Free BEER cam question.

[postal]

I'll buy some one a free beer or two at the nationals if they can answere this cam question at the end of this post.

For a naturally aspirated motor a cam usually has more exhaust duration than intake. What goes in has to go out, if you dont get it all out then next time you cant draw as much back in. A N/A cam usually runs higher exhaust duration because entire exhaust track is more restrictive than the intake track.

For a supercharged motor you tend to run an even heavier split in intake to exhaust duration. The supercharger is forcing alot more air than normal into the motor, and therefore you need to open the exhaust valve even longer to get rid of it all.

The turbo cams that are available are ether ground even up or have more intake duration than exhaust. Now the intake track has a compressor trying to force the air into the motor, and the exhaust has heavy back pressure on it do to driving the turbine section. To me it seams that getting the exhaust out of the cylinder on a turbo motor is a bigger challenge than getting fresh air into it.

Why aren't turbo cams ground with more exhaust than intake duration?


TIA: Jason

 

[Lee Thompson]

I don/t drink MUCH beer but a few will be fine. To try and give an answer. The specs. on a stock Buick turbo cam has more ex. duration than intake. Also more lift on ex. Most of the cams I see guys running now have a split heavy on exhaust. Mine is a 218-224 with .500---.524 lift. I believe the early CLUB and maybe current cams were something in the area of 205-210. Quite possibly just a carb. cam. Schneider Cams has a great flat tappet for a turbo ---208-224.
Coors Lite or Miller Lite will be fine. Could you through in a slice of pizza?

 

[BlownV6]

cam question

i think that the duration issue becomes a problem with bigger cams. if you have 30# of boost you might have 60# exhaust back pressure trying to back into the cylinder,so there becomes a balance issue of needing more air in the cylinder just to overcome the exhaust back pressure, it becomes a bigger issue with more duration. im no expert, but thats my understanding. mike booher

 

[ijames]

When you say "a naturally aspirated motor a cam usually has more exhaust duration than intake" I bet you are thinking small block Chevy which traditionally has a better flowing intake port than exhaust port, so it needs help on the exhaust side to get the proper balance between intake and exhaust flow. Also, don't forget that on a n/a motor the cylinder pressure is well over 100 psi when the exhaust valve cracks open so it blows down much faster than intake charge gets sucked in, even with smaller valves and lower flowing ports. Anyway, if you look at cams for lots of brands of motors, some will have more intake lift/duration, some will be even, and some will have more on the exhaust side. It just depends on the relative port flow and valve sizing between the intake and exhaust sides. For a supercharged or nitrous motor you are greatly increasing the flow on the intake side, so now you need lots more help on the exhaust side to get back to a balance, so you are right, those cams tend to have lots more lift/duration on the exhaust side than the intake. Some guys even stay with a small intake valve and go much bigger than normal on the exhaust valve, compared to a typical n/a head, to help even more. With a turbo the answer is not nearly as clearcut. As you said, you have boost pushing the intake charge in, and backpressure resisting the exhaust flow, and it comes down to how restrictive the turbo/exhaust system is - if it is really free flowing and the backpressure/boost ratio is pretty low, then cams with equal intake and exhaust lift/duration seem to work well. If the backpressure is really bad then the cam needs to emphasize the exhaust side to give it all the help it can. There just isn't a "one size fits all" answer with a turbo. David Vizard discusses this in one of his books, and says that in general a good turbo starting point is with the same intake/exhaust balance as a cam for the same motor in n/a form, assuming a good exhaust and reasonable turbo combination.

How bout Diet Coke instead of beer?

 

[lrayzer]

I'll stab at this one also, and I like sam Adams if no home brew is around...

Stock turbo, stock cam..... restrictive exhaust, = more duration to compensate.

Aftermarket cam will have a tubo upgrade first, and of course a free flowing exhaust tract, therefore the need for increased exhaust duration is reduced.

An aftermarket cam on a stock turbo and exhaust would be a real dog.

Also don't forget that most people have upgraded the cyl heads before a cam upgrade.
Just my 02, based upon logic learned watching Star Trek as a child <grin>

 

[postal]

When you say "a naturally aspirated motor a cam usually has more exhaust duration than intake" I bet you are thinking small block Chevy

I'm that transparent?

Most of the cams I see guys running now have a split heavy on exhaust.

I must be behind the times. I was basing the even up or split intake heavy based on the popular cams I've heard of. The only one I've been seeing people try is that 204/214 (is it a coincedence that those are the same specs for the Edelbrock performer plus cam for our motors?)

Could you through in a slice of pizza?

I don't know about pizza but Roger and I are known to visit the waffle house when we're in Bowling Green.

Thanks for the inputs, if you guys are at the nationals find the metro and you'll find me. I keep hanging around Roger hoping some of his coolness will wear off on me!

Jason

 

[Bad as L]

When you grind a custom cam you have the option of choosing lobe centers also. Hi back pressure engines dont like or need overlap so its easiest to choose a smaller exhaust lobe and then advance it. This kills two birds with one stone, you get the exhaust opening where you want it for good blow down before bottome dead center and also you kill overlap. If you do it right you don't loose anything. But it is still better to just get rid of the back pressure in the first place.
Dale

 

[postal]

http://www.turbobuick.com/forums/showthread.php?s=&threadid=144094

It seams that more boost than back pressure is possible. That would really effect what kind of cam specs I could pick.

I've talked to Jack Cotton and I'm going to order a GT42 through him. If my math is right this is a 74.5mm compressor and a turbine wheel thats a little bigger than the old Q-trim. The turbine housing a/r I go with is going to effect the cam specs I go with.

Anybody got some more feed back before I call Jack and order this cam?

Jason

 

[chevyII]

So would it be safe to say if you are using a P trim wheel go with a even duration/longer exhaust duration. And if using a Q trim go with a longer intake duration??Postal what size engine CI/and what stall speed for the GT 42. Im very intresed in the GT40 and 42 turbos but theres not alot of info right now.

 

[bruce]

Originally posted by postal

For a naturally aspirated motor a cam usually has more exhaust duration than intake.

Actually, that's not universally true. Some engines have rather miserable exhaust ports so they crutch up that shortcoming, with adding exhaust duration/lift.

With other then the stock heads, or stockers with the exhaust ports filled in, so they at least try to work right, IMO, even splits will work out fine, up until a point. *Point* being when you really get serious about going fast.

The one thing I really like about the econo roller 206 profile was the limited amount of overlap. I've tried several crane/comp cam roller 206s and none of them match the econo. Maybe someday someone will say what profile/blank was used for it.

 

[postal]

So would it be safe to say if you are using a P trim wheel go with a even duration/longer exhaust duration. And if using a Q trim go with a longer intake duration??Postal what size engine CI/and what stall speed for the GT 42. Im very intresed in the GT40 and 42 turbos but theres not alot of info right now.

I think the statement that I would try to make is that with bigger turbine wheels and housings you should be able to run a cam thats ground closer to what you would have for a N/A motor. The more corked up the exhaust is the wider the lobe separation angle should be to do a couple of things:

1. open the exhaust valve a little earlier, when the cylinder is at a higher pressure to help force the exh out better.

2. close the exh valve earlier. This is to minimize overlap, and to help prevent high exh manifold pressure from flowing into the intake and getting the fresh air in the intake headed in the wrong direction.

3. open the intake valve later. This can allow the piston to move down the bore a bit and will help the high pressure that was trapped in the cylinder to drop a bit before the int valve opens. It is easier to take relatively slow moving intake manifold air and speed it up than it is to get it to stop flowing in the wrong direction, stop, and then accelerate it in the right direction.

4. the bad thing: close the intake valve later. This can be a big help at higher rpm's but it is a prime example of diminishing returns and can get to a point where it really starts to hurt you.

The better the exhaust BP to boost ratio is the less you have to go nuts over this and can start looking at opening and closing points that would normally kill a turbo motor.

Jack Cotton has talked me into a GT42. I've looked at the compressor maps and think it will work nice for me. I think this turbo will help me run a good boost to back pressure ratio (I guess they call it TIP ratio).

Jason

 

[Jerryl]

ttt

 

[Alky V6]

Several people have brought up the point that the better the exhaust back pressure to intake boost pressure ratio, the more you can move towards a n/a spec sort of cam. This is true.

 

[turbobitt]

There is generally more energy available to drive the exhaust out, even with lesser flowing exhaust ports. The exhaust gasses will be driven out by the moving piston. On a N/A application, it becomes difficult to fill a cylinder when your depending on the vacuum created by the piston. Vacuum is less effecient than boost pressure. Many modern NHRA Pro stock engines are using massive intakes and smaller exhaust valves to take advantage of some of these things.

Allan G.