The rumor is that LT1's can and do run higher compression ratios than other SBC's past and present. And it can do it because of reverse cooling.

My question is in regards to running high SCR on pump gas. Trying to collect data here.

I've seen a few guys here and there say that they are running 13:1 or higher, but never do they say that they are also on pump gas (although if they are E85 it does say that).

So I'm starting this thread to see what SCR and DCR everyone is running without detonation issues on pump gas. Can you provide displacement, SCR, DCR, and octane you are running? Thanks in advance!

I'll start.

355 LT1
11.8:1 SCR, 9.32:1 DCR, 91 Octane.

 

350 LT1 11.5 scr 91 octane

 

355 lt4
11:1 93 dcr

 

355ci 11.7:1 SCR

 

350 LT1
12.0 SCR, 9.6 DCR, 93 octane
I mix half 110 octane race gas when roadracing (extended high power running at higher coolant temp)

 

I don't make the list

383ci 9.5 SCR, no clue on DCR, and 93 octane.

 

383 cubic inches, 12.41 to 1 SCR, 91 octane...but at 5280' feet above sea-level. Higher altitude reduces octane requirements.

 

How'd you manage to get the SCR so low on a 383? Blower pistons or bigger head CC?

 

What cam are you running? If you post your IVC event and the rod length I can figure out DCR.

 

12:1 on 92 here 355.

 

Lloyd Elliott's old car was a hair over 13:1. Last build I did was a 355 with 12.4:1 on 93 and ran great

 

Man-crush guy is a F/I ****.

 

Static compression is almost a useless number. Its only real use is to assist the cam designer. Lt1's do tend to easily run 1 full point more dynamic compression. More than that and it seems to get into a more case by case basis. I remember Mike who has the 9 second LT1 AI build saying his dynamic is in the 9.5:1 area and it detonates without race fuel when the engine is under load. On the other hand Pat is running 9.6:1 and only needs 110 when road racing. Volumetric efficiency also comes into play here where DC is calculated assuming 100% VE - Most motors won't see 100% VE....

I saw one member claim over 10:1 dynamic and people believed him without question. It ended up he was using @.050 numbers to calculate and never actually degreed the cam when installed......

To much talk about these calculations in my opinion. They aren't going to make an engine any better if you have a knowledgable person/people assisting with the details of your motor.

 

-27cc pistons, thick *** HG, and a stock 54cc head.

 

396 ci, 12.3:1 SCR, DCR unknown, 93 octane

 

Not sure of my exact compression. 56cc chambers, stock untouched shortblock, .026 gasket. I figure it's somewhere around 11:1

 

350ci 11:1 with 93 octane

 

I understand the theories, and I understand the debate among them. What I'm asking is who's actually doing what? 13:1 SCR is often repeated, so it seems common. But, it's looking like it's not common at all....

SCR, DCR, and VE all play a factor. A bigger cam will lower low rpm torque due to the later IVC event. But, raising the SCR will raise cylinder pressures in that rpm range negating, and sometimes even exceeding, the torque loss.

However, once the engine gets into the meat of the cams power band VE plays a huge role in cylinder pressures because the ram effect is in full force. One thing to note here is that SCR and DCR do not change. But cylinder pressures do.

Saying that SCR or DCR isn't important is wrong IMO. If it was wrong, we wouldn't be so focused on them. They are used so widely because they represent a multitude of different combined specs and give us a quick interpretation of what should be expected or adjusted in the engine.

Having said that, when it comes to power and detonation, I've learned the more important stuff is pressure, temperature, hot spots, and quench.

Pressure has a direct effect on temperature and vis a versa (SP??), lower the SCR or DCR and you lower the temperature. Pressure equals power, so we want high pressure and low temps, because ultimately it's not the pressure that pre-ignites the fuel, it's the temperature. Quench cools hot spots, relocates the fuel/air to the combustion chamber near the spark plug for a better flame front and spark travel, and mixes it for a better faster more complete burn.

Obviously we don't want to go too cool on the intake charge because we want the fuel as close to spontaneous ignition as possible right before spark. This is because the fuel will burn more completely from the spark while still in the cylinder, so thermal efficiency is raised.

The other thing that seems to be ignored is the expansion ratio (or ER), which is the opposite of CR. So an SCR/SER of 12:1 which leads to a DCR of 9:1 can have an DER of 7:1 or 9:1 depending on the EVO event of the cam. And power comes from the expansion stroke and the actual cylinder pressures involved, which are created by the DCR and intake charge velocities, which is effected by the SCR, which effects temperature, which effects efficiency and pre-ignition/detonation......

 

I have a 385, 13.30 static/10.68 dynamic (using https://www.uempistons.com), custom LE cam, SRP flat top pistons, EB Ported heads ~50cc, E85.