head shim or no head shim

[06 apex yamcharger]

Hi just building a motor for my 2012 apex putting 10:1 carillo pistons and carillo rods and arp head studs. It has a 315 mcx kit on it and just wondering what i should do.With the 10:1 pistons if i should run a head shim or just a head gasket or maybe 2 head gaskets its going to be a trail sled but race sometimes i don't mined putting race fuel in it when I'm going to race it but for trail driving i only want to run 91 octane. Thanks

 

[mbarryracing]

Im pretty sure you are all set with just those 10:1 pistons for pump gas but the level of boost able to be safely run is really going to depend on a host of other variables, other than the compression ratio... As we've learned that all boost is not created equal.
What limit does MCX recommend? What boost level do you want to target for trail riding?

I also run a 10:1 CR, but with Wiseco's. Before I upgraded to a MoTeC ECU and was still running the stock ECU (stock timing) with a FPP piggyback fuel controller, I ran 12-14 psi boost on the trails with 91-93 R+M/2 trail gas. My GT2860 turbo is very happy at those boost levels and not at the edge of the compressor efficiency, so it's not really beating up / heating up the charge air. I think the Turbo on the 315 kit is going to be good there too.
To run any higher boost levels with pump gas will require good timing control or lower CR (head shim). I'm not a fan of shimming, but be warned that an even lower CR will adversely affect the engine response on the trail.

 

[06 apex yamcharger]

Im pretty sure they recommend 9:1 with the stock pistons and the 315 kit runs 17lbs.

 

[mbarryracing]

Ok there you go.
Depends on what you want to do with it. Running that much boost for the trail is going to be a trade-off. I can appreciate wanting to run more and more HP, but I've started to come full circle on what is more practical for me anyways. Mostly trail riding...
My opinion is 9:1 is going to be lazy in tight trails, throttle response, etc unless your WFO making 17 lbs because you don't have control to advance the ignition timing down low to compensate. Combine the additional heavier clutching to hold back 17 lbs boost with poorer throttle response, the difference in lag will be noticeable.
Now wide open high speed trails where you are in the boost most of the time, or maybe if you were drag racing or boondocking all the time, it would be a different story.
Just food for thought keep in mind, just because it can run 17 lbs boost doesn't always mean it will be fun to drive it there all the time. There is a reason the trend for trail purpose turbo machines is going towards keeping stock compression for improved throttle response while controlling ignition timing instead. AKA reflashes, etc

 

[lakercr]

^^very good advice here^^

I'll ask Rx Fern to comment on this for you. I just had his sled apart in the fall to take it back to a single stock gasket (and some other stuff I don't think he wants mentioned). I think he's happy with having some compression back. I'll let him comment on the specifics; I know he's been out doing some testing.

 

[06 apex yamcharger]

Thanks for the replies. It will be trail sled 90% of the time and i don't really want it to be lazy down low thats why i thought with the 10:1 pistons and 2 head gaskets would be better then the 10:1 pistons and the head shim

 

[mbarryracing]

I think you will find that just the 10:1 pistons with stock head gasket at 12-14 lbs will be more fun and responsive, you might even find it will be quicker on the trails, than 9:1 at 17 lbs... Maybe not in a drag race, but honest trail riding all day long...

 

[lakercr]

I personally wouldn't run the LC pistons with two gaskets for all the reasons MBarry listed.

 

[rx1 fern]

I prefer a sled without the shim. Thanks Jim for the excellent work on my sled . The sled starts better , runs smoother and the throttle response is night and day difference .the benifits of the shim is good for a trail sled on pump gas but even with the shim and lower boost levels it still feels and pulls better with the shim at higher boost levels . If it was me I wouldn't shim the motor . I went 10 years with the shim . Jim had done the sled 10 years ago and has been beaten on for that many years . I took it to him to finish my msd mc4 and he told me to get rid of shim . Glad I did too . He was impressed with the sled for the 10 year old install on it