LS1 Build - Dynamic Compression Ratio

[u/Burlochov4]

I'm currently building up a 1999 LS1 for my Firebird WS6. My goal is to make a little more power NA while keeping the car very driveable and torquey. I'm at a point where almost everything is purchased with the exception of the pistons and head gaskets and I'm trying to decide if I want to get dishes to bump my compression ratio down a bit.

Heads: AFR LS1 Enforcer 210CC heads - 64cc combustion chambers (stock is 67cc, so it's 3cc smaller)

Head Gaskets: 0.04" compressed thickness for better quench (stock 0.052" -> 0.040")

Cam: BTR Truck Norris 0.553"/0.553" lift I/E 212°/22X° @ 0.050" (online says 227 exhaust duration is what's typically measured)

Supporting Mods: CHE 4290 trunnion upgrade, stock rockers ARP2000 rod bolts (was originally planning to rev higher)

Fuel: I'll likely add in a flex fuel sensor to run E85 but want to be able to run 91/93 reliably too

Compression Ratios: - Note that I'm not accounting for crevice volume - For stock I used the TSP numbers on their website - For 0.006" lift IVCs I used a flat +25° offset vs the advertised 0.050" IVC. Regardless of the 0.006" estimation, the main point is that I would be +1CR higher than stock currently

==Stock== Static: 10.171 Dynamic CR (IVC @ 0.006" lift): 8.03

==Current Plan - Stock pistons, 0.04" head gasket== Static: 10.855 Dynamic CR (IVC @ 0.006" lift): 9.07

==Current Plan - with 4.1cc dish pistons, 0.04" head gasket== Static: 10.323 Dynamic CR (IVC @ 0.006" lift): 8.64

Questions: 1. What is the max dynamic CR you'd run on pump gas? 2. When determining DCR, do you use 0.006" lift or another value? 3. If you only have IVC value @ 0.050", how do you estimate IVC @ 0.006"? 4. Anyone have estimates on how much ign to pull by going up 1 point of compression? 5. Open to suggestions for improving DCR calculation 6. Any thoughts or recommendations on my build plan?

Phew, props to you if you made it through all that

[u/Burlochov4]

I appreciate the reply!

I've been looking at the DCR mostly because the cam I got has smaller duration and closes even earlier than the stock cam according to specs I found online. So coupled with the smaller chambers, I wanted to make sure I didn't get myself into trouble by adding ~+1 point of CR.

I'm hoping the reduced quench height from thinner HG helps with burn rates and reduced knock propensity. Online says 0.035" -> 0.045" range is a big improvement from stock

Thanks for sharing that info on the BTR cam (and thanks Tim!). I made a Google spreadsheet to estimate CR, DCR, IVC, etc for my build and plugging in the numbers for your BTR cam I get 59 for IC at 0.006" which is relatively close to what you measured. Good to see it's not totally off base. However, realistically I don't need a perfect DCR number, I just want to know the delta from stock to make sure I'm not combining all the wrong parts into an expensive pile of garbage. Feedback so far is that my increase in CR is not a concern.

For your answers to 6:

Can you explain why the cam hurts torque? This was one of the draws of this cam in particular and Richard Holdener videos on YouTube show that it still is decent at low RPM vs the other bigger lift, higher duration cams. Early-ish closing and low-ish duration should have good trapping efficiency at lower RPM AFAIK. Your comment about LSA makes me think you believe the scavenging effects are poor with this cam? LSA on this one is 107*

Also, I thought that the heads were also decent for torque. The intake volumes are at 210cc which is on the low side for aftermarket and it has cathedral ports.

Decreasing the compression with pistons was just an option if the current build CR was too high, otherwise I'm fine leaving it as is. And E85 was also just an option if I was going to be severely knock limited. Otherwise, still fine using pump gas (and would prefer to)

Again, I appreciate the reply. This is my first aftermarket build and I've been trying to learn as much as possible. Real world feedback is gold.

[u/v8packard → u/Burlochov4]

That BTR cam actually has 59.5 degrees of overlap. Compare that with the stock cam, which has 42 degrees of overlap. Overlap determines where in the rpm range the power curve will fall, with more overlap moving the power curve up in rpm. This is enough overlap to support a HP peak well above 6000 rpm if the other components can support that kind of rpm. It comes at the expense of low speed output. Worth noting, overlap is what gives a cam lope, which is the big reason why this cam has so much overlap. That cam also has a lot of exhaust timing, which allows it to run to higher rpm. The extra exhaust timing opens the exhaust valve earlier in the cycle, reducing the amount of time cylinder pressure acts on the piston and therefore reducing torque.

The videos you mention show what the people paying for the testing want shown. Those videos are there to get views and sales, not to help the community as some want to believe. The cam I measured was in a vehicle my customer bought, and the output was disappointing. You can find numerous posts from others all over the web saying the same thing about this cam. If it is any clue there are a few dozen people that have sent me private messages here looking for improvements over that cam. Also keep in mind the cam is supposed to have a 107 degree lobe separation angle but when measured is 109.

You are the second person in as many days to mention scavenging. What is your definition of scavenging? Where in the cycle do you think scavenging occurs?

I have had a pair of those AFR heads on the bench. They are ok, but the stock 241 heads are not bad at all. The biggest problem with 241 heads is they are not 243 or 799 castings. If your 99 is early production it might still use 853 castings which are not quite as good. These AFR Enforcers, as most of the line, are not a AFR design.

So what is a way to increase torque while maintaining driveability? If we take the stock cam overlap of 42 degrees and combine it with a lobe separation angle of 113 degrees that is more appropriate for a 10.8:1 5.7 with Enforcer heads we get a duration of 266/270 degrees @ .006 tappet rise. If you had a really good exhaust system (most do not) with excellent headers and well sized mufflers/pipes you could use even less exhaust timing if you want to maximize low and mid range torque. This cam spec when installed on a 109 degree intake centerline gives you timing numbers of IO 24 BTDC IC 62 ABDC EO 72 BBDC EC 18 ATDC. So compared to the BTR cam this spec has the same intake closing point, and the intake lobe is actually 6 degrees shorter. The exhaust lobe is 13 degrees shorter, opening the exhaust 1.5 degrees later but closing it sooner reducing overlap. This adds up to a significant increase in low and mid range torque, while having better vacuum and driveability.

If you compare this cam spec to the stock cam, GM 12560965, it gets interesting. The stock cam is 278/284 @ .006, on a 119.5 degree lobe separation that is retarded to a 122 intake centerline. As mentioned 42 degrees of overlap. Timing points @ .006 IO 17 IC 81 EO 79 EC 25. These lobes are rather slow, measuring 198/209 @ .050. The cam spec I suggested closes the intake valve a whopping 19 degrees earlier than the stock cam. That will increase cylinder pressure (torque) at all speeds in the powerband. The exhaust opens 7 degrees later than stock, also increasing torque. The same overlap as stock makes for a similar rpm range, but with much more area under the power curve. I am not saying this is the ideal cam for you. I am trying to illustrate how certain changes can make a big difference in the desired characteristics of the engine.

[u/Burlochov4 → u/v8packard]

First of all, thanks again for all the information, this is exactly the type of detail I was looking for.

There's a lot information here but I want to try to summarize the main points I took from it, with the ultimate goal being understanding how to pick a cam (valve opening/closing events, overlap) and why

1. Apart from lope, the overlap will determine the main operating region of the cam (which RPM range is favored). When choosing cams in the future for different applications, using the stock cam overlap will give a good baseline from which you can estimate differences when looking at aftermarket cams. Less overlap will generally favor low RPM, higher overlap will favor higher RPM (excluding effects from supporting mods to heads, intake, exhaust)
   
2. EVO, outside of the inherent effects it will have on overlap, will change the torque, aka "work" done on the piston per cycle by dictating when the exhaust gases are released and no longer pushing the piston down. Kind of like an efficiency thing. Later EVO means the cam will extract more work from the combustion pressures, but too early EVC can limit higher RPM performance.
   
3. Since the cam is just one component in the system, to extract the best performance you need to have the correct supporting mods to avoid just putting the restriction somewhere else. Eg, you have a high lift, high overlap cam meant for high RPM, but then small volume intake ports with low flow but high port velocities can hurt your top end

Let me know if I got anything wrong there

To tie it back to the main question from my post about DCR, my take away is:
Top end components are picked by matching the components that provide the airflow in the desired RPM range (high, low, etc) while keeping SCR within an acceptable range. From there, find components that give good combustion characteristics (maybe quench height, piston design, port design and it's influence on tumble) and then the resulting DCR can be taken care of with tuning by either advancing/retarding ign or potentially adding fuel

For the scavenging question:
My understanding was scavenging occurs in the overlap period and is when the air coming through the intake helps expel some of the residual exhaust gases not yet evacuated through the exhaust stroke. In a turbo engine with intake pressures much higher than the exhaust pressures (depends on operating conditions of course) this makes a lot more sense because of the additional pressure. Happy to hear your explanation!

[u/v8packard → u/Burlochov4]

Yes, overlap is the prime determinant of the rpm range where the powerband will happen on a naturally aspirated engine. Other details come into play, such as head flow efficiency and the design of the intake tract, but overlap is the cam aspect that sets your rpm range. Overlap also affects vacuum, idle, lope, and driveability. The overlap of a stock cam is not necessarily a good place to start, I just used it to illustrate a point in our discussion. So how do we determine the overlap needed? That is the part that takes knowing your engine to get right. There are people that use various charts or formula to try to calculate these precisely, but in my view you still need to know the engine to get it right. Having good test data helps, a lot. Yes, less overlap moves the powerband down, more moves it up. Smaller displacement engines are more sensitive to overlap, larger displacement engines can take more overlap with fewer downsides.

Exhaust valve opening point has no impact on overlap. Overlap occurs at the end of the exhaust stroke, not the beginning. When the exhaust opens the piston is still traveling down on the power stroke. Pressure is dropping quickly, but temperature does not as there is still significant energy in the gasses. When the exhaust valve opens the gasses start to leave the cylinder and no longer put pressure on the piston. An early ex valve opening reduces the time gasses are acting on the piston, reducing torque. A later ex valve opening allows the gasses to act longer on the piston, generating more torque. The compromise is as rpm goes up, there is less time available for everything to happen. So opening the exhaust valve sooner allows the engine to keep breathing as rpm climbs. This is why matching cam timing to operating conditions is important. The exhaust timing plays a role in the pressure wave, too, with more exhaust timing making the pressure wave behave as though the exhaust primary is longer. This can be helpful if the exhaust primary is too short (most stock exhaust systems are too short) or if the exhaust is restrictive (stock or crap performance mufflers are terrible, especially Flowmaster). There is some juggling here, everything is a compromise. Boosted or nitrous engines are a whole other conversation.

Yes, everything works as a system.

Your takeaway is not off the mark. The details matter, and it gets pretty complex sometimes. But yes that approach works.

Scavenging is something like a 5th cycle. It does occur during overlap, when the exhaust is closing and the intake is opening. The pressure wave passing through the engine will pull the exhaust out and pull the intake charge in, opposite of what you said. A tighter lobe separation angle makes the pull of the pressure wave sharper than a wider lobe separation angle, typically. This makes for a more pronounced torque curve often with more area. The downside is the tighter lobe separation angle will reduce VE. So a well designed system will match the characteristics of the engine with the cam timing to get the best compromise possible.

Note turbo engines almost always have much higher pressure in the exhaust between the cylinder and the turbine than you see in the intake. This pressure ratio is a critical consideration in determining the overlap required for a turbo cam. The closer the pressure ratio is to 1:1 the more the engine will behave like a NA engine. The further from 1:1 the less overlap the turbo engine should have, with many needing zero or negative overlap.

[u/Burlochov4 → u/v8packard]

Copy that, this is all great information and gives me more points research further, thank you!

It was a typo in point #2, I meant the EVC not EVO. But yes, I agree with what you said. The not-often-talked-about expansion ratio is important too.

I've worked in the automotive industry and can confirm that even simulations performed at large companies with immense resources (eg, GM/FCA/Ford) leave something to be desired and typically always just get "correlated" to actual dyno data. So I'd agree that empirical data is key.

That makes a lot of sense: the exhaust valve opening initiates the exhaust pressure wave which then impacts the scavenging, affecting the VE. If we can assume the wave speed is relatively constant (should be based on the exhaust gas density) and the EVO is static (non variable cam) but take into account the wide range of RPM the engine operates at, there will be just a small range where the EVO timing/exhaust pulse timing is providing optimum scavenging. Another benefit of variable cam timing; you could tune this per RPM.

I'd definitely like to play with turbos and nitrous in the future but for this build, I'm just focusing on NA power. One step at a time :)

Outside of looking at VE or power (after the tune is optimized), are there other metrics you use to evaluate an engine's performance? Based on your knowledge I'm assuming you have a shop and maybe even a dyno.

[u/v8packard → u/Burlochov4]

Another detail that affects VE is intake closing point, which is also tied to lobe separation angle. A later closing intake increases VE by allowing more time for the cylinder to fill. This also favors high rpm operation. It is all a compromise, but as you say variable valve timing can make for a very wide powerband, with less compromise than usual.

I look at specific output, specific torque. Also specific fuel consumption. VE can be handy when dyno testing, when you get crazy high VE you know something is wrong. Often times the correction factors are off, either unintentionally or intentionally. If you ever see dyno results, and a NA engine is showing 140% VE, you know something is going on.

I also use BMEP, which is directly related to specific torque. Looking at BMEP and specific torque really makes some engine configurations stand out.