SStrokerAce

iTrader: (2)

Interesting numbers but.... them ain't gonna be real world lobes in hyd roller unless you have mondo lift, with that it's doable just don't know where your going to get economical springs for that.

Bret

HTMtrSprt

Well, our ported LS1 heads (190cc intakes btw) on a stock 346 with a FAST 90/90 makes 470-480 rwhp and 440-450 rwtq with only 3 deg overlap @ .050! The torque curve is the nicest I've seen on just about any LS1 and the car is very fast and yet also very driveable. When we tested our 408 on the ENGINE dyno so we could monitor air and fuel efficiencies, we tried cams that had up to 30 deg overlap and all we accomplished was a terrible torque curve and an incredible amount of airflow going through the engine....900 cfm @ 7000 RPM. The BSAC numbers were terrible and the torque/displacement ratio showed that even though the dyno said the engine had 120% VE, there was no torque production to back up anything even close to that VE number. This proved the engine WAS pumping alot of air (as it should with heads this good) but it wasn't trapping it in the cylinder to make any power. Now with the L92 heads excessive valve size, port area and abundance of low lift airflow, I still recommend a cam with about 0 overlap at .050 to make a great power curve.

Brian
HiTech Motorsport

fast98

iTrader: (16)

i got them right out of the compcams master lobe catalog online the other day, i forgot to right down the lobe numbers and now i went back and they just updated the catalog. just great.....................

ok i think i found them. the intake lobe is 3194 its a xe lobe for a sbc, i enlarged it 6 degrees across the board, lift on it with a 1.7 rocker is .661 now this brings up a question i have, does lift grow to as you move from a sbc core to a ls1 core? ok the exhaust lobe is 3318 its also a xe lobe for a sbc it lift is .638, its also 6 degrees bigger across the board due to core size differences

doable meaning good? this is the cam i was thinking about going with in my 408 with l92's. i dont want to pry your brain for knowledge here and i'll respect you if you decline, cause i dont want to steal your secrets but would you change anything in my cam? thanks for your response bret

JakeFusion

iTrader: (7)

Wouldn't running a much higher static compression ratio help out those huge overlap cams? I mean, they are "bleeding off" compression, so they simply need more. I wouldn't run a 232/234 114 in a 408 at 12.5:1, because it doesn't need it to make a good torque curve, but a 244/248 112 would. The DCR then becomes an important function. However, in an iron block, I know really high compression hurts the ability to run high timing as well, so it may be moot. But in an LS2 402 or one of the L92 416s, 12.1:1 with a healthy cam in the all aluminum engine would make the torque of a smaller cam in an 11.1:1 motor. With the proper tune, those engines can run on 93 octane, be streetable, and still produce more power.

SStrokerAce

iTrader: (2)

I knew what lobes you were talking about once I looked at the specs imediately. Doable cam lobes if you have that much lift otherwise they will be a bitch to control although a larger core does help.

Duration grows when you move a SBC core to a LS1 core, or any smaller core moving to a larger core.

Bret

SStrokerAce

iTrader: (2)

Brian,

Understandable.... so for instance a 226/228 112LSA cam has 3° @ .050", but to get 4 times that you would have to go to a 226/228 108LSA or 232/240112LSA... two completely different cams. I don't want to know what type of street cam you were running with 30°, if it was on a 408, that's one thing but something smaller ouch. Then again your statement is hard to quantify without knowing other criteria of the cam other than overlap, either way i'm the last guy to whine about that!. Pumping that much into the motor and not trapping it seems to be a waste to me, you already found out how to get it in there now your job is to trap it.

One thing I suggest you do is look at your valve jobs a little closer. Overlap is not your enemy, you just have to know how to control it. Also a exhaust system can throw things out of wack as well. LSA and Centerlines aren't just for determining overlap of a motor but getting the valve in the most effective spot for cylinder filling.

Jake,

Overlap doesn't bleed off compression, overlap takes place 360° before the spark is ignited.

Bret

LSX-Racer

I was wondering when someone was going to bring up this point about overlap!

HTMtrSprt

It was one of many cams we tested on the dyno...
Obviously.........

My valve jobs seal just fine, thank you....
It can be.....
Preaching to the choir.....
I'm pretty sure that was the point of my answering the question in this thread

Ditto......

BTW- In my opinion, people are putting WAY to much weight in 'dynamic compression ratio' numbers.... they are only a small part of the equation. For instance, a recent magazine article tested a 4.6 2v ford w/ comp cams in them. They tested them in the advanced position for power and cranking compression (a good indicator of 'dynamic compression ratio) and then retarded the cams and retested. The engine lost 20 PSI cranking compression and yet picked up power through the entire RPM range. There's alot more to it than DCR......

Brian
HiTech Motorsport

LSX-Racer

Another valid point and you're not the only one with that opinion.

V6 Bird

iTrader: (15)

Yes but you dont spec a cam based on LSA...Valve events are there first...LSA is the by product of the proper valve events.

JakeFusion

iTrader: (7)

I know that technically, overlap does not bleed compression and that such a phenomenon doesn't exist, which is why I put it in quotes (I'm not sure what to call the effect), but the fact is, at TDC when both the intake and exhaust valves are open for scavenging, having too much overlap allows the intake to flow out the exhaust unspent.

I'm trying to understand why compression helps with such a big camshaft. I know on the first cycle of a four-stroke cycle is for induction, third is for combustion, and fourth is for the exhaust. But the second is for compression, which happens 180 degrees after TDC. Could it be that very high compression engines seem to be able to get away with too much overlap by making up for the wasted intake by more fully igniting what does make it into the cylinder? I know higher octane gas burns slower but more completely and is necessary with extremely high compression engines. Could it be that the excessive "induction bleeding," helps somehow with completely burning the air/fuel mix? Too much air and the higher octane fuels burn slower, allowing unignited fuel into the exhaust. I believe race cars do this to cool the exhaust valve and reduce detonation.

SStrokerAce

iTrader: (2)

Brian,

I think you totally missed the part about the valve job and how it can help you and it's connection to overlap. Oh well.... Maybe go read some of the stuff that Windmer has written on his theories of cylinder head and camshaft design. Or don't it's not my mind or opinion.

Jake,

Your issue is not just letting intake charge pass thru the cylider out the exhaust, it's that the RPM and the amount of overlap you have combined with the intake tuing and exhaust tuning are not match appropriately.

Compression and big cams is usually due to late IVC's and that whole effect that DCR has... people do over emphize DCR because they think about it the wrong way. Makes me think that I should have never really talked a lot about it 5 years ago on the forums because it's spread like the plauge since then. The way you are thinking about it makes sense, more cam needs more compression. Or to more properly term it, later IVC needs more SCR up until the point where the VE% is creating too high of cylinder pressures for the octane and burn ability of the engine.

The octane of gas does not effect its burn speed. Gasoline burns at a set speed.

Bret

BlackHawk T/A

iTrader: (3)

Telling us things we already know man

JakeFusion

iTrader: (7)

Thanks Bret. I knew DCR was a component, but I was trying to understand beyond that why compression matched well with high overlap, but the late IVC makes perfect sense. On a small duration wide LSA cam, higher SCR is required to bring the DCR back up, because of the late IVC.

Usually the high overlap is a by-product of a tight LSA large duration cam. I know overlap is described as power, but it's also the reason cams like that can be a bitch to tune.

This has me thinking: if overlap is generally referred to as power, why is that? Because it means more duration and generally tighter LSAs to bring the IVC back in line with a usable powerband? Am I on the right track?

HTMtrSprt

The one phenomenon your forgetting is exhaust reversion. This occurs when there is insufficient exhaust gas port velocity at the end of the exhaust cycle to keep the gasses moving away from the cylinder during overlap. This causes the exhaust to stall and be drawn back into the cylinder, displacing fresh air and fuel. When this happens, you can get away with an abnormally high compression ratio because without enough fresh air and fuel in the cylinder, it's hard to generate enough heat and pressure to cause detonation or pre-ignition. Getting enough exhaust port speed at the end of the cycle to prevent reversion and actually create a low pressure wave behind the exhaust to help jump start the motion of air in the intake port (scavenging) requires a fairly high VE condition to begin with, meaning only at or near WOT. It is NORMAL for there to be reversion at part throttle and idle. It is for this reason that engines with big cams idle so poorly and don't run well at low RPM and throttle angles..... the cylinder is contaminated with too much exhaust gas. This is why GM no longer needed and EGR valve on the LT4 engine and on the later LS1's through LS2's and LS7's. The larger cams put in these motors created enough exhaust gas reversion at low loads to accomplish the same effect as the EGR valve making it redundant. Reversion can also happen at WOT if the exhaust valve opens too early, too fast, or the port flows too well at low lifts causing the cylinder to blow down too rapidly. The gasses cool quickly in the header, losing velocity too soon and then stalling at the tail end of the exhaust cycle. A good indicator of this is spark timing... an engine with excessive reversion will need more timing to make maximum power due to charge contamination. A clean cylinder fill needs less timing. When you see a dyno graph that has a poor torque number down low and seems really lazy, then suddenly jumps up in torque, it's usually because of a bad combination causing reversion at the lower engine speed. Because of this, current racing head design is toward smaller exhaust valves in efficient port designs that control the low lift flow numbers but still flow well at higher lifts and maintain higher port velocity during overlap.

Brian
HiTech Motorsport

BlackHawk T/A

iTrader: (3)

Right, but a ProStock motor is turning a much higher RPM and with a much different intake manifold so its breathing requirements can't even be compared. On a street LS2 we are talking a totally different animal, and the powerband would be tuned differently with duration and overlap for a street car with a plastic intake manifold. I'm pretty sure I don't need to tell you this though.
Now I won't argue with a knowledgeable engine builder but its my belief that 224 degrees of intake duration would really undercam this motor. I would hazard a guess around 235/243 .646/.653 112+0 (comp LSK lobes) with some of those new PRC/Patriot springs rated to .660 lift that bind at .710. They should handle it fine, and it would drive like a beast. Again though its only a guess and speccing a cam over the internet isn't the easiest thing to do (or most accurate).

V6 Bird

iTrader: (15)

Im almost positive its not the seal he is talking about..its the angles at which the seats are cut as to what benefits the goals that you are trying to achieve.

WizeAss

iTrader: (5)

so It is now assumed that 10 degrees duration on the exhaust and wider LSA's are good for these heads.... no need to go sub 112 on the LSA's considering most will be doing a stroker setup.... do yall agree?

also has someone determined if more than .630 lift is doable with the heavy *** 2.16 intake valve...... wonder if aftermarket valves will be available anytime soon?

Since I dont want a cam with high lift and the stock intake valve I think I will just do a cam/heads swap after the fact...... keep the intake side stock and just do a bit of exhaust work and a small mill on the heads.

What do yall think?

So far the numbers I have seen 495/49X at low rpm on a close to stock type cam...... says that ya may not have to go all TREX on the cam to make these things perform??? do yall agree?????

dame

iTrader: (3)

You never said (sorry if i missed it) what you want to do with the car.......

Is it a specific "dyno" number your're looking for? is this a weekend warrior? full out drag car?

My suggestion would be to figure out what YOU want to do with the car, and then build it (in terms of cam/exhaust selection) at that time. Me personally - once the new cam goes in and my factory exhaust goes back on... i know im gonna be a happy camper (regardless of what my dyno numbers are)

Good luck with your build by the way.

WizeAss

iTrader: (5)

Personally I am looking for 80% street use with around 200-300 miles per month and the other 20% on the track making passes.

To give you an idea..... Trex cam in a 346 is good for me.