Exhaust Duration, Boost and Elevation

[RevTheory]

I've got two engines on the drawing board at the moment. One is a 4.03 x 3.5 SBC running twins at maybe 12 psi and two is a 4.03 x 3.75 and after spending nearly a week at 6,900 feet, I think I may switch it up a little and do a little 177 roots blower on the 383 for maybe 8 psi. I was amazed at how much power our car dropped going from 3,300 feet to 6,900.

My question is regarding the exhaust duration. On the turbo 350, I'd planned on doing a single-pattern, symmetrical-lobe cam and going with a slightly longer lobe with less rocker on the exhaust side of the 383 but now that I'm thinking about boosting it, would you run the standard probably 12* split or less because of the high elevation?

You wouldn't run a reverse-split on the turbo would you? I've never had to factor in elevation before and I don't know how much that changes when you boost it, much less boosting it from two different ways; turbo and roots.

The turbo 350 is a daily driver and the 383 is a summer toy; both 6k max rpm and no "must come off hard and hang on past the flag" or any of that sort of stuff. Just generalities here.

[CamKing]

With a Turbo, or a blower, the engine doesn't know it's at high altitude. It only knows the "Altitude" in the plenum.
You would cam them the same way you would at sea level.
If the pressure in the plenum is 46", that's what the engine sees.

[RevTheory]

Good deal. Much appreciated, Mike.

Another question gnawing at my brain is regarding seat angles. I've been itching to try 50s and I don't know if being under boost adds to or takes from that particular argument but I could always save that battle for later

The lift on the turbo cam was right around .25/D and the other was closer to .3/D: 2.02 and 2.05 intake valves, respectively.

[BLSTIC]

With a Turbo, or a blower, the engine doesn't know it's at high altitude. It only knows the "Altitude" in the plenum.
You would cam them the same way you would at sea level.
If the pressure in the plenum is 46", that's what the engine sees.

Would the lower pressure seen in the exhaust system at altitude affect much for the blower engine?

And would the cam affect turbo spool at altitude? I could imagine the act of getting on to boost being both harder and more important at altitude. Assuming the relationship between torque converter stall and boost threshold was marginal anyway. With an engine that was already super-responsive you might not notice it, but I've driven one car that was moderately hard to get boosting on the stall at sea level and almost impossible on a bad day at the 1800ft track.

[RevTheory]

^^^That's a fine question I hadn't thought of^^^

[hoodeng]

Elevation is self compensating with a turbo charger, that is,,,the higher you go the thinner the air, this works in two ways, one, it provides less resistance to the exhaust gas at the outlet of the turbine/pipe allowing the turbine/ compressor to spin faster, and two, seeing as how now the compressor is spinning faster it introduces a compensated amount of air from the thinner air provided at the inlet. [I have no idea just how much this effect would be of influence at 1800ft?]

This was proven during WW2 with the Lockheed P38 Lightning that was powered by two Allison V1710 that were turbo-supercharged, that is, the engine in its normal configuration was mechanically supercharged, then there was a GE turbocharger mounted behind the engine, this fed boosted air into the supercharger to compensate for altitude [ there were engine operation procedures to save from over boosting through atmospheric pressure variance].

Rolls Royce went about altitude compensation a different way, they retained superchargers only, in latter iterations they were two stage two speed driven units. That is one supercharger fed into a second, then into the manifold, there was a two speed drive gearbox that was aneroid controlled, that is, at take off and low altitude the low gear was used to run the compressors, as altitude climbed through a given point the high gear was engaged and the compressors run harder to compensate for loss of air density.
The reason RR went down this route and not the turbo supercharger route was because they wanted to retain the feature of rear facing exhaust stubs/ejectors boost to speed.

This is the long version of what Mike said.

Cheers.

[vannik]

Controlling boost to compensate for high altitude requires keeping the plenum absolute pressure the same, not the gauge pressure. This will usually eventually lead to a turbo overspeed case so you need to also measure turbo speed to open the wastegate to control turbo speed when you get to the limiting speed.

(I live at 6000ft )

[F-BIRD'88]

Instead of the Weiand 177 blower consider the TBS 192 billet blower.. Its a bit more money but every thing about it is better.. billet case, end cases/gears, seals, 10 rib/16 rib, or cogbelt drive, inlet adapter 4-71 type allows single, dual 4bbl or 4500 series carbs or single /dual EFI vs just a cast in single carb flange.
Much better appearance.
Bigger blower cid, more power on a 383 cid engine.
Look this one over before choosing.

[BLSTIC]

Controlling boost to compensate for high altitude requires keeping the plenum absolute pressure the same, not the gauge pressure. This will usually eventually lead to a turbo overspeed case so you need to also measure turbo speed to open the wastegate to control turbo speed when you get to the limiting speed.

(I live at 6000ft )

So the correct answer is a compound turbo then? Something mind knumbingly large at the back of the car that flows enough air for 600hp at 4psi... A pair of Chinese GT45 or something :p

[F-BIRD'88]

Here's your cam.. low cr 383 cid 6000 rpm. supercharged street power where it matters on a street car.
Comp XE284H-10 12-250-3 Can use 1.6/1.5 rockers.
Install on 104/116 centers (110LSA)

[RevTheory]

Boost was never a consideration on the summer toy 383 until I felt with the old, trusty butt dyno just how much power is lost at that elevation. I just want it back without making much of an ordeal out of it. I suppose I could always just go a little larger but it bugs the hell out of me having to build a 400 just feel a 350, you know? Plus, who doesn't like a little blower whine to get the blood flowing?

My mind is kind of seeing a boosted charge preferring the path through the throat and around the valve when a 5-angle seat configuration is based on a 50 but I don't know if that's accurate. Any comments there?

[F-BIRD'88]

Don't get all lost in stuff that makes little overall difference to the end game. K.I.S.S.

What heads you you got for the 383?

[ptuomov]

I agree with almost everything that was said.

I see there being some hardware questions and then some control questions.

For high altitude operation, I think you need a big compressor inlet piping and large air filters. You likely also need big intercoolers, at least for the ambient air flow side. The engine cooling system needs to be able to run at a high gauge pressure and a big radiator and low temp thermostat would likely also make sense for a turbo compensated engine.

For low altitude operation, I think you want to have big and well flowing exhaust pipes after the compressor.

The high altitude spooling question that was mentioned earlier is a relevant one. Based on that thinking, I’d go a little smaller with the cams. However, with a twin turbo V8 street car, likely all sensible street cams would likely work just fine.

On the control logic: I haven’t ever seen a mechanical wastegate actuator that has sealed backside canister for standard pressure. Does one exist? Because such thing does exist, then a purely mechanical control system could get pretty close to a turbo compensation. There would still be a different wastegate pressure differential that would change the mechanical control a little.

Does anyone in the aftermarket successfully run a fully closed loop electronic boost control system? Or all they all still open loop bleeds on the air pressure operated mechanical wastegate system?

In any case, I think the turbo speed can be managed with open loop tables, but calibration will be a lot easier with turbo speed sensors.

This was written before my first morning coffee so reader beware.

[CamKing]

Here's your cam.. low cr 383 cid 6000 rpm. supercharged street power where it matters on a street car.
Comp XE284H-10 12-250-3 Can use 1.6/1.5 rockers.
Install on 104/116 centers (110LSA)

That is a ridiculously horrible recommendation.

[CamKing]

With a Turbo, or a blower, the engine doesn't know it's at high altitude. It only knows the "Altitude" in the plenum.
You would cam them the same way you would at sea level.
If the pressure in the plenum is 46", that's what the engine sees.

Would the lower pressure seen in the exhaust system at altitude affect much for the blower engine?

If you overdrive the blower to get the same pressure in the intake manifold, the pressure in the exhaust would be the same.