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Triumph 1500 to 1700 stroker?

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openfold Oli Penfold
Fareham, Hampshire, UK   GBR
Hi, im new to this forum and i have to say, I've really enjoyed reading through all the posts. I have seen a couple of people are running stroked 1500 engines to 1600 or either 1700 using 1300 rods with the crank turned down and with gt6 pistons. I know the 1500 was known for throwing a rod but is this modification easy to do and reliable if treated well, is there anything else that needs to be done that has been missed off...? I like the idea of having something a bit different. I plan of fitting fitting a hotter cam along with a head skim and a bottom end balance with a lightened flywheel and originally i was going to bore it .040 over with flat top pistons and H beam rods but it seems like the 1600 or 1700 would be cheaper because all the parts are standard from other models but would it be a reliable street engine?

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billspit Bill Kea
Moore, Spartanburg, SC, USA   USA
Well since stroking from 1300 to 1500 turned out so miserably, I wouldn't think going to 1700 would be all that great. One of triumphs problems though was they scrimped on quality to save money. Maybe if you use good quality parts it would work.

90632D Avatar
90632D Fox Trapper
Various, USA   USA
Been done, and works well. Used to have a nice description of doing it from John Kipping, but lost it years ago in a hard drive crash.

The 1500 has problems largely because of the quite large bottom end size. The 1300 had smaller diameter, so the surface speed on the bearings was lower. Turning the large diametered 1500 crank to the 1300 size, offset from center for stroking, apparently works well. I honestly do not know why it is not a more popular modification, other than it's a bit unknown.

Carter will likely pop in with a suggestion on alternative connecting rods. Can't remember off-hand which he recommends (sourced from something readily available). Listen to his comments on the subject of what you propose.

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grumpicus Steve Jackson
Leicester, Leicestershire, UK   GBR
At one time, these conversions were available via the TSSC Club Shop in the UK - not sure if there were many takers, as the adverts stopped after a while..... The pistons to use would not be GT6 pistons (at least with standard connecting rods) as the compression heights of the 1300 and 2 litre pistons are the same, and greater than the 1500. More likely would be the flat top TR6 pistons, which are designed for the long stroke of the 2.5 litre engine. My guess is the long stroke Spit engines used the TR6 pistons and the 1300 small crank conrods, the capacity being determined by the amount of offset applied to the grinding of the big end journals. With standard size TR6 pistons and 95mm stroke, the capacity would be around 1660cc. You might get slightly more by using undersize big end shells and increasing the offset of the grinding.
I doubt that the resulting engine would produce much more power than a 1500, but the torque would be higher, and for a road engine, it might work well. I would expect maximum engine RPM to be in the region of 5,000 to 5,500. You would be advised to spend some effort to get the engine lubrication system working well, and fit a thermostatic oil cooler to give the bearings the best chance of survival!

openfold Oli Penfold
Fareham, Hampshire, UK   GBR
Thanks for the replies guys! So I've got that id have to offset the crank grind and sort out the oiling to the bottom end. I'm already running an oil cooler so that's sorted. And the problem with those engines were bottom end weight so this mod actually lightens it, obviously id get it balanced as well to give it the best chance. But I thought that the standard TR6 pistons would be oversize to a standard 1500 one. I cant remember the numbers but something like +.040 or +.060 bigger, so would i have to overbore too? Fox Trapper, you said GT6 pistons work for this mod here any more insight you have would be appreciated grinning smiley

Also, would the con rods be from a MkIV 1300 or a MK3?



Edited 1 time(s). Last edit at 2014-06-10 06:01 PM by openfold.

clshore Carter Shore
Beverly Hills, FL, USA   USA
The problem with stroking is geometric.
Rod bearing loads come from two sources:
Combustion gas pressure, and inertia forces, ie the rods spinning around on the crank.
The gas pressure is proportional to the volume of charge that is ignited, and the surface area of the piston,
ie 250 psi from burning mixture on a 3" diameter piston will generate 1767 lb of force on the conrod bearing.
You will often see this expressed as BMEP, Brake Mean Effective Pressure.
The inertia forces are purely proportional to the square of the RPM, there's a rather ugly equation that relates stroke and conrod length
to get G of force, you plug in the weights of piston, rod, and rod centroid to yield actual force on the conrod bearing.
On automobile motors, the inertia loading exceeds the BMEP loading at perhaps 4K - 5K RPM.
Point is, the inertia forces get a lot higher a lot quicker as the stroke increases, and depend entirely on RPM, no matter how much HP is generated.
So stroking REDUCES the safe RPM that you can run the motor.
Meanwhile, perversely, HP increases with RPM, since it's the product of RPM and Torque (torque is generated by BMEP).
So, it's entirely possible to ADD displacement by stroking, and actually LOSE HP because your safe RPM has dropped.

Now, you have to make some hard choices, what are you trying to accomplish, increase torque, or increase HP?

If all you want is torque (good on a street motor), stroke it.
If OTOH you want HP, decrease the stroke to increase safe RPM, and add more air/fuel mixture by port/polish/cam/header/carbs, or super/turbo charging.

For my Spitfire Turbo project, I will be running a 1296 Big Journal crank on purpose.
The short stroke to enable safe higher RPM operation (7500).
The big journals to yield a stiffer stronger crank.
The big journal rods are HEAVY, 750+ grams.
Which is why I will be running ex-IRL H-beam rods, under 500 grams.
Have to grind the crank 0.025" to 1.850" from 1.875", no biggie, but MUCH easier than stroking.
Meanwhile, will be running up to 15 psi boost.
The models say 200+ HP, but I will start at 7 psi and creep up to that in baby steps.
Nay-sayers point to the three main crank, but I have some tricks up my sleeve.
For a given power output, safer and easier to limit the RPM (relieving stress on the crank and rods), and crank up the boost to make more power.



Edited 1 time(s). Last edit at 2014-06-10 06:42 PM by clshore.

grumpicus Steve Jackson
Leicester, Leicestershire, UK   GBR
In reply to # 1214867 by openfold Thanks for the replies guys! So I've got that id have to offset the crank grind and sort out the oiling to the bottom end. I'm already running an oil cooler so that's sorted. And the problem with those engines were bottom end weight so this mod actually lightens it, obviously id get it balanced as well to give it the best chance. But I thought that the standard TR6 pistons would be oversize to a standard 1500 one. I cant remember the numbers but something like +.040 or +.060 bigger, so would i have to overbore too? Fox Trapper, you said GT6 pistons work for this mod here any more insight you have would be appreciated grinning smiley

Also, would the con rods be from a MkIV 1300 or a MK3?

TR6 pistons are indeed oversize for a 1500 block - the TR6 bore is 74.7mm, and the Spit 1500 is 73.7mm. Note also that some people do not like boring the 1500 block to 40 thou (1mm) oversize, as it can cause issues with the gasket. The problem seems to be that the recessed blocks of the late Mk4 1300 and all 1500 engines rely on the fire ring of the matching head gasket, and boring the block reduces the sealing area. Some people deck the block to remove the recess and use the earlier flat 1300 gasket. Others, myself included, bore the block and fit +60 thou pistons and get away with it. For what it's worth, +60 thou pistons on a 1500 will give 1550cc.

No, once again, the pistons to use are not the 1300 ones, or the 2000 ones, as both of these have the same compression height (centre of gudgeon pin to piston crown), and as I said earlier, it's greater than on the 1500 piston. Just try putting a 1300 piston into a 1500 block with crank & conrods installed, and turn it to TDC - you'll see that the piston crown is above the top of the block by about 5mm. Stroke the engine, and it will be even higher! You need the 2.5 TR6 flat top piston, as that is designed to work with the 95mm stroke, uses the same conrod as the GT6 & Spit 1500, and fits into what is essentially the same block as the GT6. On the subject of reciprocating weight, the TR6 pistons (being shorter) are lighter than the other pistons.

Conrods for the Mk4 1300 Spit and the 1500 are identical - it's the Mk3 1300 rods which have the smaller diameter big end bearing.

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90632D Avatar
90632D Fox Trapper
Various, USA   USA
In reply to # 1214875 by clshore So stroking REDUCES the safe RPM that you can run the motor.

If all one did was weld up the outside of the crank, grind it to shape, and move the stock rod over that would indeed be the case and I'd agree with you.

However, that is not what is happening in this case. Instead, the stock crank is being ground down, reducing mass, and the rods will be changed to smaller lighter rods. Going this route does not inherently decrease the safe maximum rpm, and without doing the math, it could even increase it (doubt it though).

Quote: Meanwhile, perversely, HP increases with RPM, since it's the product of RPM and Torque (torque is generated by BMEP).

HP does not follow a simple linear increase with rpm. Very few engines actually produce peak horsepower at redline. Most produce it several thousand rpm below redline actually. I don't disagree that there is a relationship between HP and rpm, after all HP is a measurement of work, and without motion there is no work, only force (which is part of the problem with calculating torque on a spinning engine).

Having said that, I'll also acknowledge that I do not ever recall seeing a stroked 1500 engine running around race tracks. Instead, there are small bearing 1300 engines, almost exclusively. I do suspect a stroked 1500 might well make a nice street engine.

When it's all said and done, this is a three bearing crankshaft, and nothing can be done about that. That is the big inherent limiting factor to rpm and performance. Flexure of that crankshaft because of inadequate bearing support.

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1214939 by 90632D
In reply to # 1214875 by clshore So stroking REDUCES the safe RPM that you can run the motor.

If all one did was weld up the outside of the crank, grind it to shape, and move the stock rod over that would indeed be the case and I'd agree with you.

However, that is not what is happening in this case. Instead, the stock crank is being ground down, reducing mass, and the rods will be changed to smaller lighter rods. Going this route does not inherently decrease the safe maximum rpm, and without doing the math, it could even increase it (doubt it though).

Quote: Meanwhile, perversely, HP increases with RPM, since it's the product of RPM and Torque (torque is generated by BMEP).

HP does not follow a simple linear increase with rpm. Very few engines actually produce peak horsepower at redline. Most produce it several thousand rpm below redline actually. I don't disagree that there is a relationship between HP and rpm, after all HP is a measurement of work, and without motion there is no work, only force (which is part of the problem with calculating torque on a spinning engine).

Having said that, I'll also acknowledge that I do not ever recall seeing a stroked 1500 engine running around race tracks. Instead, there are small bearing 1300 engines, almost exclusively. I do suspect a stroked 1500 might well make a nice street engine.

When it's all said and done, this is a three bearing crankshaft, and nothing can be done about that. That is the big inherent limiting factor to rpm and performance. Flexure of that crankshaft because of inadequate bearing support.

Reducing the mass of the crankshaft does nothing the reduce conrod bearing inertia loads.
It is true that using the lighter small journal rods reduces the conrod bearing inertia loads.

Big journal = 750 g
Small journal = 600 g
So about 80% lighter.

Meanwhile, stroking 0.25" (1.875"-1.625"winking smiley increases bearing loads by 15% over 1500, and a whopping 52% over a 1296!

But even the small journal conrod bearings are marginal.
On our G-Prod regional racecar, routine race weekend maintenance was to drop the pan and inspect rod bearings *each day*.
Usually they were OK, but often, NOT.
This was with full race prepped rods, polished, lightened, and balanced, 375 g, running 7500-8000 RPM.
The crank was nitrided and micro-polished.
The oil pan was fully baffled, with an oil cooler, and system capacity was 6+ quarts.
Each daily session was typically 2-3 hours of runtime at racing speeds (practice/qualifying on Saturday, races on Sunday)

Please review my posting.

I didn't say that HP increased *linearly* with RPM.
HP is calculated as torque x RPM, not measured.
The 'meters' on a dyno conveniently do this calculation for the benefit of the human beings.
So HP is linear with torque, and with RPM.
Torque is the product of BMEP and piston area and the stroke.
Same torque at double RPM is double the HP, etc.

An interesting byproduct of the units of measure that we use is that the raw 'numbers' of torque and HP cross at 5250 RPM.

So I will revise my statement to say that stroking a 1500, and using small journal rods is *no worse* than a 1500 vs RPM.
It's just that a 1500 still sucks at high RPM.

(read that as 'on a street motor, how often are you willing to drop the pan and inspect/replace the rod bearings?')

Is that better? winking smiley

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claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
Never tried it, but the prospect of a low rpm hi torque street engine does have some appeal.

My numbers say that a stroked 1700 ish engine would be OK to just over 4800 rpm to keep bearing loads in line with a 1300 engine at 6000.

With a 25/65 mk3 cam, that should equate to around 100 ftlb, at under 3000 rpm.

A tall diff with overdrive would make for a pleasant tourer, but I reckon the engine would feel a lot like an MGB.

Some like them a lot, but they are not my cup of tea. Triumph did a lot better with the TR2/3/4 engine which has some real torque.

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1215059 by claytoncnc Never tried it, but the prospect of a low rpm hi torque street engine does have some appeal.

My numbers say that a stroked 1700 ish engine would be OK to just over 4800 rpm to keep bearing loads in line with a 1300 engine at 6000.

With a 25/65 mk3 cam, that should equate to around 100 ftlb, at under 3000 rpm.

A tall diff with overdrive would make for a pleasant tourer, but I reckon the engine would feel a lot like an MGB.

Some like them a lot, but they are not my cup of tea. Triumph did a lot better with the TR2/3/4 engine which has some real torque.

Yah,
John Kipping recommended the HS4/6 on a single carb manifold for that purpose, low end 'grunt' for the street.
Long stroke motors are known for torque.

grumpicus Steve Jackson
Leicester, Leicestershire, UK   GBR
Carter - It's a bit too late for my current 1500 engine build (as I've recently had the crank reground & tuftrided), but on the basis that lighter is better - what are your views on keeping the same stroke (87.5mm), and regrinding the conrod journals to 1.625" & fitting the Mk3 Spit conrods?

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1215131 by grumpicus Carter - It's a bit too late for my current 1500 engine build (as I've recently had the crank reground & tuftrided), but on the basis that lighter is better - what are your views on keeping the same stroke (87.5mm), and regrinding the conrod journals to 1.625" & fitting the Mk3 Spit conrods?

Interesting, I had not thought of that one.
Only side effect I'd be concerned with is reduction in the overall torsional stiffness of the crank, and shift of it's resonant frequency.
Might be worthwhile to fit a crank damper.
When you compare the small and big journal cranks, the massive bulk of the connector between 1-2 and 3-4 becomes evident.
On a SJ crank it's kind of a parallelogram shape, like a rubber eraser, the 'wedges' or tapered sections at each crankpin nearly overlap.
On a BJ crank, it's a huge rectangular chunk, with rounded ends at the crankpins, like a big 'S'.
Yet, contemporary crankshafts that transition from one crankpin to another typically adopt a 'constant section' approach, rather than a big 'S'.
The connector uses a cross sectional area approximately same as the crankpin, and travels the shortest path. Certainly, modern Finite Element analysis was performed to arrive at these optimal forms, rather than what was likely an 'intuitive' approach when our engines were laid out.
In MG Midget/Sprite world, they use the term 'wedging' to describe reducing the material at the crankpins by grinding a sharper angle into the connector.
Contemporary high performance V-8 actually remove material from inside the center of the crankpins, as it contributes little strength.
The original Saturn crankshafts were cast using the 'lost styrofoam' method, and used hollow sections in the centers.

Reducing crankpin diameter from 1.875" to 1.625" means that the sides of the crankpins are far wider than needed to support side loading of the rods. In fact, Google for 'Piston Guided Rods', you can eliminate that stress inducing ridge altogether.

But back to reality:
Combined with 'wedging', and alteration of the connector profile from rectangular to rounded, a lot of weight and material can be eliminated from our BJ crankshafts without compromising strength.



Edited 1 time(s). Last edit at 2014-06-12 09:27 AM by clshore.

openfold Oli Penfold
Fareham, Hampshire, UK   GBR
In reply to # 1215059 by claytoncnc Never tried it, but the prospect of a low rpm hi torque street engine does have some appeal.

My numbers say that a stroked 1700 ish engine would be OK to just over 4800 rpm to keep bearing loads in line with a 1300 engine at 6000.

With a 25/65 mk3 cam, that should equate to around 100 ftlb, at under 3000 rpm.

A tall diff with overdrive would make for a pleasant tourer, but I reckon the engine would feel a lot like an MGB.

Some like them a lot, but they are not my cup of tea. Triumph did a lot better with the TR2/3/4 engine which has some real torque.

is there any stroke that would work to get the redline up to about 5200 to 5300? what would the size of the engine be to get that. im putting a ford type 9 5 speed in at the same time as the motor rebuild. It needs to be able to run at 70mph plus for highway cruising. also what gears would be good to put in to get the best of low end and high end, would tr7 gears work? ive heard that people put these in the spit but im unsure what the ratios are...

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1215264 by openfold
In reply to # 1215059 by claytoncnc Never tried it, but the prospect of a low rpm hi torque street engine does have some appeal.

My numbers say that a stroked 1700 ish engine would be OK to just over 4800 rpm to keep bearing loads in line with a 1300 engine at 6000.

With a 25/65 mk3 cam, that should equate to around 100 ftlb, at under 3000 rpm.

A tall diff with overdrive would make for a pleasant tourer, but I reckon the engine would feel a lot like an MGB.

Some like them a lot, but they are not my cup of tea. Triumph did a lot better with the TR2/3/4 engine which has some real torque.

is there any stroke that would work to get the redline up to about 5200 to 5300? what would the size of the engine be to get that. im putting a ford type 9 5 speed in at the same time as the motor rebuild. It needs to be able to run at 70mph plus for highway cruising. also what gears would be good to put in to get the best of low end and high end, would tr7 gears work? ive heard that people put these in the spit but im unsure what the ratios are...

The stock 1500, or stroked 1700 can each handle 5300.
As for the type 9, which version, i.e. what ratio for the gears?
A wide ratio box can give acceptable low end performance when coupled with a 3.27 cruising diff ratio.
Close ratio box, not so much.

IMHO, stick with the stock 1500, fit a 3.27, and find a type 9 with wide ratio (likely fitted to a heavier saloon or estate, rather than a light sports model).
An added advantage is that the wide ratio are not 'favored', and so should be cheaper.

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