Head shim?
- Thread starter SAWYER
- Start date
B
BigFish BC
Well-known member
oh oh sawyers coming to the dark side
pretty much what remmy said if you run really high elevation above 8000ft you dont need the head shim but below its recommended so you can run pump fuel without detonation probs ,deto is really bad for a 4stroke.
Similar to a thicker base gasket, in some ways, which also will lower compression.
From 2 stroke thinking:
Its like installing lower compression domes/head.
Without the head shim, its like higher compression domes/head.
I think MCX shim turns compression to 9:1 or so. Least thats the one I skimmed through and saw. I'm unsure of stock compression..
Another twist is, the turbo/sc is 'compressing' the air which in a sense increases compression.
Example 1 (by no means to scale), say you have stock 12:1 compression, you are running 10 lbs of boost, compression would increase (at full boost) to 16:1 (again, no idea what or how exactly it works out)..
Example 2, say you have head shim compression of 9:1, running 10 lbs of boost, compression would increase to 13:1
Example 1 would essentially require lets say straight (at least 50/50?) race/AV fuel to keep running w/o detonation/pinging, etc
Example 2 would be safe on 91 octane pump fuel.
Which essentially is also why running higher boost requires higher octane fuel, in a backwards (well, maybe it is direct) way, your increasing compression and need to combat pre-ignition/deto with higher quality fuel.
And I could be off on how the 2 come together, but this is how I've understood it, which at least in my own head makes some sense. Someone who really knows the mechanics of it can surely get into more detail of boost and head compression relations, I'd definately be interested in an explanation.
From 2 stroke thinking:
Its like installing lower compression domes/head.
Without the head shim, its like higher compression domes/head.
I think MCX shim turns compression to 9:1 or so. Least thats the one I skimmed through and saw. I'm unsure of stock compression..
Another twist is, the turbo/sc is 'compressing' the air which in a sense increases compression.
Example 1 (by no means to scale), say you have stock 12:1 compression, you are running 10 lbs of boost, compression would increase (at full boost) to 16:1 (again, no idea what or how exactly it works out)..
Example 2, say you have head shim compression of 9:1, running 10 lbs of boost, compression would increase to 13:1
Example 1 would essentially require lets say straight (at least 50/50?) race/AV fuel to keep running w/o detonation/pinging, etc
Example 2 would be safe on 91 octane pump fuel.
Which essentially is also why running higher boost requires higher octane fuel, in a backwards (well, maybe it is direct) way, your increasing compression and need to combat pre-ignition/deto with higher quality fuel.
And I could be off on how the 2 come together, but this is how I've understood it, which at least in my own head makes some sense.
Someone who really knows the mechanics of it can surely get into more detail of boost and head compression relations, I'd definately be interested in an explanation.psi vs comp
Does anyone have a good idea what makes more HP--boost or comp. EG: will an engine with 12:1 at 15lbs make more or less Hp than a 9:1 engine at say 18lbs. If anyone has actual numbers please post them. As for the fuel requirements, compression or boost, when you reach a certain HP level you will require better fuel regardless. If you can run a shim at 12lbs down low, you can run a stock engine you will just have to lower the boost, HP should be the same.
Does anyone have a good idea what makes more HP--boost or comp. EG: will an engine with 12:1 at 15lbs make more or less Hp than a 9:1 engine at say 18lbs. If anyone has actual numbers please post them. As for the fuel requirements, compression or boost, when you reach a certain HP level you will require better fuel regardless. If you can run a shim at 12lbs down low, you can run a stock engine you will just have to lower the boost, HP should be the same.
hey buddy, would you ever consider putting a head shim on a two stroke?? It is going to affect the motor the same. buck up and buy the race gas or leave the boost down.
check out my new baby, I also went to the dark side, this season
check out my new baby, I also went to the dark side, this season
Head shim on a two stroke is really not an option. Most modern 2 strokes have o rings to seal the head which won't work. You are better off the install a spacer plate or 2 base gaskets at the cylinder base.
B
BigFish BC
Well-known member
ok here you go
at 11.8:1 witch is stock for a apex with a base horsepower of 150hp at 0ft elevation with a base octane of 87-88 at 15lbs boost you will need to run 104 oct & will make just over 300hp.
at 9.1 with the same motor at 18lbs boost you will make about 320+hp & will have to run 100oct.
these are minimal oct # its always better to run a little higher to be safe
hope this helps.
I don't think you will notice much difference if you put a shim in with the impulse kit you are going to purchase. It is a mid mount and spools very quickly taking care of any lowered compression lag you might experience otherwise.
I went with a shim in my RX to save a little at the pump. It was also a pain in the a$$ to keep race gas around. I noticed a small increase in lag but was able to clutch most of it out.
Lower the compression a little so you can run 12 #'s or so on premium. Then when you want to turn it up put a little 110 in and you will be off to the races.
J.
I went with a shim in my RX to save a little at the pump. It was also a pain in the a$$ to keep race gas around. I noticed a small increase in lag but was able to clutch most of it out.
Lower the compression a little so you can run 12 #'s or so on premium. Then when you want to turn it up put a little 110 in and you will be off to the races.
J.
ok here you go
at 11.8:1 witch is stock for a apex with a base horsepower of 150hp at 0ft elevation with a base octane of 87-88 at 15lbs boost you will need to run 104 oct & will make just over 300hp.
at 9.1 with the same motor at 18lbs boost you will make about 320+hp & will have to run 100oct.
these are minimal oct # its always better to run a little higher to be safe
hope this helps.
Thanks a lot, this is good info. Will the boost level help out response or is compression the only way. Also, if you didn't want to run race fuel why couldn't you just run lower boost without a shim?
once you turn the boost up, you will not want to turn it back down, you will want more. I think most will agree.
R
Remmy
Well-known member
Ya, that seems to be a problem, I started running boost consistently in the 18-20 lb. range and now 14-15 lbs. feels....underpowered, I'm kind of scared of whats happening to me. LOL That 110 isn't cheap either.
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BigFish BC
Well-known member
compression, timing & header length is what affect your throttle response & the thing with using timing is you need advance as you get in the throttle then you need to start pulling timing as your boost goes up.right now only a rb3 box will do that it will run you a extra 700$.
What iamvector said...timing control. Works similar to a higher octane fuel but also enables you to use a faster burn rate fuel with less octane and gain more ponies with less boost. RB3 box lets you do it as well as Powercommander 3 box. The downside is if the timing box fails...
What iamvector said...timing control. Works similar to a higher octane fuel but also enables you to use a faster burn rate fuel with less octane and gain more ponies with less boost. RB3 box lets you do it as well as Powercommander 3 box. The downside is if the timing box fails...
plus having to make changes via the laptop isnt as nice but thats the reason this box has the coolest features!
A
agent orange
Member
sawyer......................sounds like Motleyone should be running this weekend................Big Bull's early next week.............Mine wont be done for about a week or week and 1/2..................welcome to the darkside
B
BigFish BC
Well-known member
i beleave the rb3 hold 4 different tunes & you can adjust some on each one.
J
JSCC
Well-known member
At elevation, every sled I have ran up against with a head shim has required about 3lbs more boost to equal the power mine makes with stock compression. However, the higher I turn mine up, the bigger the gap has to be.
The only difference you will notice with or without the head shim is under low, or no boost. Once boost comes on, it is just a matter of who has more, although keeping with the gap I mentioned earlier.
I tend to compare it to a front mount versus a rear mount. Both tuned equal. The front mount will come to full boost first, so will the sled with higher compression, all things being equal.
If at high elevation, do not let anyone talk you into the shim. Yes, you will be required to run better fuel. However, with the huge loss of air up high, you will want all the compression you can get. Imo.....
The only difference you will notice with or without the head shim is under low, or no boost. Once boost comes on, it is just a matter of who has more, although keeping with the gap I mentioned earlier.
I tend to compare it to a front mount versus a rear mount. Both tuned equal. The front mount will come to full boost first, so will the sled with higher compression, all things being equal.
If at high elevation, do not let anyone talk you into the shim. Yes, you will be required to run better fuel. However, with the huge loss of air up high, you will want all the compression you can get. Imo.....
