This has baffled me for years. It seems like it should be a fundamental design element of any turbo system, yet I've only seen it once...

Lag, lag, lag... That's all we ever hear about turbos..... So, what does it take to spin a turbo?... Exhaust flow/pressure, right. And what does it take to create exhaust flow/pressure?... Intake flow (and combustion), right. And what does it take to create gobs of instant intake flow?........ A wide open path to the atmosphere!....... 14.7 lbs of glorious INSTANT (absolute) boost pressure!, right.

So, why then do all turbo systems basically put a big throttle body on an intake and then plumb it straight to a restrictive turbo, which probably only flows about 30 cfm through the compressor housing (before the impeller starts to spin and speed things up a bit....) Basically, how fast do you think Husain Bolt would be if he had to suck his first FULL breath... through a drinking straw!

Anyway, my basic (rhetorical) question is... Why don't turbo systems provide a means of direct flow into the intake as in naturally aspirated engines, and then provide a flapper valve to block that path off when the turbo kicks in? The only system I've ever seen this on was a Martin turbo intake manifold from the 80s. It was a two plane intake with the carb on top. The upper plane drew from the carb and was plumbed to the compressor inlet. The lower plane was plumbed to the compressor outlet and to intake runners. The turbo basically drew in from the upper plane and boosted the lower plane. What was unique was that it had a big flapper valve between the upper and lower chambers that allowed the initial air/fuel mixture to flow directly to the intake ports, bypassing the turbo. This created a much better throttle response, and, in turn, better turbo response.

A classic example of a poor design was the GMC Syclone. I logged 130k miles on one of these. Same thing. GM TPI throttle body plumbed through a small air to liquid heat exchanger, and then straight to the turbo (do not pass go, do not collect $200). The throttle response was terrible. Yeah, once the turbo spooled up it set you back pretty good, but you could play a game of Monopoly while waiting for that to happen. I drove a normally aspirated 4.3 liter V-6 S10 once and thought, WOW, this thing is really peppy! Of course, it didn't have the power of the Syclone, but if the contest was G-forces with a throttle tap, the naturally aspirated S10 would have smoked the turbocharged Syclone.

I've built a couple barn-yard turbo systems, and I'm building another one now. The first one had a vacuum controlled butterfly valve that allowed intake air to bypass the turbo until the manifold pressure was near zero, and then it would close. It gave the engine a big gulp of air to get the turbo spun up, and then it would close when the boost started. It worked pretty well. The system I'm building now has a GM TPI, and I will have a flapper valve in front of the throttle body that opens to an air cleaner. The turbo will feed in between the flapper valve and the throttle body. Basically, air will flow straight into the throttle body until the turbo flow comes in, closes the flapper and creates boost.

Oh, and by the way. In this process of giving the engine a big gulp of air, to create a big blast of exhaust, to spin up the turbo faster,.. the engine actually produces more instant power, which makes IT spin up faster, gulp more air, make more exhaust, spin turbo faster, etc.... AND the engine is producing more power that is starting to move the vehicle sooner. Just my $.03 worth...

 

Sounds interesting. A control other than vacuum would possibly be needed.

 

You can do similar with a two port BOV. Hold BOV open and rpm up. Switch the singal at the BOV and it slams shut w instant boost. This wears out the turbo faster and you have to keep an eye out for shaft speed on the turbo sincethere is no resistance to keep it from going crazy.

 

Smaller A/R? Nitrous works well too.

 

Thanks for the responses! All good ideas. I have thought about this for a long time and I've always wanted to see what other turbo enthusiests have done to address turbo lag.

 

Not sure what your seeing for lag. To give you an idea, the two curves are from the same motor combo. These are two different turbos, with similar curves. All curves are with the throttle fully open at all times and no nitrous applied.

Red is an old journal bearing turbo with a 40 AR
White is a modern ceramic bearing with a 60 AR

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Looking at the same ceramic turbo/motor combo, the Red is the manifold pressure while White is exhaust pressure. Obviously everything is in PSIG.

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For this setup, the motor made more power with the smaller AR than the chassis could support. So maybe a smaller AR would be a better match. I use the nitrous to cool the charge anyway so no big deal for this setup to overcome effects of lag.

IMO, size the turbo for what your doing.

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Stick a vacuum cleaner hose on the compressor outlet if a turbo charger and tell me what happens.

 

This sounds like a recipe for damaging turbos. The reason you have a blow off valve it to prevent high load being suddenly put on the turbo by closing the throttle blade (surge).
From Garrett: "Surge can eventually lead to thrust bearing failure due to the high loads associated with it."
Your suggesting doing just that...freewheeling the turbo up to speed then suddenly introducing load (boost)...seems like a bad idea to me.

 

It looks like turbo lag is not much of a problem anymore. Its been years since I've messed around with turbos and it looks like things are a lot better now, especially with the ceramic bearings. The turbo I just bought is a journal bearing, but the A/R is on the small side so it may just be the ticket. It is a driver, so I want it to be responsive during street driving.

The BOV idea may be hard on the bearings, I don't know. The flapper valve that I'm planning to make will not cause any major pressure spikes to speak of. I basically want it to breath as though it is normally aspirated (with air flow bypassing the turbo) and then gently transition over to boost.

 

Flapper or bov if you are pushing exhaust through the turbo it will be free wheeling.

 

Would depend on how quickly you shut the BOV.

 

Some comments on your deal

Gale Banks currently offers a EFI Turbo system using one of his patented "Shuttle-Valve" By-pass bonnets.

He used the same bonnet on his carb set-ups too.

The shuttle is open at low speeds so the intake sees lots of air to the throttle body or carb at low speeds before the turbos have spooled up.

When the boost comes on (Vacuum drops in the intake) the shuttle valve closes and now the turbo supplies the air thru the normal plumbing.

So your idea is a good one, just Banks already figured out the deal. Shuttle valves work great because they can progressively close where a throttle blade valve is pretty crappy on areas between wot and part throttle.

Hope this helps.

Tom Vaught

 

Not sure how far back your talking but the older turbo that I showed out pacing that new ceramic unit was a 1970's Rajay. The ceramic bearing is not much of a help for lag. Garrett shows some plots comparing the two bearings for lag. AR will make a big difference.

Should mention those plots I posted were in seconds. So the first graph shows the old 1970's Rajay going from 12 to 24 PSIG in half a second with this motor. It ramps up quicker at the lower pressure levels.

 

The Ceramic Ball Bearing technology is primarily used for High Thrust Loads with High Boost turbos.
The normal Thrust Load oiled "bronze washer" systems do not do well in that area.

The Ball bearing technology is not there for Time to Torque, it is there for Air Power side loading, think 80+ lb/min turbos.

Tom Vaught

 

And 2-1 back pressure. And retards doing this
http://m.youtube.com/watch?v=VajkyCtlBug

 

page 33
http://www.full-race.com/articles/efrturbotechbrief.pdf


Not to hijack the thread, but not sure I understand the 2-step video. Why would you retard? Would be interested in knowing where your at (timing) when you launch.

 

I remember that Banks system. I always thought it was strange that it drew in unfiltered air through the shuttle valve. I guess there may have been a screen in the bonnet though. I always dreamed of having that system but it was over my budget.

Below is a picture of my turbo system on a Monza from about 1982. It shows the bypass tube going around the turbo. The buttterfly is in that tube near the turbo exit tube. The vacuum soleniod is underneith. The tube could have been a little larger, but it worked well. Also, note the BOV which relieved any over pressure when the throttle was closed (between gear shifts.)


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The butterfly valve was controlled by a spring loaded vacuum solenoid connected to the intake manifold. It was a light spring and would progressivly close near zero psig to allow boost to build. It would reopen as soon as the carb was closed and it saw vacuum in the manifold again. This would bleed off the extra boost pressure too. I think the Banks shuttle valve basically did the same thing. Butterfly, flapper or shuttle will all do basically the same thing, that is allow that initial gulp of air to get the turbo(s) spooled up quicker (although the flapper will not bleed off the boost pressure and would need a BOV too.)

My frustration with this is that it doesn't seem to be recognized by the OEMs. That Syclone had terrible throttle response, and I always thought about rigging up a bypass valve of some sort, but I never did. I'm sure it would have made a big difference.

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The diesel pullers supposedly use BOVs in the way mentioned where they snap shut at a certain point. Probably hard on turbos, some Supras and RX7s used a staged sequential setup that would use valves to "bring in" the second turbo at higher RPMs. They are known for losing their secondary turbos for this reason. It's not the best for the shaft.

Besides that, the reason for timing retard is simple; it dumps heat into the exhaust by letting fuel burn out the cylinders with the valve open. By increasing the EGTs, you increase the amount of gas volume (PV=NRT) and in turn your turbo will spool faster.

In regards to the Syclone, I bet a different converter would make it behave like a different beast. Yeah, they used a 2200 RPM stall converter but it's still an OEM grade unit. Then again these trucks were
A)never really meant to be layed into from a stop (they were meant to be loaded up against the TC on the brake) and
B)I suspect part of the choices in using the turbo/IC parts they did was that the stock 700R4 transmission was rated to 360 ft-lbs. The "B" VIN HO cam was way better than the one used, and it would have been available at the time.
C)I imagine a lot of the tuning tricks used on a vehicle like this to make it run harder would be considered emissions unfriendly. Dumping more fuel/retarding timing in the areas of boost threshold would probably be considered a no-no for HC at the very least, and bumping timing up would be the wrong side of NOX emissions.
D)Until boost hits, you're down torque due to a full point of compression difference.
E)There's a huge difference between "feeling fast" and "being fast." A lot of things like gearing, manual vs automatic etc. come into play. A high geared manual could "feel" faster than a slushbox with a lower numerical gear laying down a lot more power.

 

I know your last point was not directed to the timing but maybe the most important. When adding the retard to get your turbo "spooling faster", how did it effect your short times compared with advancing the timing?

Do you have any numbers to where you are setting the timing? My motors don't make much power with the low compression. Can't see adding retard helping.

Again, sorry for the hijack.

OP, like your picture of the setup, especially that old E/E Rajay. I would still run one if I could find parts for them. Do you have any that show your valve?

 

Addressing the intake side of a turbo system to allow the engine to breathe easy is only half of the equation, for a NA engine response I am sure you wouldn't want a turbine wheel in the exhaust path either.

If a compressor wheel is open to atmosphere it will not overspeed, it will be moving allot of air at a low pressure. If it is pushing against a closed valve it will heat the air and possibly surge.

Variable Geometry Turbine is possibly the best solution to the issue. The problem being the technology is not available in popular sizes we use.