Head shim vs LC pistons (Real life comparison)

[DooAssassin]

This winter I had the opertunity (unfortunatly) to have both types of motor in my 07 apex running the exact same set up. This is what happened.

LC built up motor :

Rev-10700 RPM
Grams in weights :106
Psi @14

Head shim motor:

Rev 10100 RPM
Grams 106
Psi @14

To acheve 10500 RPM on the head shim I had to bring down the weight to 99 grams


Conclusion: Built up motor does make more power compared to a head shim motor with the same set up. Im estimating 40-50HP because each pound of boost would create about 10 hp and increase the rpms 100.

Whats your opinion? Builders out there is this what you have noticed also? Die hard head shim runners ?

 

[DooAssassin]

I should note the specs of the built motor

Carrillo rods
Balanced crank,piston
JE pistons 10:1
Fuel Mapping raised because this motor wants more fuel ( head shim motor runs rich on the same map)

 

[DC5]

What was the compression ratio on the head shimmed motor? What did you do with the cam timing?

 

[DooAssassin]

MCX head shim (A bit higher comp than the other motor) and the cam timing was done to the specs MCX wanted it. Both motors have the same cam timing.

 

[DC5]

I have done both and didn't notice much of a difference. I think there was something else going on there.

 

[DooAssassin]

How was your LC motor set up? And was it in the same sled with nothing else touched? My sled was running the same trail with the same weather 3 weeks apart and nothing else was changed.

 

[DC5]

I don't remember I have built so many different setups but I did have one sled that had many different motors in it and if anything I think the headshimmed motors were a little stronger.

 

[DooAssassin]

You are not the first to say this, I still am not convinced tho. I had 3 weeks between motors and nothing else was touched. One thing I think the head shim motor has over the LC motor is low end torque. This would make the motor feel more powerful....but when the LC motor opened up it pulled harder and should because its pushing more weight

 

[Stigapex]

There must be a difference in squish ?

 

[DooAssassin]

Yes, the head shim has more squish (higher compression) But still didnt make more power like the therory said is should. Or am I mistaken by what you mean by squish?

 

[kinger]

Ulmer dynoed a shimmed mpi stage 1 and a LC stage 1 and I thought they were around 3hp difference.

I think the rods, balancing, etc all helps play a role in more revs.

 

[DooAssassin]

More rev= more power, if the clutch weights havent changed. Are you saying all that extra power is from the rods and balance?

 

[kinger]

Not saying anything for sure but I know that people knife edge and balance a engine for the reason of reducing friction in the oil bath. If your rotating assembly is lighter then stock (is it?) it will rev easier.

More rev = more power is not 100% true but its close. You have 600 rpm difference which is the difference of 3 grams in a clutch which in your case is less then 3% change.

Bottom line I dont think there is a ton of difference.

 

[DooAssassin]

the crank, rods and pistons were balanced as a whole. BRB Im trying to figure out how much weigh I actually pulled out of the machine to get it to rev 10500. It was more than 3 grams more like 10. but I will look into it to be exact.

 

[DooAssassin]

I went from this:

heel
B B ----------- B B
B B ---- TO ---4 B
4 4 ------------ 2 4
tip

# being washers
B being the big lead washer. I dont know how much the weigh