Boost Level Saftey?

[RACINSTATION]

I just wanted to start a thread to get some input on boost levels. We have always run a head shim or low compression pistons when going over about 13lbs of boost. I would like to build a stock engined Nytro that will run 13-17lbs with stock pistons and rods as well as stock compression. I like the stock compression for bottom end in the trees, but don't want to jack a rod through the front of the case. Race gas or octane is not an issue as we will run whatever is necessary.

How many pounds?

Stock engine?

Fuel octane level?

 

[JSCC]

I personally will not lower compression to add more boost, it is counter productive.
So both my 4 stokes have been stock compression, and once I decide that my Apex has enough miles to warrant taking her down, I will do as you are and build a stock or higher compression turbo motor. I really cannot understand why people take compression away except to use lesser fuel.
As for your question, on our Nytro we ran up to 16lbs on a completely stock engine. Typically though it was ran at 12lbs for many miles.
18lbs on the Apex for races, 15 lbs for fun and thousands of miles at 12lbs.

 

[RACINSTATION]

What octane fuel were you running on the Nytro at 16lbs? Also stock cam timing? I agree that it is counterproductive, and I also think that the boost is more of an octane issue than a compression issue.

 

[zebiraross]

I don't have anything against stock compression. In fact if I wasn't going to run higher boost that require beefed up rods and pistons I would run stock compression. Imo if your running stock internals, run stock compression. The big IF is are you willing to run the proper octane.
In my opinion I would run up to 12 psi on stock compression on 100 octane, 13-14psi 110 octane ,15-16psi 112 octane 17-18psi 114 octane, anything over that I would run 116 octane plus. I personally wouldn't run stock compression stock internals on any octane for more then a quick run over 18 psi.

 

[RACINSTATION]

Yes, will be running straight 114 at 15psi on this engine. I would just rather put rods and pistons in now versus later, but want to give it a shot.

 

[Matt@MPI]

I get this call a couple of times a day. We don't recommend the head shim. The calls I get go something like this, Hey last year my sled ran prefect but this year it runs like crap. The RPM fell off and the AFR's are all over the place. I think your superchargers not working right. I ask them what they did to the sled this year and they tell me "nothing just a head shim. If you dont want to pay for race fuel lower the boost.

 

[BLUE BLOOD]

MATT, HAS MPI EVER TESTED A HEAD SHIM OR CAM TIMING CHANGES?

 

[roughrider99]

would stock piston aftermarket rod combo work for high boost? or does anyone make a piston stong enough so that u can run more boost on stock compression? If a guy were to run stock compression with aftermarket internals and arp studs is there any other weak point a guy would have to worry about?

 

[JSCC]

High boost and lower compression is an oxy moron, simple as that. It is a easy way to say you run more boost. One fact I do know is that it will take a ton of boost to overcome the loss of cylinder pressure by cutting your compression. Cylinder pressure = Hp. If you want to bleed of some pressure to add extremely high amounts of air wither it be by turbo or Nitrous, have some one cut you some cams that can do this.....that is all your getting out of me on this subject though


As far as fuel. we ran Sunoco 112 at high altitude in the Nytro at 15-16lbs. I would not dare to push this motor any higher without rods.
If my memory serves me right, JE will build custom pistons in batches of 4. So if you had the desire, you could have them build you a stock or higher compression piston and sell the other sets.
I dont know of anyone twisting the crank or lifting the heads yet. I would say lifting the head is going to be the weak link, if you have done rods and pistons.....just a guess though.

 

[RACINSTATION]

The killer is detonation. The stock pistons are as good as anything out there. I see rods start to fail around 18-20 psi of boost, but again detonation is the core problem. I appreciate the input and want a lot more. I just don't want to be the guinea pig. You can check plugs and watch afr #s, but most of the time when you see signs of detonation you are too late.

 

[philsummers21]

So how much more hp is 12# with no headshim vs 12# with a headshim.

 

[Matt@MPI]

MATT, HAS MPI EVER TESTED A HEAD SHIM OR CAM TIMING CHANGES?

Yes. It doesn't work. It runs. But it doesn't run very good.

 

[kinger9]

There is a nice octane boost calculator on TY that has been validated on dynos. Saying reducing compression for more boost is counter productive are WRONG, imo.

I want to run 91 octane ok so max boost is 5 psi on my apex for about 200 hp. I installed a head shim two years ago to lower it to 10:1 CR and now I run 10-11 psi on 91 and dynoed 241hp.

So if fuel is my limiting factor reducing CR and upping boost adds 40+hp to my sled on the same gas.

No idea why MPI doesn't reccomend a shim, maybe because they are talking 6000ft+? I am running at sea level.

I also love the sled, a SC shimmed apex for the trails is the envy of everyone that takes a pull on it. I get around 9-10mpg and sore shoulders every time all on 91 available at any gas station on the trail.

My $0.02

 

[Matt@MPI]

I don't think in the mountains its necessary. But the flat land guy is different. They ride their sleds to the gas station. I don't doubt the HP increase running more boost with shim and pump gas. But what I see and hear on the phone every day is the AFR's change in the mid range with the shim and it hard to correct with tuning. I’m not saying its wrong we just don’t recommend it.<O</O
<O</O

 

[BLUE BLOOD]

I DON'T THINK IT'S THE HEAD SHIM THAT IS THE PROBLEM IN THE MID RANGE. I FEEL THAT IT'S THE FUEL CONTROLER THAT LIMITS THE TUNING.SO WHAT WE NEED IS BETTER FUEL CONTROL.

MATT, HAS MPI TESTED THERE CLOSED LOOP SYSTEM WITH A HEAD SHIM?

 

[kinger9]

How many pounds?

Stock engine?

Fuel octane level?

Using the afformentioned excel spreadsheet and a altitude of 5000ft you get this:

12lbs = 99 octane = 216hp
14lbs = 101 octane = 234hp
16lbs = 104 octane = 252hp
20lbs = 108 octane = 287hp

These are rough but within 5-8hp and octane within .5 pt.

My dyno was off by .5hp compared to this spreadsheet on my shimmed supercharged apex, always used 91 and never a issue, even on long 1+ miles lake runs at WOT.

Hope this helps.

 

[PistonBroke]

View attachment Engine boostcalculator.xls

I think this is the one you guys are referring to?

It's fun to screw with all the variables and see what the effects are!

 

[LOYTOY]

I don't have anything against stock compression. In fact if I wasn't going to run higher boost that require beefed up rods and pistons I would run stock compression. Imo if your running stock internals, run stock compression. The big IF is are you willing to run the proper octane.
In my opinion I would run up to 12 psi on stock compression on 100 octane, 13-14psi 110 octane ,15-16psi 112 octane 17-18psi 114 octane, anything over that I would run 116 octane plus. I personally wouldn't run stock compression stock internals on any octane for more then a quick run over 18 psi.

i ran my apex with stock compression at 15-16lbs boost on vp c16 strait for a year... one day i turned it up to 18lbs boost and blew the head gasket right away... just a FYI

 

[JSCC]

There is a nice octane boost calculator on TY that has been validated on dynos. Saying reducing compression for more boost is counter productive are WRONG, imo.

I want to run 91 octane ok so max boost is 5 psi on my apex for about 200 hp. I installed a head shim two years ago to lower it to 10:1 CR and now I run 10-11 psi on 91 and dynoed 241hp.

So if fuel is my limiting factor reducing CR and upping boost adds 40+hp to my sled on the same gas.

No idea why MPI doesn't reccomend a shim, maybe because they are talking 6000ft+? I am running at sea level.

I also love the sled, a SC shimmed apex for the trails is the envy of everyone that takes a pull on it. I get around 9-10mpg and sore shoulders every time all on 91 available at any gas station on the trail.

My $0.02

This is the problem everyone gets caught up in. If you want to run pump gas or cheap out of fuel fine. The original poster wants to know how much boost he can run on a stock engine, and not cheap out on the fuel. Completely different.
We are talking making power not saving money.

 

[RACINSTATION]

This is the problem everyone gets caught up in. If you want to run pump gas or cheap out of fuel fine. The original poster wants to know how much boost he can run on a stock engine, and not cheap out on the fuel. Completely different.
We are talking making power not saving money.

SOMEBODY GETS IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!