Increasing Exhaust Valve size (Head)

[BarnesGN]

Instead of installing a 1.77 intake valve why not install a larger exhaust valve?

Has anybody tried keeping the stock intake valve and going with a larger exhaust valve?

 

[Joe Tiano]

yep..I cant think of the part# but its a pontiac exhaust valve...w/a 11/32 stem

 

[Ormand]

Has anybody tried keeping the stock intake valve and going with a larger exhaust valve?

Not sure why anybody would want to do that. The turbo engine has higher pressure in the exhaust system than in the intake, unlike a normally aspirated engine, and very different from a supercharged engine. Because the exhaust is pressurized, it takes up less volume, so for similar flows, the turbo engine would need a SMALLER exhaust valve, not a bigger one. Seems to me that the turbo engine benefits more from a bigger intake valve, stock exhausts should be fine.

 

[BarnesGN]

Ormand, Do you mean to say that the exhaust has more back pressure on the exhaust side because it has a turbo impeding flow? You speak of the exhaust being pressurized but from what source, Overlap?

I was thinking that by having a larger exhaust valve scavenging effect would increase maybe giving quicker turbo spoolup.

 

[buickpower]

don't think a larger exhaust is necessary or worth the trouble for the amount of exhaust flow you gain, IMO you'd be better off just putting in the larger intake valve, its generally said to be more beneficial. On the intake side most of the flow gain comes from the shape (back cut, swirl polished) of the 1.77 valve, not from the fact that is a measly .06" larger in diameter. We had a discussion about this recently on the before black section of the other board that has some explanation. as far as the exhaust being pressurized, yes that is because of the restriction of the turbo. it's generally said in various threads that the exhaust under boost is under roughly double the pressure that is in the intake.

 

[Ormand]

The exhaust is pressurized because the turbine that drives the compressor requires pressure. Something like twice the boost, for most turbos used on the street. So, if you're running 15 lbs of boost, the pressure in the exhaust headers will be about 30 lbs. The compressed exhaust gas doesn't have as much volume, relative to intake volume, as in non-turbo engine, so the valves don't need to be as big, relative to intake valves.

 

[Joe Tiano]

well ok then...whats up with the trend to go to split pattern cams with more exhaust lift and duration TIA joe

 

[Ormand]

whats up with the trend to go to split pattern cams

Some people just like them, because they work so well on small block chebbies. If you look at the catalog of most cam grinders, they have special "turbo" cams, which are sometimes split with more duration on the intake. They usually have less overlap than other cams, too, to prevent the pressurized exhaust from pushing back into the cylinders. It's hard to find a comparison where somebody just swapped cams, without changing other things, so it's tough to say that this cam is worth 40 hp over that cam. Just easy to use whatever works for other cars with similar turbos, heads, etc. For example, at the RJC web site, Tom Cramer's car is in the 10s with a Competition Cams 218/218. It seems like the turbo Buick is just not real sensitive to cam timing. Lots of different cams used, and the complaints are mostly about them being "noisy", and setting off the knock detector.

 

[Bryan Weber]

For years we have been providing cams with either single patterns or split patterns with more intake duration. We did much R&D years ago and that is what these cars wanted. As far as valve sizes the benefits were much greater with the larger intake valves.

 

[SloGN]

valve sizes

it is my understanding that champion GN1's use a larger exhaust valve in there heads. if the above is true why didn't they just use the same exhaust valve size. instead of making the space between the guides a little father aprts to be able to put big valves on both sides.

i think a larger valve on the exhaust side and it being a flat backed valved also to help flow.

Also why do we try to the the exhaust ports so smooth when porting them to help the flow.

The actual problem is the cam since it open a the exhaust valve towards the bottom of the cylinder to help get rid of pressure in the cylinder before the piston gets to the bottom of the bore.

In my thinking you would want to keep the valve closed as long as possible to take full advantage of the cylinder charge. So a bigger valve would over come the pressure when it is opened as the cylinder starts up the bore on the exhaust stroke. instead of it being opened before the piston is almost at the bottom of the bore on the firing stroke.

But i'm not that smart on this stuff. i have just been dreaming on this thought.

TIA Pat B

 

[10SECONDVSIX]

Well, I guess I have to add my 2 sense here. I have been Porting Buick V-6 Heads for about 10 years now. I have done two heads with bigger Exhaust Valves 1.71" & 1.56" Exhaust. One Set with S/S Valves are on my Engine, and the other with regular Valves on another car. I personally think that they work better since the Air is forced Induction, the Intake has all the Flow it needs. Adding a bigger exhaust we are increasing Exhaust Volume = FASTER Spool up. I have seen a few CFM gain on the exhaust while retaining the same on the Intake by leaving the 1.71" over the 1.77". I have noticed more low end torque and so has the other fellow. But once again, different set-ups will make a difference on the reaction of the engine performance.

Joe

 

[Bryan Weber]

Heads & Cams

You have some very good points and there are several theories about which of these factors of the combustion process is more important to focus and improve on. As with many things in engines and in life there are trade-offs. When making a change to improve one area it is not uncommon to create a negative effect in another area. It is true that there is still pressure in the cylinder when the exhaust valve opens before the piston reaches bottom dead center on the combustion stroke that may still have some ability to push the piston down but the trade off is that the piston would then be pushing against more pressure as it started up the exhaust stroke. There is a trade off at this point.
The exhaust valve is opened against cylinder pressure but not enough to cause damaging loads on the valve train like a top fuel engine.
As far as relationship between valve sizes and cam duration, the exhaust is pushed out of the cylinder by the piston which can create a lot of pressure so it is important to have a clean flowing port to minimize the pressure which increases effiiciency and clears the cylinder as best as possible to minimize the egr effect. The intake charge however is pushed into the cylinder by atmospheric pressure and boost created by the turbo. The more air and fuel you can get into the cylinder and burn the more power you can make. The turbo makes this much easier to get the air into the cylinder. If you want more air you just turn up the boost but that heats the intake charge which costs you power. So with this in mind it is much better to get the most air into the cylinder you can with least amount of boost to keep the air cooler. As far as filling and emptying the cylinders if you could open the valves intantaniously and close them the same way it would be ideal because as you metioned the valve is opened under pressure so if you could have the valve full open immediately instead of opening it gradually you could get more air through the port. This could eliminate the inneficiencies of overlap and increase the efficiency of the combustion process. Unfortunately with the currently affordable technology this is not possible but the roller cam gets you much closer than a flat lifter cam does. This is why a properly chosen roller cam always makes more power than a flat lifter cam.
As far as the GN1 heads having a larger exhaust valve, they do but the size of both valves increased. The area of the exhaust valve by just under 14% but the area of the intake valve increased by just under 18%.
This is my theory abbreviated and two cents.

 

[SloGN]

valve sizes

Thanks bryan for the info it makes for a good read. I understand what you are talking about completely . i can't explain things very well. even tho i know what i'm talking about.


yeah i want a roller cam but big bucks$$$$$$

 

[BarnesGN]

Weber Roller cam= Me.

 

[Bryan Weber]

Sorry for lengthy reply. I started off just wanting to give a quick reply and then I realized a 30 minutes later I was still pecking away at the keyboard.

The roller cams are pricey and that is the trade-off there.

 

[Ormand]

Cam tests.

We did much R&D years ago and that is what these cars wanted

Bryan, did you do back-to-back dyno runs, and check the horsepower? I've seen many, many claims on these boards that "X" works better than "Y", but very rarely is there any data to back up the statement. I can justify, logically, why a turbo engine doesn't need/want a long duration exhaust event. So it will take data, and some more logic, to change my mind. As I noted above, the big grinders, like Crane and Competition Cams and Crower have special "turbo" grinds. The Crower "Turbomaster" is typical, and the specs on it are:
Intake- 212@0.050" and lift of 0.432"
Exhaust- 200@0.050" and lift of 0.401"
Split pattern, Ok, but more lift/duration on INTAKE, not on exhaust.
So there seems to be a difference of opinion here. Bruce Crower has always done a good job explaining things, I've seen several articles written by him in various car magazines, so I understand at least some of his thinking.

 

[ijames]

Bryan, first, welcome to turbobuick.com! Second, don't ever apologize for a lengthy technical post; lot's of people are starved for just that . Worst thing that could happen is that some people won't finish reading it, but more will, and think about it, and learn.

 

[Bryan Weber]

Cam Duration

I'm not sure what you're getting at George W. but in the same post that I mentioned the cams we provide I mentioned ours as having more intake duration on the split cams. Please let me know if you are for or against this theory.

 

[JDEstill]

Here's a theory for ya. The high backpressure situation really occurs once you've got the revs up and boost up. When you are at idle or just cruising along the backpressure is much lower, and you're running a lot more like a n/a car. So I can see how in that situation, when you first punch it, that a bigger exhaust valve or more exhaust duration might help some, but then once you've got it all wound up the benefit of those changes goes away due to the backpressure effects. So perhaps such a change would help spoolup, or low end performance, but have no effect on peak torque and hp. Just thought I'd throw that idea out there for discussion, could be wrong.

John

 

[Bryan Weber]

I believe we share that theory John and agree with you completely.