Ok, say you have two engines...one's a 5 liter and the other is a 7 liter. BOTH have single turbos, optimized combos and tunes and make 1000hp at the crank. If they are both for the exact same application, whatever it is, would they require the same turbine A/R???

I have heard a few different folks say stuff like "that's too many cubic inches for a XX A/R" or "that's too many cubic inches for a XX turbine housing"...ect. This has always confused me. The way I see it, if the engines make the same hp, they consume the same amount of air, and therefore they exhaust the same amount of air regardless of how many cubic inches they displace. If this is true, why would they have different turbine requirements?

Someone please straighten me out on this...

 

Read Up..

https://www.theturboforums.com/content/113-knowledge-base

 

I, too, have a hard time understanding that. I read that website and it doesn't explain effects of engine displacement on turbine sizing.

It seems to me that X hp requires X airflow, so why would 302 cid need a Q trim while a 351 might need an S trim turbine wheel. The only reason I see to do this is to get better spool-up characteristics from the 302.

One point to note is that the 8 second Supras (180 cid) use a PT-76 GTS, while the 8 second Mustangs (331 cid) use a PT-76 GTS. Both use the same size turbine wheel even though one is almost double the displacement of the other.

On the other hand, Brent is using an 88 mm compressor with only a T4 turbine on 302 cid, which is a very small exhaust side for a mid-frame compressor. At 1200 rwhp, the small turbine doesn't seem to be hurting him.

 

I've done my fair share of reading including your link, corky's book, hugh's book, and some other random sites I've come accross on the net. Maybe I'm missing something but I don't see where this is covered in any of these.

I guess I see the reasoning behind this, ie the 7 liter is moving more air at and just off idle which gets the turbo spooling quicker given the same turbine housing or with a larger housing on the 7 liter it would allow for more hp potential while having similar spool characteristics as the 5 liter with a smaller housing. I feel this might have me headed in the right direction, but I'm still struggling with this...

 

I'm just gonna get in trouble

 

same here

 

Mike
In your example....I don't think the sbf moves air nearly as efficiently as the 4 valve high rpm motor does. In other words the ford is almost twice as big but probably half as efficient.
My .02
Dale

 

"moving air" and "efficiency" are 2 different things.

If a 180 cid engine at 8000 rpm moves the same air as a 331 cid engine at 5000 rpm and their thermal efficiencies are the same, then they should make the same power using the same turbine and compressor.

However. . . the thermal efficiency (usually measured in BSFC) of the Supra engine is probably a little higher than the Ford due to the better combustion chamber shape, rod/stroke ratio, etc . . . means the Supra can make a little more power from the same turbo assuming the same airflow and boost level.

My point was, that with such a drastic displacement difference, they don't need a different turbine side because the power level is nearly the same.

Mike

 

Something just occurred to me. . .

Let's say 2 engines (the same 180 cid and 331 cid we were discussing earlier) made the same amount of power at the same boost level. Let's also assume they have the same thermal efficiency, so they both flow the same amount of air. Therefore, the backpressure imposed by the turbo should be the same. HOWEVER. . . on the smaller motor, the backpressure is acting over less piston area and the crank has more mechanical advantage on the exhaust stroke due to less stroke. So. . . the 180 cid motor has to do about half the work during the exhaust stroke.

Does this make sense???

Mike

 

edit

 

engineermike

I think in your example the work would be less on the 180CI motor per exhaust stroke.

However, isn't the power required to overcome the backpressure really what matters in comparing the two engines?

To do this wouldn't you also have to take into account the 6 vs 8 cylinders, the different pison surface areas, and the difference in piston distance traveled per unit time with the different RPM/strokes?

At least these variables pop into my mind for comparing the power required for these two engines to combat the exhaust pressure in the above mentioned scenarios. There are probably more.

 

I was merely theorizing on why larger motors need larger A/R's and turbine wheels for the same power level.

Of course. . . I made alot of assumptions in my theory. However, the theory still stands.

One assumption is that the rpm is the same on both motors. However, if you spin the 180 cid motor twice as fast as the 331 with the same backpressure, then it takes the same hp from both to overcome it.

I agree that "the power required to overcome the backpressure really what matters in comparing the two engines". Sorry if I didn't make that clear in my earlier post.

BOS"s5.0, your "above statements" are incomprehensable.

Mike

 

So this more or less rules out the theory. The 180 does less work per revolution, but equal work per period of time if it's flowing the same amount of air at the same boost level. This is right in line with how I understand it to work and consequently why this has me baffled...still. Two engines may take entirely different paths on the way to flowing x amount of lbs./min at x psi, but in the end they're really doing the same thing.

Just so things don't get too sidetracked, I suggest we not get too carried away with things like 2v vs. 4v and the like. This obviously affects what an engine needs/wants in order to make power, but it doesn't directly relate to the question at hand and seems to complicate things more then necessary. Displacement is the variable in question, and changing it obviously doesn't necessitate changing anything else if power levels are desired to be equal.

Well, I'm still confused...

 

Its quite simple guys. Large displacment motors produce tons of low rpm cfm airflow.

Tons of pressure will be build at a very early rpm thus spooling the turbo quicker. Thats why you choose a big turbine a/r for a big motor.

A smaller motor will not produce that much cfm at lower rpm air/flow. Thus you choose a smaller turbine a/r to minimize the turbo lag.

2 many of you are looking at peak cfm airflow and yes even though the supra and mustang motor may have the same cfm airflow at redline the turbo spool up will be a lot sooner on the bigger displacement motor assuming both mustang and supra are running same turbo size.

Thats why supras take 4k to build boost while mustangs take 2.5k to build boost

 

I totally understand that you don't need as low of an A/R / trim on a big motor due to inherent quicker spooling, as I've stated above.

But. . . the question remains, at the same power level (thus peak cfm), why does a larger engine need a larger turbine if they flow the same air?

 

the bigger motor is moving more air at less rpm.

on a bigger motor you can get away with more a/r and make more power up top cause you don't have to worry about spool time as much.

RPM is the big difference in the senario

 

To say that you can "get away with" more A/R with a larger motor is totally different than saying you "need" more A/R with a larger motor.

I agree that you can get away with more A/R, but I'm trying to figure out if you need it and why.

Does anyone here contend that any given A/R will cause higher backpressure on a larger motor than a small one at the same power level?

Mike

 

Good question!


If it helps clear up any confusion, I happened to find a post in the classifieds section that has dialog of the type that I don't quite follow:

Why would this turbo make more power on a 302 because of it's turbine size?? If the compressor is capable of 1100hp, then it would seem obvious it would need to be paired up with a turbine that wouldn't choke 1100hp worth of flow. Sounds like at least the guy quoted above doesn't feel that's the case. It also sounds like he feels that a smaller engine producing the same hp (or more) wouldn't be choked by the same turbine. This is a perfect example of the logic that I don't follow...

 

I had Turbonectics to tell me that their T76 Q-Trim with a .96 A/R housing and a single 3 inch down pipe.. Works really well in big block applications..

I got this answer when I inquired with them about getting a bigger exhaust housing for my 351 T76 combo.. They said I didn't need a larger A/R exhaust..