What affects boost?

Hi Guys!

Had a brilliant time on Saturday, was quite chuffed with my best time of 13.4 something before lunch.

However I noticed something strange. I have a Blitz Dual-SBC that's "supposed" to be set to 0.9 bar and I have it set to beep madly at me when I hit 0.95 so I get advance warning of hitting Fuel Cut.

Not *once* did I get any warning during my 15 odd drag runs, and boost never went over 0.91 according to peak hold.

But, on the way home on the motorway, I'd consistently get 0.95 or more on the motorway in 3rd/4th gear.

The only difference being on the way home I had a passenger. Does the extra load mean more boost to compensate?

Check the hose connections on your BC.

If one has popped then it will give a high reading.

Andy

Check the hose connections on your BC.

If one has popped then it will give a high reading.

 

Andy

Cheers - they seem fine, but the strange thing is, this is quite reproducable.

First noticed it when I went camping after picking the car up - same setup, another adult in the car, and camping gear in the back. Hit 1.01 bar and got fuel cut. Seems like the more loaded I am, the more boost I get.

Is this normal guys?

That`s a strange one on me mate.....gotta be a logical answer somewhere.

Andy

P.S. camping gear in the back......what was it, a wendy house

boost is normaly in proportion to throttal prosision so the more right foot the more boost

boost is normaly in proportion to throttal prosision so the more right foot the more boost

In all cases foot has been to the floor, well certainly was at the Pod

P.S. camping gear in the back......what was it, a wendy house

Haha well yes it was a tight fit but managed it! Clothes, airbed, groundsheet, 2 folding chairs and a 3 man tent and all my laptop stuff!

Who says it's not a practical car

Boost is also affected by the engine load - thats why in higher gears (when the engine is under more load due to drive ratio) boost increases slightly.

This is why, when you had the car fully laden, the boost was higher due to the engine having to do more 'work'.

On a drag run the car isn't really getting onto full boost until you're in 4th gear (and heading towards the finish line).

Also, the boost tends to be a little lower on the drag strip as there will be some element of heat soak from the burnout and sitting in a queue of traffic for a few minutes.

Boost is also affected by the engine load - thats why in higher gears (when the engine is under more load due to drive ratio) boost increases slightly.

 

This is why, when you had the car fully laden, the boost was higher due to the engine having to do more 'work'.

That does make a lot of sense, and it's the confirmation I was hoping for, thanks

On a drag run the car isn't really getting onto full boost until you're in 4th gear (and heading towards the finish line).

 

Also, the boost tends to be a little lower on the drag strip as there will be some element of heat soak from the burnout and sitting in a queue of traffic for a few minutes.

If I go full BPU and my boost increases to 1.2, will it actually get there on the drag strip and improve my time? Logic says Yes but what you said up there says No in my mind

Yup, not making enough load. Most of my runs were 1.15 bar max even though my boost is set higher with quite high gain/duty

Yup, not making enough load. Most of my runs were 1.15 bar max even though my boost is set higher with quite high gain/duty

*boggle*

Maybe that explains why my post-lunch times were WORSE than my pre-lunch times, after I'd taken out the spare wheel, and most of the carpet from the car to save weight!

OK, me and my friends were just getting into the spirit of things and having a laugh, but I thought taking the ashtray out was going a bit far. Didn't rattle as much though

Could the cruising at motorway speeds as opposed to gunning it from standstill improve cooling charge density through continual airflow thru IC.

Just a thought as i was under impression if you sit in traffic say (or sit in fire-up lane running not moving or burning out on wet patch) your charge temps will rise so IC less efficient until you get constant air flowing through it for a reasonable period of time, thought that was why some 1/4mile runners used Co2 kits on their IC's before running, i'm probly wrong maybe it's just to reduce charge temps/increase density and has no effect on boost pressure.

So don't jump on me dudes, just an absent minded thought based on no facts whatsoever!

Would like to hear the answer to that, makes sense Stevie Boy.

Good thing traffic wasn't exactly a problem on Saturday - max queue was 3 cars when I was doing my runs!

Maybe that explains why my post-lunch times were WORSE than my pre-lunch times

Mine were too mate. 20 runs and my 4th was the best, with numbers 2 and 3 close behind. I put it down to general heat soak and the day warmed up too, so the effect of the boosted air is reduced.

Mine were too mate. 20 runs and my 4th was the best, with numbers 2 and 3 close behind. I put it down to general heat soak and the day warmed up too, so the effect of the boosted air is reduced.

Interesting stuff, every time i've run my car the1/4mile it's been events with only 3 runs except when i went Rotorstock at Shakespeare Raceway which was unlimited and did bou 5.

Must of my runs at the Pod in past (didn't run on Sat) usually the last of 3runs was the best but this was probly down to 1st and 2nd runs just getting back into grips with all the things i'd have to remember to do like turning TC off, A/C blah, blah, blah and i guess at those events there was such a long wait between runs and i'd end up pushing car most of way down fire-up lane to stop it getting too hot in the large que so may have given it time to cool down between runs although weather was always scorching too.

Could the cruising at motorway speeds as opposed to gunning it from standstill improve cooling charge density through continual airflow thru IC.

 

Just a thought as i was under impression if you sit in traffic say (or sit in fire-up lane running not moving or burning out on wet patch) your charge temps will rise so IC less efficient until you get constant air flowing through it for a reasonable period of time, thought that was why some 1/4mile runners used Co2 kits on their IC's before running, i'm probly wrong maybe it's just to reduce charge temps/increase density and has no effect on boost pressure.

I think you've hit the proverbial nail on the head there. Drag racing is mostly low speeds until 1/3 down the track, well speeds not high enough to force a decent airflow through an intercooler and start the heat exchange process really working.

This means that the initial heat soak from previous runs, heat put into the inlet system from a burn out & the initial acceleration off the line all combine to make the charge air entering the cylinders not very dense at all!

This hot air is very compressible, but if its not cooled effectively then the turbo could be working at maximum, but not achieving the boost setting as all its doing is drawing in hot air, and heating it up further by trying to compress it.

When the IC starts working, about 1/3 down the track, the charge temps will probably start to come down, and the boost pressure go up as the temperatures drop and density increases.

An ideal solution would be water or CO2 spray on the IC, ducted air filter and electric fans to draw air through the FMIC (& rad) for a few minutes before the run.

I think you've hit the proverbial nail on the head there. Drag racing is mostly low speeds until 1/3 down the track, well speeds not high enough to force a decent airflow through an intercooler and start the heat exchange process really working.

This means that the initial heat soak from previous runs, heat put into the inlet system from a burn out & the initial acceleration off the line all combine to make the charge air entering the cylinders not very dense at all!

 

This hot air is very compressible, but if its not cooled effectively then the turbo could be working at maximum, but not achieving the boost setting as all its doing is drawing in hot air, and heating it up further by trying to compress it.

 

When the IC starts working, about 1/3 down the track, the charge temps will probably start to come down, and the boost pressure go up as the temperatures drop and density increases.

 

An ideal solution would be water or CO2 spray on the IC, ducted air filter and electric fans to draw air through the FMIC (& rad) for a few minutes before the run.

Cool........ I love hitting proverbial nails on their heads!

That is what i was getting at though, i haven't got an intake charge temp guage but i'm sure if i did they'd be a significant drop during lowish rpm motorway cruising speeds then at low speed traffic light runs.

Similarly, i'm sure on turbo'd motors i've had in the past (as Supes got so much grunt it's a little more difficult to notice) i always had a noticeable performance improvement and higher boost creep then when high speed driving on cool dusky nights/mornings than in the middle of a hot sunny day.

Maybe i'll invest in an ict guage, could prove helpful.