Hey guys

for those of with modified LS3s, what compression are you running? and what are the mods you did to get that compression?

TIA
Tom

 

Stock. Not enough PVC to run anymore with the size cam we have. You need at least .080 on the intake side and with the LS3s big valves, things get tight pretty quick with stock pistons.

 

Aren't some of the better running LS3's running cams with .630 plus lift with no flycutting? Seems I read the flycutting happens when you mill the heads to increas CR.

 

its a common misconception that cam lift is the reason flycutting a piston is necessary. the truth is, is that the cams duration plays a much more important role in determining PV clearance than does lift.

to answer your question, yes there are LS3 cams out there running +/- .630 lift without cut pistons. and yes, milling the heads will reduce PV clearance (of any motor).

 

As Jon says lift plays little role. The reason why is that at peak lift the piston is at the bottom of the cylinder. The biggest issues are duration and ramp rate. Agressive lobes snap an intake valve open really fast as the piston is just clearing the top of the cylinder and on its way down. Piston to valve interference happens when that valves snaps open faster than the piston drops. On the initial opening side of the intake cam lobe, its far from its peak rated lift. Most agressive cams like a 228/232 XER lobes on a 114LSA no advance (not that big huh?) is already off the seat .050" at TDC and the valve drop of a .030" milled LS3 head is about .140" leaving you with about .090". Now interference happens in that first 15 degrees past TDC so the valve is being snapped open as the piston drops and the clearances as stated are at TDC making those measurements the best case scenario and thats scary since TDC isnt the danger zone yet your only .090" away at that point.

If you increase the size of the cam to 234-238 and lose .006" per degree and more for advancing degrees and LSA narrowing to keep the power band down in a usable range, you can see that not milling gives you back .030" and you lose .060" on a 234 cam on a 112LSA. Some really big cams out there are installed with no clearance and considered 'cam only' yet the clearance as checked by some shops state they have seen some LS3's running .037" clearance with big cams. They are hollow stems right?

Want it more scary? The stock valves have .030 less on the margins (the top facing the piston) so if you want to run stainless one peice valves, you lose .030" in clearance from that. Anyone wanting to run even this size cam with decent DCR will have to flycut and thats why i'm not the biggest fan of big cams with LS3 heads or L92's. I dont like cutting 1/3 through a piston to run a 230 cam with great compression. Cars not running the compression rarely break the 400rwtq mark at 4000rpms on an LS2 while a stock cammed/stock head LS2 with a FAST hits 400rwtq easy with the usual bolt-ons. I've seen 425rwtq at 4000rpms with ETP's and trick flows on the LS2. 30rwtq more with the same cam.

The LS3 is a different story. The added displacement makes more compression and the bigger bore unshrouds the valve. You get .3 more static compression than an LS2 right out the gate. The LS2 with a stock LS3 head and gasket is 10.4:1 comrpession and the LS3 is at 10.7:1. The LS3 is also making more TQ as a result of the added compression and displacement so its far less critical with TQ production at low rpm's.

These are crude measurements and are meant to illustrate how tight things are in there. Please dont think that cam to cam you can use these values to determine what fits and what doesnt. Also dont think anyone is agreeing to take on liability because they shared basic info. Stay with trusted tuner's cams and dont try to reinvent the wheel.

 

Good things guys.

Reason I asked is I know of several LS3 motors( think it is 7 now) that have blown heads. All road race motor set ups and tunes. Five were stock and two modifed

Three were valve / oiling issues issues and two were compression issues with the GMPP Cup cam ( race rules required this cam)

 

What do you mean by "blown heads"? Was it blown gaskets or actual cylinder head cracking/failure? Also, not following you on the "compression issues" with the GMPP cup cam. Just need a little clarification (defuzz it a little so my fuzzed up brain can take it in). Thanks

 

Well that sucks. I guess you need to be careful when you do mods.

 

Stock valve failure on three early LS3s and the head itself cracking around the valves.

The Cup Cam is GM- Grand am cup 88958606 239/251 570/570 106 LSA.

 

I believe that cam was designed for LS1 style heads. It also has about 3 times the overlap of what I would consider a "big cam."

 

Yes it is, but one of the only 'approved' road racing cams by several sanctioning bodies.

 

Think we may have found the answer or most likely cause. Updates soon.

might be something to do with tearing the engine down and blue printing and balancing the whole thing then not put back together properly, or that cylinder hoanning (sp?) not quite done right

 

Is the overlap because of the narrow LSA? Doesn't seem to be that big of a cam. Why is it causing the problems?

 

7 engines??? Balance issues and blueprinting is causing head gasket issues??? I would be interested to see what you guys find out. It's hard to say when everything was high speed dissassembled.


Randy
PS Who has to run that cam anyways??? Is that one of the GT2 rules?

 

I recall this topic upsetting some tuners but just to throw it out there with no disrespect intended and simply discussing a point, would any poorly designed engine (even though I dont think they were poorly designed) have failed if they used a one piece stainless valve? Even if everyone agrees with the engines having been built wrong, the heads of the valves wouldnt break if they were not welded hollow stems.

You seem to feel that if properly built, a weaker valve design could be tolerated but I would over design the valve strength variable. Stainless valves dont break like that even when they are in a bad situation. I've seen timing chain breaks that result in bent valves and they never separated. Even in a poor valve spring situation that results in valve float the, hollow stem/welded valves are in danger where a stronger stainless piece isnt.

For those not familiar with the thread in question, I took the position that since there existed a situation where breaks were occuring and a statistic of a number of breaks were recorded that it was better to chose a stronger valve when doing upgrades. I was slammed for warning of potential breaks for cam swap people based on the number of stock valve breaks. I even stated that Ramp Chevy in NY had several stock motors with valve heads comming off with no mods done to the engine. The springs were fine and timing chains intact. GM then stopped using the LS6's hollow stem for the 243 head in 2005. Since most cam installs arent done with springs being tested for open and closed pressure, nor is any blueprinting done when you buy a cam swap kit, it would be safest to watch the growing number of failures and not assume youre immune and use a valve that wont break as easy. Thats a stainless valve.

Also, anyone with input, aside from spring tension what variables do you think there are when doing a cam swap using stock heads a stock valves? I am interested in hearing what blueprinting would prevent an instability. Rockers are Tq'ed down and there is no adjustment in an LS valvetrain. Rocker arm's lengths are chosen to adjust preload. What other mistakes make for a situation that causes a hollow stem to break?

Randy, you're a first rate tuner that has offed great tuning tips (for free) especially on suspension and I'm only having a discussion because I like the tech topic. I hope you see it as such and not that you're being called out.

 

IIRC YUP GT2 rules at Road Atlanta, two engines blew. SCCA Run offs have had 2 LS3s so far blow up, and some drivers have had more then 2 LS3s create motors blow up too this year.

When I asked the powers that be farther up the food chain, all I got was 'they are working on it.?


For those guys running stock, there is no problems, it is that constant 3000 to red line issues.

 

sorry. i went a little off topic here. overlap is a function of intake duration, exhaust duration, and lobe seperation angle. with everything else being equal, less LSA = more overlap.

p.s. i dont think its the cam that is causing the problems
sorry to the OP for going off subject.
old motorhead - PM me if you want to discuss cams

Jon

 

First off it's getting pretty late, so I may have to finish this reply later.
2nd off, my hats go off to all that are out road racing a new engine.

Head gaskets, damn things. They have a huge role in any engine. Why is the ls3 different then the ls2 or even the ls1-6. Well back in 1997 the first ls1 hit the showroom floors. This was the birth of a new era of small blocks, that has sent a shock way across the global. Small bore small block 3.899. When I first heard of the 6.0 ls2, I said "this should be fun" The 4.00 inch bore has been a good time, big cubic inch for all. But with a 4.00 bore you get smaller water passages. A couple years went by, hopped up engines taking out number 7 piston like they were on fire.

Then the LS3 came along. New 4.06 bore. New head. We all know the bore size getting larger is going to help head gaskets. The new head has a huge intake valve that still fits in the same place as a smaller one did before. Which means less cooling around the valve seat because of the less space. The huge ports have left less room in the water jacket also.

How to make things better,
1. coatings are a great thing in this department. Very little draw backs with good results and wear. It's kinda spendy, but so is gas.
2. improve water flow in the block-heads. Kinda interesting to see big water plugs on the latest race blocks. I don't know what they are using them for. Water log, swirl tank, and/or extra. Stem vents must be flowing, maybe some bigger ports??
3. be very carefull of core shift and other casting tolerences in the production heads. These are production heads, don't go for every last cfm, they flow damn good already.


Valve train comes in many forms and can vary depending on the end users goals.
I personally tested 400 ls6 valve springs in 2003, there was a pretty wide spread of pressures. I sorted them from light-mid-heavy on the seat, then reset the tester to mid open pressure. I haven't heard too many people that talk about inbetween open and close pressure. It's funny because that is were the most amount of wear on the camshaft is, highest load on the lifter and it's bore. Then got them sorted again to the highest open pressure. By this time I had 12 boxes of different weight springs tested. Which ones did we use in the engine??? (After I looked at them for defects.)

Good night
Randy

 

I would rather use a valve that isnt so sensitive to the bad things around it. Lots of engines run the heavier stainless 2.16 valves out to 7200 with no instability so why bother with hollow stems? Additionally, at some point we have to say, "there are some good engine builders still having failures so (at some point) not everyone is an idiot and the part may actually be a bad idea", rather than blaming other things for a poorly designed valve. I havent heard any fables of stainless valve heads breaking off when there was no other failure such as a timing chain or valve float. Just about all the time there is an issue with a stainless valve I hear it bent, not broke into two peices.

 

Between Randy and Spin, at least I am starting to get a better understanding ( learning all the time )

Have a funny feeling that if an LS3 wins, there might be a $500 protest for a tear down.

and personally I would be worried about an engine builder who has never had a failure. Heck we all have to know our limits, then just dont quite go there next time