Ambros303

For the engine experts/modders/tuners:

What is the rule for deck height in relation to percentage of nitro?

I found this chart (ad 0,006” for every 10% nitro):

0% = 0.006” = 0.15mm
10% = 0.012” = 0.31mm
20% = 0.018” = 0.46mm
25% = 0.021” = 0.53mm
30% = 0.024” = 0.61mm

These are supposed to be conservative (safe) settings for a .21 engine (sea level)

Is this chart correct and what are your thoughts on this?

Also, if the above is true, then why do most factory engines come “over shimmed” by a large margin?
Some reasons I found: engine will last longer with lower compression and is easier to tune (tuning window is bigger), it will also have higher top speed (but less low end). On the other hand, with higher compression you will have more power…

I measured some engines yesterday:

Stock deck height Sirio CL7B 2010 = 0.75mm (also has 0.75mm shims, so there is no step up in the button)
Stock deck height GRP tuned = 0.70mm (has 0.4mm shims so there is a step up of 0.3mm in the button)
I know Nova engines come shimmed at 0.8mm – not sure if this is actual deck height aswell (don’t know if there is a step up in the button)

By the looks of it one could remove a 0.10mm shim out of each engine, also will it affect longlivety much or its just marginal? What are your experiences?

PowerHouse

iTrader: (66)

Most engine's are shimmed conservative mostly for liability reasons. They have to make something that will be fairly reliable based on the simple fact they don't know who is getting it and what they will try to do with it. A good majority of users improperly care for and tune their engines and having something that is borderline on compression isn't something most companies want to risk with their product.

As far as your question, I wouldn't worry about the shimming as much as I would worry about overall compression ratio and squish band. The squish is pretty critical and ideally you would want the band to be about 30% of the radius of the bore, have about a 1-2 degree rise from the taper of the piston and different application will want different squish but since it is the offroad threads, a typical offroad engine likes around a .018-.020" squish and most stock engines average between .022-.024" and a .1mm shim is equal to .004" and that's why most guys will pull it out as it will set up the squish band very nicely. Just be sure to run a cooler plug and make sure your tune is spot on if you run a higher nitro percentage with the added compression and reduced squish band.

Rule of thumb:

Smaller squish band/more overall cranking compression- If running this set up and using a higher percentage of nitro, use a medium or colder plug to help with pre-ignition issues. If you decide you want to stay with a hotter range plug, reduce nitro percentage to help with pre-ignition issues. This is recommended for advanced tuners.

Larger squish band/less overall cranking compression- If running this setup (usually stock shimming or more), engine can run leaner without as much risk of pre-ignition. Most people will run a hotter plug in this situation to help clean up the idle and low speed response but remember, the hotter the plug the sooner the charge will reach it's flash point and you always want the engine to roll over the top just as it reaches it's flash point so the charge burns, not explodes.

Alot of people think that the charge is supposed to start it's ignition sequence at the top of the compression stroke and that isn't true. Ideally you want the sequence to start a couple degrees past TDC or at the very beginning of the power stroke. When the iginition sequence begins before the piston reaches TDC, it explodes because the piston is still travelling upwards increasing the compression ratio while it is trying to burn off the charge working against the piston's travel so as it trys to impeed the piston's energy, it also sends a shock wave down the piston into the rod and down into to crankcase which is a big reason for premature rod bushing and/or bearing failures. When the piston rolls overs and the ignition sequence begins, the charge burns at a high rate of speed generating a new energy source as the charge burns and the gases expand under heat and pressure pushing the piston downward for what is called the power stroke. During the time when the ignition sequence starts and the exhaust port opens, the piston will have enough energy to not only move the object it is mounted to but it will also have enough energy to overcome the compression stroke for the next complete cycle giving us a constant and sustainable driving force.

Maximo

iTrader: (11)

I have run my engines with a piston/head clearance of .35 mm and using 30% nitro....

NitroVein

I´m no expert, but the shimming is only true to one specific engine.
The volume in the head might be different, then for instance the duration on the exhaust will play a huge roll in the compression also. Then there is the mean squish velocity that will be affected by different squish clearance, and that will also differ upon angels of the squish band and intended rpm.
This is only some of the things that will affect what shimming that you should/could use. I use 0.4-0.45mm total clearance with 30% fuel with no problem...

I´m sure some of the more experienced moders will clear this up for you in a better way.

As they did...

NitroVein

I´m not trying to step on any toes here. But the way I have learned it, it´s when the burn pressure curve don´t come in the right spot on the mechanical pressure curve (which is also affected by crank geometry when it comes to time rate versus burn rate) that the pressure rises in a way that the fuel might burn in a uncontrolled way (explosion instead of burning, or multiple flame fronts). The time to burn the fuel is seldom enough if it ignites past TDC, that´s why the squish band is so important to help with the burn rate in time.
Sorry for my "slightly" bad English, hope I made some sense...

Ambros303

Very interesting stuff so far.
It seems tho most stock engines are shimmed very conservative.

Btw what is the exact diff between deck height (space between piston at TDC and button) and squish band?

Thats some agressive shimming
I have seen people post about this in the on road section. I take it this needs a (much) cooler plug.

Also, will higher compression wear out the engine noticably sooner? i.e. the rod, crankpin and bearing will have to deal with some extra force/power.

NitroVein

If you have the engine mounted upright in the car, the deck height is vertical distance between the top of piston and the head-button, the squish band is when you look at it horizontal. The surface that´s parallel to the top of piston before the dome in the button.

PowerHouse

iTrader: (66)

You are not stepping on anyone's toes, you just elaborated on what I was saying. I was just keeping things simple as most people are not up to speed on the terminology and what you explained is similar to what I explained.

As far as post TDC burn, lets keep in mind that the piston remains at TDC and BDC momentarilly while the rod changes angle so when I say it should fire off post TDC, I mean just after the rod changes angle during rotation. If you degree the engine, you will see there is a good 15-20 degrees where the crank rotates and the rod changes deflection but piston remains stationary at both TDC and BDC.

NitroVein

Yes, that´s correct, there is a several cranking degrees that the piston remains at TDC. The ignition should start in the beginning so that the MEP is reached somewhere after TDC, when is depending on crank-geometry in relation to desired rpm range/power curve.

Ambros303

Thanks ,

I found this aswell, visualizes it:

Maximo

iTrader: (11)

Yes, too much compression can damage an engine..but you can run a tight head clearance without having too much compression for the engine...Just depends on the exhaust timing and button volume....Much of the time finding the sweet spot of the head button shimming comes from trial and error...Different fuels, different glowplugs, clutch setup, tune pipe will all affect where the sweet spot of the head button needs to be..so in the end it comes down to spending a little time with the engine and finding its sweet spot.....No amount of math or calculations can predict 100% the ideal head clearance and IMO the best way is just old fashioned trial and error...there are just too many variables involved to rely on any set of formulas....

NitroVein

And that´s why they come shimmed the way they do from the factory.
The manual would be an impressive book no doubt, but it would take longer time to read it than the trial and error method takes (if you want the last drop of performance out of you´re engine). But it always helps to know a little about what does what whit the engine.

Ambros303

Will do some testing in the weekend

About squish band, looking at the pic:

There's squish band width

and squish band thickness (at edge) = deck height

Roelof

If you want to start the ignition beyond TDC you are to late, if it did not ignite at the highest point where the maximum compression is then there will be no ignition after TDC.

The start of the ignition is always just a fraction before TDC. There is a small time before the combution takes place (lot of youtube slowmotion movies to find about ignition and combustion. If the ignition is on the right time before TDC then the combustion is on time after TDC.

OK, then there is the need for an earlier shift with the ignition timing at higher rpm's to be sure the combustion does not come to late. The cooldown time of the plug will get shorter at higher RPM's which gives more restheat in the glowwire for the next ignition what makes the shift. Also flex in the crankshaft and rod will made the piston go higher making more compression making the ignition sooner.

Headclearance is normally needed to be sure the piston is not touching the head at high rpm when play and flex is playing a huge role. Beside that there must be also some play to play arround with the nitro content and the altitude. If you do need more space then use a smaller chamber:
http://events.redrc.net/2010/07/oxyg...-engine-power/

To give a small idea, with F1 engines it is possible to squeeze an hair between piston and head without touching each other at max rpm.

To come back on shimming.... The Italian and Asian .21 engines comes mostly set to 25% because that is the maximum what may be used, .12 engines are standard set to 16% wile some American .12 models are set to 25~30% (like the JL red dot)
Mostly the advise is to change 0.1mm per 10% nitro change comming from stock but you can always play with 0.1mm extra or less to see what the engine does with the same nitrocontent, it does set the ignition timing..

Ambros303

I don't know if onroad engines come shimmed more aggresively, but Nova offroad engines (for example), come shimmed with 0,8mm for 25% nitro. This seems to be very conservative (for reasons mentioned earlier in this thread) as 0,6mm is still regarded as a safe setting for 30% (but sounds like a cooler plug is probably needed).