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Strengthen the crankshaft

Scotty

Silver Level Sponsor
The late 90s to mid 2000s au mod is Mitsubishi starion turbo pistons and rods with a 110 overbore to get a 2120cc based 1725

Do you know if that 2120 is obtained with the assistance of turbo and how reliable would this engine be at that overbore? Is there anything special the engine needs beyond those components? (Crank, rods, etc)

I’m kicking the can on ideas but I’m dead set to build some sort of ‘goer’ out of a 1725 and want to try something either proven or practical. I don’t want to build a grenade lol.
 

Barry

Diamond Level Sponsor
Do you know if that 2120 is obtained with the assistance of turbo and how reliable would this engine be at that overbore? Is there anything special the engine needs beyond those components? (Crank, rods, etc)

I’m kicking the can on ideas but I’m dead set to build some sort of ‘goer’ out of a 1725 and want to try something either proven or practical. I don’t want to build a grenade lol.




2120 cc refers to engine displacement which is a function of bore, stroke and the number of cylinders. Engine displacement has nothing to do "... with the assistance of turbo ...".

WSM-145 states that the maximum overbore for a 1725 engine is 0.040". An overbore of 0.110" strikes me as being iffy with regards to cylinder wall thickness and head gasket longevity, but it's your engine and your money.

FWIW, an overbore of 0.110" with a factory 1725 crankshaft would only result in a displacement of about 1844 cc. It would take an overbore of 0.110" AND a stroke increase from 3.25" to about 3.74" to produce a displacement of 2120 cc.
 

puff4

Platinum Level Sponsor
Shot-peening will help relieve internal stresses and reduce the chance of fracture.
 

husky drvr

Platinum Level Sponsor
Scotty,

This is Brian's (TIGRETR) link to the story of his Alpine and what he did to his engine to support his turbo installation. Very impressive.

http://www.turbosunbeamalpine.com/

As far as building an Alpine engine, the basic "have to do's" are:

> Engine breathing improvements

> Increase displacement

> Increase compression ratio - both static and effective - to the limit of the fuel you will use

> maybe the most important - Decrease both rotating and reciprocating weights as much as possible while maintaining reliability and safety

Keep in mind the limits of modifying an engine are effectively, the further from stock configuration the engine is modified, the more custom the parts become and the sooner you'll find the next weakest link. The fact that the Alpine engine has barely any support in the performance area doesn't help.

Hope this helps,
 

alpine_64

Donation Time
2120 cc refers to engine displacement which is a function of bore, stroke and the number of cylinders. Engine displacement has nothing to do "... with the assistance of turbo ...".

WSM-145 states that the maximum overbore for a 1725 engine is 0.040". An overbore of 0.110" strikes me as being iffy with regards to cylinder wall thickness and head gasket longevity, but it's your engine and your money.

FWIW, an overbore of 0.110" with a factory 1725 crankshaft would only result in a displacement of about 1844 cc. It would take an overbore of 0.110" AND a stroke increase from 3.25" to about 3.74" to produce a displacement of 2120 cc.
Barry,

The blocks can take it but best to sonic test the walls.

Regarding headgaskets, the stock ones can not be used as the bore execeeds the firing ring of the stock gasket.

The longevity is fine, iirc the rods and pistons are lighter and stronger than the stock rootes parts ( though thats quite comon with more modern and after market parts)

The pistons if using the mistu ones are best to be machined down little as the turbo ones are heavily dished for the original boosted application.

Also regarding the advice of the stock 1725 motors overbore being a max of .040.. That seems very odd as the bore in the 1725 is the same as 1592 amd both were rebuilt in period with .060 pistons... Infact thats the easiest size to get and still works with stock head gaskets.
 

alpine_64

Donation Time
Do you know if that 2120 is obtained with the assistance of turbo and how reliable would this engine be at that overbore? Is there anything special the engine needs beyond those components? (Crank, rods, etc)

I’m kicking the can on ideas but I’m dead set to build some sort of ‘goer’ out of a 1725 and want to try something either proven or practical. I don’t want to build a grenade lol.
Scotty,

With no disrespect to you, there seems to be some confussion your side as to the internals of the motors and terminology.

Per my post the 2120cc motor requires the mistubishi pistons and rods and of course the required over bore and stroking of the rootes crank.

The motor is N/A not boosted.. The rotating parts are from a boosted car. The 2120cc is the displacement .. As barry explained nothing to do with induction system.

Given some of the above.. You might be best to do a chev rod 1725.. There are many members who have done it and can help.. There are guides nd writeups and it provides a reliable and peppy motor. That with the head mods like vizard chambers.. Bigger valves and porting along with bigger cam and carbs.. Electronic igntion.. You will have a fun ride

Just my .02
 
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Bill Blue

Platinum Level Sponsor
Scotty,

With no disrespect to you, there seems to be some confussion your side as to the internals of the motors and terminology.

Per my post the 2120cc motor requires the mistubishi pistons and rods and of course the required over bore and stroking of the rootes crank.

The motor is N/A not boosted.. The rotating parts are from a boosted car. The 2120cc is the displacement .. As barry explained nothing to do with induction system.

Given some of the above.. You might be best to do a chev rod 1725.. There are many members who have done it and can help.. There are guides nd writeups and it provides a reliable and peppy motor. That with the head mods like vizard chambers.. Bigger valves and porting along with bigger cam and carbs.. Electronic igntion.. You will have a fun ride

Just my .02
So how does one go about stroking and living with a weak `1725 crank stroked 1/2 inch? Very carefully, I assume.
Bill
 

Barry

Diamond Level Sponsor
So how does one go about stroking and living with a weak `1725 crank stroked 1/2 inch? Very carefully, I assume.
Bill



Very carefully indeed. Increasing the stroke of a 1725 crank from 3.25" to 3.75" by offset grinding would mean reducing the rod journal diameter from 2.125" to 1.625".
 

Scotty

Silver Level Sponsor
Scotty,

With no disrespect to you, there seems to be some confussion your side as to the internals of the motors and terminology.

Per my post the 2120cc motor requires the mistubishi pistons and rods and of course the required over bore and stroking of the rootes crank.

The motor is N/A not boosted.. The rotating parts are from a boosted car. The 2120cc is the displacement .. As barry explained nothing to do with induction system.

Given some of the above.. You might be best to do a chev rod 1725.. There are many members who have done it and can help.. There are guides nd writeups and it provides a reliable and peppy motor. That with the head mods like vizard chambers.. Bigger valves and porting along with bigger cam and carbs.. Electronic igntion.. You will have a fun ride

Just my .02

No disrespect at all, I appreciate your and others advice and taking the time to talk to me about Alpines in general.
Scotty,

With no disrespect to you, there seems to be some confussion your side as to the internals of the motors and terminology.

Per my post the 2120cc motor requires the mistubishi pistons and rods and of course the required over bore and stroking of the rootes crank.

The motor is N/A not boosted.. The rotating parts are from a boosted car. The 2120cc is the displacement .. As barry explained nothing to do with induction system.

Given some of the above.. You might be best to do a chev rod 1725.. There are many members who have done it and can help.. There are guides nd writeups and it provides a reliable and peppy motor. That with the head mods like vizard chambers.. Bigger valves and porting along with bigger cam and carbs.. Electronic igntion.. You will have a fun ride

Just my .02

No disrespect at all. There's still a lot I'm learned and a lot I'm being far too overly cautious about because I'm concerned about somehow damaging my Alpine, why I ask some of the dumb questions that I do because... you never know and some of what I've learned has really surprised me.

I really appreciate you and others taking the time out to explain things like this to me. I'm in the process of going to school for automotive and I'm at the start of that journey, so my jargon and terminology are terrible, I know this, and I appreciate the patience I'm being shown by this group.

Like I said earlier, I'm kicking around ideas. The Alpines are really a 'meat and potatoes' kind of car that you can work on for the most part yourself and get good results. I'm pretty much a shade mechanic at this point and I got a pretty good reputation for patience and research, maybe too much of that. My uncertainty comes from lack of experience and lacking the ego to go 'oh I know this because it's like that'. I know better lol.

It's funny you say Chevy rods because that's what my machinist put in my Alpine. Was going through the recepits today for my machinist (Carl Stirtz) and he had installed Chevy Rods and a few other things. I had told him I wanted a reliable Alpine motor and I guess he delivered. I kept hearing how 'gutless' Alpines are supposed to be from others in the real world and I just don't see it with mine. She's not a fast car but she goes when I want her to and she's really nimble and I'm just pleased with the car. She don't look like much, but she's a go-er, not a show-er... at least for now.

I intend to build a second engine this year mainly to show people around where I live that it can be done and Alpines aren't some scary-complicated thing to work on. I get that from most of the mechanics here and I'm sure it's due entirely to ignorance. Plus I want a second motor anyway, something special.

So I'm curious: If the Alpine 5 main bearing crank is made weaker than the 3 bearing cranks, why didn't they or someone fabricate a steel 5 main bearing crank yet? Or is it more complicated than that? Just curious.
 

alpine_64

Donation Time
So I'm curious: If the Alpine 5 main bearing crank is made weaker than the 3 bearing cranks, why didn't they or someone fabricate a steel 5 main bearing crank yet? Or is it more complicated than that? Just curious.

The 5 main hs an oiling issue at #3 and this can be resolved with some mods.
The 3 main revs better as its got less beatings to provide resistance.. Its my understanding that the 1725 stroked crank gained the 2 extra bearings for smoothness over the 3 main due to the stroke...
MGB b series engine 1798cc had both 3 and 5 main verisons.. 3 is known as the revvier one.. But with the B its the same displacement... Rootes upped the displacement with stroke...

As For better cranks... I know of 3 Alpines that had custom billet cranks for racing... High dollar motors.. Crank alone more than most peoples full motor rebuild....

The demand isnt there for such parts in the normal market.

my machinist (Carl Stirtz) and he had installed Chevy Rods and a few other things.

When your mechanic did your motor what was done to the head? What about the flywheel?

If you really want to wake the motor up the head is the key.. Porting.. Combustion chamber.. Valve size.. Exhaust and bump the carb spec... And up the cam spec too.

These will give power... The bottom end needs to be built well and balanced to keep it together...

If you lighten the flywheel you will get a more responsive motor... If you lighten And balance all the rotating assembly....more so
 

husky drvr

Platinum Level Sponsor
The 3 main revs better as its got less beatings to provide resistance.. Its my understanding that the 1725 stroked crank gained the 2 extra bearings for smoothness over the 3 main due to the stroke...

Michael,

Smoothness might have been a part of the reason for the change. I've an idea that the biggest reason for the change is "cost cutting". Rootes ( Chrysler ) also changed from a forged three main crank to a cast crank for the five main engines. It would have been nice if the forged crank would have remained available in the five main Alpines.

Just a thought,
 

Alpine 1789

SAOCA President
Diamond Level Sponsor
...I'm pretty much a shade mechanic at this point and I got a pretty good reputation for patience and research...
Scotty: I have seen (and ridden in) Bryan's car and his "engine" is an incredible piece of engineering. I put engine in quotes, because what he as done is so much more than just turbocharge an engine. He has completely re-engineered the car, which is appropriate, since he is an engineer. There may be a way to turbocharge a 1725 without doing as much as Bryan (as I recall, his goal was to create the most powerful engine possible with the original block), but I think what he has done is probably well beyond the vast majority of us.

And, truthfully, if you are willing to consider an engine swap, it isn't necessary to achieve neck-snapping performance. The sky is the limit here and most of what has been done by others should be well within your reach. My SV (Chevy rods, modified head, +60 pistons, lightened flywheel, Weber 36/36 carb) isn't neck-snapping by any means, but it is reasonably quick and fun to drive on country roads and keeps up on the highway. When I finally work my way to restoring my S3, I will be building a bigger, more powerful engine for that. I just have to decide how big and how powerful I want. And, of course, how much I want to spend. :)
 

spmdr

Diamond Level Sponsor
So, to get back to the OP, Crank weakness,

And how to make is stronger.

I think it can be assumed we are only talking about 1725 cast cranks.


Dave Anderson broke a 1725 cast crank, in his Vintage Race engine.

BUT, that crank was heavily stressed with high RPMs and power.

And, without giving up any National secrets, the crank Dave broke was no longer within factory STOCK dimensions...


So, How far down on the Alpine engine "Weak parts" list is the crank?

How many stock spec Alpine engines have had crank failure?

I would suggest few if any.


BUT, as you move up the performance ladder, the crank strength comes into play.

Also, in the performance window is displacement increases, via cranks with reduced dimensions (offset ground stroker cranks).

I can't see the point of taking a known weak part and making it weaker by stroking it.


I'm beginning to conclude the practical engine to build for performance is a 1592 with a steel crank.


Even though you start with a shorter stroke, the ability to stroke a 1592 is there.

And, at the end of it, you still have the steel crank!


At this point I get to over use the term "Win, Win".

DW
 

Shannon Boal

Platinum Level Sponsor
Crank weakness; isn't the big danger torsional vibration? If all four cylinders have equal power, this is the lowest stress for the crankshaft at a given power output. So, equalize chamber volumes, piston dishes, deck heights, port flow rates and manifold flow.
 
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Five Reviver

Silver Level Sponsor
As For better cranks... I know of 3 Alpines that had custom billet cranks for racing... High dollar motors.. Crank alone more than most peoples full motor rebuild....

The demand isnt there for such parts in the normal market.

Out of interest I enquired of one US crank specialist, who advised they have done a 5 bearing Alpine crank, current price US $2850, 14 week delivery time.
Not for me - I have spare bottom ends and the first one is still holding up - but I can share manufacturer details if asked.

David
 

alpine_64

Donation Time
Out of interest I enquired of one US crank specialist, who advised they have done a 5 bearing Alpine crank, current price US $2850, 14 week delivery time.
Not for me - I have spare bottom ends and the first one is still holding up - but I can share manufacturer details if asked.

David
That a billet crank?
 
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