Ok, check this out:

Joe LS1 has an LS1 power'd F-body with a turbotech single turbo kit on it. His car is his daily transportation, so it needs to be reliable as hell. His goal is 450 RWHP, no more. With the stock heads, Joe makes 450 RWHP with 6 psi. Joe then removes the heads, and sends them to TEA for a good port job. Apon reinstallation and dyno tuning, Joe now makes his desired HP level (450) at 3 psi.

Did Joe actually gain ANYTHING as far as reliability goes? It would seem that 450 RWHP would be the same stress on the bottom end, regardless of how it was produced?

What gives, thanks! ;D

 

Joe gained a bunch of efficiency. He may be able to use a slightly lower octane gas, but yes, when he's making 450 hp the loads on the bottom end are the same, assuming the hp curve is the same and peak hp is at the same rpm.

With a programmable turbo boost controller, Joe could get max torque at a low rpm like say 2000 and hold it fairly flat to 5000 or so, and have a very driveable car. He might also be able to use less cam than he used to get 450 with 6 psi. This is about what Audi does on their 1.8 and 2.7 turbo engines. They get a lot higher than 6 psi, however.

FWIW, my choice for low boost, fat torque curve and 450 hp would be a positive displacement blower (eg. Magnason) rather than a turbo. Fattens up the torque curve all over.

My $.02

 

The 3psi reduction in boost represents elimination of pressure drop in the intake runners. The benefit to the engine is the reduced compressor outlet temperatures, cooler inlet air charge, as a result of the lowered boost ratio.

I would also think that since the turbo is putting less load on the engine, there is also a freer flowing exhaust. That would tend to reduce backpressure in the cylinders on the exhaust stroke. I guess I have this mental block, when people say turbocharging is "free", since the added pressure in the exhaust system has got to "use" power from the engine. You can't spool up a turbine and a compressor, and create energy that is required to compress the air, without taking it from somehwere else. Am I off base here????

 

I think you are right on base, Fred. Of course there is never a free lunch but turbos come pretty close. The turbo is a demand system, so even fairly high part throttle operation doesn't generate much boost, not back pressure.

However, the Eaton (Magnason) postive displacement blowers use a bypass valve so that when no boost is demanded, the output air is plumbed back into the blower intake so it "bootstraps" itself and takes very few hp to drive at cruise. I llike instant and nearly linear boost also.

 

[EDIT] Offending "off topic" comment, and followup post by Rich deleted.

I wonder if the turbo guys have measured backpressure in the headers under various boost conditions?

 

That's great, but can we kinda stay on topic?

So did Joe gain any reliability? It would seem that he did not. Am I right?

 

Yes....even though the 450rwhp will still cause the same amount of stress on the bottom end at 3psi or 6psi the big difference here is in the reliability in the cylinder head area. I'm always a fan of less boost and more power because even though big boost is cool, it just likes to break parts.

Ya gotta look at the whole picture

BUT

since you have a turbo crank the boost back up to your original 6psi and see how much more power your making now

 

the only person ive seen post anything about it......NoSlix said in another post:

I've datalogged exhaust backpressure on several of our NMRA turbo motors. My drag radial car with a single T76 with a 3.5" exhaust had 32psi of backpressure at 18psi of boost. That was at the 900hp level. My Street Outlaw motor(that made 1718 crank hp when it was owned by last year's champion) with a PTE 91.5mm(actually a cut down 98mm) with a 5" exhaust had 24psi of backpressure at 28psi.

 

I don't see how cylinder head reliability is affected? Are yo talking about head gaskets?

How does 450 Boosted HP break more parts than 450 NA power? I'm not being an ***, I honestly don't know. If you keep detonation under control, and have proper fuel, why is boosted HP more dangerous?

 

That's interesting. The more efficient Higher HP motor had more boost but much less backpressure, better turbo design and much larger turbo most likely.

At what RPM and % of max power where those numbers taken? What was the curve of those plots thruout the RPM range?

Bret

 

[QUOTE]Originally posted by neat


I don't see how cylinder head reliability is affected? Are yo talking about head gaskets?[/b]

Yes: head gasket reliability, blowing out intake gaskets, things to that nature

The overall average cylinder pressure inside the chamber will more than likely be higher and causes a lot more stress on the pistons, rings, bearings, that an n/a motor would

Bret the larger motor has a thumper sized 91.5mm (cut down 98mm wheel) wheel and a very large (5") discharge on the turbine housing to allow the exhaust escape much more efficiently. The smaller turbo was a 76mm with only a 3.5" discharge out of the turbine housing with a smaller flowing turbine wheel. That is where the lower efficiency and the bigger backpressure #s come from

 

looks like 89pro already answered a bit............but for specifics, youd have to ask NoSlix............he was the one that measured all that, i was just quoting him. he posts in here sometimes.


here was the rest of his post, when i asked about why they were so different in backpressure ratio:

The difference is the two turbines. We're not purposely trying to get a specific backpressure #. You'll always see backpressure. If you can keep it 1:1, you're doing great. That's the nice thing about the Big Thumpers with those huge turbines. I have yet to see any Big Thumper motor with more than a 1.1:1 ratio. But any To4 based turbo(which all of the T series turbos up through the GT80 are) will commonly have 1.5:1 ratios. The turbine on R trim 76 mm is starting to show itself as the bottleneck. At the 1000hp+ level, I've actually seen cars with almost 2:1 backpressure to boost ratios. This is terrible on the motors and leads to blown head gaskets, burnt valves, higher coolant temps, and even all out engine failures. With the Drag Radial motor, we actually noticed at the end of the season when I pulled it apart that the head gaskets were blown down towards the exhaust port on every cylinder, even though the heads were oringed. The lower head studs were less than 50ft lbs as opposed to the 100ft lbs that they started at. We directly attribute this to the heat retained in the exhaust port due to the backpressure. We never saw this with the Outlaw motor in it's previous home, but then again......it was maintained much more proactively.

 

Even though both 450 rwhp cases put the same amount of stress on the bottom end, the 3 psi boost case will have much lower inlet air temperatures and, thus, a much lower chance of detonation. So, better heads and low boost is probably more reliable.

Mike

 

that is not necessarily...it depends on where the compressor lies on its map. You can have compressor surge

 

Compressor surge occurs at low flow / high pressure conditions, not vice versa.

Another thing is that the compressor may drop a couple % points in efficiency, but the fact that the air is only compressed 3 psi instead of 6 psi means that the adiabatic temperature rise will be much lower, thereby safer.

My point was: If you build two engines as follows-

1) Stock 305 - 200 hp. Add a 300 hp shot of nitrous and you get 500 hp.

2) Stock 350 bottom end with AFR heads, a mild Extreme cam, Edelbrock intake - 400 hp. Add a 100 hp shot of nitrous and you get 500 hp.

Which do YOU think is more reliable? They both make the same power and have comparable bottom-end parts.

Mike