Head clearance
10-18-2006, 06:39 PM
#1
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
10-18-2006, 08:30 PM
#2
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. 
10-19-2006, 02:49 AM
#3
Originally Posted by mugenb46
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.
10-19-2006, 06:22 AM
#4
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.
10-19-2006, 10:32 AM
#6
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.
10-19-2006, 07:40 PM
#7
Originally Posted by MugenDrew
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.
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.
10-20-2006, 08:37 AM
#8
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.
10-20-2006, 07:54 PM
#9
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
*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.
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
*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.
10-21-2006, 12:02 AM
#10
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).
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).
10-21-2006, 07:56 AM
#11
Originally Posted by Corse-R
Maximo:
Combustion chamber height is mandated by many parameters, let's talk about them:
- Relative humidity on ambient. The more humidity (more water on the air), more chamber is needed.
Combustion chamber height is mandated by many parameters, let's talk about them:
- Relative humidity on ambient. The more humidity (more water on the air), more chamber is needed.
Originally Posted by Corse-R
Maximo:
- 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).
- 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).
Originally Posted by Corse-R
Maximo:
- 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.
the altitude theory makes sense, but again, contradicts the humidity theory?
- 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.
the altitude theory makes sense, but again, contradicts the humidity theory?
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.
10-21-2006, 11:28 AM
#12
Originally Posted by cdelong
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?
this makes sense, but contradicts your humidity theory below?
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.
Originally Posted by cdelong
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.
Originally Posted by cdelong
You need to lower the stack height when humidity is this high if you generally run at a lower humidity.
10-21-2006, 02:14 PM
#13
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.
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.
10-21-2006, 02:58 PM
#14
Originally Posted by cdelong
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.
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.
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.
10-23-2006, 11:08 AM
#15
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?