Camaro9969- Love the flex fuel sticker! Steaksauce!

 

Ive been known to do a cam or two.

Bro its really about balancing your overlap and EVO. Evo is more important to have than overlap to me. If i have to pull back a cam because the turbine isnt sufficient for the motor... overlap is the first thing i limit...

If a lopey idle iss a requirement, I can take some evo out and leave the overlap in.

First thing i try and do is make the customer happy. Even if a cam makes more power but the owner is unhappy.. the power numbers dont matter.

 

LilJohn--Props again on some more posts in this thread with kickass tech content!

My question referring to the concepts in your posts above, do you think that the way many people look at this topic is sort of like the tail wagging the dog? In other words, do you think there are a lot of generalizations about cams floating around that are potentially misleading because people get locked in by a certain turbo, or at least by a standard turbine size?

A more aggressive cam (that will usually have additional overlap to go along with a longer exhaust event) will probably make better power if the turbo is sized appropriately because the efficiency range is moved to a higher RPM range. At the same time, the problem of overdriving the turbine is compounded by the additional revs, so the need for a higher flowing turbine becomes even more accute.

On the other hand though, is it true that the additional overlap and particularly the exhaust flow can help to spool a laggy turbo, thus adding a bit of a win-win situation to a setup that has an aggressive cam, an appropriately sized turbo, and the rest of the setup needed to handle the revs?

 

That's what I'm getting out of everything I've been hearing regarding turbo cams lately. The cam needs to be made the last piece of the puzzle instead of the first.

 

I dont know that id call it misleading. Id call it an incomplete picture. I think theres a big issue in the LS world with under turbining for sure. Theres two thought processes and approaches that can be used. Ive tried them both and prefer a bigger turbine

A turbo motor is a system from the tips of the impeller to the tips of the turbine wheels. As we raise pressure in this system.. its going to attempt to be at equal pressure from beginning to end. Now if we have an under turbined engine that has a 2:1 backpressure to boost ratio... which way is the pressure going to go to try and equalize? Exhaust is going to fight to get toward the intake... so we kill the overlap to minimize that pressure backwash. Thus eliminates all the benefits we get from overlap and creates alot of pumping losses driving the efficiency down. NOW what happens if its the other way around? With more boost to backpressure? The pressure will want to equalize the same way as the intended flow in the system.. its stopped fighting itself. We can put the overlap back in the motor so we can take advantage of and tune it just like an N/a motor does. Our pumping losses go down and efficiency goes up. Theoretically an engine should cam almost exactly like it would NA if you can keep the boost to backpressure ratio at 1:1 or below. The only thing that should get adjusted is overlap imo. As we raise the pressure in the system.. the frequency of pressure waves in the engine changes. At 1 atm it may take 26 degrees at .050 to "tune in" the overlap. At 2 atm that might only be 21. At 3, 16.. so on an so for. Boost is. Nothing more than increased air density. Its comparable to how sound waves can move through water so much easier than air.

If you have a Laggy turbo motor.. youve done something wrong. More times than not, its incorrect valve events. Ott308 had an s480 with an 83/1.10 turbine combo, 383 LS stroker and a 235/231 cam. He wasnt really happy with it and we changed it to a 96/1.32 combo with a 234/239 cam. He says the car spools quicker to the point its almost violent. Rolling burnouts at 60mph now where it wouldnt do that before. His 4200 pound 61 impala has also ran 5 mph faster on 4 pounds less boost.. Remember what i said about efficiency?

Montgomery Ragland has an X275 mustang with a 312 (iirc) inch mod motor... car has a Promod 88mm with a 112mm turbine and a 1.15 ar housing. It will make 7 psi footbraking at roughly 3100 rpms in less than a second. At 29.5 psi down track.. its got 26 psi of drive. Has 23 degrees of overlap and a 59 evo event. This is an example of whats possible if you try and make everything perfectly matched and as efficient as possible.

Big turbines exert more torque and extract more work energy from the exgaust in my opinion as well. Picture each blade as a lever as it turns the shaft. Further away from the fulcrum point. The more leverage the wheel has on the shaft. Thats why i love big wheels and tight AR housings.

Picking the cam you are going to run before you even have a turbo is putting the cart before the horse. Guys have seen XX cam make XX power,buy it...then wonder why it wont fall out of a tree for them... Problem is they didnt put any thought into the fact they picked it based on the results from say a 5.3with a TC76.... and they have a 402 stroker with a GT5533. (Yes the disparity between those is quite exaggerated but was done so for effect).

Cam should be thought about with the turbo and CI of the motor. Those two things affect the exhaust lobe the most. In an ideal world each combo would have a custom stick done...BUT if guys do choose a shelf type cam, they need to have a good understanding of how that all works together so they can pick the proper components to work together and not against each other.

 

LilJohn knows his ****. Took the big chev out last night and now it runs 7mph quicker with 4psi less boost and a 10.9 with a lazy 1.87 60ft haha. 4210lb chev with a glide and no intercooler. I have meth inj but was not activating at the drags. This is also on stupid low timing still. If I changed diff gears or to a TH400 and used the meth inj with some decent timing, your guess is as good as mine.

 

Wow, I think that whole post needs to be in a sticky somewhere or something!!!

Regarding the part I quoted and especially the part I highlighted above though, am I understanding you correctly that you prefer a larger turbine wheel to take advantage of the torque it has to offer, along with a smaller A/R number than would be used with a smaller turbine wheel on the same setup? I guess the smaller A/R housing would squeeze extra action out of the larger turbine wheel, while still tending toward being able to handle more total exhaust volume before choking. Is that what you're getting at?

I think it's pretty clear from your post that you find the benefits of a bigger turbine wheel (within reason) to outweigh the potential down side of the higher rotational inertia the larger wheel will naturally have. It makes sense to me FWIW...

Thanks again for taking the time to put all of those thoughts down on the screen for us to ponder!

 

Big turbine=harder to choke and cause excessive drive pressure issues
+
Tight AR housing=faster spool combined with the larger turbine
+
The right valve events
__________________________________________________ _______
Is what Lil John is referring to as to what he prefers and has found to work well.

 

Here is a prime example of what is being discussed.

 

Hey that looks familiar....

 

So what size turbine is that? (with a 76mm compressor wheel?)

 

That is a local x275 drag radial car that I posted a graph of a long time ago to prove the big turbine tight AR method I subscribe to.

That car is a 363 cubic inch sbf with trick flow R heads, a sheet metal intake and a borg warner s500sx88 with a 111mm turbine. Far from a 76mm compressor.

 

Haha, okay, I guess I didn't bother looking at the actual HP numbers, just the shape of the curve and the guy's sig who posted it..

Anyway, thanks!

 

Lol it's fine.

 

Martin I think I got the graph from reed at works turbo. Pretty sure it's from Jamie hackings syclone.

 

Agreed.

So many combinations put together piece by piece when they could be so much more efficient if someone with experience spec'd out -everything-.

 

This graph shows exactly what one would expect to happen, I would say that a 1.47 housing on a 360ci engine is boardering on the upper limits of actual usefulness.

 

That graph can be deceiving depending how the test was run and the rate of acceleration. A/R testing is best done in the car for reliable results as heat can change the rate at which the turbo "comes on", and fast rate can move the high rpm drop point out.

Kurt

 

It would be nice if that was an RPM x HP graph... Anybody got one of those to post up for viewing pleasure?

 

So where would a pt7675 fall into place in all of this with a 5.3? Is that considered a tight ar large turbine?