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View Full Version : The real reason why Skylines use less boost



Benny
17th July 2000, 08:54
There seems to be a lot of people unsure of how the Skylines produce more power from less boost so to speak. The answers are involved but I shall simplify things somewhat.

The 3 main reasons are;

1. 2 extra cylinders
2. Shorter conrods = longer stroke
3. 8,000 rpm limit

Let me explain. If a 4 cylinder motor is producing 100bhp then each piston is responsible for 25bhp. To achieve 100bhp from a 6 cylinder motor each cylinder need only produce 16.7bhp or 33.2% less than the 4 cylinder motor. From this you can deduce the amount of effort required is 33.2% less for the 6 cylinder car.

Therefore, less pressure is needed to produce 16.7bhp which means less boost pressure. The pressure (pressure x area) x the stroke = torque. If you increase the stroke you get more torque. Power = torque x rpm.

Pressure is over an area and so, if you increase the bore, you can also gain power. I have illustrated the point with the stroke and so if we had a 2 ltr 6 then there are obviously no extra gains from extra capacity.

Lastly, since power = torque x rpm then due to the high revving nature of the Skyline's motor you can see how with an 8,000rpm limit the Skyline has another small advantage.

And another thing, the Skyline has twin turbos. Now, I haven't thought about this too much but 2 turbos doing the job of one to my mind means that there is less effort required from each one although I haven't come to the conclusion how that might reduce the Manifold Absolute Pressure (Boost + Atmospheric).

Questions on a postcard to the usual address.

Laters

Benny

[This message has been edited by Benny (edited 17 July 2000).]

Harj
17th July 2000, 09:09
Good one Benny! I learn something every day http://bbs.22b.com/ubb/wink.gif

Whats the reason behind people having 1 large turbo on the skylines? Would it not be easier to have 2 large ones?

Harj...

[This message has been edited by Harj (edited 17 July 2000).]

Benny
17th July 2000, 11:54
I can guess Harj but do not know the definitive answer. I would reckon on overcoming inertial loads is easier at high power with a single rather than twin.

Mine has monstrous lag that you learn to drive around but even at high rpm, a then flooring of the throttle results in a split second delay. This might be due to the front mounted intercooler though. Every R32 I have driven does this and so is not particular to mine.

If anybody has a definitive answer rather than supposition, I would be as interested as Harj would on the reason for a single turbo. They do it on the Supras as well but then their turbos are sequential I understand.

Laters

Ben

Branners
17th July 2000, 19:48
Yeah, Supras and I think the RX7 have sequential turbos. It uses various bits of equipment to limit flow to just the first turbo until you get to 3500rpm then it pre-spools the 2nd turbo (to avoid slamming it with air), and at 4000rpm the second one comes on-line alongside the first to give twin turbo power.

We had this debate a while back on the Supra list and to be honest I dont think we ever decided whether 2 turbos are better than one (when not running sequential). They say 2 smaller turbos will spool up quicker than a single, but when youthink about it its still going to need the same amount of air flow to push 1 big turbo as it does to move 2 small ones (if producing the same power).

I still believe 2 smaller turbos are better than 1 big one and thats the route I would take if I had the choice.

Perhaps somebody can give us the definative answer...

JB

iwatkins
17th July 2000, 20:08
Seeing as those Supra boys are jumping in on this one (Hi Branners http://bbs.22b.com/ubb/smile.gif) thought I might put in a little input...

I know two owners of big power Supras in the US. They have both gone for big front mounts and use the cars mainly for drag racing but are still daily drivers.

I dropped them an email about this and they both went from twin to single for the simple reasons of cost and ease of fittement. In one case the guy is running a T67 turbo and the cost wasn't cheap. $$$$$$$$$$$$

There is also the issue of twin vs. single in pipe dimensions, if one turbo can supply the same amount of air as two then it makes sense to use one. With two turbos you have (almost) double the amount of complexity of pipework plus you also have to think about pipewall surface area (drag inducing) plus every bend in that pipe work will add to the pressure drop and double in the case of two lots of pipework. I have a fellow GT4 owner who is sh*t-hot on all this pipework biz if anybody is super interested. (J, if you are there, jump in any time)

As they say, Keep It Simple...

Again, not a definative answer but maybe more food for thought.

Cheers

Ian

[This message has been edited by iwatkins (edited 17 July 2000).]

Mike Rainbird
18th July 2000, 09:23
Benny,
They say a little knowledge is a dangerous thing! http://bbs.22b.com/ubb/wink.gif I only wish it were as simple as your statement.

For example, how do you explain 185bhp from a four cylinder 1.3 engine (normally aspirated engine) care of Suzuki http://bbs.22b.com/ubb/wink.gif etc?

The reason why they run so little boost is that they are unreliable with lots, because the engines weren't designed for it. They get their extra performance from the increase in cubic-capacity, more than through the engine configuration. And although you are on the right lines, it is due to the increased valve area that more cylinders provide (as in more air in = more power) than for the reasons you state....

However, with regard to your turbo question. What you have to do is look at an engine as nothing more than a big air handling unit. The more air you get in (either by forced induction or larger capacity - think of it as bigger lungs http://bbs.22b.com/ubb/wink.gif), as long as this is matched with a corresponding increase in fuel, then you get more power.

With a turbo forced induction engine it still remains the same, so the bigger the turbo, the more air it can push into the engine. However, as you know a turbo is exhaust gas driven so the bigger it is, the longer / harder it is for the engine to spin it up to it's optimum speed. This is felt as a delay and is known as "lag". Obviously the manufacturers carefully match the turbo characteristics to the engine size to get the best compromise of performance / pickup etc. If you then change the turbo you change the character of the engine. A smaller one will pick up more quickly, but will not be able to provide the engine with enough air at the top end (the engine will effectively "consume" all the air the turbo can provide). When this happens (you will notice as the boost tails of considerably), you know you have to get a bigger turbo for more power. You then start to notice the trade off in response, but the power will stay there longer at the top end. It is all to do with compromising on the trade-offs and deciding what you want from an engine. A small turbo application would be no good on a circuit / road if you spent the majority of your time at high revs, as it would not be able to provide the engine with enough air. Conversely, a big turbo would be a waste of time on a twisty circuit / back road, as it would never come on boost before you would be having to brake for the next corner etc...

Anyway, the MAIN reason for fitting one big one and not two big ones is that there are not enough exhaust gasses coming out of the engine to spin up two big turbos at slow engine speeds. A small engine has a hard enough time spinning up one big one (humungous lag), just imagine how horrible it would be to drive if the exhaust gasses were split in two and had to then spin up two large turbos. The lag would be phenominal (and the car undriveable).

This is why they have sequential ones. A little one for the bottom end (which spins up really quickly) and then a bigger one to take over at the top when the exhaust gasses have got up to speed and are man enough to spin the larger turbo. This way avoids the lag associated with one big turbo or the tail off in power of a small one.

The best application for a twin turbo configuration would be on a V engine - like a nice V8. A small block alloy engine (2 valves per cylinder) of 5.0 litres with two TO3s on it and running about 1 bar of boost would make 600bhp all day long for over 100,000 miles. This way it would be working like two straight four 2.5 litre engines, and there would be no plumbing problems. No lag, instant response and torque to pull tree stumps out with.

As the Yanks say, there is NO substitute for cubic inches.....
Best regards
Mike R

Benny
19th July 2000, 07:18
Mike

Not surprisingly you are talking total nonsense to have disagreed with my efficiency statements. Increased capacity is only responsible for a small percentage in the extra power. The key is the amount of work each cylinder has to do and therefore the amount of "boost" each one needs to run that work.

Sorry to disappoint you but you are now disagreeing with people who have written several books on the subject and for you disagree with what I have written just goes to illustrate that you are either more knowledgeable than everybody else in the world on the subject of turbocharging or, as I suspect, a little knowledge is a dangerous thing in your case.

To have copied verbatim what has been written on this subject would have been pointless because you can go and buy the necessary text books and so it easy to put it into laymans terms.

Thank you for your OPINIONS on the single turbo though.

Laters

Benny

[This message has been edited by Benny (edited 19 July 2000).]

dumped
19th July 2000, 07:35
Another main reason for a single intead of twin is that efficiency losses are greater the smaller the turbo so the end result of a single turbo is a much more efficient machine.
One good thing about the Skyline engine is the fact it has a throttle body per cylinder which will help it produce power as it is easier to get more mass into the engine. I think I saw this under the bonnet of my friends.
Cheers
Martin

Mike Rainbird
19th July 2000, 07:39
Benny,
If you read my post, you will note that I said that it wasn't quite as simple as that. You forgot to include so many other variables, that homing in on one particular thing may be misleading. I am not disputing the efficiency theory, it is just that the increase in performance has also alot to do with the increase in valve area that more cylinders provide, and you neglected to mention this aspect....

Also the cylinders can be smaller meaning less reciprocal weight, meaning higher rev limits, it goes on and on and is not just down to "one" thing, there are knock on effects that should be considered / mentioned...

Also, changing bore and stroke might increase torque, but it can also increase / decrease the rev ceiling... In fact you can tailer the bore and stroke of an engine to produce the peak torque / power where you want it to a certain degree...

Didn't mean to offend, but that is the trouble from only quoting certain bits out of books....
Best regards
Mike

Benny
19th July 2000, 21:21
No offence taken Mike. Just trying to help others understand why American V8s run low revs and 2 valves etc...... 'coz they don't need to do anything else!!

Laters

Ben

Branners
19th July 2000, 22:00
but wasnt the original question 'is a twin setup as good as a single'? We are talking about 1 big single against medium sized twins. Would the bigger impeller wheels of a single take more effort to move than 2 smaller size wheels but with the same overall flow capacity?

Just for info, the Supra has 2 turbos the same size, it just pushes all airflow through turbo1 until a set point when it opens up airflow to both. It also only has 1 wastegate which can be a limitation when running big boost and no cats...

Anybody else going to Donnington track day this Friday evening?

MorayMackenzie
20th July 2000, 16:04
Benny,

Sorry if anyone's asked this before... but...

Surely the engine's stroke is dictated by the crank arm length, not the length of the conrods. http://bbs.22b.com/ubb/confused.gif

The swept volume depends on the stroke, which depends on the crank arm length.

Moray

Mike Rainbird
20th July 2000, 16:15
Moray,
I totally missed that... http://bbs.22b.com/ubb/wink.gif

Benny,
Also with the V8s they tend to be over 5 litres which helps. I.E. No substitute for cubic capacity (said in southern drawl)... http://bbs.22b.com/ubb/biggrin.gif
LOL
Mike

MorayMackenzie
20th July 2000, 16:25
Changing conrod lengths would affect compression ratio, not sweep. http://bbs.22b.com/ubb/smile.gif

Firefox
28th July 2000, 21:11
Wow.. I should check this forum more often.. http://bbs.22b.com/ubb/smile.gif

yep..spot on Moray... stroke = crank offset to the pin

As for turbos... effeciency will play an important part...

you will get higher losses using two turbos...

The issue of exhaust gasses going to two turbos also comes into it.

Some of the drag guys dont run exhausts... so u can rule that out..

u cant beat capacity.. one thing me and Mike agree on.. lol

Anyway..its 10pm Friday night.. time for bed..

I will continue this tomorrow.. if i remember..

cya..

J.

Benny
29th July 2000, 18:25
Gosh we are all very clever. Try changing one without the other.

Who would change conrod length to affect compression I wonder?

dumped
30th July 2000, 23:33
What do you mean by that about the conrods benny?
Cheers
Martin

MorayMackenzie
31st July 2000, 00:56
Why thanks Benny, that's so nice of you to say that... I'm touched!

psquire
29th December 2000, 12:10
and here i was thinking that because of the gentlmans agreement between japanese car manufacturers that no car shall put out over 208kw (ok i know that some do) but if you had a 2.6l 6 cyl running at the same boost as a 2l 4 cylinder there is no way that they would make the same power .and i dont need to get into physics to prove this . it just logical if you dont know that more capacity makes more power you need to find a new hobby other than cars like knitting or something
as for turbo's one large instead of two large or one small one large etc it because small turbo's lose boost at hi engine speeds because they cant spin fast enough to push enough air in and its basically only drag racers that will go that route or professional (see japanese) racing teams because the larger the turbo the more rpm needed to produce boost which not much power down low lots of power up high as for not going two large turbos have you seen the size of a t88 ummm where are you going to put it harj the boot?????

Harj
30th December 2000, 02:22
Nice posting Squire Junior http://bbs.22b.com/ubb/wink.gif Seems your dad has taught you something or was that MRT? http://bbs.22b.com/ubb/wink.gif

Harj.....

psquire
30th December 2000, 03:18
nah im just talking from my fast years of experience

or maybe they just grow em smarter in this part of the world????

my dad taught me some thing????
he thinks you can get 600hp from a 1.3 lt charade http://bbs.22b.com/ubb/smile.gif

[This message has been edited by psquire (edited 30 December 2000).]