To shim or not to shim; that is the question....

[ghost rider]

I run 8-10 lbs boost (race gas) on a stock motor... Ie) no head shim.
With the front mount turbo I get instant throttle responce and a ton
Of torque which is fantastic on the trails - which is 90% of my ridding.

So if I shim I can stop running the 10 buck a gallon fuel every weekend
and save money ...Plus jack up the boost to 12-14 with race gas for some
High speed lake runs.
My question is how a shim behaves under normal trail ridding
I hate lag with a passion and currently have zero lag.... Will a shim give me lag
And poor throttle responce?????? I have read this is so.... But I have no experience with
A head shim.....coming from the automotive world of turbos and super chargers
It feels like an odd thing to do.... In fact it's never done on a car that's worth it's salt.
But this is no car so I just don't know ........

Thoughts??????

Hear is my sled in action running trails with 8-9lbs boost

http://youtu.be/jRAeDB_1RqU

 

[A.Granath]

Hard to explain how a engine "feels" after a headshim, best i think would be if you where able to throw a leg over a sled with a shim installed and try it before pulling the trigger.
I installed a shim on my supercharged nytro and now i run pumpgas @ 10-13psi on sea level.

 

[Riceburner]

My answer is "it depends".

We head shimmed the 2005 MPI Supercharged RX 1 and it has no lag.

My head shimmed Apex, a 2011 MCX 290 rear mount Turbo has some lag but not near as much as a stock Turbo Cat.

The headshimmed 2009 Doo 1200 in our group with a Powderlites front mount Turbo is better than my Apex but not as good as the Supercharged RX 1.

I believe you are correct, that the best way is low compression pistons.

I think with your front mount , based on my experience, and what the guys with the new Turboed Vipers are saying, that you will get a little lag but not enough to bother you.

If I add boost to my 2013 Apex it will likely be head shimmed if I Supercharge it & low compression pistons if I go with the MCX Rear mount Turbo again.

Race gas not an option for me as I usually run more than 100 miles a day and race gas on the trail is not available.

Hope this helps.

 

[ghost rider]

Helps a lot.. Thanks. I have an innate dislike for any type of turbo lag.
Having run porsche cup cars in the. Early eighties turbo lag was the norm.
Now running supercharged cars with 23-25 lbs boost and zero lag there is no going back
I'm not a straight line guy but prefer the road course.... Or in sled terms the trail ride experience. I'm not the fastest guy but I can't run fast enough to please my inner insanity.... However I have more control and security without the dreaded lag.
The front mount tubo gives me the feel of a no lag super charger and allows for greater corner speed in, through and out..... Lag has always been the deciding factor between 1st place and third.....that's why the shim has me worried... Unfortunately i don't have the ability to ride a shimmed sled and compare......

Barry

 

[Yamadog]

I went with low compression wiseco pistons in my viper and could not feel any diff at 6#'s of boost. We did change to a quick spool waste gate which made up for any difference. At 11 lbs it is crazy and builds boost so quick.

At an estimated 2 seconds I am reading 9 lbs and 6 is almost instantaneous

 

[ghost rider]

Thanks yamadog.... I'm starting to think that pistons and rods are the answer.
I would think that quench is adversely affected by a shim....?
But I don't know .... I have never shimmed a motor... Only heard about its use
For sled engines it's not something you would do to a v8
At the end of the day pistons and rods are top shelf..... But not cheap....
It's hard to afford go fast on a sled and also in a car... Adds up quickly......

The shim is definitely affordable but I still wonder about lag and overal trailability?


Barry

 

[Yamadog]

You are correct!!! it is expensive but we are talking about a 1059 CC motor that is making 240 hp or more at 9000 rpms. The risks are that you will end up like me and have to replace the block, rods, pistons and head work if it goes wrong!

The shim is one way of reducing compression but with the pistons you are assured that octane will not be an issue and the motor is not adversely stressed.

There is several ways to achieve what you are looking to do inluding changing the cam timing, shim or pistons but I now sleep well at night knowing that I am not even close to the limits of the motor with this boost level at the low altitude I am running.

 

[Yamadog]

Also I know what you mean as far as $/speed. I have spent thousands on my cars only to gain tenths in the 1/4 mile. This is expensive but you don't need a stopwatch to see the gains.

From stock which I did not even want to ride to 6#'s was great but pushing it nearer to 10 is AWESOME!!!!

 

[nate007]

As most have said, it depends, but on other threads where this has been discussed in the past, the general consensus is this..
For a trail ridden sled that sees more miles, keep the stock compression for the lower end power and rideabiliy, and run lower boost. If you start running more boost on a stock compression motor, invest in the timing control system to retard the timing under boost.
If you're not running as many miles, or run more wide open than anything, adding a shim or lowering compression will help with combustion temps and allow higher boost levels.
Either way, I feel strongly about retarding timing at anything over 5-6psi, or 7+/- on lower compression, but every setup is diferent.

 

[ghost rider]

If you start running more boost on a stock compression motor, invest in the timing control system to retard the timing under boost.

Thanks Nate; great advise….what kind of timing control works on a RX1? -im familiar with these systems but only for Automotive
applications.

 

[mbarryracing]

ghost rider, for timing control on a RX1 go with a MSD MC4 conversion, that has boost reference capability along with 2-step anti-lag, etc.

 

[ghost rider]

ghost rider, for timing control on a RX1 go with a MSD MC4 conversion, that has boost reference capability along with 2-step anti-lag, etc.

Thanks Murray....you have been very helpful this year for me getting the bugs out of this machine....
below is a "trail ripping video"

http://youtu.be/jRAeDB_1RqU


Barry

 

[nate007]

ghost rider, for timing control on a RX1 go with a MSD MC4 conversion, that has boost reference capability along with 2-step anti-lag, etc.

X2 on this one.. I'm a little less familiar with the Rx-1 than I am the newer stuff, but the MSD MC4 is the best coice for that sled. Someone chime in here if I'm wrong on the RX1 stuff, but as a general rule, I'd start with pulling one degree of timing out per pound of boost after 3psi, and I'd go 2degree per after 8 or 9 psi to start. That sould equate to 10 degrees at 10psi, 12degrees at 11, etc. That should be safe to start, and you can add back in a degree or two here and there while tuning. Without knock sensors it's harder to tune sled motors, so it's best to start with aggressive compensations and gradually advance timing and lean the afr's under a variety of operating conditions.
The cooler you can run your engine, the better everything will work, so be sure you've got all of that taken care of first. I've seen guys blow motors at 160f under boost, so when I hear people running them hotter than that, it makes me cringe a little. Remote oil filters, inline oil coolers, etc, will help cool the motor as well even simply by adding oil capacity.
If you run a good timing control, and can keep your engine temps (and boost temps too!) down, you should be able to build a pretty aggressive and reliable system, even on 91 octane.

 

[kinger]

Been shimmed on supercharged 10-11 psi apex for most of its life has over 6500 miles now. Run stock timing on 91 pee water and the GEMS unit. It just runs and scares the sh** out of anybody I put on it including myself on occasion. Its fun to overthink things sometimes but I see absolutely no reason to NOT shim the 4 holer. 6 psi is fun but 10-11 is where things get epic.

As for why shimming, the cylinders being lined with Nikasil is a major one, they need to diamond honed for new pistons and rings that takes over $1000, then add to the fact these are low volume and pistons are 2X that of the automotive variety. When you add it all up the 'proper' way is to use pistons but people just say that, in reality the shim has shown no issues for many boosted sleds and its hard to beat $60 in parts to pull up to a 91 pump on a trail in the middle of BFE gas and go!

That said I never ran a shimmed turbo sled so take it with a grain of salt.

 

[MrSled]

Been shimmed on supercharged 10-11 psi apex for most of its life has over 6500 miles now. Run stock timing on 91 pee water and the GEMS unit. It just runs and scares the sh** out of anybody I put on it including myself on occasion. Its fun to overthink things sometimes but I see absolutely no reason to NOT shim the 4 holer. 6 psi is fun but 10-11 is where things get epic.

As for why shimming, the cylinders being lined with Nikasil is a major one, they need to diamond honed for new pistons and rings that takes over $1000, then add to the fact these are low volume and pistons are 2X that of the automotive variety. When you add it all up the 'proper' way is to use pistons but people just say that, in reality the shim has shown no issues for many boosted sleds and its hard to beat $60 in parts to pull up to a 91 pump on a trail in the middle of BFE gas and go!

That said I never ran a shimmed turbo sled so take it with a grain of salt.

Right on!! I love mine with more boost and head shim...