After searching and searching the past two weeks to find a good turbo cam, I finally called Comp Cams and they suggested this grind:

224/222 568/566 114

What do you guys think? I'm building a forged 6.0 block with stock crank, .060 over (375ci), ARE stage III heads, 9.5CR

 

what kind of boost are you gonna be running? Why the 9.5:1? I would go 9.0 or 8.8:1 if I were you, that is just me though.

Comp Cams usually does a damn good job on recomendations.

 

Living in Denver @5800ft, we can go a little higher on compression here without detonation. 9.5:1 here is like 8.5:1 at sea level. Planning on running 10psi street and 15 psi at the track with race gas.

 

the cam sounds good. BUt id still use around 9:1 CR. i never heard of diffent CR at different altitudes. BUt i duno, its posible.

 

I guess I am still not understanding the logic of running an intake biased cam. I am pretty sure that the air doesn't need help getting in, and scavenging on a boosted car means you are more likely to blow your charge straight through. Anyone have any experience with this, or even a desktop dyno simulation?

-Geoff

 

I've been told by many to use a reverse split pattern cam with a turbo setup and thats what that is. This is what Comp Cams suggested for a turbo LS1. I wouldn't mind having a tad more duration and lift though.

Harlan is running a 228/224 .537/.534 reverse split pattern and we all know the power he is making. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

 

I was told by alot of peps to run revers split, Speed inc, Thunder, Charged Air, AMS, East Side, And Im sure more than that. Im making decent power on mine 222/216 563/563 @116.
Im doing a new fuel system and retune next week if weather permits so I'll keep you posted!

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Noyzee:
<strong> I was told by alot of peps to run revers split, Speed inc, Thunder, Charged Air, AMS, East Side, And Im sure more than that</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Building engines is science, not some kind of voodoo. I was really hoping someone had some scientific reasoning about why you need more intake on a boosted car. It seems intuitive to me that it would be just the opposite - air is getting pushed in, but resisted on the way out, especially on a turbo. Intake biased is new on the LS1's, and I would hate to see people jump on this bandwagon for an FI car without any evidence it is better. Has anyone run back to back dyno's with cams like this on a boosted car? If the amswer is no then you can't say for sure it is the best cam, it might just be adequate.

By the way, if you look at Thunder's supercharger cam, it is exhaust biased (224/227). Plus Lingenfelter runs a 207/220 cam with a 117.5 LSA in all their twin turbos. So all the vendors do not say to run intake bias. In fact, two of the most reputable ones actually run exhaust biased cams.

I am not trying to start a war, but I think people might be making decisions based on what's the fad in cams rather than what is the best for their combo.

-Geoff

 

Turbo cams are different than blower cams. There is a concern of burning exhaust valves with turbos. The heat doesn't get away as fast. I'm with WhiteHawk. Before anything, I would read a $12 trubo book that has more exact theory than any BB.

 

I agree with the "building engines in science" bit, but there's not a whole lot of research data available for turbo applications. You can only gather so much info from the generic turbo advice you find in books. Apart from actual experimental data(often yours, or other peoples experiences) there's not much info out there. I'm gonna be using a 226/230/114LSA in my 383 turbo motor, so i let you all know how that works out. If it works, great if not, I'll swap it out an try a reverse split cam. The money spent will be well worth the first hand learning experience.

 

On the CR topic. If the waste gate is set to 10lbs or whatever it shouldn't matter what the ambient air preasure is. Does that make sence or am I missing something? With computer control where it is now, people are getting away with running alot of boost and CR. I think air temp will become a concern. Air temp rises in relation to how much it is compressed. Next stop detoNATION.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by White_Hawk:
<strong>
By the way, if you look at Thunder's supercharger cam, it is exhaust biased (224/227). Plus Lingenfelter runs a 207/220 cam with a 117.5 LSA in all their twin turbos. So all the vendors do not say to run intake bias. In fact, two of the most reputable ones actually run exhaust biased cams.

I am not trying to start a war, but I think people might be making decisions based on what's the fad in cams rather than what is the best for their combo.

-Geoff </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I know you're not trying start a war but I don't think this is some kind of fad. This has been proven by a few already. Thunder doesn't know what to suggest. I called them and they weren't sure what was best for a turbo (they do offer a reverse pattern BTW). Lingenfelter I'm not too impressed with the turbo kits they have and the hp figures. Granted, Lingenfelter has one of the fastest C5's out there but look how much money went in that car (over $150k) and thats also using the C5-R block. I think Comp Cams knows what their doing and I'd rather trust them than anyone else in the cam industry. I'm pretty sure LS1 Motorsports is also running a reverse pattern cam. I tried to call Rob and get his specs but he won't be back till Monday. I'm not saying one is better than the other but seeing some of the figures that Harlan and Rob are putting out, it shows me the reverse cams do well in turbo applications.

I just called Comp Cams and they basically said that it all depends on your head flow and boost. They have found that when a turbo starts spooling that a vacuum is created and too much scavenging takes place. Hope that kind of makes sense.

As for CR in high altutude, you still have to consider oxygen. Turbo, SC or NA, we're still slower up here no matter how much boost you push. I attribute that to the lack of oxygen (not pressure). The top dyno here in Denver (for street) belongs to a guy that runs a blown 427 using a D1SC, 10.5:1 CR and 8lbs of boost on pump 91 gas www.dynopro.com . Its been proven that you can get away with more CR up here with pump gas than you can down at sealevel being NA or FI.

<small>[ February 21, 2003, 04:54 PM: Message edited by: rodent ]</small>

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by rodent:
<strong> As for CR in high altutude, you still have to consider oxygen. Turbo, SC or NA, we're still slower up here no matter how much boost you push. I attribute that to the lack of oxygen (not pressure).</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Huhhh....Actually, your stochiometric ratio does not change at a higher altitude, therefore at any given psi, your amount of oxygen would be the same. IOW, an engine seeing 8psi at sea level will have the same amount of oxygen as an egine seeing 8psi at 5500ft. As long as the pressure measurement device is measuring absolute pressure and not relative pressure, the amounts of oxygen will be the same.

 

Then how come a turbo car running the SAME psi p(boost) up here runs slower compared to sea level?

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by rodent:
<strong> Then how come a turbo car running the SAME psi p(boost) up here runs slower compared to sea level? </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">How are you quantifying that? Don't take this as a flame, I just want to understand your conclusion. Did you run the car at your home track and a track at sea level, or are you just comparing it to similar power number cars and their times at sea level?

I would guess that the reason the turbo sees a loss in performance is due to the method of pressure regulation used in turbos. A turbo bleeds off pressure over a certain psi (max pressure) which is controlled by a mechanical valve. This valve is calibrated using an ambient 14.7 psi pressure. When a lower ambient pressure is seen, the calibration is no longer 100% accurate...it will not sustain as high of a max pressure, because it won't have as much force pressing it closed. Granted, this will probably make a VERY little change in peak boost, it might be the loss of performance you see.

Out of the three choices: NA, SC and TC a turbo should see the least amount of change.

I guess I should have clarified in the last post that at higher altitudes, your peak boost will still suffer...just not as dramatically as with a NA application.

 

Its a fact that any turbo charged car will run slower up here than at sealevel with the same boost. A stock DSM, Sy/Ty, Subaru, Supra, etc all run slower at 5800ft (Bandimere Speedway) then they do at sealevel. Ask anyone with a turbo car that lives at high altitude.

From NOS:
"Nitrous Oxide was discovered by the German's during WWII. German fighter and reconassance aircraft were equipped with the so-called "GM-1" system which added nitrous oxide to the intake charge to compensate for reduced air density and less oxygen high altitude. "

From DSM.org:
"For those not operating near sea level, the 'safe' boost you can run seems to increase by about 1 psi (0.07 bar) for each 1000' over sea level. For example, a car at 3000' over sea level can safely run as much as 18 psi of boost."

From KB Pistons:
"High altitude reduces cylinder pressure so if you only drive at high (above 4500') altitude, a 10:1 engine can be substituted for a 9:1 compression engine. "

Having lived in Denver all my life and the engines I have built in the past (283, 350, 454, 496, 4.3 Typhoon, soon to be LS1) I know I can get away with higher compression up here Turbo or not. Just my experiance and I've talked a few engine builders in the area that say the same thing.

<small>[ February 24, 2003, 07:58 PM: Message edited by: rodent ]</small>

 

I don't think we're arguing the same thing...I might have misread your post that I quoted. My first post was just to clarify that the pressure percentage of Oxygen at higher altitudes was the same as at sea level (~21%). Due to lower overall pressure, the absolute amount present is less...so that part of your statement is true. I thought you were trying to say that the amount of oxygen present did not depend on pressure, as obviously it does.

As for why performance suffers at high altitudes and why you can get away with higher compression is still due to ambient pressure. For any given volume of air at high altitude, there are fewer oxygen atoms (total atoms as well)...since the energy produced by an engine is directly proportional to the amount of oxygen/fuel it effectively burns, this causes a lower output. Since "safe" operating pressure is an absolute pressure inside the engine, compressing a set volume of gas at high altitude will yield a lower absolute pressure than at sea level...hence the ability to increase CR at higher alt.

These points are all outlined in the quotes you just posted, I just wanted to make them a little more clear to some readers who might not understand why the statements hold true. This has been a good discussion....let's get back to your topic concerning cam selection. (mostly because I am also interested in finding out what cam is best suited for FI) <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

Cheers

 

I wasn't sure what you trying to point out but now it makes sense. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

I'm suprised nobody replied about what Comp Cams said about the turbo creating a vacuum thus the reason why a shorter duration/lift is suggested on exhaust.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by rodent:
<strong> I wasn't sure what you trying to point out but now it makes sense. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

I'm suprised nobody replied about what Comp Cams said about the turbo creating a vacuum thus the reason why a shorter duration/lift is suggested on exhaust. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">No way in hell the turbo is going to create a vacuum in your exhaust system. That would mean the turbo is self-propelled.

Depending on total duration, you usually want to have shorter duration on the exhaust to minimize overlap. A naturally aspirated engine can benefit from overlap because the exhaust slug pulls in the intake charge during overlap.

If you want to run a healthy amount of intake duration without overlap, you will need to shorten the exhaust duration (if you stay with a reasonable LSA). Since you are only looking for the high energy exhaust gases, holding the exhaust valve open for an extended period is not beneficial. (While having the intake open longer is). And while you may have negative overlap @.050, you still have some seat to seat overlap, and reversion can happen during this time. (Most street turbo systems will have more backpressure than intake pressure).

When looking for a turbo cam, don't just look for one with a reverse split duration. Look also at valve opening/closing points, and overlap, to see if it makes sense.

The 207/220 117.5 cam will work well with a turbo because the intake duration is so short that having a traditional split duration cam doesn't cause it to have alot of overlap. This is also a good choice because of emissions/driveability, but it is NOT the highest HP cam for the application.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by INTMD8:
<strong> No way in hell the turbo is going to create a vacuum in your exhaust system. That would mean the turbo is self-propelled.

Depending on total duration, you usually want to have shorter duration on the exhaust to minimize overlap. A naturally aspirated engine can benefit from overlap because the exhaust slug pulls in the intake charge during overlap.

If you want to run a healthy amount of intake duration without overlap, you will need to shorten the exhaust duration (if you stay with a reasonable LSA). Since you are only looking for the high energy exhaust gases, holding the exhaust valve open for an extended period is not beneficial. (While having the intake open longer is). And while you may have negative overlap @.050, you still have some seat to seat overlap, and reversion can happen during this time. (Most street turbo systems will have more backpressure than intake pressure).

When looking for a turbo cam, don't just look for one with a reverse split duration. Look also at valve opening/closing points, and overlap, to see if it makes sense.

The 207/220 117.5 cam will work well with a turbo because the intake duration is so short that having a traditional split duration cam doesn't cause it to have alot of overlap. This is also a good choice because of emissions/driveability, but it is NOT the highest HP cam for the application. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Good info!

Can I ask what cam would you guys suggest for a 375ci, stage II heads and 10-15psi? This would be biased for more HP and less emissions/driveability.

<small>[ February 24, 2003, 10:57 PM: Message edited by: rodent ]</small>