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Spitfire & GT6 Forum

1296 SMALL CRANK OR 1500

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2manyspits Avatar
2manyspits Thomas Schlundt
south bend, IN, USA   USA
if you had a choice which would you prefer ?

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billspit Bill Kea
Moore, Spartanburg, SC, USA   USA
Depends on the car. A small crank 1296 for any round tail. 1500 for a square tail.

mkivmarty Avatar
mkivmarty Marty Yanik
N.E.Ohio, USA   USA
From what I've learned over the years. the small journal 1296 is the most powerful engine Triumph put in a Spitfire; all 75 horses of it!!

It's what I have in my MKIV. Runs great when built well.

Marty

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clshore Carter Shore
Beverly Hills, Florida, USA   USA
In reply to # 1497873 by mkivmarty From what I've learned over the years. the small journal 1296 is the most powerful engine Triumph put in a Spitfire; all 75 horses of it!!

It's what I have in my MKIV. Runs great when built well.

Marty

So your original MkIV 1296 big journal motor was swapped out at some point?

I think it depends on what you want from the car.
For 'normal' performance, the SJ 1296 is best.
But the superior torque of the 1500 can be an advantage if you want to cruise.

I deliberately chose the BJ 1296 as the basis for my Turbo project.
It has the strongest, stiffest crankshaft.
We will see how well that works out ...

claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
Hey Carter,

Do you know if there is any significant weight difference between a BJ and SJ 1300 crank.

I am considering my next engine build, and as I have never laid eyes on a BJ 1300 crank, am curious, as I also want the stiffest I can find, but mine is for a non turbo application.

clshore Carter Shore
Beverly Hills, Florida, USA   USA
The journals are larger, but the counterweights and connecting 'bridge' between the 1&2 and 3&4 rod journals are about the same.
So the BJ is heavier than the SJ, but not tremendously so.

I just weighed a stock BJ 1296 crank, 29.5 lb.
Don't have a SJ 1296 crank at hand, but I'm guessing 25 lb.

The crank stiffness comes from 2 factors:
First, the journal ID, torsional stiffness goes up as the 4th power of diameter: (1.875/1.625)^4 = 1.772
Second, the 'section overlap' between the main journal and rod journal, greater overlap yields greater bending strength.

It's worthwhile to note that .50 mm undersize of the Honda 1.889 journal is about 1.869, just a 0.007" custom crankshaft grind away.
Many racing rods can use the Honda bearing, most of them weigh around 530g, vs the 710g of the stock Triumph rods.
Fit custom pin pushings for the Spitfire 0.8125 pins
Or, you could resize the racing rod bigends to 2.021" and use stock Spitfire rod bearings.

If one were to fit a 6.000 c-c rod to a BJ 1296 crank, along with TR-6 pistons, then would require skimming the block 0.125"
to achieve zero deck height. With the added advantage of removing the bore recess, you could run a flat SJ 1296 head gasket.
Or even a solid copper head gasket with these compression rings:

http://www.headgasket.com/compresring.html

I mean, if someone was so inclined.

mkivmarty Avatar
mkivmarty Marty Yanik
N.E.Ohio, USA   USA
[/quote]

So your original MkIV 1296 big journal motor was swapped out at some point?

I[/quote]

I did a light rebuild on the original FK engine; new .010 rings, bearings the TWs. A light hone the break the glaze on the cylinder walls. Lapped the valves, etc. No machine work, just getting it running. I was never happy with the engine though.

I searched for a couple of years for the FD engine. I did the full machine work on it, had it balanced and assembled with all new parts. Got my hands on a Euro exhaust manifold and twin HS2s. I've been VERY happy with this one. I keep the FK around incase I blow the FD up!!

Marty

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claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
Hmmm,

I am thinking along the lines of an offset grind of the big ends, to SJ size, yielding a .25" increase stroke to 82.35mm (87.5 on the 1500), which, with 75mm bore will end up at 1455cc.
The lightweight rods are 540g, and the plan is to keep the porting and cam conservative, but use E85 to introduce the extra oxygen required for the larger capacity motor.

I think that this will result in a high winding motor with plenty of torque, using lightweight valve gear with gentle acceleration ramps, as the fuel is 30% oxygen, so the need for airflow is reduced.
I will be using the 6 port head.
The block is 1147, and still have not decided whether to use line bore and use stock BJ main journals, or to grind BJ to SJ main journal size.

The preparation of the fuel system will be important, but relatively straight forward, the selection of needle in the carbs may be more challenging, and may well be simplified by using the .100" jet assembly from the larger SU model in place of the .090 HS4 jet.

Using E85 a CR of 11.5 :1 is very safe, and of course a wide band O2 sensor will be essential for setting up purposes.

I may have to consider some extra tankage, as I regularly drive 300 km on a Sunday, but have no plans to install anything until I have actual data



Edited 2 time(s). Last edit at 2017-11-14 11:37 PM by claytoncnc.

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
Getting back to the question.

In reply to # 1497866 by 2manyspits if you had a choice which would you prefer ?

Before I got a 1500, I used to look down on them.

After I got a US spec 1500 I hated it. BUT after a rebuild and bringing it up the euro spec plus. It became a very nice engine.

However, the small crank 1300 and the sorted 1500 are still very different (feeling) engines.

As the saying goes 'Different horses for different courses'
The 1300 needs to rev, it feels best zipping up throught the gears, double de-clutching to downchange and power through corners.
But it can feel gutless if it's not on the boil.

The 1500 has more low end torque, it WILL rev to the red line, but you will be more inclined to shift before that.

On the road, both engines are equally quick. The 1300 a boy racer (which can get tiresome if just keeping up with modern traffic) The 1500 the tourer, sporty but less frantic.

It all depends what you want.

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claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
In reply to # 1497957 by Tonyfixit Getting back to the question.
Yes,

Sorry Tony

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
In reply to # 1497961 by claytoncnc
In reply to # 1497957 by Tonyfixit Getting back to the question.
Yes,

Sorry Tony

No problems :-)

I found your project very interesting, and would love to hear more and ask questions.

But this is Thomas's thread.

65or66 Gold Member Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
Tom, is this for your autocross project?

spitlist Avatar
spitlist Joe Curry
Sahuarita, Sahuarita, AZ, USA   USA
My answer is in what I did. When I first built Huxley (my red Mk1) I built a 1500 engine for it mainly because it had the largest displacement. Once I learned more about the difference in these engines, I built a 1296 small journal engine and sold the 1500 to a friend.

Joe

2manyspits Avatar
2manyspits Thomas Schlundt
south bend, IN, USA   USA
not really i wouldnt have a 1500 as an albatross around my neck. I've put too many number three rods thru the side of them. never had that happen to a 1296 and we rev to 7500 all the time.

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
I'm confused, it sonds like you have clear ideas of which engine you want.

But in your first post you are asking the question as if you have yet to decide??

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