87Porsche951

I was wondering whats the maximum displacement the 951 engine can be increased to? What is the maximum displacement the motor can be increased to safely without major reliability issues?

I appoligize if this question has been asked before!

Also i want to ask who sells brand new reproduction 951 blocks? Reason i ask this is i want to increase displacement but i dont want to have my car in peices for along time so i can get a block worked on and build from that engine and then take the bolt ons from my current engine onto the new engine so that way the car see's little down time.

rage2

I bought a used bare block for $50 and been building my motor on the side. 106mm bore for 2.8L. If I go with the 968 or 944S2 crank, It'd bump it up to 3.1L. That's probably the biggest size I'd go.

I remember Huntley talking about 3.3L 944 motors, but don't remember much about the thread.

Matt H

I have seen reference to 3.4L and I would LOVE to know the answer to that question as well. I have never received a straight answer. I think that is because no one really knows, most people dont go beyond 2.8 on the stock block even though I have seen a few 3.0's.
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J Chen

Theres this thing called the
point of diminishing returns
guys. Has anyone of you ever
given the thought to the
following questions :
1. The weight of the pistons
2. Secondary harmonics issues
3. With such a big surface area,
is combustion as efficient.
4. What about detonation.

To date, I have not recall reading
any threads about power output
or reliabilty issues with big bore
conversions. Maybe someone can
shed some light on this.

NZ951

J, I am pretty sure there are posts on HP gains... How can you measure combustion efficiency and harmonics? The weight is surely a non issue...

J Chen

Hi NZ,
At what boost level ? Anyone don't
an A/B comparison between 2.5L
vs 2.8L ? Obviously torque will
increase but is it proportionate
in terms of capacity increment ?
Personally, going big bore would
be better served if the 16V heads
are used as well.

rage2

There's no replacement for displacement! More cc's mean more power on the same pump gas. As to your questions...

1. The 106mm JE pistons feels a lot lighter than the 100mm stock pistons. I haven't officially weighed them though, but my guess would be the JE's are about 30% lighter.
2. Secondary harmonics? Care to explain more?
3. Not sure about that, but the 300cc increase should overcome any cylinder head inefficiencies, etc.
4. What about detonation? Any motor can detonate if not tuned right.

I haven't seen many threads regarding 2.8L reliability either (other than tuner's shop cars). I hope to be one of the few that make it out OK . I still have my 2.5L block as backup if I fail...

NZ951

Can someone simply explain the easiest way/s to get a 3.0 in my current 2.5?

J Chen

Hi Rage,
Agreed, more CC's equal more power
on the same pump gas. But at the
same time, it it also depends on the
energy that's created during the
combustion process. I don't think that
there is another 2.5L or 3L 4 cylinder
engine out there. Porsche did it for
cost reasons. If big 4's are the way to
go, why are'nt other manufacturers
persuing it as well ?
By defination, detonation means an
uncontrol explosion. Yes all engines will
detonate if improperly tune. What I
saying is at what ignition timing,
compression ratio & boost level
will it tolerate before detonation
occurs ?
The above mentioned will also have
to do with the head design & the
distance the flame front will have to
travel plus the time needed for
complete combustion. I think that by
increasing the diameter of the piston
in theory may be compounding the
problems even further
Secondary harmonics, that report will
have to come from you once your engine
is up & running. This is the reliablity factor
that I'm talking about. Lighter pistons
should help matters.
Please let us know how your engine turns
out. Yes what I'm saying are all speculations
but it's not without any foundation.

87Porsche951

A 3.1L+ Turbocharged Inline 4 would be a beast. I saw the dyno chart for John Anderson built all bore 2.8L 951 motor and it made some crazy tq. wonder what the power & tq numbers would like like w/ 3.1L and bigger.

I think companies dont make big displacement 4 bangers cause its easier & to make the same amount of power with more pistons. I would guess that a 6 cylinder at the same displacement as a 4 cylinder would make its tq lower in the rpm too.

Im just a fan off BIG displacement 4 cylinder engines.

87Porsche951

If you have the engine balanced wouldnt that help with harmonics? I would think anybody doing a rebuild or building a engine would balance the engine anyway.

J Chen

Static balancing & dynamic
balancing is a totally different
kettle of fish. Dynamic balancing
is very difficult to achieve when
you take into account the force
exerted during each power stroke.
That force in itself creates a huge
amount of harmonics which needs
to be dampen. It is even more
difficult on an auto engine because
the rpm of the engine is never
constant. This why you see the
balance shaft being adopted but
it is only able to cancel vibration
at a certain rpm

J Chen

Oh I forgot, crazy torque but
how long does it last. This
question to date has not been
answered. 5K/10K rebuilds
are not what you & I are looking
for right ?

BTW Rage,
How much did it cost you for the
sleeve & pistons ?

Danno

"2. Secondary harmonics? Care to explain more?"
"If you have the engine balanced wouldnt that help with harmonics?"

No, balancing parts will have zero effects on secondary harmonics. Ok, we're back to primary & secondary moments and forces on the crank again. Please review this thread where I threw up a couple of pictures on how that works: Balance Shafts. I'll expand a little on the secondary inertial force here and explain how weight and balancing has very little to do with secondary harmonics.

First, the assumption is that you're gonna balance all the pistons to each other and all the rods to each other, that's a piece of cake, and a lot of people don't even do that!

Then you have to balance the various parts together so that they work in harmony. Let's go over primary static balance:

A seemingly easy balancing act on a per-journal basis such that the counterweight applies the exact torque on the crank as the other side with the rod-journal, rods and pistons.

However, if you get lighter pistons and rods, the assembly is no longer in balance. You then have to lighten the counterweight to match:

But if you lighten the crank and the counterweights even more, then you no longer have balance on a per-journal basis because the pistons & rods will have higher torque than the counterweight. This would be the case if you got 1-lb lighter pistons & rods per jjournal, but removed 4-lbs per journal from the counterweights.

However, in that case, you can still achieve good balance between pairs of journals:

In effect, you have two imbalances that cancel each other out, at least statically. But when you start spining it, this causes a wobbling motion that adds torsional stress on the crank. It might be worthwhile in a racing engine where every little gram of high-speed rotational mass counts, but you have to balance it with durability.

Ok, so you've got good static balance, so let's start spinning the thing around:

When the adjacent rods and pistons are opposed to each horizontally, the forces are fairly well matched. This is where balancing and matching parts makes a difference because the forces are in opposition and cancel each other out (or not cancel if the parts weigh differently).

However, the problem comes in when the journals are moving horizontally:

The shaded areas represent the swept path of the con-rods as it swings from 45-degrees BTDC to 45-degrees ATDC. As you can see, they cover different pathes through space in relation to the crank-center. Additionally, the vertical movement of the piston is different depending upon whether it's at TDC or BDC for the same 90-degree angular sweep of the crank (distance X). One result of this difference in motion is that identical-weight pistons have different accelerations and motions simultaneously on adjacent crank journals. This results in different forces on the crank, thus an imbalance, but it's small due to the minute motion of the pistons at TDC and BDC.

The other more significant effect is the forces from the con-rods themselves, which moves in completely different motions:

To figure out the difference in force from the con-rod at TDC vs. BDC, you have to integrate the polar-moment of inertia of the con-rod itself as it sweeps through a certain angular rotation. In the above example, I just made it easier to illustrate by calculating the polar moment inertia of the centroid:
.
Even if the two con-rods at TDC and BDC are the same mass, they swing through at different distances from the crank center, thus exert different forces and torque.

In this case, there is absolutely no way to counteract the different paths and motions by balancing. It's like trying to twirl a barbell with identical weights, but your hands are placed at 1/4 the distance from one ball, and 3/4 the distance from the other ball. Having the ***** be exactly the same weight will be of no help.

Now the problem comes in when you make a 4-banger of larger displacement. The heavier pistons magnifies the differences in piston speeds at TDC vs BDC. The longer crank-throws exacerbates the differences in horizontal forces of the TDC vs BDC con-rods (heavier too).

BTW - Back to the original question, you can make a 3.3L 951 engine. This is a with a 110mm bore using the S2/968 crank. You'll need to grind little channels on the side of the block to clear the wider crank throws. The problem with such big-bore engines is that the flame-front has to travel so far to get to the edges of the combustion chamber. The BANG of fully-combusted gases is lessened because by the time it reaches the far corners, because the initial burn will have been over. This can be improved by a centrally-located spark-plugs with 4V heads, or a twin-plug set-up. But the most optimal shape for a combustion chamber is a perfect sphere. Wide flat pancake ones are not so good. Even worse are crescent shaped chambers which are caused by large included valve-angles on DOHC engines with convex piston domes reaching into the chamber .

"Can someone simply explain the easiest way/s to get a 3.0 in my current 2.5?"

Bore it out to 110mm and get larger pistons. Can use the same rods, same crank. The really cool part about this is you're not going to increase piston speeds. And you can expand the combustion chambers in the head for more flow around the outsides of the valves, AND fit larger valves too! Big-bore engines are easier to get high-HP from than stroker motors.

87Porsche951

Whats the max the bore can be increased to? What do you estimate the hp & tq would be with a 3.0L & 3.3L running no more than 18psi boost? I know there really isnt no way of really knowing without actually dyno but just want to know a questimate!

If you go with stock stroke is there anyone who makes a lightweight aftermarket crank at the stock stroke for the 951?

If you go with stock rod length it would make more sense to go with say Carrilo rods with stock length correct? I would think they would be stronger.

Im just thinking about long term reliability!