Engine/Transmission (1994 - 1998) Paging Mr. Donnelly..... Blow off valve vs Wastegate....need an education!

[RSchwarzli]

I am having a internal debate as to if I go wastegate or blow off valve for the W350 running a 1997 12V with compounds... . HX35 and HT3B.



I had a wastegate on the engine originally but removed it due to needing that extra clearance for the downpipe and also thinking originally that like my 1st gens, who needs a form of pressure control! . Ellimination of it got me an extra inch and a bit so thats the way I went.



However, reality is I still do require some form of pressure control. So I figured blow off valve may be the way to go.



Only issue is, I am not very versed on the subject.



So I though of you.



Can you educate me a bit of the pros and cons of each style and give your opinion on what style is the better choice for my application? Thanks!



Robert

 

[RSchwarzli]

Oh, open to everyone btw. If you have input feel free!

 

[fox]

A wastegate is on the hot side, a blow off valve is on the cold side.
By letting air out of the cold side you are slowing the cooling of exhaust gasses. Let that cooler air thru the motor to cool things off.
If you have issues with barking, address that issue.

 

[Joseph Donnelly]

If your issue is total boost vs. headgasket strength, I would go to a larger exhaust housing, probably the one on the secondary turbo was the wastegated unit so change that one. Wastegates are not the best idea as I outlined in Issue 65, p. 90:

"Some of us think about the exhaust or turbine housing of the turbocharger, and wonder why it is so small. Here our diesels may make 400, 500, or more horsepower, and the cross sectional area of the turbine housing is only 12 or 16 square centimeters, or well under that of a single 2” diameter exhaust pipe. How can we make such power with 1-3/4” single exhaust? When we compare a small exhaust housing with a big wastegate to a non-wastegated larger exhaust housing, we find that the spool-up and power when first getting on the accelerator pedal is a lot better with the small housing, but it seems to “choke” the engine (from reaching full power potential) at high boost and high power, compared to the bigger exhaust housing.

The secret, and the answer to all the above questions on backpressure, is in that turbocharger system and what it does on the intake side of the engine. The turbo has given the engine a lot more air, so it thinks it is twice or three times as big as it was with no boost. With a big single turbo we can try a fairly big exhaust wheel and housing to keep boost pressure above or close to the amount of backpressure, but responsiveness at lower power and rpm have suffered. We don't like the poor drivability nor the smoke while the big turbo tries to spool up on a small engine. So, we go to a smaller exhaust housing with a wastegate. The smaller housing give us the spool up we want, and boost pressure vs. backpressure is still good, until the wastegate opens. Why does giving a bigger exhaust flow path to the engine hurt back pressure? As we open the wastegate, some of the exhaust energy bypasses the turbine wheel so boost drops, and the boost pressure just got too low compared to the backpressure. If we increase fueling and power, we are no better off, because we are making more exhaust backpressure from burning more fuel into exhaust gas, and from heating the gas more. We aren't increasing boost because the wastegate is open. That is why a small housing with a nice wastegate is still not ideal.

The Cummins factory has used wastegated turbochargers on our Dodge applications since 1994, in part because they are balancing power and emissions. To get good power and sell engines, they need responsiveness. To keep smoke down, meaning to get that good response without the belch of smoke, they need a small exhaust housing. Because of engine design limits such as maximum turbo wheel speed and head gasket sealing, they use the wastegate to limit boost pressure. They also try to size the turbo so that the wastegate is open only a tiny fraction of the time the engine is running, meaning only when it is at full power (in stock form). Then we come along and increase that maximum power level greatly, and the wastegate is open much more of the time if we are using the increased power. "

While a blow-off valve may be a band-aid type of fix, I would not prefer it either because once again you are upsetting the backpressure vs. boost balance.

 

[RSchwarzli]

Ok..... So what do you suggest then? Neither? Is it even required? My head is fire ringed and I run studs. I am mildly fueled but not excessive.

Sorry. Was just not following where your thoughts were leading!

 

[cerberusiam]

Monitor the drive pressure and see where it is, that will define whether you need a WG. A blow off vlave is a waste of time and energy as they never seem to work consistenly. If you are not pushing lots of fuel you likely won't even need the WG.

 

[AH64ID]

Is the HX35 not gated at all?

A wastegate regulates pressure while fueling and BOV regulates pressure when you stop fueling. Opening a BOV while fueling, especially heavily, will unload and overload/damage the turbo very quickly.

They have 2 very different purposes and a set of compounds will require a WG on the secondary turbo, and some run them on the primary but that's usually in towing applications with smaller twins.

 

[RSchwarzli]

No. The HX35 is a PDR 16cm non WG housing with the ht3b being a 26cm also non WG.

If memory serves, they are DDP stage 2 (60hp??) injectors, 3200 spring, 181 taper cut DVs, and the fuel plate I think is a 10 (I think).

 

[AH64ID]

If you get much above 35 psi your going to need to add an external gate or your drive pressure will be thru the roof.

Your spool is also going to be considerably slower with that 16cm housing and twins. Have you considered a 12cm gated housing? That would be much better suited for towing and daily driving.

 

[cerberusiam]

With that large of a housing and not a lot of fuel, not sure you will have a problem. I would add a drive pressure gauge and see what it does before digging in to far.

 

[RSchwarzli]

Sooooooo, what exactly would I see in drive pressure issues exactly?

For example the 1990. There is normally about a 3 psi difference between the two sides. And that is a HX35 non gated 18 CM.....


cerberusiam

The only reason I am trying to be 100% sure now is the fact that the WG housing is 1. 5" thicker. If I need to change after a few weeks I will have to refab the downpipe.

 

[AH64ID]

Ideally you want drive pressure to be equal to, or less than boost.

With a large housing you won't have an issue at lower boost, but if you get that HT3B moving some air the HX35 will be the choke point.

 

[JJPage]

a set of compounds will require a WG on the secondary turbo

I have one, but I do not require it... have it blocked. There are many different possibilities/configurations, depending on fueling.

I sometimes believe, if there is a need to wastegate, or bov, then the system is not efficiently engineered...

This is not from a total performance stand point.

 

[AH64ID]

If your not fueling enough to need a WG on the secondary then you don't need twins or are running too large of a secondary for the setup/fuel. Trying to put the volume of air of the larger primary thru the turbine of the secondary, without a WG, is going to cause drive pressure issues. There is only so much air they can move.

Sometimes that statement about engineering is correct. I never use the WG on my turbo, and have considered swapping the housing for a non-gated one of the same size, but that's more work than it is worth.

Stock turbo's use WG's to allow for super quick spool and to relieve DP at higher boost, and to limit boost. Most journal bearing turbo's that have quick spool will need a WG to limit peak boost. You can get away with out one on some BB setups with larger housings due to quick spool.

An HX35 with a 16cm housing may not need a gate, but it's not going to spool very quick and be smokey if towing. But just because it doesn't need a WG as a single doesn't mean a thing when you compound the turbos. A HX35 moves about 50 lb/min, so anything above that and your drive pressure is going to get high very quick, which leads to lower performance, decreased airflow, and increased EGT's. An HT3B is about a 90 lb/min turbo, so unless your only using 50% of it's air your going to outflow the HX35, which is going to be only a hair more than stock fueling, which leads back to the question of "why twins" when you could run a single with better spool and less money.

I saw a video recently of a guy who blocked the WG on his R700 twins, the DP pegged the 100 psi gauge in no time flat, over a 2:1 ratio. You simply cannot move the air flow from a large frame turbo thru a small frame turbo without restriction.

So on a single, yes you can engineer a fast/efficient setup without a WG; however, on a properly engineered set of compounds you need one, and I stand by that comment.

I am not talking all out performance, simple physical limitations.

 

[cerberusiam]

Ideally, boost to DP should be 1:1 across the range of rpm. You won't get that without some type of VGT in the mix but its a realtionship to look for.

What you don't want is DP 1/2 again as much or even double boost, then you have a choke point for the fueling. I kinda doubt you will see that without some more fuel as a 16 cm housing is going to flow a LOT before becoming a choke point. I think your DP will be under boost with that setup but thats why gauges are used, guessing at times is inaccurate. If the DP is too far under boost at low rpms then your loosing spool and air delivery potential.

 

[AH64ID]

A 1:1, or slightly better, doesn't require a VGT. My BB Garrett does it quite nicely. It's always just under 1:1 except WOT runs above 2500 rpms where it peaks at about a 1. 2:1. Pulling grades it likes about . 85:1, much better than the stock 1. 5-2. 0:1.

A properly matched and wastegated set of twins can also run a 1:1 pretty easily, just build them for how you use the motor. . don't use the motor for the twins you think sound cool on paper.

Another important reason for a slightly smaller and gated housing on the secondary is the efficiency of the primary. We all know that EGT's have a huge temp drop across the turbo (think pre-turbo vs post-turbo EGT'). So when all of the air is routed thru the secondary it's a cooler and smaller volume of air to spin the larger, heavier, primary turbo. When the WG opens on the secondary its diverting manifold temperature air, with a higher volume, to the primary. This makes the primary spool faster and more efficiently. If all the air is routed thru the secondary then the pressure builds in the manifold, but doesn't get to the primary very efficiently and you lose airflow.

 

[cerberusiam]

It's always just under 1:1 except WOT runs above 2500 rpms where it peaks at about a 1. 2:1. Pulling grades it likes about . 85:1, much better than the stock 1. 5-2. 0:1.

Exactly, its not 1:1 across the rpm range and load. To do that would require a VGT tuned correctly. That was the whole point of the VGT comparison.

Whether or not the range you see is good or bad is beyond this discussion. Likely, it is as good as can be achieved for a balance, but, by the numbers not optimal. Then, the discussion becomes what IS optimal at different loads and rpms, again beyond this discussion.

Like I said, without a baseline of existing performance it is hard to tell what, if anything, needs to be done. C'mon Robert, you can ALWAYS use another gauge. :-laf

 

[JJPage]

I sometimes believe, if there is a need to wastegate, or bov, then the system is not efficiently engineered...

I knew this comment would spark some debate :-laf

 

[AH64ID]

Exactly, its not 1:1 across the rpm range and load. To do that would require a VGT tuned correctly. That was the whole point of the VGT comparison.

Don't forget a properly sized VGT. The 6. 7 VGT won't hold a 1:1, hence the appearance of external gates (which shouldn't be needed on a VGT).

There is also a big difference in a DP:Boost ratio on short WOT bursts vs sustained power at any level. A WOT run with a cruise temp turbine will create less boost per psi of DP than a run with a load and a hot turbine will (assume WG is closed). My ratio might be . 9:1 at the bottom of a hill, but can drop to . 8:1 by the top with a hot turbine. Same thing with high rpms, if I am tuning and seeing how the truck run WOT thru redline the first run has a lower ratio than the last.

 

[cerberusiam]

Don't forget a properly sized VGT. The 6. 7 VGT won't hold a 1:1, hence the appearance of external gates (which shouldn't be needed on a VGT).

Yep. The comparison was for academic reasons only. Actual implementation is likely a lot harder due to just what you pointed out, plus all the other variables.