For any discussion not specifically related to your project
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By PSIG
#52489
boroko wrote: Fri Aug 13, 2021 11:42 am... I would have thought that knock control would have been a big deal, but I'm learning, maybe it's not.
...This application is for a rally car that sees severe duty, so it a seems prudent thing to have. Now, I understand that if you plan and tune properly, you should not need it as much, but I'm still puzzled.
In order to tune your rally car fully, you need to understand what causes knock (detonation or det). Search here for many threads about it beginning with basics like this.

For a racing engine, you should not be using the lowest grade fuel you can find, but rather the correct fuel for your engine to reach best performance. If you do, then you will not have knock, and have no need for a knock sensor. However, if you want to use low-grade fuel, then you accept lower performance and possibility of knock. You will need to know how to detect and verify knock in multiple ways, and a knock sensor is one tool that may be used with others to do so.

While you did not ask; knock has absolutely nothing to do with correct ignition timing or fueling. The power levels or severity of duty is irrelevant, except as factors to help set the grade of fuel required. Knock is a limitation to maximum engine performance caused by the choice of fuel. If you wish to use low-grade fuel for some reason, you should understand why and how to de-tune the engine in order to allow use with minimal damage, verifying knock with more than one detection method.

Knock sensors were designed for low-performance engines to run on low-grade and low-quality fuel at low power levels. You need to know more than the average racing tuner to do otherwise, with some sense of safety and reliability. Good luck!
By boroko
#52492
PSIG, thanks for the answer.
I understand the need for proper fueling for a specified motor, and I'm learning deeper details
every chance I get. The car is currently running a different motor and on a modified tune. The new motor is higher compression and being pushed a bit further. I currently run good fuel and have no problems, but with higher compression, I wanted to have any safety margin that I can. Being that rallys cover a lot of territory, there is always the possibility that you have to accept fuel that is not perfect, and I would like to have that protection, or at least be aware when I'm having problems. The last one was around 400 miles of racing and transits and sometimes you have to stop at a regular gas station and fill up along the way. I just found it curious, even though I realize its a crutch.
User avatar
By PSIG
#52529
No problem, and I'm not arguing against knock sensors in-general, but presenting a reality check. We have been in the same situation with fuels like many others, and the easy answer is to test it real-world. Fuel-up and drive it. Using the traditional detection methods such as electronic ears and spark plug reading, see if you have any signs of det at all, before you stick yourself out in the race with no recourse.

Relying on a knock sensor to tell you you're screwed after it's too late to do anything about it... sucks. Detecting det before the sensor can hear it is much more satisfying. :lol:

As a general approach -- if you are working with det-limited fuel, you should be cautiously working-up on the tune while testing for det constantly using multiple methods, so you know your limitations. For example, I typically plan to use 3 to 4 sets of spark plugs before it's fully tuned, because I use them as disposable or recyclable sensors.

You should always avoid being in uncharted territory. This way you create the best tune and maximum performance without random retard or excessive fuel, and also know when and how you'll see det, and exactly how you will de-tune to effectively counter it. Det becomes a part of life for you like diabetes, and just part of pre-race prep. Have fun! 8-)
User avatar
By Chris Wolfson
#52530
There is another problem with a (real) Rally car. As stock engine mounts will not last for more than a stage or two, usually stiffer ones are used. This will trigger false knock alarm on the sensor. Another problem are small stones hitting the engine at the oil pan and gearbox, with the same result.
On a highly tuned engine, constant over fueling and retarded sparks (if used as knock suppressant) lead to severe heat problems on pistons and valves. So the cure for knock may be the cause for similar defects on your engine.
One solution ist to use knock control only on fast tarmac stages where you need maximum engine power and disable it on tough stages.
On turbo engines knock control has an easy job: It only has to reduce boost for maybe 0.4 bar and you can run 95 octane instead of 98. It does not work that easy on aspirated engines.

For your "400 Kilometer Rally Stage" problem just keep two bottles of Octane Boost in the trunk. That will do it for the next 400 km.
By boroko
#52572
I appreciate the reality check guys. I hadn't thought about the rocks or solid engine mount. I can definitely understand where that would trigger a detect event. It usually sounds like you're going through an asteroid field on stages. Since this sport is usually traction limited, a few hp here of there doesn't matter really. As it is, the new motor will already be significantly stronger than the current one. A craftsman never blames his tools, right?

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