Dynamic Compression Ratios... and blown engines

[sidekick]

Originally Posted by Lex I believe OEM VVT is too advanced if you're modifying the car or even stock in the gen1 car. You can tone it down a bit and still get great results. The K04 can achieve 20psi of boost at around 2000 RPM. Spool will not be an issue but torque down low will be with less VVT. Any way you cut it, the OEM turbo spools so quickly it is almost counter productive. I am making more peak torque on a larger turbo than OEM yet have much better traction. Ramp up in boost/torque and the ability to modulate this is important and difficult with the K04 but easier with a larger turbocharger.

Now I know this is going to vary by car/mods, but what do you think is a good "safe zone"? Maybe pull about 10* of advance out the whole table? Or at least in the low end? That's about where my custom VVT setting were, but with small changes here and there.

[Dano]

Generally speaking the trouble comes in with low load, low RPM, high VVT advance when mashing the gas after the engine is heatsoaked.

Trouble is, to decrease spool time and add TQ that is smack dab where you run 35s. The way around it is to downshift your car before mashing on the throttle. RPMs go up and shift you to the right of the table out of the 35s zone.

Bottom line, drive your stick like its supposed to be driven and you have no worries unless you totally screw up your VVT table.


Tappin

[btstarcher]

I never mashed the throttle and I still blew, twice. I gently pressed it down, slowly.

What about keeping it around 35 at higher loads and tapering off at low loads?

[Dano]

any mash on the go peddle induces a huge pressure spike in the cylinder on an already heated up motor if you don't down shift. not saying that was your problem but generally that's what causes the huge spike in compression thus huge KR thus over time a weak rod.

you need that 35 down low to flow through the cylinder it just needs to be very strategically tweaked.

here lemee show you my table. The red box indicates the area you want to be careful in for highway cruise situations especially. Lets hope nobody mashes on the gas at 2K rpms in 5th 6th gear which is outside the box.

My table is even a bit too aggressive in the bottom left hand corner if you are lazy and don't down shift.

Lex has done way more research and infinitely more knowledgeable than I on this so don't take my example as the gospel...it is just my understanding of it and I would consider my table somewhat aggressive for sure. This was my setup for the GT28 so it will likely get modified for the GTX30

[btstarcher]

That's exactly the same group of cells I had decided to modify!




After I blew LOL

[Dano]

meaning they had higher VVT values before??

[btstarcher]

Those are OTS values right? It looks like what I had before adjusting.

[btstarcher]

Yeah, those were pre-blow numbers. I lowered them a bit in my next revision.

[sidekick]

This thread has inspired me to just run a map with no VVT. That 20ft/lbs isn't worth an engine

Edit: @06Speed6; Wouldn't you get the same result by filling the whole VVT table with 15° of advance?

[06Speed6]

I think you guys are reading way too much into the VVT table, it alone isnt going to blow an engine regardless of what value is in the cell.

15* across the board is probably too high for the cells at the end of the table.

My VVT table differs significantly from oem, it is extremely agressive across all loads and rpms and I ran just fine for thousands of miles. Im fairly certain my engine bent a rod because I was doing some cdfp testing for a company and the pump was acting up and it acted up at the wrong instant. Low rail pressure ftw.

[Lex]

I feel that overly aggressive VVT, especially at lower loads or as boost ramps up with the OEM turbo is what caused a lot of zzb in the gen1 cars on the hwy when the engine is heatsoaked and engine enters boost. Many cars blew this way and had the huge KR stutter just as they got on the gas. This was cured with custom tuning.

To support this Mazda has changed this significantly for the gen2 cars and a lot less zzb occurred.

[sidekick]

Originally Posted by Lex I feel that overly aggressive VVT, especially at lower loads or as boost ramps up with the OEM turbo is what caused a lot of zzb in the gen1 cars on the hwy when the engine is heatsoaked and engine enters boost. Many cars blew this way and had the huge KR stutter just as they got on the gas. This was cured with custom tuning. To support this Mazda has changed this significantly for the gen2 cars and a lot less zzb occurred.

Do you have a screen shot of a Gen2 VVT table vs a Gen1 VVT table? Or are you saying other changes in the tune helped prevent this? Like lowering the High BAT flags or something?

Originally Posted by 06Speed6 I think you guys are reading way too much into the VVT table, it alone isnt going to blow an engine regardless of what value is in the cell. 15* across the board is probably too high for the cells at the end of the table.

But if you welded the VVT actuator at 15° of advance, you would end up with the same problem, no?

[Lex]

Here are the OEM values:

Gen1 VVT



Gen2 VVT

[sidekick]

Originally Posted by Lex Here are the OEM values: Gen1 VVT http://www.mazdaspeedforum.org/forum/forum/attachment.php?attachmentid=89142&stc=1&d=13583648 38[/IMG] Gen2 VVT http://www.mazdaspeedforum.org/forum/forum/attachment.php?attachmentid=89143&stc=1&d=13583648 38[/IMG]

That's a pretty big difference. Maybe I'll try running the Gen 2 table and see how that goes. The theory behind the VVT causing ZZB makes a lot of sense, but it is just a theory.

Has Mazda ever made any sort of statement about ZZB? Or have they just chosen to ignore it?

[DSIT995]

There is a fairly big difference in those tables exactly where low load cruising would be...

I might give the genpu tables a try to, or at least lower the values in the crushing zone

[Lex]

My custom tables aren't like the gen1 or gen2 tables - VVT doesn't have to be set right at 0 but the gen1 tables were too aggressive in the lower load regions.

[Ziggo]

So are we talking specifically for K04 setups here? Cause I am a huge fan of buckets of VVT. I run 35s throughout the spool range of the turbo. My tuning goal is to get the fattest 350wtq curve possible and that allows me to extend it downwards. I dislike downshifting to second on the track and the the TQ increase from VVT allows me to run less timing so the car can still dig out of the low RPMs while feeding the turbo nice toasty retarded timing exhaust.

Zigatapatalka

[sidekick]

Originally Posted by Ziggo So are we talking specifically for K04 setups here? Cause I am a huge fan of buckets of VVT. I run 35s throughout the spool range of the turbo. My tuning goal is to get the fattest 350wtq curve possible and that allows me to extend it downwards. I dislike downshifting to second on the track and the the TQ increase from VVT allows me to run less timing so the car can still dig out of the low RPMs while feeding the turbo nice toasty retarded timing exhaust. Zigatapatalka

You're on stock internals right? This ZZB shit is confusing. Some people blow up with minimal mods and a decent tune and then there are people like you, who are fully bolted+turbo upgrades and track your car regularly without any problems.

[Ziggo]

Originally Posted by boost_addict You're on stock internals right? This ZZB shit is confusing. Some people blow up with minimal mods and a decent tune and then there are people like you, who are fully bolted+turbo upgrades and track your car regularly without any problems.

Yes, stock internals. Who blows up with a "decent" tune? From what I can tell the ZZB that @Lex; was talking about referred to the stock tune. I have not seen any ZZB recently on a properly tuned and modded car that were a mystery.

[Lex]

Originally Posted by Ziggo So are we talking specifically for K04 setups here? Cause I am a huge fan of buckets of VVT. I run 35s throughout the spool range of the turbo. My tuning goal is to get the fattest 350wtq curve possible and that allows me to extend it downwards. I dislike downshifting to second on the track and the the TQ increase from VVT allows me to run less timing so the car can still dig out of the low RPMs while feeding the turbo nice toasty retarded timing exhaust. Zigatapatalka

There are several aspects of the stock tune that was questionable and they had to do with hot engine, slow OL transition, early RPM spool at low throttle angles for the K04 and lots of VVT on top of it.

[mrQQ]

is KR a necessity for ZZB?

let me rephrase:

if I don't get KR at low rpm high gear acceleration, then I don't need to touch VVT table?

[silvapain]

Originally Posted by mrQQ is KR a necessity for ZZB? let me rephrase: if I don't get KR at low rpm high gear acceleration, then I don't need to touch VVT table?

Not necessarily; I blew my engine on E85, where there is no possibility of KR.

It's high cylinder pressure that causes ZZB. It can come from KR, timing advanced past MBT, or simply too much torque down in the lower RPMs.

[btstarcher]

My second motor also blew with e50 in the tank, so no KR there either.

[Dano]

Knock causes pressure spikes in the chamber as the piston is coming up thus resulting in a ton of opposite force applied to the rod.

excessive VVT can produce more in-cylinder pressure as well but may not lead to knock but will put additional stress on the rods.

it has been discussed here or in the VVT thread like 15 times but low load/low RPM high VVT advance and a lazy driver can put extreme pressure on an already heat soaked motor in highway cruise situations. This can likely lead to a ton of KR and or high cylinder pressure at low RPMs. This lead to quite a few of the early ZZB scenarios.

current maps have safer VVT advance settings but you can avoid any issues by downshifting to raise RPMs and get you out of any high VVT/ low load/rpm cells.

[cld12pk2go]

Originally Posted by silvapain Not necessarily; I blew my engine on E85, where there is no possibility of KR. It's high cylinder pressure that causes ZZB. It can come from KR, timing advanced past MBT, or simply too much torque down in the lower RPMs.

Originally Posted by btstarcher My second motor also blew with e50 in the tank, so no KR there either.

How much timing were yall running prior to failure in the 3000-5000 RPM range were the KO4 can actually make some torque?

I have been running the following with the GTX3071 while aiming for ~400wtq:

4.00	6.00	8.00	10.00	12.00

[silvapain]

Originally Posted by cld12pk2go How much timing were yall running prior to failure in the 3000-5000 RPM range were the KO4 can actually make some torque? I have been running the following with the GTX3071 while aiming for ~400wtq: 4.00 6.00 8.00 10.00 12.00

This is what I was running 3000-5000 on E85 when I blew:

5.50	7.50	9.50	12.00	14.25

[vtdone]

Apology for resurrecting this but according to the DISI OEM Specifications post the overlap is negative so there is no overlap at all between EVC and IVO.

Am I reading the table wrong?