_Wayne_

On my set up, with my current heads, I am looking at around 11:1 static CR. With my cam, the Dynamic CR is around 9.2. I want to run <20psi and I plan on running e85 most of the time but on the off chance I get caught and need to run 93 I don't want to have issues. Do you guys see an issue with this? I don't want to change pistons (-2.7cc) I could change heads and drop the SCR to ~10.5:1 maybe play around with some different gasket thickness. Any ideas are appreciated, Thanks!

C5 Pete

With E85, I don't see an issue with your current CR. Obviously with a good tune.

If you know there will be occasions that you'll need to run pump gas, it would probably be best to have a different tune for that.

We have an LSA block with TFS220 heads at 11:1 on E85 pushing 25+psi with no issues. Again, (from the "no ****" file) it has a rock solid tune.

Best of success to you!

Ant-Man

I'm aiming for 11.5-12:1 going to run flex fuel, obviously if I have 93 I'm not going to push it too hard but I also have meth for that. 20+psi and conservative timing.

old motorhead

9.2:1 is a low DCR?

_Wayne_

I guess low in comparison to the static, I know ideally it'd be lower.

Yeah I think I can get the tune pretty dead on. I'm currently running a flex fuel sensor, Should I just eliminate that and tune on straight e85? or can I keep it and just not push it on 93. I do have a meth kit I could install as a safety net as well.

Detoxx03

Install the meth kit but either way when on 93 take it easy. I don’t but mine isn’t as high as yours.

_Wayne_

Okay sounds like that's the plan!

old motorhead

I'm just curious what kind of cam specs you have that would contribute to a DCR that high. That sounds just short of outrageous to me.

old motorhead

I checked DCR's on three of Cam Motion's "shelf" centri blower cams. On a stock LS3, the small one would have a DCR of 8.35, medium one 8.15, and the large one 7.7:1.

You absolutely can lower DCR with cam choice. You need to find out actually where you are now DCR wise before going further. You may be just fine. Your calcs must be off somewhere to come up with a 9.2:1 DCR.

Example: Raise the static CR in a stock LS3 to 11:1 and use Cam Motion's 224/240 116+5 centri cam. The DCR goes from 8.15 at 10.7 SCR to 8.38 at 11 SCR. Grind the same cam with 3* less advance and you're back down to 8.15 DCR even with the 11:1 SCR. You can widen the LSA and have the same effect. I'd get a pro to spec you a different cam long before changing pistons or fooling with head gaskets.

TurboLX

I love how people talk about DCR on here like it's science. I have been in exactly ZERO engine design meetings at the OEM level where DCR was even mentioned.

The trick is, you're just shifting when and how the cylinder fills and subsequently compresses the charge. This changes with RPM, Baro, temp, and a few other factors. Making blanket statements about it based on cam events vs clearance volume for a given displacement is an oversimplification and misleading.

Pekka_Perkeles

Thank you, once again.

I doubt SAE has anything at all about this..

https://en.wikipedia.org/wiki/SAE_International

old motorhead

If nothing about static compression ratio and valve timing was discussed at your meetings, that means you weren't at the meeting where they were discussed. Probably that stuff left up to those low level "grunt" engineers. Probably the same guys that fool with variable valve timing and such.

schpenxel

Maybe only the German OEM's talk about that kind of thing?

TurboLX

Weird, I would think it might come up in the meeting with the engine's chief engineer, the base engine testing/calibration team, or perhaps the driveability and emissions guys, all of whom were in the room with me...
I'm pretty sure variablische nockenwellen didn't change the discussion, but my German is a little rusty in the time since I used to also work for a German ECU supplier.

old motorhead

It must be those sneaky night shift guys deciding on all of that. Since every different LS and LT engine that I'm aware of has different camshafts (valve timing) and static compression ratios.....I wonder who made those decisions on where the intake valve was supposed to close on the compression stroke. And on the LT motors with variable valve timing, who jumped out there and decided when to advance valve timing and when to retard it. All of that, static compression ratio, and connecting rod angularity contribute to dynamic compression ratio. Obviously, all of that appeared out of thin air without any thought and discussion by the GM guys.

TurboLX

You are confusing "dynamic compression ratio" (the voodoo mumbo jumbo BS here) with cylinder filling efficiency. We spend all kinds of time fine tuning cylinder fill and charge motion to improve combustion quality and efficiency. But nowhere in that discussion are we talking about how it "changes the dynamic compression ratio". (Unless you're talking an intentional Miller/Atkinson cycle, typically on a hybrid) We use variable valve events to get take advantage of port velocity and charge flow to better fill the cylinders or create more turbulence in the cylinder (which, in turn affects burn time and required spark advance), but I still play that same game in the performance aftermarket world too with non-stock cam/heads/engines.

My point being: at an engineering level, we don't design an engine with an excessively high static CR and then say "it'll be OK because the valve events make it dynamically lower". Because at some speed/load/boost condition, we would just be ramming that air into the cylinder anyway and the static CR is still there. Worse yet, the "bleedoff" of cylinder filling that gets pushed back up into the intake with a late IVC event mostly happens at low RPM, where we are less knock prone than up near peak torque RPM. At the elevated RPM and high load, the incoming aircharge has lots of velocity and inertia that help it pack the cylinder more full and all that air/fuel charge still gets subject to the same static CR and knock potential for the given conditions (temps, AFR, charge motion, fuel quality, etc...)

TL;DR: pick a reasonable static CR for the engine and fuel spec and just calibrate your airflow and spark values accordingly to optimize what you have. Don't worry about "DCR".

schpenxel

I'm just busting your *****--I don't doubt your credentials or abilities.

old motorhead

If you're only dealing with GM camshafts, then no, you probably don't have to worry about DCR. There's a whole wild crazy world of aftermarket camshafts out there that can make things way more difficult. If there is a cam that can create a 9.2:1 DCR in a 11:1 SCR supercharged engine, the greatest tuner in the world (maybe you) with access to race gas and meth is going to have a really hard time with a force inducted engine. A cam swap resulting in a DCR at 8 or less, with 93 octane and meth would make things way more livable on the same engine. Lots of those running around these pages.

TurboLX

sigh... I don't think we are ever going to be able to agree here. I am no stranger to calibrating cars with big cams and boost. Wait until you see our GENV tuning video where we used Howard's 1300hp Procharged LT4. I can assure you that's nowhere near a stock GM cam profile, but yet cylinder filling in the midrange is still crazy high regardless of "DCR" and we must still respect the knock limit there accordingly. What you are preaching is simply not the science we use to design and control engines and I'm not sure how to relate that here for you. Time to move on to something else for me, I guess...

@schpenxel no worries, you'll have to try harder to offend me.

Pekka_Perkeles

Pre-ordered. Just kidding, but I will purchase it, no doubt.

I always thougt DCR is based on, well, something. As an engineer, I have doubts now.