[MeanGreen1LE]

I’m in Oklahoma and pretty much all I can get is 91 octane and no e85 even remotely close.

The heads I have coming have slightly larger chambers so I’m down a smidge from factory. I was thinking about going .040 or maybe even the .028 to raise it a little. But again I know I’m limited with 91 just don’t know how much with being DI. So what’s ideal with 91 pump for LT1?

[6spdhyperblue]

You could roll this into your cam thread because it’s based on your cam timing

[MeanGreen1LE]

I’ll admit to being a novice here but doesn’t cam timing affect dynamic cr? Which I believe you can play with that to make up for your static ratio? But where should I start with my static compression ratio for best results?

[95 imp]

What 6spdhyperblue said....

Do you plan to run your motor?

Then static ratio is kinda meaningless if you're trying to figure out true cr so you have an *idea* of what octane fuel you'll need.

If you plan to just crank the motor over by hand and never run it, go with static cr as your base.

[laynlo15]

Did you talk about this with the company your getting the heads from? I wouldn't exceed the 11.5 ratio that the stock Lt1 comes with. GM builds the engines to work down to 87 octane but no one should ever run that in a 11.5 CR engine. I'd stay around the compression ratio.

[MeanGreen1LE]

Originally Posted by laynlo15

Did you talk about this with the company your getting the heads from? I wouldn't exceed the 11.5 ratio that the stock Lt1 comes with. GM builds the engines to work down to 87 octane but no one should ever run that in a 11.5 CR engine. I'd stay around the compression ratio.

I’m trying to find a chance to call their tech department. These heads with 59.8 cc chambers will be down on static CR which basically limits your starting point as far as dynamic or effective CR goes. I’ve had some trouble making static compression calculators work with stock lt1 specs but worked backwards to get the numbers right,[anyone know what’s up with that? Tsp even has one that pre populates for LT1 and it’s not right compared to commonly accepted 11.5]. Anyhow best I can tell that change in chamber brings it down to about 11.3:1 scr. Going with gaskets at .040 thick and 4.1” bore raise it up to about 11.8

I also found a formula online to estimate dcr utilizing your scr and cam specs. https://uempistons.com/p-28-effectiv...alculator.html
Factory LT1 including cam shows dcr of right around 10:1
Using the .040 gaskets and the GPI ss3 cam it shows dcr of around 9.8:1. Being that is 99% of factor spec it should probably be a decent mid range performer.
Like you mentioned factory allows using 87 without damage and I could probably go with .028 for higher dcr with same cam or even more overlap on more aggressive came while still having good pressure. However so far GPI has said I’d have to fly cut.
I plan to see if the thicker deck on these heads allow for anymore leeway for valve clearance.

[95 imp]

^^ This sounds good to me. If you're around 99% of original DCR you should be fine.

At the end of the day, the knock sensors are going to tell you what octane this particular motor wants to be fed.

[cmitchell17]

I thought we have no way of calculating dynamic compression because we don't know true cam timing? We don't know true seat to seat timing on the stock cam and aftermarket cams don't release this info. So unless anyone has taken the time to degree several examples in their LT1's how do we know? I have heard dynamic compression has to be taken from seat to seat timing and not from timing at .050.

[MeanGreen1LE]

Originally Posted by cmitchell17

I thought we have no way of calculating dynamic compression because we don't know true cam timing? We don't know true seat to seat timing on the stock cam and aftermarket cams don't release this info. So unless anyone has taken the time to degree several examples in their LT1's how do we know? I have heard dynamic compression has to be taken from seat to seat timing and not from timing at .050.

I think it’s definitely an estimation for sure. I didn’t bother with vvt and just used with 0 advance with earlier calculation.
However I’ve read that vvt goes from as advanced as Much as 8.5 at idle and as high as 22.5 retard somewhere higher rpm. (Anyone know these for sure? I had trouble finding consistent numbers). But with this dcr goes as high as 10.4 to as low as 8.4:1

[cmitchell17]

Originally Posted by MeanGreen1LE

I think it’s definitely an estimation for sure. I didn’t bother with vvt and just used with 0 advance with earlier calculation.
However I’ve read that vvt goes from as advanced as Much as 8.5 at idle and as high as 22.5 retard somewhere higher rpm. (Anyone know these for sure? I had trouble finding consistent numbers). But with this dcr goes as high as 10.4 to as low as 8.4:1

So you are saying DCR is in the 8.4 to 10.4 range stock when you account for VVT?

I used DynoSim to estimate the cam profile and therefore estimate seat to seat timing for the stock cam and my new TSP Stage 1/2 cam. According to what it says the stock cam intake valve closing is 46.2 degrees after bottom dead center, which yields 10.31 DCR. DynoSim estimates 61.4 degrees ABDC, which yields 9.98 DCR. I also have TSP ported and milled heads (.030) which I calculate my new compression ratio to be 12.25:1, but even with this it looks like I have lost about .3 points in dynamic compression even with milling.

I am not sure how accurate DynoSim's estimate of the seat to seat cam timing is. I believe it just uses your normal advertised @ .050 duration, LSA, and lift valves and tries to curve fit it. There is a setting to change the unitless "cam acceleration ramp rate" but it appears you can estimate seat to seat without having to select a cam acceleration ramp rate value.

[MeanGreen1LE]

Originally Posted by cmitchell17

So you are saying DCR is in the 8.4 to 10.4 range stock when you account for VVT?

I used DynoSim to estimate the cam profile and therefore estimate seat to seat timing for the stock cam and my new TSP Stage 1/2 cam. According to what it says the stock cam intake valve closing is 46.2 degrees after bottom dead center, which yields 10.31 DCR. DynoSim estimates 61.4 degrees ABDC, which yields 9.98 DCR. I also have TSP ported and milled heads (.030) which I calculate my new compression ratio to be 12.25:1, but even with this it looks like I have lost about .3 points in dynamic compression even with milling.

I am not sure how accurate DynoSim's estimate of the seat to seat cam timing is. I believe it just uses your normal advertised @ .050 duration, LSA, and lift valves and tries to curve fit it. There is a setting to change the unitless "cam acceleration ramp rate" but it appears you can estimate seat to seat without having to select a cam acceleration ramp rate value.

Yeah that’s what I’m saying but please don’t bet the farm on anything I say lol. I’m just trying to figure this out enough to make as much power as I can without creating a detonation machine or a valve slapper.

I failed to link this earlier that shows the formula.

http://angryengineer.blogspot.com/20...ion-ratio.html

I assume you used LT1 cam specs without any advance or retard?

[cmitchell17]

Originally Posted by MeanGreen1LE

Yeah that’s what I’m saying but please don’t bet the farm on anything I say lol. I’m just trying to figure this out enough to make as much power as I can without creating a detonation machine or a valve slapper.

I failed to link this earlier that shows the formula.

http://angryengineer.blogspot.com/20...ion-ratio.html

I assume you used LT1 cam specs without any advance or retard?

Yes I used the specs here that don't say anything about advance or retard:
https://www.onallcylinders.com/2018/...am-specs-more/

But thats the thing, does the stock cam have a built in advance or retard? or is it accounted for in those specs? I know the stock cam sits in a full advanced? or retarded? position and the cam phaser moves it only one way. I've never understood this, and I wonder if this is the reason I can't get the peak DynoSim numbers to align with the stock advertised HP and torque numbers for the LT1.

[MeanGreen1LE]

Originally Posted by cmitchell17

Yes I used the specs here that don't say anything about advance or retard:
https://www.onallcylinders.com/2018/...am-specs-more/

But thats the thing, does the stock cam have a built in advance or retard? or is it accounted for in those specs? I know the stock cam sits in a full advanced? or retarded? position and the cam phaser moves it only one way. I've never understood this, and I wonder if this is the reason I can't get the peak DynoSim numbers to align with the stock advertised HP and torque numbers for the LT1.

I believe it sits at 7 or one place I read 8.5 advance and only retards when it moves. I remember lots of excitement about tun-ability early on with this engine but a couple years in it was discovered that the phaser is unstable at higher rpms and now it seems most tune it out completely (And usually Do lock out kit) when changing cams even for cams originally ground to take advantage of them. Comp is the only one I’ve seen that makes one still and don’t know anything on them. Maybe gm hotcam uses vvt?

[cmitchell17]

Originally Posted by MeanGreen1LE

I believe it sits at 7 or one place I read 8.5 advance and only retards when it moves. I remember lots of excitement about tun-ability early on with this engine but a couple years in it was discovered that the phaser is unstable at higher rpms and now it seems most tune it out completely (And usually Do lock out kit) when changing cams even for cams originally ground to take advantage of them. Comp is the only one I’ve seen that makes one still and don’t know anything on them. Maybe gm hotcam uses vvt?

Yes the GMPP hotcam still uses VVT, and on the LT535 crate motor that has the hot cam in it, I believe the tune in there still has the stock WOT VVT timing and eliminates the high advance at part throttle (mainly for emission and egr effect).

I would be really interested in trying to use my VVT again with my 228/236-114 cam to see if I could improve idle combustion stability, and maybe get some more bottom end. But I have the zero degree lockout.