Doc

Just curious since I've heard variing answers to this over the years. What I want to know is, with a stock turbo using a dual port wastegate and adjustable boost, what is the rule for each pound of boost added over stock setting. e.g. +1lbs of boost = +20 hp. I'd imagine that there's someone out there who's done dyno runs that would know for sure.

Thanks in advance.

P.S. If it makes a difference, assume using 50/50 race gas and pump 92.

Mike1982

From what I have read on here, it is just an guess that every 1psi on the stock turbo adds about 10hp at the flywheel.

billgot

A dyno could tell you.The only thing I know is there is a drop off at a point.I asked David raines from Powerhaus this question once.He puts everything in RPM's you can say at 70,000 turbo rpm you make 20 more HP then at 80,000 you make say 15 HP then 90,000 say 5More Hp there is a limit on the Turbo's design.I am told by all turbo people its not boost that is important its Volume.David from Powerhaus could probably give you these numbers but I am sure he would say dont go above 14psi.

SeaCay

To put it another way, PCA gives us (very graciously I might add) a 1.3 multiplier to equalize the turbos to the NA's. I know it doesn't have anything to do with the level of boost, but I figured it may help anyway.

Danno

There's is no general rule. It comes down to the specific VE-volumetric efficiency of your particular engine. The stock 951 is about 49hp/ltr•bar which is less efficient than a 944NA @ about 80hp/ltr•bar. So in the stock 951, each psi boost is only worth about 8-9hp. This scales pretty linearly with boost within the efficiency sweet-spot but goes down once you run towards the edge of the compressor map.

Now if you were to have VE mods like headwork, cams & adjustable timing etc, then you can increase the efficienty to 80-90 hp/ltr•bar which would give you 15-16hp per psi. This would be like the Scott Gomes engine I have on my RacerX website which made 458rwhp@20psi. Doing the math we get:

(20.0psi/14.5) = 2.38 bar absolute boost
458rwhp/0.85 = 538 bhp

So the efficiency is:

538bhp / (2.5liters * 2.38bar) = 90.4 hp/ltr•bar

To see what this engine would do at 21psi, we go the other way:

21.0psi/14.5 = 2.45 bar absolute
90.4hp/ltr•bar * 2.5liters * 2.45bar = 554bhp

or an increase of 16hp per 1psi provided that the turbo or injectors aren't maxed out yet.

NZ951

Danno, where is the cam you were gonna make with good lift, no extra duration???

Danno

Actually extra lift won't do much for our cars since it's close to the limit already. Generally, max usable lift is 25% of the valve-diameter and we're about close to that. What really is important is the intake-valve closing. Timing this event makes the biggest difference in determining where the torque & HP peaks are in the engine and how high they are. An early intake- closing typically favors low-end torque at the expense of high-end HP. Conversely, a late intake-closing favors high-RPM HP, but kills low-end torque. Thus the need for VarioCam. But even then, that's just a two-step on/off transition like VTEC. Best to have a continously variable cam-timing like VANOS or Vtt. Even better to have continously variable timing and duration and lift like SuperVANOS or Vtt-i, etc.

So the cam I've got is done and matched to head, but I need to finish up on the adjustable cam-sprocket to give you the most flexibility in tuning possible.

NZ951

What advantage is there in and adjustable cam gear? I can control timing through my Link, Are you referring to gear like Lindseys cam gear?

Doc

So if I understand you correctly, given that 1 bar is about 14lbs:

16lbs = +16-18 hp
18lbs = +32-36 hp
20lbs = +48-52 hp

Still not a band gain for simply turning a ****.

Waterguy

You must have grabbed these numbers from memory Danno. Porsche was neither that good or that bad with our engines. Using SAE shaft horsepower, the actual numbers are:

Model -------Power --- Displacement -- Boost at Power Peak -- Specific output

944 US ------ 143 -------- 2.5 L ---------------1 bar --------------- 57 hp/L bar
944 Euro ---- 156 -------- 2.5 L --------------- 1 bar -------------- 62 hp/L bar
944 1988 ---- 158 -------- 2.5 L --------------- 1 bar -------------- 63 hp/L bar
944 1989 ---- 163 -------- 2.7 L --------------- 1 bar -------------- 60 hp/L bar
944 S ------- 188 -------- 2.5 L --------------- 1 bar -------------- 75 hp/L bar
944 S2 ------ 208 -------- 3.0 L --------------- 1 bar -------------- 69 hp/L bar
951 --------- 217 -------- 2.5 L ------------- 1.52 bar ------------- 57 hp/L bar
951S -------- 247 -------- 2.5 L ------------- 1.75 bar ------------- 56 hp/L bar

The high compression, 4 valve 944S wins! The 951 is similar to the low compression US 944; no big surprise there.

So a stock 951 should develop about 9.8 shaft hp per psi boost, or about 8.3 rwhp/psi. It drops off at higher boost due to the heat generated. In addition, the back pressure of the hot side (especially the K26 #6) reduces volumetric efficiency at high boost. But in theory, a stock 951 should produce about 242 rwhp at 15 psi and about 266 rwhp at 18 psi. Pretty close to what is observed.

Danno

Whoops, sorry, yes I did pull the numbers out of my head and somehow I thought that the 163bhp 944 was 2.0L, whoops . Also the Turbo and TurboS has the same max-boost of 1.75bar. The manual shows a range of 9-12psi as being within the range of acceptable Just that the TurboS doesn't drop as much as quickly. At redline, the Turbo may have dropped its 11psi (average) down to 7psi while the TurboS is still closer to 9psi. The similar boost can be seen with the similar peak torque figures. However, peak HP is dramatically different, thus showing the differences in high-RPM boost.

"But in theory, a stock 951 should produce about 242 rwhp at 15 psi "

Also there's no way the 951 is as efficient as the early 944NA, simply from the lower compression. Because the 125rwhp typical output of early 944NAs is never eclipsed by a 951 sucking in double the air-volume (15psi boost). That would require 250rwhp @ 15psi from a non-S turbo; I've haven't seen one yet. Typically APE-chip upgrades at 15psi gives 210-222rwhp. GURU Chip-upgrade gives 225-238rwhp and that's with extensive remapping of fuel & ignition curves. Although turbo-efficiency and a dropping boost-curve can be a factor here.

Non-S turbos cars are only making 260-270rwhp @18psi with VE-enhancing mods such as MAF & MAP kits and exhausts, not comparable to just bumping up the boost to 18psi with an all-stock car. So I still assert that the 944NA is more efficient than the 951 .

"What advantage is there in and adjustable cam gear? I can control timing through my Link, Are you referring to gear like Lindseys cam gear?"

I was referring to valve-timing. That's a mechanical adjustment with the camshaft. Since we don't have 2-stage VarioCam or a continuously variable mechanism to adjust the camshaft, we have to do it mechanically with an adjustable gear. Again, it's up to personal taste and how you drive the car. With a single cam, you can time the intake-valve closing to give you a power curve with low-end torque, or if you're racing, perhaps adjust it to give high-RPM HP. Plus the cam-gear I'm devising will have larger range of adjustability than the Lindsey unit for a significantly lower price.

TonyG

57Hp per liter per bar (944US & 951). That's exactly what my old 1972 Datsun pickup put out.... stock.


TonyG

Waterguy

Yes, the 951 and 951S have the same peak boost (11 psi) and the same torque. The difference is that the 951 uses the KLR and cycling valve to reduce the boost to 7.5 psi by the 5,800 rpm power peak. The 951S is programmed to hold 11 psi to 5,800 rpm, then start dropping it. I quote:

"Maximum boost pressure of 1.75 bar (absolute pressure) is reached at approx. 3,000 rpm. At higher engine speeds (engine type M 44.51) the boost pressure drops again and reaches a boost pressure of 1.52 +/- 0.03 bar (absolute pressure) at 5,800 rpm. With engine type M 44.52, the boost pressure remains constant at 5,800 rpm (1.75 bar absolute pressure) and drops only after this value has been passed."

The boost drop isn't (all) the turbo boost curve and weak wastegate. Porsche programmed dropping boost into the stock cars. Probably a good reason to bypass the CV valve when you add a manual boost controller.

So what are you making right now Tony? with all those fancy schmancy improvements, you car probably is only about 94 Hp per liter per bar. (!) I would be willing to race you, if you drive your 1972 Datsun pickup.

TonyG

Waterguy

It's approximately between >82 and <87 Hp per liter per bar depending on the boost run at the time. The lower the boost, the higher the Hp per liter per bar.

With a larger than stock intercooler would have a pretty large impact on these number bringing them up over 90 hp per liter per bar for sure.... (which is reflected in the numbers above)


TonyG