me, it costs a LOT more than $1500.

I budgeted around $8k, and went well past that. The 2.8L stroker definately a cheaper option, but has it's disadvantages.

Some notes, I strengthened the sleeves by engine cementing the bottom half of the water jackets. Also, I did not have to modify the stock rods at all to use the 106mm pistons (JE custom units from Anderson Motorworks). With iron sleeves and forged pistons, it requires a fairly large piston to wall clearance, so ring gaps are very important. This is why most 2.8L big bore engines fail, because of improper ring gaps and piston to wall clearances. Highly suggest you go to a shop that specializes in 2.8L big bore motors, because the specs to do it properly is fairly secretive. You can probably figure it out yourself, but it'd take a few tries, pulling engine, regapping the rings... etc. The reason for this is because JE's specs that they provide with the pistons are incorrect.

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rage2
87 951 2.8L Big Bore

Last edited by rage2; 12-09-2004 at 01:07 PM..
Old 12-09-2004, 12:58 PM
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