Boost Quicker With Greater Load

[salad]

howdy guys

im wonderin why a car will boost sooner with a greater load? ie in a higher gear it will hit boost sooner than in a lower gear, and very little boost at all in neutral no matter the rpm.

can anyone give me an explanation of why this is?

or link me to somewhere that can tell me as i am pretty interested in this

thanks guys

[Sass]

engine load makes exhaust gas, more gas, the faster the exhaust wheel spins, which makes more boost. also explains why you get jack all boost in 1st gear as there is engine rpm but not much load, load makes exhaust gas, which makes it spin more

[Cubes]

It will only boost quicker with regards to rpm, not so much time.

[Links]

The higher the load, the more fuel being injected to match the air flow, the more airflow, the more exhaust gas.. the faster the turbo spools up..

[salad]

how does a greater load create greater air flow in though? if you have your foot to the floor, the cylinder will be completely filled with air and fuel, so i dont see how there can be such a difference in boost response.

not arguing cos there obviously is a difference, just after a technical explanation is all.

thanks

Edited February 17, 2006 by salad

[dangerous_daveo]

This is why I was told to stay away from light weight flywheels. And in fact some of the guys I know with big power big turbo cars have gone to heavier flywheels... Odd but true.

[Trust33]

1st gear the car will be pushing the car from a dead stop so u cant expect it to just ramp up onto boost like it would when it is rolling.It is called momentum and if there is none then the car has to rev to create it.

it (250hp engine) cant rev if the thing holding it back weighs more than a tonne.the car has arround a 6:1 power to weight ratio.

Do a slow run in 1st and i bet you will be able to hit boost allot sooner than if you were at a complete hault.Why?because it doesnt have to work to pull the car allong it is allready reving,making the torque,and passing the exhaust gasses out through the ex turbine.

turbo 6cyl cars rely on the turbo to make all their torque.they have a low compression ratio so it is possible.If it were high then there would be to much pressure in the piston bore for the engine to handle,causing many comnplications leading to death for the engine.

So if boost isnt there -------------------^-^-----------flatliners for the skyline.

T-33

[Cubes]

dangerous_daveo,

with regards to keeping away from light flywheels.

I dropped a light flywheel on my Rb20DET, to my suprise it felt so so much better, quicker off boost acceleration with no difference in spool. Taking off from a standing start was so much easier as the car simply wanted to accelerate where as with the stock flywheel my guess was it also had to over come the intertia of the heavy flywheel, resulting in a dougy feeling little 2ltr.

Gear changes were quicker, free rev's were noticably quicker allowing easier rev matching when approaching corners hard under brakes.

Fuel economy didn't change, neither did drivability.

I will definitely go a light flywheel again. 99% of those that have had a light flywheel before/after will always go the light flywheel.

[Trust33]

dangerous_daveo,

with regards to keeping away from light flywheels.

I dropped a light flywheel on my Rb20DET, to my suprise it felt so so much better, quicker off boost acceleration with no difference in spool. Taking off from a standing start was so much easier as the car simply wanted to accelerate where as with the stock flywheel my guess was it also had to over come the intertia of the heavy flywheel, resulting in a dougy feeling little 2ltr.

Gear changes were quicker, free rev's were noticably quicker allowing easier rev matching when approaching corners hard under brakes.

Fuel economy didn't change, neither did drivability.

I will definitely go a light flywheel again. 99% of those that have had a light flywheel before/after will always go the light flywheel.

<{POST_SNAPBACK}>

What about top end speed?

I only know that when i upgraded to a lighter flywheel on gran turismo the car wouldnt hold top speed for long untill it started to drop off.Where as the heavier stock item would allow it to stay there for longer.

MEH!!!

[Cubes]

lmao...

Drop a diesel in it and nice high diff ratio if you want to hold a top speed.

Acceleration is where its at.

On a serious note slight inclines were not noticed.

[Trust33]

LOL

KEWL!!

Truckies powah!

[elbee111]

It will only boost quicker with regards to rpm, not so much time.

<{POST_SNAPBACK}>

I am interested in this statement and also agree with Trust33 re momentum.

I am not sure that I agree with the premise that higher gear necessarily means higher load, although I agree that it is hard to create boost in neutral.

Bogging down in First Gear is death to a good launch with load at Maximum.

Will watch this space.

El Bee

[Busky2k]

Its because at higher gears the rev climbs up not as fast as a lower gear which gives the turbo more time to spool up and produce boost. Ie it might take a few seconds for it to eventually produce 5psi at 2000rpm. But in lower gear you're not sitting for a few seconds at 2000rpm. Ie;

Nail it in 4th at 2000 rpm and it'll stay at that revs for a while lettin the turbo spool up. Nail it at 2000rpm in 1st gear and you wont see any boost until later because the revs are climbing and the turbo is spooling up trying to 'chase' the revs so to speak.

Edited February 17, 2006 by Busky2k

[salad]

ah, i think busky2k is dead on the ball.

as the revs rise the pressure in the plenum drops because more air is bein sucked in.

if revs rise too fast then, the positive pressure is bein cancelled by the negative pressure in the plenum

this sounds right to me anyway.

anyone else got any theories on this one?

and trust33, i dont think its about the momentum thing.... cos you will build boost faster in 5th than you will in 2nd, and in these gears your definitely not starting from a stop, so you have the momentum there

[Cubes]

Yep Busk2k explained it better than I.

[BHDave]

You've also got to consider that it's a bit of a circular system, that is air in air out.

If you WOT it in first it takes all of 3 seconds to go from 1000 rpm to 7000rpm. You aren't going to spin up a reasonable size turbo in that time to the point where it's making a lot of boost.

Then take the 5th gear on the freeway response trial that we all like doing. You plant it at say 2500rpm and within a second or so the car is making a couple of psi, that extra gas flow into the engine is going through the turbine side and speeding it up so at 2600rpm, 100rpm higher which in an NA car means squat in terms of extra airflow, you have started to spin the turbo at the point where it is actaully starting to push a bit more air into the system and you are making 4psi. Then at 2700 you are making 6psi and all of a sudden the turbo is helping itself make a decent amount of boost as you have more air going in, bigger bang, more exhaust, spins the turbo faster and boost starts to rise rapidly.

It's easy to see this on a typical small engine big turbo setup. It will take forever to make 5 psi but once it passes that point (or somewhere around there) the turbo is feeding itself so to speak and the boost rises exponencially (sp?).

[Trust33]

101

[Links]

yeah I think Busky2k & BHDave have a good point..

http://www.gnttype.org/techarea/turbo/turboflow.html

some interesting reading/maths... lol

it's all about how much air flow there is.. don't forget the more boost, the more positive air being pushed into the engine, the more air, more boost, so quicker a turbo spools up.. I think the speed is probably exponantial as well..

in fact this whole page has some great stuff

http://www.gnttype.org/techarea/turbo/turbopage.html

Edited February 17, 2006 by Links

[salad]

good links

very informative, a lot more technical than how stuff works or the like, which is good.

thanks

[dangerous_daveo]

dangerous_daveo,

with regards to keeping away from light flywheels.

I dropped a light flywheel on my Rb20DET, to my suprise it felt so so much better, quicker off boost acceleration with no difference in spool. Taking off from a standing start was so much easier as the car simply wanted to accelerate where as with the stock flywheel my guess was it also had to over come the intertia of the heavy flywheel, resulting in a dougy feeling little 2ltr.

Gear changes were quicker, free rev's were noticably quicker allowing easier rev matching when approaching corners hard under brakes.

Fuel economy didn't change, neither did drivability.

I will definitely go a light flywheel again. 99% of those that have had a light flywheel before/after will always go the light flywheel.

<{POST_SNAPBACK}>

I actually found my better once I went back to a stock one. (my light weight was about 3.5kgs). Thing is I also got the rb25box and 1 peice tailshaft in at the same time. So that might have had some affect.