Headshim vs Stock compression

[LuckydawgAK]

What do you feel?

Debating staying around 8-9 psi on stock compression vs doing an Stage I kit with a headshim? I don't want to lose on the bottom end. Who has experience on both on the Genesis motor?

I realize the other advantages with the Stage I but is it worth $1500 if I'm shooting for 8-9 psi?

This will be a 2015 Viper Mountain.

 

[FLYBYU]

8 to 9 psi is really pushing it on stock compression unless your running race fuel. I wouldn't do anymore than 7 psi.

 

[A.Granath]

I first did the 180hp (6psi) for one year.. then i mounted the headshim and upgraded the fuel system for 12-13psi for this season.

On stock compression the engine have a more raw feeling i think, it's hard to explain but now with the headshim it feels more like a big block 2-stroke engine.

I have done a lot more beside just changing the headshim i also mounted a inter cooler, rebuild plenum, new clutching ect

The added boost makes the sled launch hard! When lining up vs other nytros with turbo applications is't faster out of the hole!

Overall i feel confident to recommend adding a headshim on a supercharged nytro engine, the boost comes on so fast anyway and compensate the lost compression.

//AG

 

[HURRICANE]

If you are going to add a head shim,I would recommend that you reset the lobe centres of your cams and then turn up the boost to about 10lbs.We have just tested this once again on the Hurricane Viper 190hp kit.We make 192.7hp@6lbs boost with stock engine.We added a head ship but set the cams and we then made 215hp@6lbs boost.The gains definately due to less valve overlap.We then turned up the boost to 10lbs and made 240hp where the turbo starts to run out.The turbo still spools instantly

 

[Riceburner]

4 lbs more boost only added 25 HP. [ Seems low ] Was the Turbo getting inefficient above 6 psi?

Adding head shim same boost added 18 hp. I get the valve overlap change but that seems like a lot.

Not questioning, just looking for a little more explanation of the #'s.

 

[HURRICANE]

The turbo is efficient till 240hp. We actually made 252 at same boost with a leaner tune but I rather add fuel to insure the the engine is well in the safe zone.

 

[TurboTime]

The turbo is efficient till 240hp. We actually made 252 at same boost with a leaner tune but I rather add fuel to insure the the engine is well in the safe zone.

Amen to that quote. To lean to long to bad

 

[Yamadog]

On the viper to go to 9 lbs you will need at least a head shim and fuel pump upgrade. The stock pump cannot push enough fuel for anything over 6 lbs. Remember it is a CAT pump!!!!

I am running carillo rods, Wiseco low comp pistons and the MPI kit with upgraded wastegate and can now go safely to 12 lbs. I have it switched at 6 and 9 for trail riding now. I rode it once at 11 lbs and it was too crazy at that time. Now with over 800 miles on this set up I am thinking of pushing it back up to 11 but octane is a great concern. I am carrying Torco booster with me cause I don't trust the stations in the UP that say 91 non eth. 1 bottle will push 10 gals up about 7 octane pts so I add 1/2 to each fill up. I carry 110 race gas with me in the trailer and blend that with 91 in the mornings when I leave the shack.

 

[LuckydawgAK]

Evolution changes the timing and has a stock compression kit to 11 psi. More details will be out soon.

It seems like a lot without a head shim but it is what it is. Boost-it out of Canada does the same thing. They both are successful and sound like great kits. Boost-it is only in CA. Both kits come with an upgraded fuel pump and air-air or air-water intercooler

I start off at SL so I don't think I would lose much on the bottom end like those shimming at higher elevations. Having stock compression and 8-10 psi is appealing but it might not matter for those at SL.

 

[Remmy]

Stock compression should make more power at equivalent boost levels with snappier throttle response.

I've also been debating this same issue (Viper mtx) Pretty sure I'm going to run stock compression and av gas up to 9-10 lbs of boost. 110 octane if I want to push it to 12. Can't wait.....

 

[mbarryracing]

Guys, keep in mind boost is just a number, doesn't always direcly correlate to HP... so don't be fooled by a higher boost pressure number, as proven that you can make more HP with lower boost with a more efficient turbo on the same engine that you'd have to run more boost otherwise.
When talking timing and boost on a stock motor, I am confident that you'll find that pulling the ignition timing out and running higher boost pressure typically will make near the same HP as stock timing with lower boost pressures...
Unless you are provided with some data that shows a power advantage, or you just like bragging that you are running bigger boost, then I'd prefer the lower boost pressures because it makes less intake charge heat, less heat means less you have to try to pull out to gain a margin of safety for detonation. Just my 2 sense...

 

[ghost rider]

Guys, keep in mind boost is just a number, doesn't always direcly correlate to HP... so don't be fooled by a higher boost pressure number, as proven that you can make more HP with lower boost with a more efficient turbo on the same engine that you'd have to run more boost otherwise.
When talking timing and boost on a stock motor, I am confident that you'll find that pulling the ignition timing out and running higher boost pressure typically will make near the same HP as stock timing with lower boost pressures...
Unless you are provided with some data that shows a power advantage, or you just like bragging that you are running bigger boost, then I'd prefer the lower boost pressures because it makes less intake charge heat, less heat means less you have to try to pull out to gain a margin of safety for detonation. Just my 2 sense...

X10