MX-5 SP Stock v T-Piece Mod

[lambertius]

Completely stock MX-5 SP, standard clutch/diff/final drive etc. Peak power was 116wkW (155whp).

I ran the waste gate reference directly into the manifold (same effect as the T-Piece mod but I don't need the T-Piece). There are plenty of spare vacuum line connections. Peak power was 134wkW (180whp).

Torque is wheel torque.

Stock AFR was ~11:1. Changing the boost reference improved the AFR to 12.4:1. This resulted in a noticeable improvement in the smoothness of driving, particularly when lifting off the throttle.

[ManiacLachy]

That's a healthy bump in torque across the board! I bet it feels stronger as well as faster. Nice result.

[lambertius]

It is a very noticeable improvement, though I think a lot of the improvement was due to having a more rational AFR.

[Slow Yank]

Late to the party - but, isn't 12.4:1 AFR in a 10:1 compression engine pushing it a bit?! Admittedly no experience with turbo MX-5's, but my experience with heavily modified Mitsu 4g63's, Toyota 2JZ's (and a dabble of Subaru EJ's) has always led me to understand that 11.5:1 was ideal for high compression, high boost. Anything in the 12's was wayyyyy too lean for an engine without either E85 or lower compression.

I would think that the ECU is pulling quiet a bit of timing at those AFR's..that or SP really has done so already in the ECU tuning they did.

Even if you're OK on the street on an average day, I reckon on a hot day, or certainly at the track where the IC could potentially become a bit heat-soaked, you would be right on - if not over - the ragged edge of detonating your pistons to bits.

[bruce]

SPs have a habit of bending rods on stock boost (as the motor is un-modded from factory and is stock NB with a turbo bolted on).

[lambertius]

None of that sounds right to me at all.

11:1 is overly conservative, and its borderline that they were even able to get the car to run. 11.5-12.5:1 would be 'normal' for that generation of boosted cars (no DI, and other modern fun).

I've never heard of an SP, or any other NB bending rods that wasn't modified substantially. When I spoke to NitroDann, he was saying that 200rwkW/10PSI was about the edge before the engines start to suffer. That lines up with the experience I've had as well. I'm not even at 7PSI and the fuelling isn't lean, though I wouldn't want to put in cheap fuel.

[bear2230]

I would say it would be very rare to find a SP with straight rods. Even the T-Piece kid would put enough extra strain on those pissy stock rods if given a good thrashing.
These are mine at 160kw
Only 28000Km on the car when I did this.
Carrillo H beam rods in now.

[bruce]

Yes, one of many examples from this forum I enquired about a stock car for sale many years ago which also had an engine rebuild under Mazda warranty.

[Slow Yank]

None of that sounds right to me at all.

11:1 is overly conservative, and its borderline that they were even able to get the car to run. 11.5-12.5:1 would be 'normal' for that generation of boosted cars (no DI, and other modern fun).

I've never heard of an SP, or any other NB bending rods that wasn't modified substantially. When I spoke to NitroDann, he was saying that 200rwkW/10PSI was about the edge before the engines start to suffer. That lines up with the experience I've had as well. I'm not even at 7PSI and the fuelling isn't lean, though I wouldn't want to put in cheap fuel.

Search around online outside of the MX5 forums and you'll find tons of data to back up what I'm saying.

11:1 is not overly conservative, and nowhere near what a car will run at.

A stock USDM 1995 Mitsubishi Eclipse GSX 4G63 has a WOT AFR of about 9.5:1, with a compression ratio of 8.5:1
A stock USDM 1992 Subary Liberty SS EJ22T has a WOT AFR of about 9.5:1, with a compression ratio of 8.17:1
A stock JDM 1995 Supra 2JZ-GTE has a WOT AFR of about 10.5:1, with a compression ratio of 8.5:1

NO manufacturer without DI ever built a factory car with anything cose to 10:1 compression and an AFR in the mid-12's!

There are many measures as to what limits an engine can be pushed to. Absolute torque ("200rwkW") is one measure of what the rods can withstand, but you can destory rods and pistons at far less once you start to detonate.

Again, you are absolutely on the ragged edge of binning your engine. As noted - a hot day, bad fuel, one pull that's just a bit too long - it's gone. If you're extremely lucky, your head-gasket will act as a fuse and blow first.

[Slow Yank]

I would say it would be very rare to find a SP with straight rods. Even the T-Piece kid would put enough extra strain on those pissy stock rods if given a good thrashing.
These are mine at 160kw
Only 28000Km on the car when I did this.
Carrillo H beam rods in now.

Do you recall roughly what mods were done to your SP when you bent your rods?

[lambertius]

I would say it would be very rare to find a SP with straight rods. Even the T-Piece kid would put enough extra strain on those pissy stock rods if given a good thrashing.
These are mine at 160kw
Only 28000Km on the car when I did this.
Carrillo H beam rods in now.

Do you recall roughly what mods were done to your SP when you bent your rods?

I asked him, the car was running a new ECU and injectors and the tuner ran the car up to 170wkW before it went.

There are two other SPs near me, one has near 200 000km on the clock and is regularly tracked. None of our cars have any signs of issues. I doubt very much that anyone has bent any rods without there being a specific reason, like running the power up or money shifting the car.

NO manufacturer without DI ever built a factory car with anything cose to 10:1 compression and an AFR in the mid-12's!

There are many measures as to what limits an engine can be pushed to. Absolute torque ("200rwkW") is one measure of what the rods can withstand, but you can destory rods and pistons at far less once you start to detonate.

Again, you are absolutely on the ragged edge of binning your engine. As noted - a hot day, bad fuel, one pull that's just a bit too long - it's gone. If you're extremely lucky, your head-gasket will act as a fuse and blow first.

I assume you mean there are no FI cars, not that there are no factory cars? I'm really not worried about binning the engine with how it is currently running. That AFR value was just a number I picked off the screen at the time, and overall the car is still running rich. I might have a dig around and see if I can find the print out.

[Slow Yank]

There are two other SPs near me, one has near 200 000km on the clock and is regularly tracked. None of our cars have any signs of issues. I doubt very much that anyone has bent any rods without there being a specific reason, like running the power up or money shifting the car.

But you have a very small data pool with the SPs. If it were a 10% probability of failure with a typical car model after doing a said mod, the reputation would quickly build and nobody in their right mind would do it. With an SP, you would have to intimately know what was happening with 10-15 other SPs doing exactly as you are (as it is doubtful that everyone who is damaging there engine is broadcasting it...), and when one failed might you still might just chalk it up to bad luck.

What I'm saying is, if you look at the broader pool of data - any make/model with boost and relatively high (9.5:1 or greater) compression, a mid-12 AFR can cause trouble in a hurry, and you really don't have any factor of safety with the factory BP6D; cast pistons, weak rods. You've REALLY got to have timing under control.

I assume you mean there are no FI cars, not that there are no factory cars? I'm really not worried about binning the engine with how it is currently running. That AFR value was just a number I picked off the screen at the time, and overall the car is still running rich. I might have a dig around and see if I can find the print out.

Yes of course, no factory FI cars.

If it were my car, I'd install a wideband and I'd be logging it vs. rpm when doing a dyno run. In fact, my general rule is that I never touch a boosted cars engine without first installing a wideband and boost gauge.

[Daz]

I was around a lot of SP owners back when they were new and all the early talk of mods was going on. I never heard of a single rod bent on a car with a stock ECU with or without T-piece. There was however one incident I know of where that reference pipe melted and essentially stopped opening the gate.... that car had ongoing issues, no fault of the owner and many of us replaced that hose with silicon (or did a heat sleeve on it) which I am sure saved a lot of SP's from the same demise.

I can attest to the stock ECU pulling timing in the heat, on a 40 degree day with the AC on it is a total slug, but bounces right back when it cools off at night.... no bent rods though. I would be very surprised to hear of any SP owner with a stock ECU bending rods on 98 octane even with the T-mod.... but there are a lot of boogie man stories about these cars out there from people that don't own them.

[lambertius]

The performance difference between a hot day and cold day on my car and my friends' cars is super noticeable! I should try and get it onto a dyno when it is heat soaked, it has gotta be ~10kW down when it is 35+ outside!

[Slow Yank]

The performance difference between a hot day and cold day on my car and my friends' cars is super noticeable! I should try and get it onto a dyno when it is heat soaked, it has gotta be ~10kW down when it is 35+ outside!

Common-place on any turbo car with an air-to-air IC. It's not necessarily timing advance, but the fact that you're effectively increasing the boost (boost itself says the same, but considerably more air at the same psi).