Twincharging camshaft

Just for the sheer fun and wanting a challenge I'm twincharging a ford 2.3 SOHC. Turbo spec engine from an xr4ti with crower rods and wiseco pistons. I'm blowing the turbo into the charger for compound effect. My end goal is to have more boost at the inlet valve than back pressure at the turbo thus changing the fundamental flow of a turbo only motor. Anyone care to suggest a camshaft spec for what would essentially be a supercharged engine with a turbo?! Duration, overlap etc?

I can give you extremely general guidelines, but I'm no expert.

Reduce overlap. This is generally done by a slight reduction in duration and widening the LSA. You don't want to be squirting all your yummy intake charge out the exhaust during overlap.

That's pretty much all I have. I'm not sure I have ever chosen a cam for turbo/SC applications so I hope someone else has more specific ideas for you.

Curious.

If the pressure is higher at the turbo inlet. Would that make the turbo more or less efficient?

More: Denser air means more air moved per rotation of a fan blade

Less: The turbo experiences more resistance to movement than under normal circumstances and speed of the fan blade is reduced.

Hey bill I read that as just opposit of what you said,turbo into Blower,so More pressure into turbine, there are a couple Mfg's. that have done this so look at their specs for an Idea but Curtis is right. But you also don't want the Intake air blowing the Valve off it's seat you may want a stiffer spring on the Intake and a LARGE Blowoff on the Exaust Before the Turbine. I Know Nuthing

Oh man, you're right! I skimmed right over that part and assumed "inlet" referred to "turbo inlet" because of the back pressure part.

Ok, but my curiosity is still piqued. I'm off to the googles!

I have way stiffer valve springs installed so should be ok there. Boost delivery will be controlled using an external wastegate instead of the usual BOV. It has a more progressive on/off rather than the quick action of a standard BOV. I'm probably gonna run with the stock cam at first but would like to get a grind done that suits the characteristics of the build better.

Intake valve diameter?

Stock inlet valves 1.74" if my memory is correct...

In reply to Hungary Bill:

I believe there's no (meaningful) difference. The turbo only cares about pressure ratio, not absolute pressure. That's why compressor maps show PR instead of boost, outlet pressure, or something else. Then on the X axis they show mass flow, because ultimately the only thing that the compressor blades care about is the mass of air that they're hitting, and not really how quickly it's being hit.

For the same reason the only speed you really care about while flying a plane is the indicted air speed. At sea level on a standard (68 deg F) day, IAS will be the same as the actual speed that the air is hitting the wings. Though once the temperature and your altitude change, they will no longer match. IAS is proportional to the dynamic pressure of the air hitting the front of the plane, which is the only thing you care about because at the same dynamic pressure a plane will always handle the same, regardless the actual speed of the air.

I know that Volvo has gone to this method in the new rides on the 2.0 motor. Maybe see what type of shaft they are using? I don't know if it would be meaningful or not as I imagine they are using variable timing as well.

No different than any other forced-induction camshaft - huge lift, relatively short duration (to prevent overlap).

VW also built a twin-charged solution for the euro market. It was not well-received.

Lancia used it in Group B before that to greater success.

There was a HKS twincharger kit for the Supercharged MR2 AW11.

The vw was a parallel setup where it was supercharged at low rpm.and then when the turbo spooled the charger was bypassed to let the turbo breathe. So essentially a turbo only motor up top. I'm gonna lean more to a supercharged spec cam with wide LSA

know that Volvo has gone to this method in the new rides on the 2.0 motor.

It's very different as it's not a compound setup. Volvo uses a very small supercharger that's geared very aggressively but redlines and has to be un-coupled (it has a clutch like an ac compressor) at 3000 engine rpm to avoid assploding. At that point the turbo has spun up and valves in the intake tract bypass the supercharger entirely and the engine acts as turbo-only.

My basic impression of boost and cams is that people who try aggressive cams tend to get better than expected results. The use of a roots blower in particular should be very freeing because you'd be able to use a cam that performs well at high rpm without having objectionably weak low-rpm torque. As far as the valve overlap thing, generally a turbo setup would have much higher pressure in the exhaust manifold than the intake manifold, so periods of valve overlap would actually result in exhaust reversion and 'charge contamination'. Turbo setups where boost pressure is higher than 'drive pressure' are rare, but even in those scenarios i dont think you'd lose much at all to the overlap. I wouldn't worry as much about the overlap thing and instead figure out what your blower-only boost is likely to be on a stock engine, think about that torque level, and then think about how much less than that you'd be happy with. Once you know that you can ballpark as to how aggressive of a cam profile you could be happy with. I dont actually think it has to be tailored to the turbo so much as made one or two steps MORE aggressive because you have the low rpm torque fill from the roots blower anyway. You could probably make the decision mostly off of what idle rpm/quality you're willing to tolerate!

That's the right answer. I am a cam grinder and I'll tell you that most people buy into the low duration, less overlap myth, end up running a stock cam and leave all kinds of easy power on the table. If you're adding forced induction to make power, whey would you run an economy cam? You wouldn't.

Easy answer, choose the duration for your intended RPM. The FI will buy you a little more RPM on top than normal, so you can get away with a little less duration, but for the most part choose a cam like you would for a non FI application.

Big cams and boost definitely work fine. Too much overlap is bad, but as long as you don't go crazy, there's no need for a tiny cam.

Of course, a cam with lots of lift and fast lift rates, reasonable (not massive) duration, not too much overlap combined with good flowing heads will give a much wider powerband than the typical "I want all the hp" giant cam setups that idle like crap (but sound good), etc.

Thanks for the info guys.

Vigo, the part you wrote..

generally a turbo setup would have much higher pressure in the exhaust manifold than the intake manifold, so periods of valve overlap would actually result in exhaust reversion and 'charge contamination'. Turbo setups where boost pressure is higher than 'drive pressure' are rare,

This pretty much sums up what I'm trying to achieve with this compound arrangement. I have a gauge to monitor back pressure at the turbo so that should always read lower than the boost gauge.

Looking forward to getting this thing going.

Craigorypeck wrote: I have way stiffer valve springs installed so should be ok there. Boost delivery will be controlled using an external wastegate instead of the usual BOV. It has a more progressive on/off rather than the quick action of a standard BOV. I'm probably gonna run with the stock cam at first but would like to get a grind done that suits the characteristics of the build better.

I think you're mixing the two terms here. The wastegate controls the amount of exhaust that is routed through the turbine to control boost, a blow off or bypass valve is to prevent compressor surge when the throttle closes under boost. Running without a bypass or blowoff is not recommended.

This pretty much sums up what I'm trying to achieve with this compound arrangement. I have a gauge to monitor back pressure at the turbo so that should always read lower than the boost gauge.

Normally, having a turbine design that would allow that would mean you wouldn't spool up early enough to be useful in a street car. You'd have a 'dyno queen' powerband. But, since you have the roots blower, you don't NEED the turbo to make low rpm boost to get torque and can go bigger and less restrictive on the turbine.

It looks like the setup is already assembled, but choosing a turbo on this kind of setup would be very different because of that roots blower. An extreme simplification would be if you had a roots blower and ~7psi, it would make the engine 'act' ~50% bigger, so instead of sizing your turbo to spool at 3k on a 2.3L you'd be picking it to spool at 3k on a 3.5L. And if you want very low backpressure and don't need the turbo to be online by 3k you could pick a turbo to spool at 4000rpm on a 3.5L. In short you could end up with a VERY large turbo for the power level and displacement just because the roots allows you to. That's mostly talking about the turbine side. Picking the compressor in a compound setup is a little different too because the compressor has to be relatively happy at low PR but high mass flow.

Strizzo wrote:

Craigorypeck wrote: I have way stiffer valve springs installed so should be ok there. Boost delivery will be controlled using an external wastegate instead of the usual BOV. It has a more progressive on/off rather than the quick action of a standard BOV. I'm probably gonna run with the stock cam at first but would like to get a grind done that suits the characteristics of the build better.

I think you're mixing the two terms here. The wastegate controls the amount of exhaust that is routed through the turbine to control boost, a blow off or bypass valve is to prevent compressor surge when the throttle closes under boost. Running without a bypass or blowoff is not recommended.

Indeed. I'm running two wastegates. One in the conventional way to divert exhaust gases at the turbo and the other which has been modified with a very weak spring to control boost delivery. Topside of diaphragm sees manifold vacuum and lower side sees boost from other side of throttle body. This has a more gradual on / off boost. Most oem superchargers have the throttle body before charger but with all this piping I'm keeping it close to the engine. Using a conventional piston type BOV is no good.. been there.. kangarooing down the road. The large diaphragm of the wastegate is more responsive to vacuum signal. Glides onto boost nicely. The weak spring in the wastegate can be tuned so it actually regulates how boost is delivered. A really week spring (say 1-2 psi) will take little vacuum to open so you could be at 80% throttle before the wastegate shuts giving all the boost to the inlet. A stronger spring will shut quicker and give all the boost at say 25% throttle as it will overcome the reducing vacuum signal on throttle movement easier. When fully shut the top and bottom of the diaphragm will see boost so essentially the weak spring is keeping the valve shut.

Rough instal pic. Ignore that turbo.

Vigo wrote:

This pretty much sums up what I'm trying to achieve with this compound arrangement. I have a gauge to monitor back pressure at the turbo so that should always read lower than the boost gauge.

Normally, having a turbine design that would allow that would mean you wouldn't spool up early enough to be useful in a street car. You'd have a 'dyno queen' powerband. But, since you have the roots blower, you don't NEED the turbo to make low rpm boost to get torque and can go bigger and less restrictive on the turbine. It looks like the setup is already assembled, but choosing a turbo on this kind of setup would be very different because of that roots blower. An extreme simplification would be if you had a roots blower and ~7psi, it would make the engine 'act' ~50% bigger, so instead of sizing your turbo to spool at 3k on a 2.3L you'd be picking it to spool at 3k on a 3.5L. And if you want very low backpressure and don't need the turbo to be online by 3k you could pick a turbo to spool at 4000rpm on a 3.5L. In short you could end up with a VERY large turbo for the power level and displacement just because the roots allows you to. That's mostly talking about the turbine side. Picking the compressor in a compound setup is a little different too because the compressor has to be relatively happy at low PR but high mass flow.

Now we're talking.!

Again nail on the head. This turbo is a hx35 off a 6.0 diesel and I have a choice of two turbine housings a 12cm and a 16cm. I'm guessing the turbo will be seeing an engine of 3.5l all depending on the charger pulley ratio.. I've belted it at 2:1 crank but just guessing for now.

Once spooled i may have issues keeping boost at a manageable level.

As I say this is a bit of pure fun to gimme something to think about at my boring job. I will take any criticism onboard as I'm no expert.

This setup although somewhat complex will have the best advantages of both boost suppliers with none of the drawbacks associated with either. When cruising to the track I'll still get decent mpg as it's small cc. I'll also be able to run more ignition timing as there will be a nice fresh load of boost with no residual gases hanging around pushing cylinder temps up.. the inlet charge may be higher but running two intercoolers should help.

I don't know what 2:1 drive ratio will equate to on that setup but i do know that with a compound setup you get to make decisions that don't exist in a single-turbo setup, like which combination of PRs from each 'blower' will give the best overall adiabatic efficiency (i.e. put least heat into the air charge). Roots blowers are poor in adiabatic efficiency compared to turbos, so in general you would probably want less PR across the blower and more PR across the turbo to have the best adiabatic efficiency. But, that limits you to lower 'blower-only' boost until the turbo spools up. So, for around 30 psi of boost you could do 1.7PR across the turbo and 1.7PR across the blower (14.7psi ambient X 1.7 X 1.7 = 42 - 14.7psi ambient = ~28psi boost), but it may be more 'efficient' (cooler) to do 2.0PR across the turbo and 1.4PR across the blower, if you could stand the loss of 'low end'. Exceptions to this would be if the turbo compressor is so unhappy at low PR and high mass flow that its adiabatic efficiency really isn't any better than the roots blower anyway, or if you have so much intercooling from whatever method or charge temps are low enough either way that it doesn't matter.

As for the HX35, another thing about compounds is that your max power is still limited primarily by the inducer diameter of the 'big' turbo (or in this case only turbo). All your air has to flow through there so that's your limit regardless of the fact that you have two power adders instead of one, and the compound setup CANNOT make more power than just a single-turbo HX35 setup. It will just have WAY more area under the curve and the later the hx35 would have spooled on its own, the bigger the difference would be. I actually think the HX35 is a little small for a compound setup on a 2.3L and would probably use the bigger turbine housing if i had the choice. There are compressor wheel upgrades for Holsets out there so if you were to run into its ~550-600hp limit (iirc?) you could upgrade it instead of replacing totally.

Compound setups have interested me for a while.. I collected most of the parts needed to do this same thing (turbo+roots compound) on a 3.0 mitsu v6 and just never built it.

This assume the turbo is the flow limiting factor.

I think the roots will be the strangle point even though you're spinning the dog doo out of it. The end fed design is going to limit airflow me thinks. Force feeding it will certainly help.

Logging pressures will provide answers.

As long as the bearings and rotors in the blower are up to 12k rpm (with some overhead). I expect you have already looked into this aspect before getting this far.

Vigo wrote: I don't know what 2:1 drive ratio will equate to on that setup but i do know that with a compound setup you get to make decisions that don't exist in a single-turbo setup, like which combination of PRs from each 'blower' will give the best overall adiabatic efficiency (i.e. put least heat into the air charge). Roots blowers are poor in adiabatic efficiency compared to turbos, so in general you would probably want less PR across the blower and more PR across the turbo to have the best adiabatic efficiency. But, that limits you to lower 'blower-only' boost until the turbo spools up. So, for around 30 psi of boost you could do 1.7PR across the turbo and 1.7PR across the blower (14.7psi ambient X 1.7 X 1.7 = 42 - 14.7psi ambient = ~28psi boost), but it may be more 'efficient' (cooler) to do 2.0PR across the turbo and 1.4PR across the blower, if you could stand the loss of 'low end'. Exceptions to this would be if the turbo compressor is so unhappy at low PR and high mass flow that its adiabatic efficiency really isn't any better than the roots blower anyway, or if you have so much intercooling from whatever method or charge temps are low enough either way that it doesn't matter. As for the HX35, another thing about compounds is that your max power is still limited primarily by the inducer diameter of the 'big' turbo (or in this case only turbo). All your air has to flow through there so that's your limit regardless of the fact that you have two power adders instead of one, and the compound setup CANNOT make more power than just a single-turbo HX35 setup. It will just have WAY more area under the curve and the later the hx35 would have spooled on its own, the bigger the difference would be. I actually think the HX35 is a little small for a compound setup on a 2.3L and would probably use the bigger turbine housing if i had the choice. There are compressor wheel upgrades for Holsets out there so if you were to run into its ~550-600hp limit (iirc?) you could upgrade it instead of replacing totally. Compound setups have interested me for a while.. I collected most of the parts needed to do this same thing (turbo+roots compound) on a 3.0 mitsu v6 and just never built it.

Thanks for the reply. You are very correct. I have a larger pulley for the supercharger too so I can lower the ratio if needed. I have 2 intercoolers with before and after temp sensors on each IC. This should allow me to monitor what heat each stage is putting into the boost and how efficient the cooler is and rectify if needed. But at the risk of slightly poor adiabatic efficiency, I'd rather keep the back pressure at the turbo less than the overall boost. So keeping the charger geared up may be the only option. Otherwise why bother? Id be better just running a turbo only with the nasty cylinder environment associated with a turbo only motor. I think the elevated charge temps may not as bad as the residual hot gases hanging round from back pressure pushing cylinder temps up leading to reduced timing and detonation.? The hx35 may well be small, but it was very cheap and as you say the compressor can be upgraded. As a starter unit it'll get 'er goin'!

It really will be a seat of the pants set up, I'll probably drive around with charger only for a while then introduce turbo later.