SP and SE bottom end question.

[wozzah1975]

The sump is different to suit the girdle. from memory all nbc have the right sump. Early sumps can be modified to suit. Details, along with factory and upspeced bearing support plates can be found on the miataroadster website.

Can someone please explain how with the same block height a different stroke is achieved? I'm imagining that the piston is pulled further down into the block at the bottom of the stroke? Hence more bending load on the rods?

Sent from my GT-I9100 using Tapatalk

The piston travels further up and down. The piston comp height is made shorter so the pistons don't poke out the top, and they travel further down the bore.

Put simply, engines with larger stroke are harder on rods and pistons because the accelleration is higher (piston travels further in the same amount of time) and the inertia is greater (being thrown out further) and the angles are greater (when the crank is at 90deg rotation the big end sits out further) so the rod and piston is stressed more from the great angles.

As stated earlier, the B6 rod is dimentionally the same as the BP, so in a BP the rod is stressed further than a B6. It's not an ideal way of doing it, but it was the way Mazda decided, and in standard form it doesn't cause issues, in fact even in modified form its not the end of the world.

There are a number of different theories on rod ratios etc. I find a rod ratio of about 1.6-1.8 works best on these style of engines.

A standard BP has a rod ratio of 1.5657 (133mm long rod / 85mm stroke = 1.5657)
A standard B6 has a rod ratio of 1.5909 (133mm long rod / 83.6mm stroke = 1.5909)

The B6 is closer to where it needs to be than the BP, hence less rod dramas.

Cheers
Woz

[greenMachine]

Woz, you are implying confirmation of a belief of mine, the the root of the problem is piston speed, so high revs exacerbate the 'problem', and high revs with boost being the worst case scenario.

For a standard car that's not an issue, for a high revving (say 8000+ rpm) atmo car the margins are getting slim; for a low revving boosted car, ditto; for a high revving, boosted car, maybe perilously thin?

Just generalisations, the variables are many ...

[wozzah1975]

Woz, you are implying confirmation of a belief of mine, the the root of the problem is piston speed, so high revs exacerbate the 'problem', and high revs with boost being the worst case scenario.

For a standard car that's not an issue, for a high revving (say 8000+ rpm) atmo car the margins are getting slim; for a low revving boosted car, ditto; for a high revving, boosted car, maybe perilously thin?

Just generalisations, the variables are many ...

Correct, it only becomes a problem when things are pushed. I find it interesting that even companies such as Carrillo etc to my knowledge don't make an off the shelf longer rod, so obviously not enough demand.

Having said all the above, although the rod ratio isn't ideal I have still seen BP's with standard length good quality rods turn to 8500+ in N/A form, and 300rwkw+ in turbo form without any issues, and I still run the standard length rod in my engine simply because of the cost in getting a custom longer set done and pistons done to suit. If I was going to go a "no expense spared" build I would be looking at running a custom rod at around 136mm long and pistons to suit.

Cheers
Woz

[sailaholic]

A bit off topic, but how are the stroked motors achieved then?

The mahura kit uses special rods and a special crank, so could be either?

The fm kit though is just rods and pistons. So these rods would be longer?

Would be intersting to know the ratio on both of these systems.

Sent from my GT-I9100 using Tapatalk

[NitroDann]

You cannot stroke an engine without changing the crank.

Dann

[wozzah1975]

You cannot stroke an engine without changing the crank.

Dann

Not 100% true. By using under size bearings and offset grinding the standard crank you can usually get 1+ mm extra stroke (depending on what size bearings are available) , OR by finding a rod with a slightly smaller big end journal and utilising that, again offset grinding the standard crank.

I'm not a fan of stroking the BP. It already has a less than ideal rod ratio, stroking it makes it worse. In my opinion boring a BP is better bang for buck. There are now off the shelf pistons available at 85.5mm, the right block will take these (one that sonic tests ok), and that puts you at 1952cc. Whilst stroking it gives low end torque, it also loses the ability to rev, and for a performance application that can become a problem. The other thing is that the stroker kit is super expensive, which doesn't make it good bang for buck. More power to be had elsewhere

The 85.5 mm bore is what I'm aiming at next block build for my car,

Cheers
Woz

[NitroDann]

Im familiar with offset crank grinding, but thats still changing the crank. Thats what I meant, but thanks for the long winded explanation for everyone, I couldn't be bothered lol.

Dann

[wozzah1975]

Im familiar with offset crank grinding, but thats still changing the crank. Thats what I meant, but thanks for the long winded explanation for everyone, I couldn't be bothered lol.

Dann

Now i get what you meant, changing (modifying), not changing (replacing)

My bad

Woz

[Vilko]

So just to clarify what dann was saying in the previous page, compression has no effect on max boost? I thought boost combined with high comp would put even more stress on the rods?

[NitroDann]

The boost and compression is nothing. 10:1 compression is 147psi, and 15psi and 10:1 is about spot on 300 psi. A healthy naturally aspirated engine make 2500psi during combustion.

Dann

[Vilko]

The boost and compression is nothing. 10:1 compression is 147psi, and 15psi and 10:1 is about spot on 300 psi. A healthy naturally aspirated engine make 2500psi during combustion.

Dann

So on combustion 147psi becomes 2500psi.
Wouldnt 300psi become greater than 2500 during combustion because of the greater amount of fuel being burnt?

Not sure if I fully understand lol.

[NitroDann]

Yeah it does but its not the compression its the pressure.

Pressure is torque.

So its torque not compression that bends rods.

Dann

[Vilko]

The boost and compression is nothing. 10:1 compression is 147psi, and 15psi and 10:1 is about spot on 300 psi. A healthy naturally aspirated engine make 2500psi during combustion.

Dann

So on combustion 147psi becomes 2500psi.
Wouldnt 300psi become greater than 2500 during combustion because of the greater amount of fuel being burnt?

Not sure if I fully understand lol.

I just realised how irrelevant this point is lol.

So, to summarise, B6/BP engines die from busted rods. Rods die from torque. Compression increases power, not torque. So compression is irrelevant to longevity and/or max safe boost for these motors. Correct?

So for the final piece of the puzzle; why do manufacturers run lower compression in non-turbo variants of the same engine?

[Okibi]

From memory bog stock the SP had a compression ratio of 10:1 (same as the asthmatic MX-5s) and the SE 9.5:1.

[NitroDann]

Well...

Argh.. Its late lol.

The piston gets pushed down by PSI of explosion pressure.
Force is pressure over area.
More pressure or area the more push down on the piston.
The bigger the bore the more area.
The more fuel/air/boost the more pressure.

When the piston is exactly half way, think of where the crank is, its to the side.
And the rod is pushing down on it, like a lever. The crank is a moment arm.

The bigger the stroke, the further to the side, and therefore more leverage the crank has on it.

Torque is force multiplied by leverage.


Also, the more bore and stroke you have the more capacity you have, so more air and fuel is drawn in each stroke, increasing combustion pressure.

Power is torque multiplied by revs.

If you have a 100Nm engine that spins 100rpm
And you have an other 100Nm engine spinning at 200rpm, you can gear down the second one by half, so it would be 200Nm at 100rpm. Therefore it has double the horsepower.

Hope this helps.

Dann