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Discussion Starter · #1 ·
I have been working a theory that there are big gains ahcievable by raising the shift points on the automatic transmission on the 03 and 04 SSRs.

First, notice that all the microtuners (Predator, Superchips, and Crane) change the shift point from the stock 5600 rpm to about 6200.

Secondly, my computer modeling led me to the same conclusion, although at least on MY 04 SSR, you get about 99% of the benefit available by going to 6000 rpm, which is a bit easier on the engine's valvetrain and con rods.

The reason I suspected this is that the transmission ratios on our 03s and 04s are so widely spaced that the engine literally falls off the power curve after the shift from 1st to 2nd, because the rpm drops down too far.

It is NOT that our LM4 "truck" engines make big power at 6000. They do NOT. They peak around 5000 to 5500 and fall rather rapidly after that. However, even after falling rapidly by the time they hit 6000, they are STILL making more power at 6000 rpm than they will at 3000 rpm in 2nd, which is where they land after the 1st to 2nd shift!!

By raising the shift point to 6000, you actually get a double benefit:

1. You have a LOT more power available at 6000 in 1st than at 3000 in 2nd

2. Your drop to 3200 versus 3000 after the shift, which helps a tiny bit more

Now you guys know by now that I don't just make statements like this. I actually TRY it.

I had actually asked that when my SSR was dyno tuned, right after my gearing change, that the technician would raise the shift points right then. He evidently misunderstood me or just forgot. I found out that he had forgotten only after I started doing instrumented acceleration runs with my G-Tech RR. The G-Tech clearly showed me that the transmission was shifting up at 5600 rpm.

So, purely in the interests of science (see my separate posting on the "General discussion" board on "I THOUGHT I was alone"), I did some actual instrumented teting today, where I compared the results of shifting at 5600 to those of shifting at 6000. To do the 6000 rpm shifts, I had to shift the automatic manually, so if anything I expected my results to be a little rough, as no human can shift as well as the transmission itself.

But, the results verified that you drop your 0 to 60 time and your 1/4 mile time by about 1/4 second shifting at 6000 versus 5600.

So, next week I get the shift points adjusted to 6000.

With the 6000 rpm shift points, my SSR, with only the 4.56 axle ratio and dyno tuning using HP Tuner, now consistently runs:

0 to 60 mph: 6.5 seconds (a little over a second faster than a stock SSR, even with my 235 lb AND 15 gallons of fuel aboard (YOu KNOW they did not test the factory SSRs that way!).

1/4 mile in 15.067 seconds.

My modeling program says that with a 160lb driver and 2 gallons of fuel (THAT is how cars are tested by magazines and factories), the results would be about 0.2 seconds better for both numbers, or in other words, I guess I now have a "14 second vehicle" (14.86 or thereabouts with 160 lb driver and 2 gallons fuel).

The factory stock numbers are 7.6 seconds and 15.9 seconds, so I've gotten 1.3 seconds out of the 0 to 60 time, and about 1 second out of the 1/4 mile time.

Not bad for no headers, no exhaust, no air intake system, and not even getting inside the engine yet.

These DO have potential, despite their "industrial weight".

Jim G
 
G

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Thanks Jim. I STILL don't have my gears in yet...sigh

FWIW - We have 1 ton diesel dually's running all over town that do 6 second 0-60 with only programmer, intake and exhaust mods. They leave 4 big black stripes at launch - yikes! I'm looking forward to being able to give them a go...

- psy
 

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Discussion Starter · #3 ·
psychoslaphead: Diesels are almost ashtonishingly easy to soup up, because of the way they make their power, BUT their lifetime, and that of the Allison transmissions behind them, gets exponentially shorter when you do that.

The diesel engine manufacturers know how to make way more power with their engines, but they recognize that we want them to last for a reasonable period of time. The thermal and mechanical loads in a diesel are VERY high compared to that of a gasoline engine. That's why those Oldsmobile diesels that GM briefly built using gasoline engine components self-destructed.

The Allison automatic used in those duallies is also the weakest transmission that Allison makes (their better transmissions are MUCH more expensive and MUCH heavier). It will not handle the extra power any better than the diesel engine will.

Those guys who have done those mods are going to find that their overall cost of ownership per mile and per horsepower is VERY high.

Jim G
 
G

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They also smoke like a coal stack when you turn up the hp setting...not my thing. But they sure are fun to test drive when we get done with 'em. So are Ford Lightnings :)

Lately we've been working over a lot of 05 Mustang GT's (we have one around our shop every other day) - 300+ hp in a fairly lightweight vehicle = big grins for $25k

 
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