split pattern cams

[turbov6joe]

Kind of a generic question, but which is better for use on our motors as a mostly race cam, more lift on the intake or exhaust side? At one time I seem to recall reading that more lift on the exhaust side EX. Speedpro 204-214 is better for turbo motors, yet the 210-205 being the complete opposite has gone pretty fast??? I'm getting my thoughts in line for a winter build up that will include and new cam and fully race ported heads for my TTA.

 

[TurboDave]

Those number aren't lift, they're duration.
On most N/A motors, having more duration on the exhaust side is desirable, to try and evacuate the cylinder as best a possible.

On forced induction motors, turbo's in particular it is more desirable to have longer duration on the intake side, since everything is under much more pressure, the longer exhaust durations aren't required.
The 210/205 would be the better choice
Or, cams with no splits, ie. 210/210

 

[Mike Licht]

Well IMO the 204-214 is NOT a good turbo cam. The split is way too big. Quick history. Old school turbo cams were shorter on the exhaust to kill the over lap and help with the high pressure in the headers from the turbo. Turbos get better on the exhaust side and single pattern cams become better. Now with the newext generation of turbone wheels I think we could go a little bigger on the exhaust, maybe 4 or 5 degrees but no more. The fact that almost every cam ever tried on these cars works for someone tells us that cam selection is just not a big deal on these motors they are not that sensitive to cam changes. Bigger cams need more stall and move the powerband up.
Mike

 

[JDSfastGN]

well i love the 204/214 im using, it pulls a lot harder than my old 208/201 ever did. I'm not gonna claim to know cams as well as most of the guys in this post, but i would think with a turbo cramming all the air into the cylinders you would want more exhaust to evacuate it and help spool the turbo immediatly. The factory camshaft had more exhaust than intake as well, and it seems to work fine into the 10s. I have also noticed that it seems most blower cams seem to be more exhaust than intake as well. But as stated, people have seen good results from most cams they have used. I would just continue to ask around and see what has worked for others.

 

[Blown&Injected]

Originally posted by Mike Licht
Bigger cams need more stall and move the powerband up.
Mike

The 224/224 flat -hyd you sold me did not hurt the low end for me. Maybe I have enough stall to nullify the effect

For any flat cam I would insist that the cam have the reverse taper cut. The #3 exhaust lifter bore is not right and the factory cam had a special taper cut in that lobe to help prevent wiping that lobe. I have read here that Comp Cams, Lunati, and Reed are at least three manufacturers that now do the reverse taper thing.

 

[Ormand]

On a turbo engine, the exhaust is under pressure, higher pressure than the intake. So, the gas is denser (relative to an naturally aspirated engine) and since it takes up less space, it needs less valve, and less valve timing, to get out. For example, a Buick running 15 psi boost, might have 30 psig exhaust back pressure. With no turbo, the same engine might have "0" psig at the intake, and 1 or 2 psig in the exhaust. On the other hand, a blown engine, not turbo, has no pressure on the exhaust, just like a N/A engine. But the intake flow is pressurized, so there is more gas to get out, and the blown engine would need more exhaust timing and/or bigger valves relative the the N/A engine. Depending on how the engine is set up originally, it may need split timing when it's unblown, like a chevy. But the general rule, starting from an unblown engine, is that a turbo needs less exhaust duration/lift, while a blown engine, non-turbo, needs more.

 

[REDS HOT AIR]

this should get interesting

I think Im responsible for so many people running the 204/214 cam so guess you'll know my oppinion

 

[chevyII]

wouldnt you think the more pressure in the exhaust pipe on a turbo is a good thing? More pressure to turn the wheel. Longer exhaust duration seems logical,I think

 

[87GN_70GS]

I would think you would want to open the exhaust valve sooner, so that the residual cylinder pressure is higher than the exh manifold pressure, enabling flow out of the cylinder. If the exh valve opens when the exh manifold pressure is higher than the cylinder pressure, gas will flow back into the cylinder.

 

[WarpSpeed]

If I can say, it seems more reasonable to say, since there's a turbo forcing air in the intake, it seems the exhaust should have some extra help/duration getting it out, since there's nothing pulling gasses out.
the 208 208 seemed pretty good, (3-4b tenths over stock.) but it wouldn't do 11s in testing.*(not that it can't).... the 204 214 seemed to , and proved, to make more power. Where the Power ban is is more of the issue. Overall I'd say lift is a factor, but cams don't make a different very much. It's more picking a cam with a nice lope.

 

[turbov6joe]

Dave,
Thanks for clearing that up, I meant duration but had lift on the brain when I typed it. I'd sure hate to have a cam with only 204"/214" of lift

Keep the opinions comming fellas!

 

[Ormand]

since there's a turbo forcing air in the intake, it seems the exhaust should have some extra help/duration getting it out, since there's nothing pulling gasses out

There is a turbo pressurizing the intake. But the piston moving up in the cylinder is forcing the exhaust out, with enough pressure to drive the turbo. There is no exhaust flow out of the cylinder unless the pressure in the cylinder is higher than in the headers, usually about twice the turbo boost pressure. So, if the valve stays open long, then the flow will reverse. On a normally aspirated car, or a blown non-turbo, leaving the exhaust open past TDC will allow for some scavenging, getting all the exhaust out. But in both those cases, there is little pressure in the exhaust system, you can even tune the exhaust for better scavenging. Much harder to tune exhaust with a turbo, especially if you want all the pipes to fit under a hood, with AC, etc. Maybe open soon is good, like 87GN said... open soon, close soon.??

 

[Chris McDade]

Joe,
You have to stop sniffing that racing fuel. I told you that will kill brain cells We will let you slide this time!

 

[ijames]

The trouble with opening soon is that you waste power, since that pressure could have been pushing the piston down. I think the main thing with turbo cams is to avoid overlap so exhaust doesn't flow back into the intake while both valves are open. Other than that the differences between cams gets pretty hard to see .

 

[WarpSpeed]

>>So, if the valve stays open long, then the flow will reverse.

I see your point. I might agree if the piston was stationary at somepoint with the ex valve open. But, the piston is moving at a pretty good speed, especailly at red line, and it's somewhat hard to believe the air/gases flow could reverse in the short span of time that a cylinder is on it's ex stroke.
Of course it would be helpful to know what kind of pressure the exhaust valve/exhaust port is seeing during the exhaust stroke. I'd have think there was some kind of "jetting effect" as the piston exhausts the spent gases, forming a sort of pressure sheld or bubble, momentarily, downstream of the valve.

 

[turbov6joe]

So what's the concensus with this...favor the intake (210-205) or exhaust (204-214)?

 

[TurboJim]

Typically, I have noticed people with longer exh duration cams tend to run higher boost to achieve their goals than people with the opposite. A longer exh duration cam may have some self egr which may be favorable in some cases. I believe this is due to the fact that boost must be raised to overcome the backpressure in the header/chamber.

I have a 214/210 cam and it seems to make a LOT of power. I guess we'll see how much very soon, but suffice to say it is making more power at 11psi than my old setup did at 21-22. No, thats not all the cams doing. I have a friend was running 10's at 30psi on a longer exh duration cam with open exh, and another friend running similar times at 20psi, with a similar build, but different cam. Turbo's were close enuf for me to say the cam was making most of the difference.

Mine should be pretty scary at 20psi right now... at least on the street without any vht on the ground

 

[turbov6joe]

Jim,
I also was a proponent of more duration on the intake side as that seemed to be the "norm" for SO many years with these turbo Buick motors. Now with Red A. and a few other guys running SO fast with the longer exh. duration cams I'm beginning to wonder if they didn't stumble onto something that goes against the grain so to speak. I do know for one thing, my TE45a is not very optimized anymore with the OE cam back in there. I have to run 3-5 psi more to accomplish the same goals I did with my 212-212.

 

[Ormand]

The chevy guys have it easy. There is ALWAYS somebody like Camaro Craft willing to run a cam test, comparing A to B to C and so on. For us with the Buicks, it is pretty much "word of mouth", and since there are very few apples to apples comparison tests, we pretty much go by trial and error. Farther up the thread, it is mentioned that cams don't really make a big difference in the Buick engines. (Maybe Stage II, hard to know about that) So it seems like you just get one that sounds good, and that will last a long time, and then turn up the boost!

 

[bruce]

Which is better?
for what circumstance?.
Which cylinder head?.
What's the flow chart of the head look like?.

IMO, for anything running a muffler, TR wise, then overlap and intake closing timing are the real issues.

Overlap, can in large part just be thought of as the engines way of self inducing EGR. The more overlap the more EGR.

Cam design, IMO, is always about the engine builders preferences. Some guys just are able to get what shouldn't work well, to run like gang busters. Takes trying a few, IMO, to really find out what works for you.

YMMV