8v 2l Twin Charged -my Little Project

[Alex]

Wow.

*speechless*

Which metal did Jim mill that from?

[5yearplan]

Dizam! I wonder how much those go for...

[lenus]

Seemingly if you want to do anything very special with an alfa, you need to own your own cnc.


look forward to seeing the twin charger running!

[Biba69]

I've never gotten into making major modifications to Alfas primarially from the standpoint of lack of knowledge and the $$$ factor. Some years ago a friend who (seriously) vintage raced along with building race BMW's for clients told me he felt the BMW's had the edge on Alfa's because they have a cast iron block. In essence you can modify the older BMW engines to a greater extent than you can in an Alfa. Not that one could use a TS in vintage racing - which of course isn't buzzed's goal - but I suspect the block is somewhat stronger as is the head. In any event there will always be a (or several) weak points when highly modifying a street car.

I feel safe in saying this because buzzed not only sounds very determined but also knowledgeable - but my choice would be to start with a TS and 'just' add a water type intercooler and supercharger. My problem is I love engine bays and they must be aesthetically pleasing first - then run well. Okay, not completely true, but close to it. Turbo's just aren't pretty. They're sort of like the girl down the street who is not at all attractive, but, ummm, makes up for it in other ways.

I'm sure you'll all agree this is a 'nice looking' engine bay.

Biba

[buzzed]

haha, biba69 you're a crack up.

Some blokes are tit guys and some are arse guys...maybe we're opposite...but can appreciate the merits of both. Me I want both :-). Actually for the large part, I find turbos to look hot in an engine bay, and superchargers for the most part to be a bit chunky and unatractive. Take your photo above, the exhaust is hot! But the supercharger is barely seen and nothing to write home about.

Anyway, to rebut :-), here are 3 pics in my defense of turbo's being pretty..
One from Jim stecks talented workshop...one from somehwhere in webland, and the turbo on the manifold is from my rx7 s3 turbo conversion I used to own, 270rwkw's on 17psi....it was a quick beast seeing as it weighed around 1100kgs wet.

On your point of twinspark, so are correct for sure. The only reason I don't go twin spark (and I do have a a twin spark engine in the shed) is due to its looks. Its only my opinion, but twin spark heads are seriously funtional...but seariously ugly. I LOVE the look of the classic Alfa twin cam head, so I'll happily loose the hp this would have gained me through better head chamber shape.

I'm getting excited now, the engine is pulled down and it looks like I have a very good condition 1985 engine to jump from. Head is 112.2mm deep, and from what i can glean, this means its virginal.

I've dropped my comp ratio to 8:0:1 on the piston side of things....decided that I'm better off starting with a lsightly lower comp and working with boost, than I am starting too high and finding I may as well have just run a supercharger and spun it hard with a big intercooler. This way, its boost time :-)

Speaking of boost, sounds like the limits of the stock motor are around 350flywheel hp. This brings me to a dilema, as I now have to consider monoblock above this hp...and thats big bucks, like 5k AUS big bucks depending on supplier.....so I'm chewing the fat on that one for a bit.

[Greg Gordon]

Quote:

Originally Posted by buzzed haha, biba69 you're a crack up. Take your photo above, the exhaust is hot! But the supercharger is barely seen and nothing to write home about.

[junglejustice]

Nice catch Greg! Those are intake manifolds...

[Greg Gordon]

Quote:

Originally Posted by buzzed I've dropped my comp ratio to 8:0:1 on the piston side of things....decided that I'm better off starting with a lsightly lower comp and working with boost, than I am starting too high and finding I may as well have just run a supercharger and spun it hard with a big intercooler. This way, its boost time :-)

[Greg Gordon]

As I have said, I am very excited about this project, however I do have some concerns. I won't address all of them because it would tend to take the thread off topic. Here are the two I want to touch on.

1. That supercharger is HUGE for the intended application. Normally when the objective is low RPM boost it's better to spin a small supercharger fast rather than a big one slowly. For example Mercedes used a 1.0L/rev supercharger on a 3.2L V6. A 1.4L/rev on a 2.0 is quite large, it's ideal for a lot of boost at high RPM. Well...it's ideal assuming the supercharger is a good design, I have very little info on the the Toyota blowers. Hopefully it doesn't have straight two lobe rotors. If it does I would never use it due to the pulse and slip issues. The drawback of a large supercharger spinning slowly is that it will have a steep boost curve. In other words very little boost down low for a give amount of peak boost. I think it would be a lot better to use a M45 from a Mini, they are cheap, very efficient and will give you a lot of boost at the low end.

2. There is no need to bypass boosted air around the supercharger. With two stage compound systems the first stage usually (a turbo in your case) provides boosted air to the second stage (a Roots blower in this case). This decreases the pressure differential across the rotors minimizing slip thus moving the second stage blower's adiabatic efficiency island up and to the right, which is exactly what you want at the high end. It also allows you to keep manifold pressure higher than the exhaust pressure between the turbine and the exhaust valves. These are two huge advantages compound systems have over other designs and I wouldn't want to give that up. Alfa Romeo used this technique to dominate in racing with the Alfetta Grand Prix car (Alfa's compound system used a large slow spinning Roots blowing into a small fast spinning unit), and it was commonly used in WW2 aircraft for the reasons I just mentioned.


Greg Gordon,
hiperformancestore.com

[buzzed]

before I read further...let me just say I must have been on drugs when writing the comments about the 'exhaust'! Sorry guys, too many reds. Let me re-phrase! 'Nice intake manifold' :-)

how embaressment

[buzzed]

Greg,

hmm very thought provokeing.

I'll start with the comment about the supercharger being too large. I certainly take your point about the smaller the better for quick boost response. There are a few reasons I think this should still be a good size to start with.

1. The toyota engine it came off was a 2l engine spinning to 5500rpm. It ran about 7psi. I plan to run it to 4500rpm (a tuneable switch point) but at around 12psi. Also remember that I doubt I will be able to run an intercooler post supercharger (ok I can water inject post). So this leaves me limited somewhat by safe intake temps. All in all I think a mini supercharger could work well, and would provide a quicker boost vs low rpm response, but I would hazard that to create the boost I desire, I would end up with unsatisfactory intake temps. So the way i see it, with intercooler, sure, I'd go smaller, without, I still choose my toyota poison :-). The toyota design is 4 lobe by the way, actually a very nice s/charger for their time. It was awarded the Japan Society of Mechanical Engineers Award for Technical Excellence in 1987. Rotors are resin coated to reduce drag also.

Your comment on not bypassing the second stage has merit in air planes and tractors, but I feel in a road car not so much. In planes, it gets you around the fact the air is far less compacted at altitude. For tractor pulls, torque is the name of the game and so long as thye can hold the head on they're laughing...for a few minutes anyway. For road going cars, a single turbo can easily supply more boost than preignition will ever let us stuff in. I assume what you suggest is to use a smaller turbo and continue to spin the supercharger to make up the difference. In my case, when I bypass the supercharger>4500rpm, 100% of the boosted inlet charge has passed via an intercooler which is a good thing. Also, it has not had to deal with the very restrictive path of the supercharger. Nor will I have the losses inherent with still spinning a supercharger.

The way I have sized my turbo is for the highest efficiecny I can find on a turbo for the mass flow in question. My mass flow is calculated on 20-25psi, 4500-7500rpm. The very small rpm range means I can pick an otherwise excessivly laggy turbo to do the job. I have however given consideration to the rotating turbo shaft asm mass, as I am still interested in good transient response at the lower end of the desired rev range.

I should point out that I started off with the aim of this being a budget build......well forget it....it could be done, but I could not help myself and before I knew it my hp desire had crept up 150hp :-). This is really becuase there is little benefit to this setup if you plan to hang around the 250flywheel hp. With that kind of hp, these days, turbo spool up times are just so good, it would hardly be worth the effort. BUT...creep up the hp to around 300+ out of an Alfa motor and you really to start jit a point where lag is a pain again....and lets face it, almost all of us will never enjoy a decent set of ratios in an Alfa drive train that would keep us away from lag...especially on the road.

Update: The cams that were in the motor when I pulled it down are not only 10548 cams, they are actually a regrind with 11mm of lift.....perfect! I was toying with the idea of higher lift cams, and now I can just run these as a good starting point.

stay tuned...should have an order placed for pistons and rods this week.

[Greg Gordon]

I think the temperatures out of the supercharger would be about the same with either a M45 or the Toyota unit. However I can't find a compressor map for the Toyota SC14, so I don't really know. Even if the adiabatic numbers are 10% different, it's only going to be about a 25F difference at the boost levels we are talking about. I suspect the numbers will favor the M45. Toyota abandon that SC14 and started using M62s (which is a M45 with slightly longer rotors).

The reason aircraft engines (and a number of road cars) used compound systems with turbos blowing through the superchargers was because it results in more power than you get with the same amount of boost an a turbo alone. This has nothing to do with the need for gobbling up massive quantities of air at altitude, and it's very applicable to a road car. This system results in more power because the pressure differential between the intake and exhaust ports allows the use of valve overlap to evacuate all exhaust and completely fill the cylinder (and combustion chamber) with air. That filling of the combustions chamber has the effect of adding 300ccs of displacement in the case of an Alfa 2.5, so it's a big deal.

Don't take my word for it, here are a couple quotes from G.E. and I don't think anyone has built more compound systems than they have:

1. Regarding the use of valve overlap: Maintaining a high manifold pressure ahead of the intake valves allows the use of "valve overlap" in the engine, so that the intake valve opens just before the exhaust valve closes at the end of the exhaust stroke. This allows the compressed mixture from the intake manifold to scavenge the spent gases out of the clearance volume of the cylinder and also tends to improve the cooling of the exhaust valve.

2. Regarding use of a turbocharger alone with no supercharger: "With no supercharging between the carburetor and the intake valves, "valve overlapping" would permit exhaust gases to flow back into the intake manifold. This would cause backfiring and dilution of the next charge of mixture with burned exhaust gases."

It's more work not to take advantage of this pressure differential, so I just can't understand why anyone wouldn't do it this way. However it's your project, and I have a lot of respect for you for taking it on. It's exciting to see someone doing this.

Greg

[buzzed]

I'm not sure I follow..

If we ignore the form of pressure delivery to intake and just talk about the pressure at the intake valve...are you saying that that is pressure (lets call is P1) when presented with valve overlap, overcomes the exhaust pressure (call it P2) and consequently scavenges the chamber? I would agree that this is desirable. what I don't yet understand is why the supercharger improves this pressure differential? Regardless of whether I run series or parallel charging (I think you are suggesting series), I will still have the same peak boost from the system, which will be P1. So I assume you are saying that with series the P2 will be less for a given boost?

PS...I disagree on the series charging in planes thing, atm pressure is a main factor for series charging. I'm not saying that the reasons you suggest are not factors either, they are.

Thanks for taking the time to help me out, it's good to chew the fat.

heres a question for you, if your proposed method is so much better, why do no auto manufacturers do it this way? All the systems I have reviewed, lancia, mazda, VW etc....they all bypass the s/charger past about 3500rpm.

[5yearplan]

Correct me if I am wrong, but I believe the idea of the charged valve overlap is that without a charger the pressure is closer to equal depending on where you are on the turbo's airflow/boost map. If your turbo is at 60% efficiency, as indicated by it's map, at the point of valve overlap... The pressure(boost) created by the charging turbine is 60% higher than the back pressure caused by the exhaust turbine. That of course is a raw number. Once you take into effect the rest of the exhaust system and the back pressure it causes (if any) as well as the pressure(boost) lost from the piping, intercooling, intake manifold, and throttle body, as well as how well the head/intake ports allow air to flow into the combustion chamber... the number is greatly reduced, and in some cases ends up being less than the pressure at the exhaust valve [again depending at what efficiency the turbo is at, etc etc.] leading to the problem Greg mentioned. If you add the supercharger the added pressure(boost) without the added exhaust pressure will overcome and/or increase that pressure difference. Which makes a big difference when you take into consideration that as 4000+ RPM the exhaust valves are opening and closing more than 60 times a second! That extra pressure will insure that all of the already combusted air is out of the cylinder in that short window.
Why don't other manufactures use the same idea? If I were to guess, I would say it's because that kind of system would take much more tuning, and therefore time and money, research and development... Also in a higher tuned system there is less of a safety zone to account for fuel quality, temperature and elevation differences, etc. A factory boosted engine for the most part, is a generalization of a very complex system. They ship the cars all around the world, and wouldn't want to take the time to tune for each demographic separately, or be concerned that the engines would develop a lean condition as time goes on. For example; the older BMW's are designed with consideration that they would run lean after several years of use. that is why the AFM needs to be adjusted periodically. I found this out when mine started running poorly [coincidentally] at about the same time as several other's with cars that had comparable age and mileage. That is also why most drag racers don't use boosted systems or even fuel injection. The more variables you add, the less consistent the cars run.

[Greg Gordon]

Nice to have some sharp people on this forum.

With a turbo alone, exhaust pressure between the exhaust valve and the turbine (p2) is always equal to, or higher than inlet pressure (P1). With a belt driven supercharger P1 is always the higher of the two (provided it's even remotely well designed). That's the reason the supercharger provides a scavenging advantage. When you combine the two you still retain the supercharged P1 and P2 differential. For example with 5psi from a supercharger, and a great exhaust system at full boost manifold pressure will be 19.7psi (with standard atmospheric pressure). The pressure in the exhaust port will be 14.7 for a 5psi differential. Add 10 psi of turbo boost and the inlet pressure will be 29.7psi but P2 will rise to 24.7psi. We still have our differential. Remove the supercharger but keep an inlet pressure of 29.7psi by spinning the turbo faster and exhaust port pressure will rise to 29.7psi at least. It will probably rise more depending on exhaust manifold design and other factors.

This is not just my opinion, it's right from G.E. They built literally tens of thousands of these systems. They say the reason they used a compound system rather than a turbo alone was for this scavenging advantage, I am inclined to believe them. They certainly could have used a big enough turbo to get the boost levels they wanted without a supercharger. In fact quite a few aircraft did this, and quite a few did this with a supercharger alone, but that's another story....

If anyone has any evidence that compound setups were used on WW2 or later aircraft because turbos couldn't deliver enough air on their own, I would love to see it.

Why don't other manufacturers do it this way. THEY DO! You mentioned Lancia, well the Delta S4 does it exactly this way. I fact I think the Delta S4 setup should be the basic guide of how to do it on an Alfa because the goals and designs of each are similar.

The VW system is totally different and is not comparable to the Delta S4's system. The Delta S4 was designed for power and winning races. The VW twincharger is designed for mass production and fuel economy. The VW's system has the supercharger BEFORE the turbocharger. This is the opposite of what G.E. and Lancia did, and the opposite of what you want for a performance application. I think it's opposite of they way you plan to set it up. Explaining the reason VW did this would take me pages and pages of writing. It was not for horsepower.

I have not seen Mazda or Audi's compound systems so I can't comment on those.

Greg