Small charger twins lag-- not what we think?

[Hohn]

OK, just a quick thought:

For the longest time we have read about how a set of twins will always spool slower than the smallest charger it has. We know this is true.

BUT-- we were theorizing that this is because the smaller charger has to pull air through the big charger on the intake side, and there's a partial vacuum in the crosstube as the smaller charger spools before the big charger.

I'm thinking that it's not the INTAKE side at all that has to do with the slower spoolup.


It has to be the exhaust side. Think about it. The small charger spools because of pressure differential (well, temp too, but mostly pressure). That why we often notice better spoolup on a single charger when you install larger exhaust-- the reduced "downstream" pressure of the larger exhaust INCREASES the pressure differential across the turbine--- thus, it spools faster.

But what if we went the opposite direction? What if we installed another restriction AFTER the single charger? What about another turbine section??

Well, now the pressure differential across the small turbo is LESS because of pressure built up in the hot crosstube between small charger turbine large charger turbine side.

So the irony here is that if you build some twins with say, an HT3B and install a really tight housing on the HT3b to help spoolup-- you may instead be actually HURTING spoolup (at least of the small turbo).

So adjusting the size of the larger charger's turbine housing is basically just shifting the spoolup lag from the small charger to the larger charger and vice versa?

Larger HT3B housing= faster small charger spoolup, slower big charger spoolup
Smaller HT3B housing= slower small turbo spoolup, faster large turbo spoolup.

I remember noting how Jim Fulmer was posting back in the day about how his 40 spooled pretty well even with a BB on bottom. It makes sense-- the BB is big enough to not be much restriction after the 40s turbine.

At least, that's how it appears to ME.



So, Jim-- you need to get a B-52 under the SPS66-- your BB may be too small

 

[SRadke]

How then do we explain the fast spoolup of the BD twins which use a tiny exhaust housing on the primary?



-Scott

 

[Jim Fulmer]

So, Jim-- you need to get a B-52 under the SPS66-- your BB may be too small



That's actually not that great of a turbo! I would rather send Industrial Injection my B/B and let them hybrid it, but first I have to find the end of the current system.



Jim

 

[RonA]

I am not sure about your theory Justin. I switched from a 15cm housing on the secondary to a . 7 and it spooled much faster. I then when from a 26 to a 23 on the primary and it spooled a little faster still.

 

[RHARVEY]

I dont know if we can compare what I am running to what a lot of guys run but my towing twins spool so much faster than my 35/14 single did. Yes I did drop the housing down to a 12 because I wanted them to spool low in the rpm range. .

This PDR Towing Twins has the throttle response of a sports car...





Rick

 

[Hohn]

So, Jim-- you need to get a B-52 under the SPS66-- your BB may be too small

That's actually not that great of a turbo! I would rather send Industrial Injection my B/B and let them hybrid it, but first I have to find the end of the current system.

Jim

I. I. certain appears to have their stuff together!

 

[Hohn]

How then do we explain the fast spoolup of the BD twins which use a tiny exhaust housing on the primary?

-Scott

How do we quantify spoolup? It's like trying to measure beauty:-{}

I'm just thinking through things out loud. I could be totally wrong.

But I'm trying to foster more discussion on twins theory.


For example, some people think that twins should be built so that one turbo lights, then the primary lights once some boost is made- much like kindling is used to start a larger fire.


Then there are those who say that twins should function as one large turbocharger-- no "handoff" per se, but rather have them both spool at the same time.


If you subscribe to the first theory, you'd build twins with a small secondary and gate it early, hoping to get into the primary ASAP.

If you subscribe to the latter theory, you'd run a larger housing on the small charger and smaller one on the big charger.



Personally, I think that twins efficiency and spoolup are at odds with each other. For maximum compounded efficiency, it seems you'd want the latter, which would be laggy.

What to do?

 

[Devan Manis]

I agree with HOHN's theory, but don't forget the other side even still, I would be willing to bet that its a combination of both the exhaust side and the intake side.

 

[Jim Fulmer]

I. I. certain appears to have their stuff together!

This is an understatement!



Thanks to them I'm back on the road! I guess the little air filter took it's toll on the B/B as it sucked a seal and started pushing oil, but a quick call to Brady and a kit was on the way for a rebuild!



I will have to admit that I couldn't find the package but a little talking with them about shipping (when and were) and I found it at the back fence..... Oklahoma wind.



Shane and I were talking about what could be done to it, that's when that conversation went to crazy!



Jim

 

[Hohn]

Jim, I have a PowerCore unit you can test if you want. It might be tough to package on your truck though, it's a little longer than a BHAF.

But it flows >1000CFM @ 8" water and is 99. 99% efficient.

 

[Jim Fulmer]

I guess I should take a picture of the new filter and post it! I ordered a 6x24 from AFE and had to modify allot of stuff to make it fit, like the turbo to intercooler pipe ect. Then had to built the two little pieces if pipe to mount it to the turbo! One side is 2" from the front cover and it goes to the right front corner and lightly touches when wiggled..... it's a monster but totally changed the sound when coming off boost!



Jim