Static CR concerns with Truck Norris cam

[carl_tone_garage]

EDIT: I've decided not to use these heads on my tow pig for the simple fact that IAT's frequently reach 130+ where I live, and it gets thrashed really hard while towing my camper. I'd rather keep the compression down and not be so octane-dependent.

And holy crap, it appears the internet hasn't changed at all since I stopped frequenting forums.

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For starters, I have a 2006 1500HD that needs to tow a lot better than it does(currently completely stock with 3.73's and 33's). My plan as of right now is to install a pair of Greg Good ported 706 heads, a Truck Norris cam, TBSS intake, 1 3/4" headers, and possibly 4.56 gears. I'll decide on the gears after I get a feel for the torque curve after all the engine mods.

My concern is being able to run 93 octane with that much dynamic compression on an engine that will be towing a camper through mountains and receiving a healthy flogging on a regular basis. Am I over-thinking this, or will 10.6:1 SC be totally safe with a cam that small on a tow pig?

[blindsquirrel]

LS2 in the TBSS is 10.8:1 with an even smaller cam. (edit: 204/211 116*)

[carl_tone_garage]

LS2 in the TBSS is 10.8:1 with an even smaller cam. (edit: 204/211 116*)

In retrospect, after I wrote this I realized I used to flogg my L92 pretty brutally with trailers behind it with no issues either.

[SiriusC1024]

Calculations based on LQ4:
truck norris lq4.png

LQ4 with 317 heads:
CR = 9.41:1 (advertised CR)
DCR = 8.24:1

LQ4 with 706 heads:
CR = 10.42:1
DCR = 9.11:1

That cam is an excellent towing cam. It's meant for stock engines, as you can see from what happens with smaller chambers than factory. 8.2:1 DCR is about the limit for 91 octane. 93 gives a little more room, but 9.11:1 no way.

I'd stick with the factory heads. Maybe port them if you're up to it. Speaking of porting...why? For towing you'll want to maximize low-mid rpm torque. This means that you have to think about port velocity. Just do some bowl work and blend the short-turn. I say 'just' but it helps quite a bit. Don't pay for hogged-out ports.

Here's an example to illustrate what I mean. Look at the lip on the factory transition from seat to port due to core shift. This is the first port of my first port job. I did a full-port, but shown here is the bowl. The heads are currently on my car. This is exhaust-side factory then rough (before fine sanding). Harbor Freight die grinder, 1/4" egg carbide, sanding mandrel and rolls.
IMAG0279.jpg IMAG0275.jpg

[blindsquirrel]

Gains from increasing compression, whether static or dynamic, virtually always more than offset any loss from lowering timing to avoid knock. Quench/squish distance is also critical. Full-round-dish pistons with high CR is bad bad bad as the dish essentially eliminates all the quench area.

[horsepowerguru427]

Gains from increasing compression, whether static or dynamic, virtually always more than offset any loss from lowering timing to avoid knock. Quench/squish distance is also critical. Full-round-dish pistons with high CR is bad bad bad as the dish essentially eliminates all the quench area.

WOO HOO!!! 12HP

[SiriusC1024]

What he's saying is you have to run garbage .040 Cometics (inferior to Genuine GM) and tear apart the bottom end to put in flat tops.

So,what we end up with is (706 heads):
CR = 11.92:1
DCR = 10.39:1

All for maybe 12hp. Good luck with that.

[SiriusC1024]

WOO HOO!!! 12HP

HAHAHA we got the same number.

[blindsquirrel]

If you ever thought you had a problem because dynamic compression was wrong, you were probably missing the actual cause. Dynamic compression is cargo cult stuff.

Most LS will have the piston out of the hole, the standard .051 gaskets work just fine. Don't go less than .040, and don't go more than .060 or so, unless you go WAY more than .060, like .120 or more. Have you guys ever built a real engine before, or only internet engines?

[SiriusC1024]

Let's just not. Can you honestly recommend the 706 heads? That's all the guy is interested in. We'll let him decide and deal with the consequences.

[blindsquirrel]

10.6:1 with that cam (or any cam) would be fine. How much would you notice the 3* or whatever lower WOT timing? Big deal.

Personally on a 4" bore I'd rather have rectangle port heads and live with the lower compression - in that case, the better heads would more than offset the loss of static CR. But then 10.6:1 with rectangle ports would only be even better.

[SiriusC1024]

Not being a smartass for this.

I don't follow your logic. CR matters, apparently, but DCR doesn't? DCR is corrected CR. Until that's rationalized, I say the 706 combo is a bad idea.

What about bore overhang/valve shrouding. Rec valves are too large for the 6.0 (almost too large for 6.2). Plus cathedral is superior to rec for low end.

[SiriusC1024]

Oh I think I know what you're saying. It's common enough for LQ4's to have 706 heads put on them. Thing is, when people do the heads it's a rare exception that they don't put in a bigger cam. Bigger cams, with their much higher duration, lower the DCR back down.

For the factory cam, DCR is nearly the same as the Truck Norris. Factory cam makes 9.2:1 DCR with 706. It's all to do with IVC angle.

Here's a couple examples from Alvin's shop. Recommended for the 6.0:

Stage 3 - DCR on the 706 combo comes out to 8.61:1.
225/235, .612/.621, @ .050″, 113 LSA
https://www.pcmofnc.com/Cam%20Sheets...0Stage%203.pdf

Stage 4 - DCR on the 706 combo comes out to 8.13:1.
242/247, .615/.624, @ .050″, 112 LSA
https://www.pcmofnc.com/Car%20Cam%20...ge%204.Car.pdf

Truck Norris - DCR on the 706 combo comes out to 9.11:1.
212/22x, .552/.552, @ .050″, 107 LSA

[SiriusC1024]

Final point. If CR can be compensated by spark then why don't auto makers use that strategy? Why not diesel-style compression of 17:1 with 1 degree of timing? That would be a lot more efficient. It's because of pre-ignition. No amount of spark retard will stop it.

DCR is the true measure of compressed volume. DCR for pump gas should max mid-8's. Much higher and no amount of spark magic will save it because pump gas will ignite from compression alone.

[blindsquirrel]

dynamic compression.png

If quench is good, and static CR is under 12, DCR does not matter. I know you think it's important because everyone says it is, but it's really not. If quench is bad and DCR is whatever the internet calculator says is perfect, you still won't be able to keep it out of detonation.

OP's engine in question is a flat top 6.0, right? Cause I don't think a dished 6.0 will get to 10.6 with just a 61cc head, it's only like 10.3 or something. Have the 706s been milled?

[Alvin]

I thought I stepped back in time 10 years ago with all this dynamic compression ratio talk.

[blindsquirrel]

And turbos need a reverse pattern cam, and...

[blindsquirrel]

Here's a horror show of an example if DCR is really a thing:
SBC. 4" x 3.48". +8cc dished piston, .020" down the hole, .041" gasket. 55cc head. Cam is 191/195 @.050, 111* LSA. I don't know the ICL/IVC but it's got to be terrible in the DRC calculator. Like horrific, if those DCR numbers mean anything. Even though quench is nearing the danger zone and total timing is 34*, it runs fine. It could probably use a freshen-up but the heads have never been off since it was first put together back in 1998 or so.

And back in the day it had countless bottles of nitrous run through it, never with a jet smaller than 100 (that's 300+ HP). Not using this as an example because it's great, but because it's probably one of the least well thought out combos I ever did yet it still worked.

[Alvin]

DCR with only things in static being considered... hummm

[SiriusC1024]

OP should be running LQ4. Dished piston 6.7cc. My calcs posted are accurate.

SBC's usually run mid-30s-40s timing. That's a misleading comparison.

Yes quench will mitigate detonation to some extent. When a gas is compressed it heats up. If it heats to auto-ignition temperature then it doesn't matter chamber geometry. The measure of compression on the gas itself is DCR. As long as the intake valve is opened compression cannot occur. It's easy to think about, and I don't understand how we're having a discussion on it. It's not something I heard or read. I don't have an opinion on physics.

I know what you're saying, Alvin. That's not complicated, either. VE is a factor no need to be vague.

Try to remember OP's purpose. Towing not racing. Extended loads, not a few send it passes. He needs to get to his destination without the engine self-destructing. Yes 706 is possible on this combo. I don't recommend it at all.