Boost Drop at High RPM

[Autopart101]

I'm trying to plan my next move with my build. Roush 2650, E-85, Fuel Sys, 2.7 Pulley, catless LTH. Stock pulley 3.0. I think the stock pulley with show 11ish PSI boost. Do you think the boost drop on the graft is belt slip or inlet restriction. Roush has a stock 87mm TB. With the 2.7 pulley i'm seeing 13.2-13.3 psi boost up top. Mid range is 14.5 psi. thinking about installing a 2.5 pulley and see if there are any gains.

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[HKusp]

I would certainly think you are running out of air up top with that throttle body.

 

[Oakley]

engines are air pumps. gen 3 coyotes are incredibly good at moving air the higher they go.
boost is just pressurized air. think about these engines and how much air they flow at high rpms.

its not a turbo so whatever boost it makes is all you get. sure it'll scale with rpm but if the engine consumes air at a rate that outstrips that scale you might see a decrease in boost pressure.

I could be totally off but I wonder if the engine moving a poopton of air naturally at the top end causes that little dip.

IMO this means there's more left in the engine which means things are safe. once you start maxing out every last little hp... you're on the ragged edge asking for trouble.

 

[blackk3389]

Try putting your cats back on and see what you get? i made more boost and HP with them on? more boost than anything, kinda weird but try it, it wont cost anything but your time. boosted engines do like a little back pressure.

 

[HKusp]

Boost is a measure of restriction. I would expect more boost, post supercharger, with cats in place. They choke the exiting air. An 87mm throttle body, prior to the compression of the air by the rotors in the supercharger, will be restricting the amount of air available to compress, hence a decrease in boost.

 

[Autopart101]

Boost is a measure of restriction. I would expect more boost, post supercharger, with cats in place. They choke the exiting air. An 87mm throttle body, prior to the compression of the air by the rotors in the supercharger, will be restricting the amount of air available to compress, hence a decrease in boost.

I had the same reaction. TB is to small. Until I saw this boost drop on a 132mm TB Whipple car.

This graph is a 2020 GT 3.0 Whipple with a 132mm TB. Same Dyno as my car. Also seeing a boost drop.

 

[HKusp]

I had the same reaction. TB is to small. Until I saw this boost drop on a 132mm TB Whipple car.

This graph is a 2020 GT 3.0 Whipple with a 132mm TB. Same Dyno as my car. Also seeing a boost drop.

I am not saying you won't see boost drop as you reach the limit of volumetric efficiency. I am saying he's seeing boost drop because he's running out of supply. I may be wrong on this, but there is a reason they make a 132mm throttle body, and twin 65's, and twin 69's, etc.

 

[Autopart101]

I am not saying you won't see boost drop as you reach the limit of volumetric efficiency. I am saying he's seeing boost drop because he's running out of supply. I may be wrong on this, but there is a reason they make a 132mm throttle body, and twin 65's, and twin 69's, etc.

I just thought the whipple graph with 132mm TB wouldn't see a boost drop similar to my car with a small TB.

 

[Andy13186]

Possible belt slip?

 

[blackk3389]

Maybe heat soak?

 

[Unas2k5]

I thought it’s best to remove cats on high hp setups to prevent destroying your engine. I removed mine idc if my boost drops at that point.

 

[robvas]

How hard would it be to post the whole damn graph...

You look to be missing about 20lb/ft (at 1,000 less RPM) than the Phase 2 pulley (2.9? 2.8?) on pump.

You should be making way more on e85

There will be belt dust all over your pulley and front of the engine if the pulley is slipping

 

[robvas]

I had the same reaction. TB is to small. Until I saw this boost drop on a 132mm TB Whipple car.

This graph is a 2020 GT 3.0 Whipple with a 132mm TB. Same Dyno as my car. Also seeing a boost drop.

That looks okay...look at this (also a whipple) 132 vs 150mm TB and air intake. It just gains all around and the boost curves stay the same. you won't just flat out hit a wall or anything (and this guy is making like 1000hp)

Posted the wrong graph..hang on

 

[Autopart101]

How hard would it be to post the whole damn graph...

You look to be missing about 20lb/ft (at 1,000 less RPM) than the Phase 2 pulley (2.9? 2.8?) on pump.

You should be making way more on e85

There will be belt dust all over your pulley and front of the engine if the pulley is slipping

That looks okay...look at this (also a whipple) 132 vs 150mm TB and air intake. It just gains all around and the boost curves stay the same. you won't just flat out hit a wall or anything (and this guy is making like 1000hp)

View attachment 804328

My point was that it looks like all PD blower have a boost drop. Doesn't matter what brand or TB size.

Roush 2650
My set up Roush 2650 on E85 smaller pulley making 13psi boost @ 775WHP. Starts out at 14.50 then drops around 5600 to 13.3 and holds it to 7500.

Whipple 3.0 132mm TB 93 pump gas
Whipple 3.0 with 132mm TB starts at 12 psi and drops to 9 psi around 7000rpms. This is a pump gas set up made 680whp. Both on the same dyno.

 

[robvas]

My point was that it looks like all PD blower have a boost drop. Doesn't matter what brand or TB size.

Roush 2650
My set up Roush 2650 on E85 smaller pulley making 13psi boost @ 775WHP. Starts out at 14.50 then drops around 5600 to 13.3 and holds it to 7500.

Whipple 3.0 132mm TB 93 pump gas
Whipple 3.0 with 132mm TB starts at 12 psi and drops to 9 psi around 7000rpms. This is a pump gas set up made 680whp. Both on the same dyno.

I posted the wrong graph before....a Whipple 3.0 should hold (and increase slightly) boost until redline. Look at these two graphs as comparison

Are you 100% sure they are graphing boost in both of the ones you posted? This is why it's nice to see the whole graph (as well as the legend). If boost is going down you will be able to tell in the power curve.

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