If this is true i should pick up some up top since the cam is 218/224@110 and i had it advanced 2*. It will be 4* retarded from where i had it previously. It should be obvious since the engine is making lot of power and a few % gain or loss will be 20-25hp. All the more reason to 4 bolt the combo from the beginning if you want to make a lot of power and not be so concerned with the cam profile. You could run more intake duration relative to LSA and increase the driveability without hurting power. Fwiw my engine is 9.3:1
Boost vs. back pressure
- Thread starter postal
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If this is true i should pick up some up top since the cam is 218/224@110 and i had it advanced 2*. It will be 4* retarded from where i had it previously. It should be obvious since the engine is making lot of power and a few % gain or loss will be 20-25hp. All the more reason to 4 bolt the combo from the beginning if you want to make a lot of power and not be so concerned with the cam profile. You could run more intake duration relative to LSA and increase the driveability without hurting power. Fwiw my engine is 9.3:1
I appears that the timing of the exhaust lobe is very important during the cam installation process since the closing event is the dominent factor in residual cylinder/chamber pressure. The intake opening will fall where it is based in LSA. Of course, this all depends on the cam itself.
For a cam with a very wide LSA, ensuring the exh lobe closing event is where the cam specs is supposed to be appears to be less critical.
Compression ratio also appears to have a positive affect on the cylinder filling.
Not much, but it does help.
All else being equal, it appears that an 8.3:1 vs a 9.1:1 compression engine will gain a theoretical 3CID with a -2deg crank rotation difference in presure equalization.
Very apparent how cam timing will make or break the performance.
Interesting is the benefit of a larger bore diameter as well in reducing required crank rotation before pressure equalization.
Like I said, it is an interesting model.
I thought about a larger turbine housing to get BP down but it would only make the HP peaker. For a track car it would be acceptable, but it would not be fun to drive on the street. I will need to run some numbers with the BP you are seeing. Interesting for sure . . . . .
Charlie,
You provide concistent valiable input and I don't think there is a ounce of ignorance in your body.
I think the longer exhaust event will allow earlier pressure bleed down and from my research it appears that the magic number for opening the exh is around 30-40 deg BBDC. No real revelation I know but the closing event is where the difference is, especially on the turbo motor.
The increased lift just ensures quick pressure bleed so the piston does not have to fight the pressure, but, if the velocity is low, the exhaust system pressure can backfeed into the cylinder.
I think too much lift (>35% of diameter) combined with extended duration may do that.
I know I have not yet told you anything you don't know yet . . . I am merely "thinking on the keyboard" . . .
Thanks for the compliment Jerryl.:redface: I just do a lot of reading of tech stuff so I can figure out the ideas I want to do.:smile:
I don't really agree on using a lower compression ration but I do understand what you're saying. The volume would be increased but I don't think it will change the power output any Jerryl. Compression is one of the proven factors that will increase cylinder pressure and since we use a turbo to increase it more I don't think it's going to make much of a difference in the long run. I do agree about the timing of the cam lobes and have been an advocate of at least a 112 Degree split. I think you could even go as high as 115 and still get a very streetable engine. The main thing is to time the exhaust valve to close about 4 to 6 degrees ATDC as the intake is opening. This will allow the lowest possible pressure for the intake charge to enter the cylinder. If the intake valve closes at 3-5 degrees ABDC then you get the best timing for a full cylinder to chage for the most power while minimizing back flow into the intake.
I don't think you're going to like it being retarded that much Brian. There's another thread over at TBs.com about this particular thing. It seems that straight up or 1-2 degrees either way effects the way the engine behaves a great deal. Two threads to be exact. they came up in the last week I was suprised how much it killed power in the engine. 2 degrees retarded totally killed spool until 3500 so you may want to reconsider.
On a NA engine if u get any back pressure u will lose power.
now in my opinion if your turbo makes 2x the exhaust backpressure vs intake boost pressure than your turbo has stopped making power earlier in the rpm range.
I wonder what the TIP at different boost levels would be , and to also see what the difference would be in the exhaust to intake pressure ratio .
now in my opinion if your turbo makes 2x the exhaust backpressure vs intake boost pressure than your turbo has stopped making power earlier in the rpm range.
I wonder what the TIP at different boost levels would be , and to also see what the difference would be in the exhaust to intake pressure ratio .
Do you have any links? How much power were these engines making? How much back pressure did they have? Did they use N2O? It will only be retarded 2* from the way it was on the cam card. I had it 2* advanced before. This engine really has nothing below 4700 anyway and i have a dry N2O setup through the xfi for spoolup on there so spoolup wont be a problem. If i had to id hit it with 100. Only stays on for about 1 sec. It already spools so fast on a 50 shot it is uncontrollable from less than a 60mph roll without a boost controller on the street even with 100-125lbs ballast in the trunk. I could easily advance it back to where i had it if need be.
I'll see about finding the link for you Brian. I was suprised at the results myself so I want to make sure. I was planning on running with 1-2 degrees advanced to get a little more on the bottom but from what I read the cam seems to work best straight up and you need to degree it to make sure that's where it is.
Here you go Brian. It took a while to find it but there's a second link in it so please read all of it.
Retarded Cam Timing = Slow Spooling Turbo? - TurboBuicks.com
Retarded Cam Timing = Slow Spooling Turbo? - TurboBuicks.com
Turbo back pressure is not dictated by wastegate size unless the wastegate is too small to bypass enough exhaust. The back pressure is dictated by the turbo.
Jerryl
Tall Unvaccinated Chinese Guy
- Joined
- Dec 14, 2004
Cal,
Does porting a given turbine housing (Increasing A/R) help BP to a point?
My thought is if the choke point was opened up, flow should increase, therefore reduce BP.
Charlie, Let me clarify my statement :redface:;
Higher CR = Good.
So going from 8.3 to 9.1, you would see an increase in effective CID with the 9.1 engine, all else being equal.
johnplogii
Senior citizen
- Joined
- Aug 13, 2005
I love this thread.
Can someone explain why I've never heard of someone blowing any exhaust gasket if the pressure is almost double?
I guess people just don't talk about it? I replaced a header gasket a few weeks ago, just didn't post anything on the internet about it.
I doubt you would be able to port it enough to change the A/R significantly. Don't forget the exhaust still has to push against the turbine blades. Spoolup backpressure can get real high.
I love this thread.
Can someone explain why I've never heard of someone blowing any exhaust gasket if the pressure is almost double?
They do. Those that choose to use gaskets where there are none needed will have them blow out eventually. Header gaskets (not needed) usually do. And for some reason there are those that insist on putting a gasket between the turbo and header (also not designed for one) will see that part blow out more often then not. Problem you run into when putting gaskets where there should be none, is that when the gasket starts getting crushed, the mating surfaces start bending and warp the flanges, making it nearly impossible to run without one from that point on.
Stick with what Buick thought best. No header gasket, not turbo flange gasket.
Exactly but if the surface area exposed to the pressure is greater than it will take less pressure to push it open.
Ive read the thread and didnt see any real verifiable data there. No back pressure datalogged or anything like that. Those examples given were for engines making half the power i am. None had a PTC 9.5" or N2O to use with spooling either. Im goign to try it and see if i pick up mph. I have a strong feeling i will. The loss at low rpm is easily offset with the N2O and converter.
yes, but that's just the releif point like a stiff vs weak spring. It's a reslult of backpressure not a cause
Did you read the link to the other thread? I think it was where the info was but It's been a while since I've read it. If you get good results please post the info. I'm still leaning toward a 2 degree retard on the cam myself but I'm not sure whether it would be worth it.
Interesting thread. I'm sorry I found it so late.
From what I understand about cam design, if you have to retard the cam below straight up, you have the wrong cam and you are bandaiding a problem with your cam choice. When people retard a cam beyond straight up it's usually to control a fuel detonation problem in the mid range, or you're lowering power in the midrange due to a traction problem with a particular track. You're cutting cylinder pressure at that point in the rpm band, and a byproduct of that would naturally be longer spool up time.
If you're retarding the cam to try and gain more top end, then you simply have the wrong camshaft for what you're trying to do.
From what I understand about cam design, if you have to retard the cam below straight up, you have the wrong cam and you are bandaiding a problem with your cam choice. When people retard a cam beyond straight up it's usually to control a fuel detonation problem in the mid range, or you're lowering power in the midrange due to a traction problem with a particular track. You're cutting cylinder pressure at that point in the rpm band, and a byproduct of that would naturally be longer spool up time.
If you're retarding the cam to try and gain more top end, then you simply have the wrong camshaft for what you're trying to do.
True but most dont realize they hit the backpressure limit of the wastegate setup they have way before they are even close to maximizing the compressor wheel in their turbos. Ill be using CO2 to hold it shut. My charge air was 11-12* over ambient at 140mph and 26psi. Comp wheel is not a problem based on that data. Exhaust pressure is a big problem with a 3 bolt when going over 700hp.
