Intake backfires during launch control with transbrake

Hi All,

I would like to understand what is causing backfires/large explosions at the intake side, when we are using only ignition retard at launch control?

So there a dragster, with a 6cylinder 3.0 liter turbocharged engined, which runs on methanol fuel.

Besides the intake port injection, it has 4 additional injectors 2200cc/min, after the turbo and before the throttle body, to cool the air charge, so it doesn't have intercooler.

It has an automatic gearbox, which has transbrake function for launching, so during launch control basically the engine has load on it.

We used only ignition retard/cut launch control, with low boost pressure at launch we didn't have issues, just that the car is not quick as it should be.

If we increase the boost pressure we get backfires at intake side, which causes explosion, and rpm fall as well.

As the injectors were operating at the throttle body as well with the ignition retard the intake air temperatures, were fine.

We tried to use fuel-cut launch control, with this method we don't have intake backfires, but the intake air temperatures now raised above 120 C degrees, as we had to turn-off the injectors at throttle body.

Kindest regards,

Qgli

Do you have any logging of this occurring that you can share? As it is impossible to give advice with no data, only a description.

There are several things that could be occurring. With the transport delay from your throttle body/post turbo injectors, you could be experiencing a lean mixture, which is likely to cause a backfire due to pre ignition.

Methanol also does not atomise particularly well, depending on the contribution from the additional injectors vs the ones in the port, you may have puddling in the intake. Seeing as you don't have backfires when these injectors are turned off, it is quite possible you are trying to inject too much of the required fuel from these upstream injectors.

Hi N8B,

I've attached a picture with the time stamp when the backfire occurs and there is a significant RPM drop could be also seen.

I've upload the log file to google drive, I was not able to attach.dat format here:

https://drive.google.com/file/d/1cnZSXSMwAD_bsCgtGEAbUgevq1VKgChl/view?usp=sharing

Thanks for your support in advance :)

Qgli

Is it waste spark or sequential? If it has a lot of overlap you can get ignition from the outgoing charge with significant retard, likewise waste spark with heavy retard is firing on the incoming charge.

I'm about 99% confident the problem is your valve springs, from what you're describing.

If you don't have enough seat force (AKA 'seat pressure') the higher boost pressure in the intake manifold can prevent the valves fully closing and on ignition the burning charge is being forced back into the intake port, igniting the charge there. There only needs to be a single 'weak' spring to cause the problem.

With a 2JZ, the stock intake is ~34mm diameter, or ~1.33". That gives a valve area of ~1 square inch, so every 10 PSI increase in manifold pressure will reduce the net seat pressure by 10 lbs. Simplified, if you've gone from 40 PSI boost to 60 PSI, and you have a nominal 70 lbs seat force, you've only 1/3rd the net force closing the valves.

Your camshaft supplier should have recommendations for minimum seat pressure, and your valve spring supplier should have a minimum inter-coil clearance/minimum height spec', and if the latter allows you may find fitting, for example, 1mm shims under the springs cures the problem.

[edit] it may be an exhaust side valve spring issue, if it's allowing leakage from unseated valves to ignite the fresh incoming charge while the inlets are open.

From my experience it is always wasted spark ignition system that creates the problem or lack of valve spring seat pressure just like it's been already explained. Never heard of any other route cause...

Hi All,

This engine has sequential ignition system with IGN1A upgraded coils.

This is a Audi v6 engine with 30 valves, so it has 3 intake the valves per cylinder, the valve springs are from Supertech, both for the intake and exhaust valves.

What I don't understand that the engine when it was tuned on hub dyno it ran on 2.6 bar boost without issues.

With the same engine and same valve spring other dragster runs on 3.8 bar boost.

So if we would have a problem with the valve seat pressure shouldn't we have some sysmptoms during a dyno session as well?

Is there a difference between launch control mode and normal operating mode from the valve spring point of view?

The hydraulic valve lash adjusters are converted to mechanical.

The camshaft's are regrinded from the factory one.

Thanks for your support and kindest regards, Qgli

I'm aware of people having issues with Supertech products, when pushing them hard - might not apply, or be a historic issue now fixed.

You cannot directly compare the two engines, and the boost they're running because they are different applications - for example, some of the things that can affect the seat pressure -

Installed height - the seat in the head may be in a different postion because of being replaced and/or recut differently; the valve seat may have been refaced; the spring seat may be slightly different in the heads; the overall valve lengths may be slightly different if from different manufacturers, even without refacing; different spring retainers/caps, both in mass (weight) and where they place the spring; different valve collets/locks/keepers may have slightly different effective taper diameters, which will place the valve retainer differently; different vlave material or design can affect their total mass that has to be controlled, etc.

Then there is the mass of the cam follower assembly and shim pack, to be added to the valve, valve spring, etc. assembly that has to be controlled.

I really dislike "reground" camshafts for several reasons but here there is a question of how accurately they were ground and the valve train accelerations, especially on the back flank, as there may be variations between the camshafts in each engine - especially if the base circles are different.

The only "Audi" valve springs listed on the Supertech official site seem to be for the 4.2 V8 exhausts, but I did find these specifications for a V6 kit - https://sportignition.com/product/audi-s4-rs4-2-7t-2-8-3-0-v6-30v-supertech-valve-spring-set-sprk-audi2-7t/ - which may give a basic installation guide. On the other hand, the same (apparently) product as advertised here - https://performancebyie.com/products/supertech-valve-spring-retainer-audi-27t - is pure advertising hype which may be worse than no data whatsoever, as it's 100% pure marketing BS that tells you nothing helpful.

No valve spring is made 100% accurate to the nominal specifications and, hopefully, they are conservative, but... You really need an accurate valve testing machine to ensure all the spec's are met, and a tool for checking the actual installed height of the valves with that particular valve (where each will be used), seat insert (you ARE using them?), retainer, collet, etc. to ensure the seat pressure is as required. You can work it out by other means, but it is complex and a real PITA(!), with potential for arithmatic errors.

Even after all that, it doesn't mean the spring/valve assembly is going to meet your needs, just that you have what's advertised.

OK, moving on...

Assuming the boost numbers are accurate, there may be pressure fluctuations with the boost as even the best controllers have some latency (delay) in activating, and there can be areas within the manifold where pressure waves exceed the nominal boost value, and if they occur at the closed inlet valve(s)...

If there is any ignition cut operating for rpm control, that will cause pressure fluctuations (see below) that can slightly increase the turbine speed and hence spike the boost pressure.

So far, I have been looking more at the intake side, but all these issues can apply to the exhaust, possibly more so as the ignition retard will tend to significantly increase the gas pressure in the exhaust manifolds and so have more affect on the net seat pressure, especially if the manifolds are restrictive. This may result in leakage of hot gas past the exhaust valve that ignites the charge in the cylinder while the inlets are open, if the engine used port injection (think that'd be direct, though) it will ignite that as well.

There's a fair chance it's something else, but those should explain my reasoning for the problem being lack valve control.