DanaT

I have too much time on my hands to think about useless stuff (i.e. this means flying on an airplane).
Anyways, I am starting to get the 951 tuned in and am trying to get it straight in my head what limits 951 power (which I guess should be the same for any engine).
There are a few common ways to make power. More displacement, turbo charging, supercharging, better flowing head, etc. However, to me they all have one tburhing in common. They are trying to get more oxygen into the engine so that more fuel can be burned. The displacement also has some other advantages but in general more air=more oxygen= more fuel burnt. More fuel burnt = higher thermal expansion of the gas (not gasoline but gas as in the invisible stuff that is most like air but may be a mixture if nitrous is involved) inside the cylinder. More thermal expansion = more cylinder pressure on the power stroke which in turn creates a higher force on the piston. More pressure on the piston creates more force on the connecting rod which in turn creates more power (f=ma).
In the case of more displacement, there is the added advantage of more surface area for the pressure to act on which in turn creates more mechanical multiplication of the pressure on the piston. For example, taking a 100mm piston/cylinder and putting in a 101mm piston/cylinder (overbore) creates 2% more surface area and displacement. This means for any given pressure, one would expect to see a 2% increase in power. Increasing the stroke may also give a higher displacement but not the added area. Anyways, there are practical limits of what a 951 can be bored to (we cant make a 5 inch bore which would make great power) so for the sake of this am ignoring it.
What we commonly see on 951s is a few different ways to generate power with the exception of 3.0L engines (again for where I am going let’s assume no change in displacement). I would say the common ways of making power with the same displacement are: more boost, better flowing head/intake system, cams, and better exhaust. The uncommon way would be nitrous. Looking at all of these, we are trying to get gas and fuel into the cylinders to make more pressure inside the cylinders on the power stroke and generate more power. However, at some point the cylinder pressure becomes to great and a few things can go wrong and hopefully it results in just a blown headgasket (I had this before).
Looking at the different options there are limitations.
1) Turbo boost can be increased to a point where the turbo becomes inefficient.
2) Exhaust increases can only flow to a maximum of what is in the cylinder (for example the super racing 8 in exhaust with 3 inch header tubes probably wont help too much but they may look cool)
3) Cylinder head/cam/intake manifolds, etc make it easier for more air to get into the cylinders at lower boost levels. This means is [slightly] more efficient than more boost as boost heats up the air and a free flowing intake adds no heat. The head work also does not come cheap.
4) Nitrous could be injected up until the user gets scared and has the added benefit of cooling the intake charge making it less dense. The biggest limitation is refills, how much can be carried, and kahones.

Now I am just making some things up but I need to for the sake of argument. Lets say I am making 300rwhp and am shooting for 400rwhp. Also, for the sake of argument lets just assume 104octane unleaded race fuel so that I could make power (and its only 40 cents a gallon more than 100 octane).
So I get cheap, and decide to crank up the boost. By cranking up the boost I can do this until a) the turbo is out of its efficiency range or 2) something inside the engine gives up. Wanting to get another 100hp means I need between 7 and 10psi more boost.

Now, I decide to go all out and call up Race Shop X because they have a great reputation and order the head and cam that they claim gives me 100hp (I don’t think any of them claim this in real life so I am just making it up). I slap on the head and cam and my wallet is now much lighter (which I guess also helps because weight savings makes a car faster).

Finally, I figure out my turbo is at its limits at 300rwhp and after looking in my wallet, I realize it is only about $600 over weight. Therefore I step up and put on a 4 cylinder nitrous kits with four foggers right at the head and slap in #26 nitrous jets. This is good for about 104hp (actually I know these jets because I ran 4 number 26 jets on a motorcycle).

For the question, wouldn’t all three methods above generate the same cylinder pressures and therefore be just as “safe” (or dangerous) for the engine at a give octane level?

This is not as silly as it seems, I want my 951 to make good power and will start playing with upping boost (I listened and I know baby steps with boost). Assume that I can flow enough air through the turbo to make linear power increases up to 20-22psi of boost. Am I better off cranking up boost to that point (boost is free and race gas isn’t so expensive)? What would I really benefit with a cam? Would I be less likely to damage something with headwork and/or cam and less boost for the same power level or since I assume cylinder pressure is about the same either way (I have simply forced more air in at a given boost level) am I going to start running into the same problems at about the same power level no matter what?

There are other ways of making power gains that help such as water injection, bigger intercoolers, etc, but these are all based upon getting denser air into the cylinder at a given boost level which I guess means one can run more boost. For example, lets say without water injection the charge temp coming out of the intercooler is 140C@15psi. The water injection cools that to 90c. Then the charge pressure is 13.18psi. I suspect people make more power by increasing the boost back to 15psi.

Where are the weakest points of a 951 engine and what would be basically an upper limit of HP that a 2.5 can generate without become a time bomb? What are the most the most cost effective ways to hit say 250rwhp, 300rwhp, 350rwhp, 400rwhp?

As you can see, I have too much time on airplanes to think…what would a 777 engine in a 951 do????

-Dana

rop3

Dana,

I think you are opening up a can of worms here, gonna get a lot of diff opinions. In my case it is not strictly how much "safe" power but for me more importantly where or what range it comes in, it's nice to claim 400/500hp but how much sacrifice for street or around town driving?

My setup I have less than 100 miles not sure if the cam and head work hurt low end or not, it's all new but I am not seeing nearly the torque you are. I have been told that a cam can really make or break the car. And turbos not a simple choice either.

I have been told I am in the minority and that most guys just want top end over 4k acceleration etc. Certianly not what I need for a street car but the opposite for a track car, then again it depends on the kind of track. Lime rock small short straights etc. you need more of what I am looking for.

rlm328

Boost + nitrous = loud bang

DanaT

Why do you say this? There are many many turbo cars using nitrous.

When the WRXs first came out I say a fast one that the owner was running just using nitrous. He didnt put on a bigger turbo, as is common, but used nitrous because he said he had the qucik spooling of the stock turbo but had much more power when he wanted it (for drag racing).

-Dana

JDS968

I think you're mixing together a few different issues and questions.

The straightforward answer is that the maximum limits on 951 engine power are how much stress (pressure, temperature, heat energy, vibration, torque output, RPM, etc) the engine can take before it grenades.

In practical terms, though, if you do it properly, you should be able to hit 400 or probably even 500 horsepower long before the engine turns to shrapnel. Probably do want a new head gasket and upgraded head studs, though.

(Oh and we ARE talking about real horsepower, not voodoo [wheel] horsepower, right?)

Anyway, you seem to be looking at power upgrades in terms of "one or the other". However, most of these upgrades (especially with turbocharged cars) work together. Cams are great, better flowing heads are great, better flowing intake and exhaust are great, increased boost is great...but a little more boost through a little better flowing intake [MAF/throttle body/manifold] into a little better flowing head opened up by a little hotter cam and pushed out a little better flowing exhaust usually works best. Add in some other things you haven't discussed, like a little more efficient and effective intercooler and water/methanol injection [which gives you a cooler, denser fuel/air charge to get more of it into the combustion chambers, plus more effective octane to allow you to run higher boost and more advanced spark timing without knocking], even things like lighter pistons and connecting rods...you can get a lot of power, and you're not going to put as much stress on the engine as you would by simply trying to force 60 pounds of boost through it, or dumping a gallon of nitrous into it.

I know I'm going to stir up a lynch mob for this, but nitrous is stupid unless you live your life a quarter mile at a time...

JDS968

Over decades of airliner development, they've gone from compact turbojets [707] to very large, very high bypass ratio turbofans [777]. I believe the 777 now has one of the biggest turbine engines ever fitted to an aircraft, many times larger than an entire 951 (almost 24 feet long and over 11 feet wide). Sooo...the 951 would go in the 777 engine

And if you had a 951 that folds space to be bigger on the outside than the inside, like the TARDIS...ummm that engine has well over 100K horsepower. So...yeah, after the entire drivetrain exploded into shrapnel and killed you, and then you got better and put in a new weapons-grade drivetrain made of carbon nanotubes woven into Chuck Norris's beard hairs and replaced the rear wheels with tank tracks...I think you would break the sound barrier on land, then you would go to ludicrous speed.

http://www.youtube.com/watch?v=mk7VWcuVOf0

TurboTommy

Unless you want to drag your car, nitrous oxide is useless. Your bottle will always be empty just when you really want the power, and your whole life will evolve around when and where you fill the bottle.
So we'll set that aside for now.
Power; you can get two ways: either increase the force on the crankshaft (torque) or increase the speed at which the torque is made (RPM), or obviously a combination of both.
Cams and generally increasing the flow through the head basically results in the higher RPM route. Limitations there are the valve lifters, springs, the oiling system, and would all have to be addressed. But cylinder pressures do not go up when increasing power this way.
Increasing boost increases the torque part of the equation. But even if your turbo compressor has some room to move the boost up efficiently, the hotside needs to be big enough (and this is often overlooked). It gets to a point that exhaust back pressure rises waay faster than the increasing boost. The exhaust gases don't get expelled properly and there are pumping loses that take power from the crankshaft. Both of these greatly lessen the effect of the higher boost you want to run. So, I'd say the limitations here (if detonation is not a factor because of your race gas), is the exhaust side which includes the header and everything back.
Experts should chime in on the strength of the case.
Also, increasing both RPM and/or torque requires the turbo size (both cold and hotside) to be addressed, since more flow is needed in either case.
Also, if mass air flow increases, intercooling and pressure drops through the intake need to be looked at.
Then you could argue that your gearing should be changed to get the most of how you added your power. If you take the RPM route, your car won't feel faster if you don't shorten up the gearing, for example.
Alot more technical discussions could come from this, I'm sure, and could go on and on.

blown 944

Dana, IMO boost overcomes the need for nitrous on these cars. I had it on mine for awhile and took it off because ultimately I didn't feel the need for it anymore. This is coming from a guy who was a total nitrous freak for quite some time. I think that once you switch to e85 and crank up the boost and then maybe even setup an anti lag(you are one of the few I think who would like it flame suit on) on there. You will see what I mean.

JDS968

What about trying one of those Aerocharger variable geometry turbos? I really want somebody here to be a guinea pig for those things, so we can find out if/how they work and then I can put them on...well...everything. Including my Land Cruiser

piperporsche180944

You know, I was always curious what a 100shot would do to a 944 na LOL!

Variable geometry turbo would be interesting on a 951.

Rogue_Ant

Good luck getting a VGT turbo to fit in the stock location... The hotsides are way to large on all the VGT stuff I've seen - they would hit the block.

toddk911

what about the new ones from Borg Warner that are on the 997TT ?

Also, once the pandora's box of nitrous is opened, it is a great idea for spool up use and that would allow the tank to run much longer than for a top end shot.

So slap on a big fat laggy turbo for the top end and a 100 shot that would come on down low, 2K-4K, for "off the line/spool up"; i.e. to fill in the area under the curve from the big laggy turbo that can run to 7K. So when the nox is done, the turbo would be at full boost or close.

So you would have an area under the curve from 2K-7K.

Or propane injection. Something I don't think has ever been discussed on these boards.

Chris White

The philosophical answer is as follows –
“the is no limit to the power a 951 can make – there is only a limit on how much you wish to spend”

DanaT – in your original post you don’t mention any engine management mods – you really need to keep the engine management in tune with your mods, otherwise things will break.

To answer the ‘weak point’ question – tuning is the weak point. I have made 370rwhp on engines with stock internals (used on the track for long periods) without a problem. That was with a stand alone, larger turbo, good wastegate and bigger injectors.

Oh yeah, and just to be a techno ***** –
Cooling the intake charge makes it more dense….but decreases the pressure. You may really want to remember that if you want to pilot an airplane, its kind of an important element of flying!

DanaT

Thanks for the catch. I don't know why i said less dense. Cooler is always more dense because of pv=nRt. That is what happens when I don't proofread what I write....

DanaT

Who needs engine management? Carbs have worked just fine for a 100 years

I guess I should have put that as an assumption that engine management would be able to take advantage of the increases. I have 83lb injectors, so I have enough to flow the required fuel and I am getting everything dialed.

I didn't respond to an earlier post, but I think one of the limits is the stock head studs. I think they stretch enough to allow the head to lift from the block ever so slightly and blow headgaskets. After putting on good aftermarkets headstud and a widefire headgasket, I haven't blown a head gasket.

-Dana