[JSH]

We're in the tuning process, the car runs well, and the most surprising data we've seen so far is a drop in absolute manifold and boost pressure from 31# in the previous build to 25# in this build.

The previous build was OEM shortblock with LME heads designed for the Hennessey 850. The new build has LME's short block and race heads.

Kong has always said that 31# was too high and that I needed a more efficient motor.

Has anyone else here seen such a drop?

[Joshinator99]

Originally Posted by JSH

We're in the tuning process, the car runs well, and the most surprising data we've seen so far is a drop in absolute manifold pressure from 31# in the previous build to 25# in this build.

The previous build was OEM shortblock with LME heads designed for the Hennessey 850. The new build has LME's short block and race heads.

Kong has always said that 31# was too high and that I needed a more efficient motor.

Has anyone else here seen such a drop?

Yeah huge drop when I went with the Mast heads and new cam vs the stock longblock. Had to make a major pulley change and spin the blower a lot harder since the motor could move a lot more air. Sounds like you’re on the right track!

[JSH]

Originally Posted by Joshinator99

Yeah huge drop when I went with the Mast heads and new cam vs the stock longblock. Had to make a major pulley change and spin the blower a lot harder since the motor could move a lot more air. Sounds like you’re on the right track!

I was using a 2.625 upper previously but put a 3" on it to start with. Sounds like I should go back to 2.625

[Joshinator99]

Originally Posted by JSH

I was using a 2.625 upper previously but put a 3" on it to start with. Sounds like I should go back to 2.625

For sure, that still might not be enough but it’s a logical next-step.

[JSH]

Originally Posted by Joshinator99

For sure, that still might not be enough but it’s a logical next-step.

Shouldn't I expect the boost to go down a little bit as my tune gets better and more efficient? This has changed my thinking about boost.

[Joshinator99]

Originally Posted by JSH

Shouldn't I expect the boost to go down a little bit as my tune gets better and more efficient? This has changed my thinking about boost.

Nothing to do with your tune.

Boost is a measure of restriction. Letting the engine move air more easily means the blower puts the air into the motor with less “boost”. This lets the blower operate at a different point in its compressor map…so less boost doesn’t mean less airflow…it just gets into AND out of the engine more easily. You can tell by your MAF values how much air you’re moving so just adjust your blower speed accordingly. If you’re trying to max out a PD blower, you can’t be trying to run 30-35 pounds of boost (unless the blower is specifically designed for that like some turbos are)… you needed more airflow but lower boost…say 22-24 pounds max.

[JSH]

Originally Posted by Joshinator99

If you’re trying to max out a PD blower, you can’t be trying to run 30-35 pounds of boost (unless the blower is specifically designed for that like some turbos are)… you needed more airflow but lower boost…say 22-24 pounds max.

When you say boost, you mean total manifold pressure (ambient + boost)?

[Joshinator99]

Originally Posted by JSH

When you say boost, you mean total manifold pressure (ambient + boost)?

Yes. Boost is just the differential between manifold and atmospheric pressures.

[ZL1Atlanta]

My last engine was a 416 ci with LME's max effort CID heads and made 29.5 psi. On the new engine (427 ci) using the same pulley combo it made 21.5 psi. The lower the better with a 2650.

[JSH]

Originally Posted by ZL1Atlanta

The lower the better with a 2650.

This has changed how I think of boost. Kong had told me all along that I needed an engine that would flow more air to take full advantage of the 2650. I was thinking that more spark advance would consume more air.

[ZL1Atlanta]

Originally Posted by JSH

This has changed how I think of boost. Kong had told me all along that I needed an engine that would flow more air to take full advantage of the 2650. I was thinking that more spark advance would consume more air.

Yep, keeping boost down has been a major focus on this build. TooHighPSI explained the importance early on with the 2650 and it definitely seems to have paid off.

[2018ZL1]

My stock LT4 engine made 21lbs of boost and when we installed the new bullet it made 15lbs! Pully'd up to get to 17lbs.

[jlaudio11]

Like others have stated, boost a measurement of resistance. A bigger engine, moving more air is going to have less resistance. Look at airflow or hp numbers if you want to compare apples to apples. It could also make a little less boost as the tune is dialed in and the engine is running more efficient.

[Joshinator99]

Originally Posted by ZL1Atlanta

My last engine was a 416 ci with LME's max effort CID heads and made 29.5 psi. On the new engine (427 ci) using the same pulley combo it made 21.5 psi. The lower the better with a 2650.

Originally Posted by ZL1Atlanta

Yep, keeping boost down has been a major focus on this build. TooHighPSI explained the importance early on with the 2650 and it definitely seems to have paid off.

Originally Posted by 2018ZL1

My stock LT4 engine made 21lbs of boost and when we installed the new bullet it made 15lbs! Pully'd up to get to 17lbs.

Originally Posted by jlaudio11

Like others have stated, boost a measurement of resistance. A bigger engine, moving more air is going to have less resistance. Look at airflow or hp numbers if you want to compare apples to apples. It could also make a little less boost as the tune is dialed in and the engine is running more efficient.