Jump to content
The mkiv Supra Owners Club

Boost Limits


Phil Wall
 Share

Recommended Posts

I believe the general feeling is that for reliability's sake, 18psi is the highest you want to go. So if you get a Pete Betts FCD you can make sure you don't pass that threshold.

 

To go higher requires ball baring turbo's which have a lower friction rating.

Link to comment
Share on other sites

depends how safe you want to be, but aparently the efficiency of the turbos also starts dropping off after 18psi, so me running 19psi is probably only as good as running 18psi.

 

You can run a bit higher if you have SUL, millers, FMIC, water injection, but really you need to start running hybrids at that point.

 

JB

Link to comment
Share on other sites

 

 

It's not so much how high the boost. It's how high the EGT's and how much detonation you want your engine to endure.

 

As for turbo efficiency, I don't have the pressure maps, but my guess is that Toyota would have chosen the turbos such that, at stock boost, the flow parameters would fall in their maximum efficiency island; which, on an axial compressor, occurs at quite a narrow range of flow at a maximum efficiency of around 73%.

 

So with a stock boost of 10 psi I would reckon on turbo efficiency falling from about 12 or 13 psi. It would be interesting to do some tests of turbo outlet temp -versus- turbo boost, as you can get a good idea of when the turbo is going well off its map. There will be a big rise in temperature for every small increase in boost.

 

Plus, I still haven't solved the mystery of how some of you guys can run boost pressures in the region of 18psi and yet not be running mad lean. I'll be doing some tests on Justin's car soon, so I'll hopefully get the answer then. I suspect it could well be the case, as I have long suspected,  that turbo inefficiency (and consequent decrease in the density of the intake charge) is cancelling out any increase in boost. If that *is* the case then no extra power can possibly be made above 1 bar - the cutoff point of the pressure sensor (or whatever cut-off is selected on your FCD).  

 

Yours,

J

Link to comment
Share on other sites

Weel basically, it is the turbo pressure, and the higher it is the more power the turbo produce. However, the higher it is, the more lag u get. And if too high, your turbos could be damaged.

 

U probably need to ask Ash for a better explaination as mine ain't very good!

Link to comment
Share on other sites

 

 

Syed is right, it is the turbo pressure expressed either in Bar (short for Barometric) or PSI (short for Pounds Per Square Inch.

 

1 Bar equals 14.7 PSI.

 

Stock turbo pressure is in the region of 10 PSI or 0.68 Bar.

 

Yours,

J

Link to comment
Share on other sites

isnt also true that a car with stock turbos running 1.2bar will be producing less power than a car with hybrids running 1.2bar due to the increased air flow from the larger compressors and stuff (sorry if I got all technical on that last word!!)

 

Which is why somebody like Leon can run 1.2bar and beat the living daylights out of me on the road or track. It really is disconcerting to be flat out runing 1.2bar and to have Leon sitting right behind youwaiting for you to get out the way...

 

JB

Link to comment
Share on other sites

 

 

The probable reason for that is primarily two-fold:

 

1) The different air density of the air entering the cylinders of each car. Bigger turbos will be operating more efficiently at 1.2 bar than the stock units.  

 

2) Bigger turbos also means less exhaust back-pressure for the same level of boost.

 

During the overlap period of the camshafts, the pressure trying to get into the cylinder is being resisted by the exhaust back-pressure. (That's one of the main reasons high-overlap cams tend not to work so well on turbo'd motors designed primarily for street use.)

 

I haven't done any testing on this so I cannot give you actual figures: with a turbo, you will always have exhaust back-pressure in the primary manifold that is above the boost pressure. The more efficient the turbos are operating then the more closely matched exhaust back-pressure is to boost pressure.  

 

Under stock conditions, you may have a situation where exhaust back-pressure is only 2 to 3 psi above stock boost of 10 psi. But as you operate the stock turbos less and less efficiently, then the higher the back-pressure becomes relative to boost. At 18 psi you might have a situation where exhaust back-pressure has risen to around 25 psi, say.  

 

The incoming 18 psi flow "sees" this 25 psi back-pressure during the cam overlap period. During which there could well be an outflow of gas from the cylinder into the intake manifold; until the exhaust valve closed whereupon flow would revert to the normal direction.

 

In Leon's case, as the turbos are operating in the area of their maximum efficiency island at full boost (or at least that's where they *should* be operating) exhaust back-pressure will be a lot less than with your car running the same boost pressure. So there won't be such a tendency for the reversion of gas into the intake manifold during the cam overlap period.

 

It's rather like if you had a length of pipe and sent air in one end at a pressure of 10 psi and left the other end open, i.e. at normal atmospheric pressure. Air would obviously flow from the high pressure end to the low pressure end. However, if you started to gradually increase the pressure at the open end, flow would gradually slow down. With 10 psi at both ends, flow would cease entirely. If you continued to increase the pressure of the originally open end, flow would begin to revert, i.e. begin to flow in the opposite direction.

 

Also, the incoming air density in Leon's engine will be higher.

 

All of which means, for the same level of boost, Leon will have a significantly greater amount of fuel/air mixture entering the cylinders. Hence his engine will make more power.

 

The main aspect of the turbos that determines back-pressure, at a given level of flow, is the A/R ratio (Area to Radius ratio) of the turbine housing.  

 

Yours,

J

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. You might also be interested in our Guidelines, Privacy Policy and Terms of Use.