Boost vs. back pressure

[TurboTR]

Hmmm... well actually if I recall the pressure I measured back then was more like 40-50 pisg. It also went up in a straight line basically with rpm. That was with an old p trim 72 if I recall. I've spent alot of time playing with Engine Analyzer sw for cam "design" (if you could call it that- lol). Not real sure what it said was even close to what the engine actually liked; I don't think so. Would have liked to try and use the professional engine design sw at the current job at the time but the engineer(s) was not too confident it could predict much useful without extensive model input that I couldn't provide. So, put that stuff away and tried to use some common sense, results and input from others, etc.

It seems to me that for hp the engine likes holding the int valve open later (no surprise). Or late enough for the target hp rpm range is a better way to put I guess.

To my eye, cylinder pressure plots seem to indicate that it would probably like some help blowing down on the exh phase as well. On mine, blowdown was not "finishing" until the piston was about halfway up on the exh stroke. It does try to blow down into some 40 psi+ backpressure after all; and our ports and valves and flow capacity are probably laughably tiny IMO for the amount of exh we're trying to blow down across only 6 exh ports, with some 700-800+ hp worth of exh going on... I mean, a BBC size valve and port x8 ports is probably getting small at that power level, much less the pea-sized holes we have x6 lol Anyway, the cylinder pressure plots seem to indicate little, if any pressure penalty from cracking it open earlier. The pressure does not drop abrubtly at EVO. But if you look, an inflection point can be seen at EVO. So, it seems an earlier opening is called for to get the blowdown started earlier.

Conversely, there doesn't seem to be any pressure beni from hanging it open past TDC for EVC. The pressure around TDC is ~ equal to the exh pressure, until the intake opens and the exh closes of course, during which it transitions over to int pressure. It looks to me like it should be ~ closed by TDC, or even a few deg earlier to allow the IVO to start earlier.

Overlap- it seems to make sense to reduce that as much as possible in our app to reduce backflow. Using typical cams, I usually see evidence of plenty of exh in the intake- gray and kinda sooty in there.

The good news is that spreading the LSA inherently does all of the above.

That leaves IVO. It doesn't seem to make sense to open it in our app until the EVC is ~ done. The EVC looks like it should be done by TDC. Somewhere around EVC then, certainly as early as possible but not too early- vague enough I guess

Bottom line, I baselined the 274" with a smallish' 224/224/110 SR. It ran ok. Low 10's at upper 130's with no boost launches. So I guess we could make the ever popular cry "it pulls like a freight train!" if we wanted and proclaim it the only way to fly (LOL), but... seemed a bit down on trap speed to me, lol. This with an old q trim, .81 4 bolt Garret hsg 76 turbo. There were some other issues as well at the time, like flexo-flyer pushrods. Never again Anyway, went up from there to a 242/242/114, along with much better pushrods, etc. With no other changes, and not touching the wg knob the engine dropped ~ 4 psi boost at wot. From 30 down to about 26. I'd say that clearly we had moved quite a bit in the increasing airflow direction on the compressor map. It certainly felt that way- it made noticeably more power Clearly it was now breathing substantially better at wot. Personally, I'd like to experiment with spreading the LSA even more, and would have picked a 116 LSA for it if I could have, but sadly, that is not available in our Buick v6 world w/o going to a billet core...

I think about the "perfect" hot'ish street/strip cam for it would be like a 248/248/116. Maybe even 118 or more, who knows. This for a target rev range to about 7000 rpm or so. Would be cool to do comparo testing of various cams and LSA's; wish I had an LSA matrix along with the time and money to test on a dyno, but don't...

But otoh, if people can also run 9's on a 218, then hey... that'll work for them too I guess Some are perfectly happy with just the 1/4 mile results... Others like to try and tinker and maximize the engine. Turbos are powerful beasts no matter what stick choice they have to put up with

Anyway, JMO- always one of my favorite topics

TurboTR

 

[HighPSI]

JDB: Two were GT 42's. Unfortunatly, I can't give out any information since all the turbos I tested were prototypes of PTE. Believe it or not, the last time I publicly tested a turbo (GMHT magazine), I opened myself up to criticism.

TurboTR: You've tempted me to take out my little cam. I've had a 248/248 on the shelf for several years, now, hmmm.

 

[TurboTR]

Hmm, well then you could dyno test the cams for us as well I'd guess your shelf cam is probably on a tight'ish LSA though?

TurboTR

 

[postal]

what about this for some .050" specs:
Int. duration 218*
valve lift .590"
Int. opens 2* ATDC
Int. Center line 111* ATDC
Int closes 40* ABDC

Exh. duration 230*
valve lift .604"
Exh. opens 52* BBDC
Exh. Center line 117* BTDC
Exh. closes 2* BTDC

LSA 114*

This would be using Comp Cams extreme energy mechanical roller lobes.

Can you guys tell me what the intake lobe centerlines are of these cams you've mentioned using. Just knowing the duration and the LSA makes me have to guess whats opening when. With the intake centerline listed and can break it down exactly.

Thanks: Jason

 

[HighPSI]

Originally posted by TurboTR
Hmm, well then you could dyno test the cams for us as well I'd guess your shelf cam is probably on a tight'ish LSA though?

TurboTR

Yeah it's a 110. Swapping cams is a lot harder than swapping turbos. If you guarantee I'll pick up over the 8.57 my wimpy 234/224 cam has gotten me, I'll do it

 

[TurboTR]

Hehe... Well personally I think I'd leave that one on the shelf then, but would be interested in a much wider lobe sep version of it Another point I guess, if I was well above say 30+ psi to run what I run, and was interested in trying to bring that back down closer within the the 3 bar MAP control range via a better breathing cam, might be another reason to check into it. Wow, what a crappy sentence <cringe>, lol...

An opinion from a friend at Lozano Bros was for me to try and run more in the 250'ish range lobes, rather than 24x I stayed smaller for now. Seems to be working well enough for my app. It has hose too, so can add 100+ hp at will. Also, if I recall right a friend up in DFW (Gene G) picked up like 150+ hp when he went up in cam size on his super nice, tt SBC '67 Camaro a few seasons back. They dyno'd it at Young's Perf. I was interested in maybe buying that car, if it became available for reasonable cash. He used to race it in the hotly contested area 10.5" tire wars... Need to run in the high 4's in the 8th mile to be a playah there. I wanted it to be a street car lol...

TurboTR

 

[postal]

Man I would love to try a big cam but if you look REAL close at specs on bigger N/A cams you will see that alot of the extra duration is in the overlap area. Alot of 6500 rpm cams have Int close at around 40* ABDC and Exh open at around 50* BBDC. If you pick a cam with a wide LSA and spec it out to have 0* overlap @.050" then the .050" specs would be:

Intake duration of 220*
Exhaust duration of 230*
LSA of 112.5*

If you draw up a 3.59" circle and a 6.5" rod and start plotting piston height/location for "X" degrees of crankshaft rotation you will see why alot of the added duration of a bigger cam goes into the overlap area around TDC.

When the rod is swung through the arc of the crank around BDC the piston doesn't move much at all for the first 20 degrees ABDC. Once you hit a certain number of degrees past BDC the piston really starts moving up. The amount of piston movement in inches compared to the number of crankshaft degrees past BDC is not even close to being linear. Basically every 5 degrees past BDC the amount of piston travel goes up and up. It doesn't look too terrible when you only look at .050" valve timing but when you add the extra 19 to 26 degrees on each side for the gross duration you will see that a intake valve that closes at 40* ABDC .050" timing, will not be closed until around 66* ABDC gross timing. I could take a pic of the drawings I have, but you would see that the intake valve is not shut until the piston is 1/4 of the way up the cylinder. That would be some pretty serious bleed off with a cam thats only 220* duration. Mechanical rollers have alot less "wasted duration" than hydraulic rollers and look alot more appealing when you really get into it.

Any one what some pics of my drawings? There not engineering caliber drawings but they get the point accross.

Jason

 

[TurboTR]

Well that's why were advocating the wider LSA than "usual" I'd guess that, for example maybe say a 242/242/118 would work better for us overall, while also having about the same IVC and EVO as a larger 250/250/112 would, for example. Have not run the actual opening and closing numbers, just roughly illustrating the point. If we want to keep the overlap to minimal and hold IVO and EVC around TDC, then we are just left with advancing the EVO and delaying the IVC as the lobes get bigger. Which also spreads the LSA out along with it I'll try and fire up the old Win95 machine soon and get the DOS Engine Analyzer sw to crank out some cam cards that result from the above. It is very good for that job at least, and draws the resulting lobe plots too The companion Drag Analyzer is pretty cool too, which takes the power curves from EA and predicts and tabulates the acceleration curve down the track....

TurboTR

 

[bruce]

Originally posted by postal
Any one what some pics of my drawings?

That's something everyone should do every once in a while. ie draw out how the piston moves in relationship to TDC, with various rod, stroke ratios. Not to mention plotting out some valve posistions.

 

[HIPSI]

I've actually done just that in Excel. Right click, save as:

http://www.2weird.com/mike/DCR with piston motion.xls

1st sheet is a DCR calculator.
2nd sheet is the graphical representation of piston motion, piston velocity, valve motion (3 different cams), cylinder pressure decay, and torque applied to the crank.
3rd sheet is a table of the above. Be warned that it is VERY difficult to change the cam timing numbers.

 

[HRpartsNstuff]

VERY INTERESTING READING !!!!

Me & bud Dave have been throwing this around for years, and because of money & time it takes to experiment with cams, we have never done any real world testing on this. In the years gone by, it seemed to stay on the small & conservative side was the way to go. Terry Houston backed this up along with Red, and performance doing this was outstanding (even by today's standard). So.. If it aint broke, why fix it?

Cause there might be another 5-10-20% more power there with the same combo?

Couple of things I have noticed that may help figure all this out are:

1- Calculators, theorys, charts, and ideas are nice, but not always do they relate to real world results.

2- Trends in the BIG HP V-8 turbo world are MORE duration and WIDER lobe separations. I believe it was Mike Moran (6 sec 200++ driver) that said "generally speaking, people tend to go with too little duration on turbo cams". That was directed to people already using way MORE than what we use in the Buick world. Cam specs in the new turbo world are HIGHLY regarded secrets, and not easily shared to anyone. Kinda like manifolds for pro-stock guys If I had to guess, they are running in the 250-270 durations at .050 with 116+ LCA, but that's to spin it 8000 rpm & make 2000+ HP. Just a guess now, only a guess

3- A 250 cu in V-6 = 41.6 cu.in per cyl, which equates to
a 333 cu.in V-8. Also, a 750 HP V-6 = a 1000 HP V-8, with same HP per cyl. I threw a cam question to a WELL known Ford V-8 engine builder based on a 1000 HP 331 V-8 Turbo motor (actually common buildup for them). Speaking generally, I said we had a dispute between lobe centers for a 4000-6500 approx power band at those HP levels. I said around 110 LSA and friend said 115 LSA, based on same duration/lift/etc would work best. The reply was definitely the 115 was better. They made no mention to even more being better or what, but gives us an idea that the Buick V-6 world is stuck in 90's technology with cams.

4- A motor (like the Turbo V-8 guys build) making over 125 HP per cyl (and generally in the 150+HP/cyl range) have WAY bigger ports, valves, headers, turbos, etc. Back to basics, it's an air pump. Get more in & out = more power.

A couple conclusions I came up with are:

A) - Back Pressure is mainly caused by an obstruction in the exh path (basics). Whether it's a bannana in the tail pipe, too small of a header, restrictive exh/mufflers, or turbocharger turbine wheel. With a turbo, the bigger the turbo (and turbine), the less restrictive it is. Yes it takes more to spool up, but once there it's lots more efficient cause it's spinning & building boost while causing less B/P or restriction. The A/R and design/# of fins plays into this as well. That's whay a stk turbo being pushed will have WAY more exh pressure than an 88. Also, if it has less B/P, then the turbo has less time to take on the heat from the gasses. That & other factors lower intake temps for same boost compared to smaller turbo. Some call it "Not working as hard", but it basically means the heat in the exh are allowed to get out of the chamber, reverse less, and out of the headers/turbo faster.

B) - Boost is created by heat, pressure, and velocity on one side of the turbo trying to make the other side draw in ambiant air and pressurize/move it into the engine (basics). I am not sure how the 100% efficiency is calc'd, but could it be based on say 800 CFM air spinning turbine = 800 CFM coming out of compressor side? If so, then the added pressure/heat/velocity of the fuel burning (added separately from compressor working, like NOS would be) would help spin the turbine wheel faster. This is basically explained in a compressor map. The sweet spot is where it builds some pressure ratio where the wheels and housing are most efficient. Go to higher pressures, they become inefficient & need more turbine power (B/P) to create boost. Cal has noticed the crossover at 16-18psi, which may be the sweet spot of efficiency. Seems to me that adding heat & pressure from burning the fuel, that you bleed off excess exh at lower boost (don't need it) and create B/P at higher boost (where efficiency drops off).

C) - Could be full of it, but I would wager a Bonneville racer with a 250 cu.in motor and an 88 turbo would have LESS B/P in exh up to say 30 psi boost (holding there, NOT accelerating to it) running at max speed. Because we are drag racing and always accelerating, we are dumping tons of air/fuel in at WOT to force the turbine to spin harder and bring in more air on compressor side. If you run 125 mph in the 1/4, you have more B/P in exh when crossing the line regardless of turbo sizing. (A smaller turbo would increase B/P, but either way it's presesnt). If you were to stablize speed & hold it at 125, you would have to back off throttle, reducing fuel & air going into the engine. Becuase all that extra heat & pressure is not being pushed thru the turbine, temps would drop & B/P would drop or be eliminated. Hmmmm.

D)- Basically a perfect world would be a staged setup with multiple turbos of the same size. Make exh pressure go thru 1 turbo up to say 2500 rpm, the thru 2 to 5000 rpm, and 3 to 7500 rpm. A variable vain or housing setup would also accomplish this. Then you have fast spoolup and minimum B/P at any given time.

How does all this play into cam selection????
Not 100% sure myself, but I am definitely leaning towards more duration and wider LSA. More duration on int vs exh may depend on turbo sizing & heads (port size) and other factors. Smaller heads with BIG turbo may work better with more int duration, where BIG heads & average turbo may need more exh duration. This would be to achieve best efficiency & power levels. Rod to stroke ratios play into this along with comp ratios, so not all combos will like the same cam. Less overlap will also reduce throttle response and make it seem more sluggish with off-idle performance, similar to a N/A setup because there is no boost yet. Might be able to be tuned out with the ECM, but it will effect more street driving than racing. Might go a little safe on street/strip apps with LSA. Also, more duration with same LSA = more overlap, so it all plays together. A fast acting (aggressive profile) cam will also make a bigger gain in HP than it would on a N/A motor. The .050 numbers don't tell it all. The valve comes off the seat a LOT different with mild & aggressive profiles, in the zero to .050 range. As important as all this becomes on a turbo motor, can't give anything up there either. So imagine a solid roller profile against a hyd one of same lift/duration @ .050. Giving up a LOT there!

REMEMBER, even though the B/P is more than boost pressure, the B/P is WAY less than the cyl pressure present. In theory, the exh contaminates the intake charge because it's higher pressure. In fact, it only does this AFTER the cyl pressure drops low enough (bleeding OUT exhaust to drive turbine) and intake opens (during overlap). It's not the B/P 100%, it's got to do with cyl pressure too. The higher B/P slows and leaves some cyl pressure/residue behind. With too small of exh duration, there will be too much left in the cyl to contaminate int charge. With too much, you get into more overlap, contaminating it too. Real world is probably a balance between it all, and conforms to port size, turbo size, usage/purpose, and rpm range.

For original question, I would maybe try a straight pattern cam in the 224-234 range, on a 114 LSA. Should be great without going too far. If your sole purpose is to experiment and race, get a billet roller made up with 260 duration on a 124 LSA and tell us how it does

PS- I believe Ken Peace of OH runs stg heads on the street with mild setup & great results/luck with it. Not sure on cam, but he knows Terry & Red, so my guess is it's small.

PPS- Sorry for length, only get to post once a month or so anymore. Tried to cover a LOT and make it easy for people to reference to and pick apart.

For anyone still living in tha past & doesn't believe all this, Dave may have a cam for sale that fit's your needs

 

[INTMD8]

FWIW, I've done some dyno testing with cam swaps in my car.

214/222 114lsa

226/218 117lsa

224/224 113lsa


The 224 cam made the most power by far (at peak, and past). It has the most overlap of the cams I've tried, and at the time, backpressure was nearly double boost pressure.

IMHO, you guys are on the right track with the cams you have been running. (As from what I've seen, you run more overlap than most other 'turbo communities')

I may be wrong, but I think if you guys start trying less overlap, the only thing that's going to happen is alot less power.

 

[TurboTR]

Yes, unfortunately we are limited in our available cam selection to more conventional choices mainly because of sheer lack of available parts. The only alternative at this point is to go custom billet, which it seems that most of us are too cheap to pony up for to experiment with lol.

It seems pretty clear that bigger than the tiny little things we have come up with historically in our turbo Buick circles makes more power. And it makes intuitive sense to try and spread out the lobe sep for 3 big reasons. 1) There appears to be no need to hang open both valves when the exh pressure is much higher than the intake pressure. This seems to only invite exh flow through into the intake. 2) Wider lobe sep natively holds the int valve open longer, which benefits high rpm power. Holding it open longer via wider lobe sep also reduces overlap (as opposed to a bigger lobe and more overlap) and should invite more power w/o most of the usual low speed tradeoff. 3) It appears to me, from looking at actual wot cyl pressure traces that the exh needs help blowing down when the cyls are so crammed full of power. Spreading the lobe sep also does this for us- opens the exh earlier, and also closes it earlier per the overlap issue above.

I'm not really able to convince myself that waiting to open the intake for the cyl pressure to go down by piston movement (expansion) is of real beni. My point is, what exh leftover is trapped in there after the exh is closed is trapped, period. That dillution will remain in the intake process, whether you then start opening the intake now or wait ~ 10 deg later... the charge dillution is still in there. Otoh starting the intake opening now gets the valve open and more out of the way for the intake process. It seems intuitive to decide on the best EVC point that traps the minimum residual charge, while also not closing too early that it causes more pumping work against an exh valve that is not holding open quite long enough.

If I had to guess, for something that maintains some streetability but provides even more power (at the already mid 8's level (and with relatively low flowing, production-based heads)... Maybe the thing to do is like ~ 242/242 lobe on an even wider lobe sep, rather than a 248 or 252 or more on like a 114. The wider lobe sep should get the inc power benis from 1-3 above, without the low speed and turbo backpressure tradeoffs of the bigger lobes. Wish we could try this. Maybe I can baseline mine on a dyno and then spend bonus money (if we get one this year lol ) on a billet 242/242/~118 and compare Or Cal, since you have the dyno and the means, I'll offer to go halfers on a custom billet cam investigation project if you can do the baseline and comparo work and share the results

TurboTR

 

[postal]

I custom ordered my solid roller from Comp Cams. Its 230/230 113 LSA which has only 4 degrees of overlap at .050". The intake CL I will install at is 113 degrees. The intake will open at 2 degrees before TDC and the exhaust will close at 2 degrees after. The intake wil close at 48 degres after BDC and I think this is late enough to pull hard to 7200 rpm with ported stage 2 heads.

Time will tell if I picked my specs wisely, but I did pick them my self and had it made. If It doesn't work good then I'll have no one to blame but myself.

Jason

 

[TurboTR]

Yes that sounds like a pretty good choice. I'm sure it will make plenty of power

TurboTR

 

[HRpartsNstuff]

<snip>
The intake CL I will install at is 113 degrees. <snip>
Jason

We found it hard to build boost off the line with a 114 cl installed on 114. Put it in around 107-108 (have to check with Dave England (underpressure) to get exact info) and had much easier time building boost.

If nothing else, make sure you have piston to valve clearance AT BOTH INSTALL CL's, so if you ever want to try it or have to go there, it will clear OK. sloooooow spoolup bites!

 

[TurboTR]

Yes good point, I found the same thing with advancing mine. IME it does make it noticeably more torquey down low, if your combo needs that.

TurboTR

 

[postal]

We found it hard to build boost off the line with a 114 cl installed on 114. Put it in around 107-108 (have to check with Dave England (underpressure) to get exact info) and had much easier time building boost.

I ordered the 230/230 113LSA solid roller for my stage 2 build up for my TTA. Spool up is going to be a BIG concern for me. I want to run a relatively tight convertor and see if the areodynamic TTA can put up some decent MPH w/o leaning on the motor. The motor will be a 275" stage 2 with ported stage 2 heads, 9 to 1 compression, the 230/230 solid roller, 3 stage dry sump oiling, and a big nasty liquid IC. The turbos are a pair of turbonetics TS04 ball bearing turbos. These are thier smallest T-series turbos. Compressor wise they are real close to a TE44 but with the big Tnetics T-series compressor housing. I ordered these with P-trim turbine wheels and .69 a/r 4 bolt tangental turbine housings.

I deffinately feel that when this thing ever gets finished I will have to change or tweak things to get the car to perform properly. How do you initially pick a cam, torque convertor, turbine housings, and gear ratio for some thing like this? My chances that I picked all this correctly right from the start are probably 0.0001% especially when it comes to spool up. BUT! I have nitrous on the car, and I'm not affraid to use it!

I will have to remember to keep the installed intake CL in mind if she spools like a pig at first.

Jason

 

[postal]

We found it hard to build boost off the line with a 114 cl installed on 114. Put it in around 107-108 (have to check with Dave England (underpressure) to get exact info) and had much easier time building boost.

Any educated guess on why that is? Does the spool up like:
1. the intake closing earlier
2. the exhaust opening earlier
3. the intake opening earlier
4. the exhaust closing earlier

If the cam was ground with a tighter LSA would it have liked being installed "straight up". My "Guess" would be that it liked the intake closing sooner. If you would have kept the exh lobe where it was and advanced only the intake lobe 6 degrees (basically reduce the LSA to 111 and install intake CL @ 108), or if you made the intake lobe 6 degrees smaller with an LSA of 112.5 and install @ the 108, would it have the same increase in spool up?

That makes my brain hurt

Jason

 

[SloGN]

back pressure

Hey guys


i dug this post up from the grave. I done some testing on the SloGN today on backpressure.


Mods to the engine/exhaust/turbo is in the sig. well here goes.


All the pressure reading were taken from the EGR port.

RUN 1 20 psi of boost in third gear i showed a BP of 33 psi made two runs to verify consistesy. this was thru the exhaust

RUN2 Next i proceeded to open the collecter on my test pipe. with the dump open @ the same 20 psi the BP was @ 36 psi Again to runs were made to show consistesy.

RUN3 i took the Test pipe competely off the car and ran just a open DWN pipe the boost went to 22 psi with 35 Psi of BP

Run4 i corrected the boost back down to 20 psi with the Dwn pipe and got 31.5 Psi of BP.

So from what i have noticed on my car just opening the dump on my test pipe yeilds no gains at all.


next. i'm going to check the Back pressure @ the o2 sensor location to see if there is an actual diffrence in pressure there than at the egr port. i doubt there will be any difference since the EGR port is tied in witht he rest of the exhaust system pre-turbo.

so 31.5psi/20 psi= 1.57 ratio just a open dwn pipe.

20psi/33psi= 1.65 ratio thru the exhaust

20psi/36psi= 1.8 ratio with the dump open on the test pipe.


The only thing that i can think of why the Backpressure went up by just opening the test pipe is that it is creating turbulence in the pipe by the air having turn and exit the dump collector. thus having to have more pressure to get the air to flow out or maybe not more pressure bu more engery to push the air out even tho it's a shorter way out.


The only thing that i can see that would make the Backpressure lower by opening the dump collector is the fact that the car has a real restrictive exhaust.


how this makes for some interesting info guys.