the 12-15psi is due mainly to the stock cam's LSA. Also, the inefficent turbo kits dont help the situation with high DP.

My cam swap alone increases the max boost, as well as a better turbocharger.

My lowered CR has less to do with it than you might think.

Cylinder pressures in my IDI are MUCH higher than a stock 21.5:1 CR @ 15-20psi.

 

agreed. The amount of extra boost you can run is only something like 2.1 lbs per CR dropped iirc. Besides all of that, the combustion of all the extra fuel is way more cylinder pressure than you would ever see running even 40lbs with a stock pump.

 

I know he has lower compression. Lol. 40psi is still a lot though even with lowered compression. A few forced induction books that I have read said that for every point you reduced compression you can add 2psi. Now I have seen quite a few trucks run a lil over 20+psi so it is my thinking that 25-30psi isn't out of the realm of possibility. Especially with fresh gaskets and overtorqued studs like Racin has. I think it would definitely be a good opportunity to test since he is planning on doing a full build in the future.

 

21.5 or 20.5 cr dropped to ~18.5 is somewhat significant. 40 psi is alot either way though. im just curious how much stud torque the block can handle. And i haven't heard of many people blowing gaskets with studs unless they installed either wrong, etc...

Either way with head gaskets n studs properly installed I'd run up to at least 25 without sweating myself

 

For sure 25psi is fully doable IMO with proper studs. There is only one person that I know of that has blown multiple gaskets with studs but who knows if they were actually replaced properly each time...

 

Im fully with Justins on this tho, the cam... is one big deal on these when it comes to boost. Then with that cam that is horrid for a boosted engine, the insanely ineficient "stocker" chargers quadruple the problem. Im planning to hit 50ish or more on mine with 17.5-18CR when competing. I have a few tricks up my sleeve to keep it all sealed up too. As racin has mentioned before, a torque plate hone will be one of those tricks.

 

Oh ya I am definitely with you guys on that too. Boost is a measure of restriction and those are both restrictive in these engines...

 

Without blowing everyones minds... let me just give a quick lesson.

The stock cams and the regrinds up until my cam was built all have on thing in common. Narrow LSA.

What this means is that both valves overlap open for a period of time.

Now, the stock turbo systems run at approx 2:1 drive pressure. That means when you have 15psi reading in your intake, you have 30psi in the manifolds/up pipe behind the turbine.

When both valves are open at the bottom of stroke... what wins.. 15psi or 30psi?

The already burnt exhaust gas is allowed back into the chamber, which increases cylinder temp, cylinder pressure, and prevents efficiency.

On top of this, the factory y-pipe allows for bank cross-fire.

My cam changes everything about a stock cam... that is why I am calling it a "turbo cam". Not only does it have much higher lift, but it has a significantly wider LSA which help prevent what is called reversion.

That is why a cam swap on its own would allow you to run more boost in a stock application.

Now even still you have a turbo system that runs @ 2:1, and you have bank crossfire. My efficient turbocharger, and running a full twin scroll setup completely eliminates bank cross fire, and gets the DP closer to 1:1.

That is the main reason my engine can handle 40+PSI boost, and Racin's can handle significantly higher than the standard "15-20psi". Not because I'm 18:1

The 18:1 actually causes higher cylinder pressure @ the same amount of boost.. 40psi @ 18:1 means I have more air in the chamber, more air from the charger = denser = more pressure....

Hope nobody heads exploded

 

That is good info. Well put man.

 

Whoa, Scary!

-Enjoy
fh : )_~

 

I meant cylinder pressures in your IDI at 18.5 versus if your IDI was at stock CR, not a stock idi. Big difference between yours and a stocker. I came off sounding like the CR was all that helped or w/e- not my intention . Obviously not. I still think stock-bolt stretch is a big part of what blows the HG, but with studs (and moreso those modifications) idis can obviously run higher boost.

I get that the twin scroll separates the mani's and that the cam grind helps performance. I've never looked at the cam profile but I've always heard stock was crap.

Thanks for the clarifications lol


pm me with info on your cam when you get another two posts

 

that would make since, considering that these engines were never really made for turbos.

won't a wider lsa decrease the torque down low? but then again, a narrow lsa would allow it to suck though at higher rpm and a na engine so it may help torque?

not sure what the comp ratio is in my dad's truck(i think 20.1:1) but he can hit 27 psi with a 3" down pipe with the ats kit, head studs, intake, and exhaust studs).
i still think there is a air leak somewhere, but he can't hear it.
and yes, it gets hot REALLY fast

i think you worded that last bit wrong didn't you nmb2?
a higher cr at the same boost will cause higher pressures because it packs all of the air into a tighter space.

 

The main reason behind dropping compression isnt to allow for less cylinder pressure, instead to make a bigger combustion chamber in order to fit more air in. This is where stroker kits really shine. Potentially same CR with more CC in the chamber. Least that is how im coming to understand alot of this.

 

Here is a really simple general overview of the differences:

Technical Specs & Information

and no, I didn't word the last portion wrong.

There is too many variables for it to be cut and dry - black and white....
However, think of it this way:

Compression = hot air. Hot air is thinner.
Boost = cold dense air.

I've seen many people claim that high CR and lower boost is the same as low CR and higher boost. This is not correct.

As I said though, my engine is WAY too different to compare. Racin's would be the closest comparison of factors.

Still, my camshaft is the heart of this thing being able to run the way it does.

 

More boost is always better since there is more oxygen in the cylinder to combust. Turtlemann what NMB2 was kinda meaning is that while there is more cylinder pressure before compression on an engine with higher compression, with his engine, there is more dense oxygen that fills the cylinder and after combustion it is has more pressure than an engine with less dense oxygen. So to sum it up, it depends on if it is before combustion or after combustion which has higher cylinder pressure. Also, you should have your dad send his turbo to Type4. He can machine the compressor side of his turbo to fit a bigger wheel. This will help a lot with heat on his setup. But the best thing he can do period to help those egts is upgrade his turbo. And maybe a cam swap... Lol