Exhaust reversion?

[Mike T]

I just pulled my motor to replace the head gaskets and noticed that the throttle body is turning dark in color. It's not oil from the turbo because both the up pipe and compressor housing are absolutely spotless. Could this be reversion?

 

[earlbrown]

Open the blade and look in the plenum. Is it dark too?

 

[Mike T]

Earl I'll have the intake off tomorrow so we'll see then. When choosing the cam I never considered reversion.

 

[charlief1]

Common issue even on a stock NA engine. There will always be some sort of carbon deposit even from a small amount of reversion.

 

[earlbrown]

Earl I'll have the intake off tomorrow so we'll see then. When choosing the cam I never considered reversion.

I wouldn't sweat it. You're always going to get some up there. There's just too many pulses, harmonics and strange stuff that happens in the engine to keep the throttle blade spotless forever.

 

[fastblackracing]

Are the Egr ports blocked or disabled somehow to prevent egr circulation? And what are your cam specs and installed at numbers?

 

[Mike T]

I removed the EGR tower and tig welded the hole. I need to find my cam card but it's a 204/214 flat tappet install slightly advanced.

 

[pacecarta]

cant expect the plenum to stay clean with a popped head gasket especially if the gasket popped into the valley as it pressurized the crankcase and will push oil under pressure up the pcv valve and will push oil past the valve guides into the intake and exhaust

 

[fastblackracing]

This is one of the reasons I have stayed away from the reverse duration splits where the exhaust is larger than the intake. advancing the cam is probly a good thing as it opens the exhaust earlier BBDC and closes it earlier when approaching tdc , but when crossover is happening near tdc, the period when both intake and exhaust are open at the same time I can see the potential for the crossover period to be biased towards the exhaust meaning more exhaust lift than intake unless the cam is advanced an additional amount. With the intake bigger than exhaust duration it is easier in my mind to bias these events toward the intake meaning intake is open more than exhaust at and near tdc. Now with that said when I install and degree a cam after I get it at the number I want, I go back and plot the lifter rise from 20* BTDC to TdC in 2* increments on both the exhaust and the intake. What I see from this is The exhaust will be closing loosing like .007" per 2* increment and the intake will be opening like.007 per 2* increment and you will be able to see the point where both lifters are open the same amount. I have done this for myself to get a better understanding of the mechanics of the timing events. We all know that our cars usually have more Exhaust back pressure than intake pressure so In my mind this is how I have done the cam timing to help scavenge the chamber at/near TDC. I am not saying I know it all only that this is my Take on things. I also believe that the boost pressure we see on our gauge is only an average reading of what is in the plenum, I think if we were able to instrument our entire intake tract with a pressure tap say every 1" all the way to the backside of the valve we would see some interesting things meaning say 25# in the plenum and maybe 23# mid intake port and maybe 21# at the valve throat. Same probly happens in the exhaust side I would think. But it is not practical for the average guy to outfit our engines with this much instrumentation. Sorry for the long winded post and I hope I have put my thoughts down in a way that is not too confusing. I encourage others to step in and share there thoughts and experiences on this subject, maybe we can all learn a little something out of this.​

 

[Mike T]

Fastblack your thoughts are well understood.... I also comprehend things better when looking at it actually happen. I chose my cam because I'm running a GTQ 70 and in my mind the bigger exhaust side may be a benefit in spooling the big turbo. It may have worked because everyone who has driven my car that knows turbo Buicks are surprised at how well the turbo responds. I know there are a lot of ways of doing things but I like experimenting once in a while.

 

[earlbrown]

You can't have scavenging when the exhaust pressure is greater than plenum pressure. Not to mention we don't have a collector in the exhaust stream that produces negative pulses, we have a cork.

Cams are black art voodoo at best and every change you make to the good takes away from somewhere else. They are a compromise at best. The trick is to get just enough lobe to do the job and nothing more. The idea of grinding a cam (or installing it) advanced is to band-aid not having enough static compression for a cam that's too big.

 

[bison]

This is exactly where you can benefit from an aggressive lobe and or solid lifters to reduce back flow during overlap. I time the cam to the ex closing @.050". I have an area where I want it. If it doesn't fall within that range then the lobes are wrong. Overlap will increase exhaust pressure and increase response but it will kill the power once the pressure ratio between intake and exhaust starts to bias toward exhaust.


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[bison]

Cams are black art voodoo at best and every change you make to the good takes away from somewhere else. They are a compromise at best. The trick is to get just enough lobe to do the job and nothing more. The idea of grinding a cam (or installing it) advanced is to band-aid not having enough static compression for a cam that's too big.

I don't agree with this if referring to a performance standpoint where $ spent isn't the hang up. You can put a cam in an engine that will have more duration at high lifts and less near the opening. It costs extra $ to do this but it's the opposite of a trade off. Your picking up duration where it counts and eliminating it where it counts.


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[earlbrown]

What I meant by trade off is every thing you do to a cam changes something somewhere else.

The trick is to maximize what what you want to happen and put the side effects where you don't mind as much.

 

[Mike T]

After taking the motor apart the intake runners, plenum and injector tips are not too bad. I still think I'm getting some reversion but the head gaskets are far worse than I expected and may be a big part of what I'm seeing. I seems like it's entering around the IAC and not going beyond the throttle plate.

 

[earlbrown]

You're really obsessing about nothing. That amount of discoloration is minute in the grand scheme of things.

 

[Mike T]

Your right....... it's just in my nature to get caught up in all of the little details.