Head clearance
 

Hello I was wondering if there is any rules of thumb regarding head clearance.... I Just got a new motor, a STS 30 amd it came with a total of .046 head clearance...the motor ran poorly and I dropped it down to .029 and its runing alot better, I am kind of curious to see what the proper clearancew should be...we are running 30% nitro......I appreciate any advice you could offer

Maximo, the lower you go on clearence the more power you will feel. but keep in mind that you are increasing compresion but also heet. i ran a picco p3-28 and new it had .028 hight and i went to .022 and it woke the motor a ton. just go very small incriments at a time get a good run after each change to find the sweet spot. i also ran 30 in it, no problems. also keep an eye on the plug, it can tell you alot on what the motor is doing with thodr changes. hope that helps. :sneaky:

not quite mugenb46. When you lower the clearance you do get more power but the engine runs cooler.

When you drop the head clearance, you infact have less air gap between piston and combustion chamber, this reduction in air gap, increases the ability for heat transfer, thus reduces temps. but do realise that when it is reduced to a point it can harm the engine through pre detonation (pinging) this will leave small pitting in either the piston top or the head button. trick is to run for short run times to and check for this detonation to achieve, the clearance required. basically you can run less clearance for less % of nitro, and more clearance for more % nitro.

:)

So I just got done setting the valve clearance on a 728 c.i. Gene Fulton built Chevy and I read this. As the piston compress the fresh charge there is a small amount of heat built, but nothing in comparison to the heat that's produced by the rapidly expanding burning charge as the piston is being pushed down the cylinder. When the transfers open and the fresh charge is pushed into the cylinder a fair percentage of heat that is produced from the power cycle is swept away with the escaping fresh mixture out of the exhaust port. These engine or any that I know of are not 100% efficient and heat is the by product of that inefficiency. So if you are making more power your making more heat.

True on face value.
A while back I turbocharged a customers car and he wasnt keen on rebuildong engine with forged pistons so we decompressed engine by offset grinding the conrod this lowered the compression to around 8:1 from a 9.5:1 std cr.
Car used to run great at 1.2 bar boost but always used to run hot.
after a few months trying different things we couldnt come right untill I read about engine efficiency and related info.
I rebuilt engine with new conrods and retarded some timing and got car running great ,imediarely we noticed on the dyno that temps were more stabil and car cooled a lot quicker as well as we were making the same power as before but at a lower boost pressure.
ie: in my opinion and experience there is a optimum compession ratio to suit the application and engine ,the only exception is at altitude you might need to add a shim to get more charge filling the combustion chamber to get power up.

anyway my 5cents.

Rite, it is generally excepted that per pound of boost total timing should be reduced 1-2 degrees. Sounds like it was very close to detonation. The charge was early and the rod did not know whether to push the crank around or push it out of the bottom of the block (lots of heat generated). So as part of this discussion we need to include our glow plug because not only does it light the charge it is also one of the contributing factors as to when the charge gets lit.

rule of thumb I've always gone by is:

0.026"- 0.028" on a .21 running 30% and a #7 plug

0.020"- 0.022" on a .12 running 30% and a #6 plug

I check with modeling clay just as I do when checking valve clearance on my real car stuff :tire:

*note- I'm on the east coast and generally conditions are 400- 1000ft above sea level. Only once or twice a year do I need to re-shim for a particular set of conditions.

Maximo:

Your .029" height is almost perfect (you can have a little more margin to continue lowering, but your actual value is very good and If I were you I'll keep around those value).

Combustion chamber height is mandated by many parameters, let's talk about them:

- Engine size (this doesn't changes as the engine is created with a fixed value).
- Fuel and percentage of Nitro used. The more nitro on the fuel, more chamber height needed. Also as more nitro used, a colder plug is needed.
- Relative humidity on ambient. The more humidity (more water on the air), more chamber is needed.
- Height over sea level. As you more approach to the sea level, you need to make more space for the additional Oxygen present in the air. The same (but inverse does for going higher over the sea level - less oxygen present on the air).

Chris (Cdelong) has posted very valuable numbers, keep it and use them as indication. Your final values can vary, but will be very near those.

One thing that people did not told here is another 'byproduct' you get meantime you're reducing your combustion chamber height. The carb adjusting starts to be a little bitchier, sure, you get more power, but the engine turns itself to be very delicate about carb tunning and their tunning window gets smaller.

On competition engines this is needed to squeeze up to the last fraction of power, but not for a recreational use, where you can trade some power for the ease of carb adjustment and spare you from a P/S grenading who is an expensive mess if you go too far (caution: piston pitting isn't only caused by an excess of compression, a too short pipe can cause pitting and grenading on your engine).

I carry a small weather station on my toolbox (just for measuring temp, barometric pressure and relative humidity on the track - seems a nonsense to carry one, but is a great tool to know the quality of the air you're breathing on your engine and isn't terribly expensive) where I race, the relative humidity goes very stable all the year between 41% and 58%, sometimes spikes to 65 or 75% (specially after rain or in foggy conditions) and I keep a running record of temps, pressures and rel. humidity, but my rule of the thumb is:

- If the relative humidity goes higher than 65%. I'll add one shim. And I'll continue adding 0.10mm for each 20% it raises over 65%.
- I run around 700m over the sea level, and unless I go sea level. I don't need to raise the combustion chamber. I my track is at the highest altitude here, so I haven't felt the need of going lower on comb chamber height due altitude.

I use a 25% Nitro fuel on my .21's but noticed that going from 25 to 30 doesn't changes so much (that's could be because I'm a little conservative shimming my engines when aren't on race spec - I give a little of power for the ease of tune).

seems opposite to me...... the more moisture, the LESS oxygen per given space of air (just as running at a higher altitude). The higher the humidity, the less shim I run.

this makes sense, but contradicts your humidity theory below?

I've drag raced for many years and humidity will slow down your car like nothing else...... why? there is less oxygen is in the air. the only thing we can do there is re-jet the carb using SMALLER jets. the less oxygen (due to moisture displacing oxygen in high humidity conditions), the less fuel you need. we don't really have the choice of lower the combustion chamber every week by changing head gaskets.

I'm a nuclear chemist by trade and pretty sure here on the shimming required when humidity is relatively high (80%- 90%+ for me).

You need to lower the stack height when humidity is this high if you generally run at a lower humidity.

Nope, are just different factors of the equation and they don't come from the hand. You could be at 2400Ft high and have 80% Rel Humidity or being at only 800Ft and have only a 40% of Rel. Humidity. One is a matter of pure barometric pressure and how much O2 you have per cfm and the other is how much myst you have on the air.

And remember that water is basically H2O, during combustion, the water (not all but a part of it) converts into Hidrogen and Oxygen breaking the molecules of water on the combustion chamber and you may have to take them into account. Maybe in a 1/1 engine, those changes on Rel. Humidity and barometric pressure doesn't affect them so much, but in our engine sizes such change is a big variation of the terms on the equation.

I've been playing all the summer with the Motec EFI and my laptop on my boat (a 540cid EFI 10.2:1 tranny muncher) and the humidity and barometric pressure didn't changed so much the fuel maps and spark timings.

Yep... true, but could be funny seeing someone waiting on the stage and wrenching their car to raise the heads. LOL!!!

Is true up to some point. Remember that our engines are another different animal than 4 cycle, gas engines. What works on big puppies should or should not apply to the small puppies. :smile:

I'm talking of a single variable changing....... a change in altitude, or a change in humidity. Not a change in both along with fuel, plugs, temperature, etc....

If you raise either factor (higher altitude or higher humidity) you plain and simple need to remove shims if that is the only thing you change.

Multiple variable changes may require a different approach, but these two factors alone are a relatively simple concept to understand.

Yep... (not trying to argue) but this isn't an ideal case of a controlled environment where only one factor varies, when you move to another place you face another conditions, of height and rel.humidity (and not necesarily moving to another place, from day to day, the weather varies), that's why you need to take all into account as a global problem, know the factors and make the decission of raising or lowering the Combustion Chamber knowing what do each time.

Is easy to ask and get an answer, but is more interesting know what to do, when and why (IMHO).

And of course, if you only change a variable, everything is easier. :rolleyes:

Cdelong is tuning to maintain, hopefully the same power he makes when the weather is good. Corse-r is not trying to cheat the weather, he is excepting a power loss that is expected. The humidity deal is based on the fact that you cant compress a liquid, however given what goes on in the chamber we can cheat a little as we know what is going to happen when the temps of the burning gas gets over a 1000 degrees. More oxygen (sea level) your adding shims because there is more fuel that will need to be compressed? The added fuel and raised density of the air advances the point at which the glow plug lights the charge, so why not switch to a colder plug? The motec deal is that system open or closed loop?

[Off-Topic On]
What I got is an Motec M48 Pro and could work in open and closed loop modes, quite puzzling and overhelming at first moments, but I'm just digesting all the you can do with the ECU and the info can throw it.
[Off-Topic Off]

I like running #7 plugs unless ambient temps go much below 60F. In high humidity I don't think a #8 would be all that beneficial to control timing and have the engine maintain it's performance. I like to run the #7 plug and just take 0.1mm shim out in real high humidity.

Low altitude AND high humidity/temp (FL) is real fun for a guy who is from NY and normally races in Canada. The altitude at sea level is the predominating factor in my decision making. More air= adding a shim to prevent detonation and keeping the #7 plug to keep the flame lit on the low end with the humidity. There are really three facors to consider in this condition...... altitude= add a shim, humidity= remove a shim, temp= lean the mixture. The best thing I have come up with during a lot of hot laps is to add a shim while keeping the #7 plug. My needles are usually with 1/4 turn based on temps.

ask some guys that travel up north to run what they do..... probably the opposite of what I do as far as shimming.

I'll add a few factoids that I have had experience with...

Closest clearance (piston to head) on a .12 engine is .4mm
Maximum effective clearance for .12 engine is .8mm

Closest clearance for .21-.32 engine .5-.55mm
Maximum effective clearance for .21-.32 engine 1.1-1.3mm

Lowering head clearance WILL raise combustion pressure and combustion temperature respectively. (Thermal efficiency may go up but negligibly)

Raising Nitro % will also have similar effect (more pressure and more heat)

Raising Nitro % requires more alcohol to keep similar burn ratios (this explains why we need to set carb richer when changine to more nitro) (Also this explains why miliage suffers when going to higher nitro)

Raising ex port will cause higher rpm effective tq peak

Widening ex port will have similar effect without losses on midrange

Water vapor DOES NOT convert itself to hydrogen and oxygen in the combustion process. (A. if it did there would be little use for fuel cell engines) (B. the combustion temp. is far too low in our engines for this to even begin to take place) (C. Water vapor in internal combustion engines only turns to steam during combustion)

The Combustion temps of our engines are much lower than that of gasoline or petrol engines. (Ex temps of gasoline engines in the 1200 deg range and Glow engines in the 400-500 deg range)

Hotter air = leaner Colder air = richer

More humidity = leaner Less humidity = richer (this is due to water vapor displacing useable fuel molecules in the intake charge requiring less fuel due to less oxygen)

BK

mx... seems like we are on the same page :nod:

I did not convert your metric units of clearance to what I measure in..... thousandths. I do know that every 0.1mm is .004", so your .55mm (0.022") is a bit lower than I shoot for on my standard set-up of 0.028". My 0.028" standard falls right in the middle of your band at 0.70mm. In FL I may add 0.1mm shim and occasionally I may remove one.

We are very close on .12 engines as well. Again, I fall right in the middle of your band at 0.022"- 0.024" (about 0.6mm) on my starting point.

I agree with all your other tuning points on humidity and temps as well. Usually there is one over-riding factor that makes the determination on how to best tune the engine at a given set of conditions. The determining factor could be different with each new set of variables, but good to see someone who is on the same page. ;)

Yes, my lowest values are what I consider to be the absolute minimum clearance before the piston makes contact with the head. This value may be runnable on some engines and not runnable on others depending on combustion chamber dome volumes. Moreso these values are just a guide on what is a mechanical possibility, before you have angry parts.

BK

Nitro 2 to 1. Methanol 6.45 to 1. Fuel to air ratio is what your speaking of in regards to burn ratio or are you talking about the speed the flame kernel reaches the cylinder wall? The temps you listed are these from an E.G.T. located in the header? I'm curious, based on known temps to reform Castor into large molecules and that the synthetics generally burn off some where above 800 degrees. we should not see the deposits on the piston if the temps are this low. Nice Dyno! Good conversation fellas... :nod:

Since I didn't find OS TZ thread I will post here regarding head clearance in this engine eheh
I own a TZ12-T3 with T-1040 52 pipe and I need more power to beat a Sirio Evo3 :D
I have thought change to a fuel that contains 30% of nitro, in the moment I'm using Byron 20%, but I had seen some people talking that I must change the Head Clearance, and I would like to know how many shims I should add
I've got excellent settings, good temp and my glow plug lasts hours.. I really want to keep this while with a 30% nitro fuel

Any extra advice is welcome :smile:

thanks a lot

Hi

Firstly addingmore nitro only slightly increases the power and has the effect of lowering the revs as it burns slower. I would not go higher than 25%

For 25% Notro you need a head clarance of around 0.4-0.5mm. Measure the head clearance by inserting a length of solder into the glow plug hole and turning the piston over so it crimps the solder on the head. Measire the flat spot.

Maybe for a .12cc engine 0.4- 0.5mm would be OK, but for a .21 you need about 0.7mm of clearance. 0.4- 0.5mm is far to little here in the states for a .21 and beyond most anyones margin of comfort for clearance at RPM issues. Remember.... the rod is stretching under racing conditions.

.. we are talking about 12 engines..

This is the most straight forward answer. :nod: