I am putting together a '96 SMPI 5.2 and I am wondering how much compression can be run on regular or premium gas. On 340s we could run 10.3:1 with premium without detonation-even with a lot of advance timing. Surely some of you Dodge Boys have tried to stretch the compression on these Magnum engines.
The stock compression was 9:1 using regular gas. However, I am using 62cc EQ heads which should raise compression to 9:4-9:6. So how much extra compression can the ECU compensate for?
Will the port injection make the air/fuel/cylinder temps noticeably cooler to aid reducing the detonation?
I will be using a cam with about 210* duration and .515 lift (no razzing about the small cam please. This is a work truck). Which LSA will help best to minimize detonation to keep the hyper pistons going in it from cracking up?
Air will flow well with a hi-performance intake and long tube headers so the engine will finally be able to breath. Should I go with one or two stages cooler spark plugs on these engines?

 

Welcome to DF!

You are going to need a custom tune to go with all those changes.... What is your truck used for??

 

Looking at your build info, why are you even worried?

 

Ya, finding a tuner in Central Florida that actually knows how to tune, not just program, is going to be a challenge.
I have a 3.9 in it now. I have the '96 engine in a van and it isn't fast enough for me, and it needs rebuilt. Because I need to be able to tow with it, I have to keep the cam smaller than I want. Still, I'll get 330-350 hp out of it, and I am trying to reach 21 mpg.
Since I am changing the top end, I should be able to squeeze a little more out of it. I just need to find out what is the limit for CR
Spending $600 for good intake pisses me off--even though I am use to paying more for parts being a Dodge Boy.

 

I know I can get the 9:1 CR, but how high can I go on regular gas? I have never seen or heard of anyone running 9.5:1 on regular and the engine holding together for over 100K miles--without running aluminum heads.

 

It's all about the entire system design. Small quench area reduces tendency for spark knock, and you can push compression/timing pretty good. It is truly unfortunate that Dodge, in their finite wisdom, didn't decide to put a knock sensor on these engines, so the computer could run max timing, and only pull as necessary.... Instead of having to program for constant 'worst case scenario'.

if you keep quench area to a minimum, and are gentle with timing, (which will rob power) you could do 9.5:1 on regular gas....

 

9:1 is factory and the factory heads are 62cc. The EQs don't raise compression at all unless you shave them or use a thinner gasket or both.


Good buddy of mine and a guy who used to be on here (Merc225hp) runs 10.5:1 with a custom grind, 1.7s, shorties with 2.25 true duals xpiped, heads comparable to the EQs, #24 injectors and 2bbl M1 ALL controlled through a MoparPerformancePCM

 

The stock bottom end will take a lot of abuse even with Boost and Spray, it's all about the tune....But again, I don't see anything you are doing to cause any concern.

I know my Dakota is running Eddy Heads, but I run 91 pump gas on 11.5:1. As said...It's all about the combo and tune

 

FlyinRyan is in Florida so you have one of the best Magnum tuners in you backyard. I can't tell you yet what grade of fuel I'm going to have to use but I'll be running about 10.2:1 CR on my rig.

21MPG is not going to happen out of a magnum engine. You will see people all the time saying they get that but I really think their math is off.

 

So true. Speedos are typically not accurate straight from the factory then factor in tires and it's a whole new ball game. Very few people take the time to actually calculate, correctly, their mileage. Although, GPS units do help a lot these days.