Don Terrill’s Speed-Talk

Ericnova wrote: ↑Sat Apr 28, 2018 6:22 pm You do realize the swap of 3.42 gears is almost worthless....with a 28" tire diameter you are barely dropping 250RPM @ 70 mph if this is the early SS 454 truck with the TH-375 trans.

If you really want to lower cruise rpm and help fuel mileage you're going to have to make either a bigger gear change or go to an overdrive trans.

if it is the later version with the overdrive trans, then the 3.73 to 3.42 swap only results in a 175rpm drop @ 70 mph.

When people talk of changing gears I generally tell them to go at least .50 of a ratio at a jump, otherwise you are wasting your time unless you are just trying to fine tune drag strip finish line rpm....on the street those little jumps like 3.73 to 3.42 or 3.55, or 3.73 to 3.90 are almost unnoticeable.

Maybe the change is worthless in your vehicles, but when I changed from a 3.73 to a 4.10, the result was quite noticeable. Unfortunately the engine didn't care for the extra 600 rpm it had to turn in high gear.

rfoll wrote: ↑Sat Apr 28, 2018 8:12 pm

Ericnova wrote: ↑Sat Apr 28, 2018 6:22 pm You do realize the swap of 3.42 gears is almost worthless....with a 28" tire diameter you are barely dropping 250RPM @ 70 mph if this is the early SS 454 truck with the TH-375 trans.

If you really want to lower cruise rpm and help fuel mileage you're going to have to make either a bigger gear change or go to an overdrive trans.

if it is the later version with the overdrive trans, then the 3.73 to 3.42 swap only results in a 175rpm drop @ 70 mph.

When people talk of changing gears I generally tell them to go at least .50 of a ratio at a jump, otherwise you are wasting your time unless you are just trying to fine tune drag strip finish line rpm....on the street those little jumps like 3.73 to 3.42 or 3.55, or 3.73 to 3.90 are almost unnoticeable.

Maybe the change is worthless in your vehicles, but when I changed from a 3.73 to a 4.10, the result was quite noticeable. Unfortunately the engine didn't care for the extra 600 rpm it had to turn in high gear.

But that is finish line rpm, like I mentioned that is a different animal. To get a 600 rpm difference between 3.73 and 4.10 with a 28-29" tall tire requires 120+ MPH.

You're not turning 600 rpm difference with that small gear change at 70 mph cruise, unless you are running golf cart tires. That's the point I was trying to make to the original poster....3.73 to 3.42 isn't going to be worth it for cruise rpm difference nor gas mileage.

Is the truck all original? Is is in great condition? If yes to both of these questions, I’d leave it alone. That truck will be worth a mint down the road. I’d by another generic short box and have at her.

I purchased my 91 GMC Sierra ST back in 1998 with 45000 miles on it. I still have it to this day and I regret playing around with it. The Sierra ST’s, as far as I know, were only produced in 1990 and 91. They were only available in 3 different colours and all have the signature grey stripe on the bottom. My reg cab short box 91 came with the needle gauges( which I’ve been told is rare for 91), buckets, chrome wheels, fog lights, 3.73 rear gears, 275r-15’s, and painted grill and bumpers.

Just saying. You’re right the 454ss was a turd, but it will be a collectors item shortly.

It won't help the mileage, but that low CR cries out for a bolt-on blower. A friend of mine has a 454 SS he bought new; he actually helped develop the Whipple SC package on it and it's easy to change out if/when the time comes.

Get the CR up around 9 to 9.5 to 1 and add an ERSON 284/218 or 220, 111/111, .542" lift hy-cam, headers, the mentioned Q-Jet on say an old Weiand 8005 or Performer intake, headers and go.

The real key here is the low CR is killing it! Put 100 cc chamber heads and shim head gaskets and it should be right at the CR I mentioned earlier, but run the numbers.

Even an old pair of -206, (96.7 to 98 cc), true closed chamber stock heads will get you there on the cheap!! I have a pair just for this in my garage..

I also have the ERSON cam in my bedroom closet!!!

pdq67

leave the gears, put a 700r in
up the cr, minor headwork conservative cam & headers, dual exhaust
Should be a night and day difference without going too deep

Were those flat tappets or rollers?

Friend bought one new yrs ago that is all he did, was cheap & effective ran really well still had the TBI on it.

I’d put closed chamber heads on it with small bumpstick and a nice worked over qjet or nice 750

The solution to the RPM/fuel economy problem is a 4L80E swap. The later version came with it. To avoid heavy modifications to the wiring, use an aftermarket controller, the stock wiring and sensors will provide the necessary signals.

The stoke compression is a joke, and a massive impediment to both power and fuel economy. The nicest solution is the Edelbrock head that has 100cc chambers, 60499 is the PN for a pair with flat tappet springs. The other good solution is an old closed chamber 396 cylinder head, but those really require hardened seats to be installed, that gets expensive. You could swap in a set of forged closed chamber dome pistons, and use a late model 049 or 781 head, if you want to go that deep into it, an open chamber head and the 454 LS-6 piston actually yields the highest compression, around 10.2-10.5:1, depending on deck height and head gaskets.

The intake is junk. Any decent aluminum high rise oval port intake (Edelbrock Performer RPM-O, etc) is far better, use an aftermarket spacer style adapter. Realize that the stock TBI only flows around 650 cfm. Now, one cool solution I saw for that was a dual quad intake and dual TBI setup. Turbo City did it years ago. They used two cheaper and more common 305/350 TBI units. I was never thrilled with any of those aftermarket "4 barrel" throttle bodies.

Some of those blocks have problems with aggressive cams, because the lifter bores aren't very true, so swapping in a flat tappet HP cam could be a problem. With EFI, you want to stay at or below 222 degrees at 0.050", and on a 110 LSA to keep the vacuum up.

Mtengines,
Help me understand the statement(better would be just a performer NON AIRGAP).I do not understand the issue with an Air Gap intrake.
Thanks
Scott

A blower & a t700,,, then it can easily be put back to stock if the value of these skyrocket

I had a 91 truck with a 454 and the ECU is the commodore 64 of ECU's It sucks to do any mods and it was very sensitive to a cam change as the vacuum signal must be way up or it will run like crap.

I sent the ECU out to several guys in a 3 year period and no one ever got it right so I changed the cam and then it was better running then it ever was with the best tune available in the ECU.

I went with just the stock heads and got the compression pu to 9.6-1 touched up the ports a little and put better exhaust on it. That made a world of difference. Then a gear vendor and then I had about 1800 rpm at 65 MPH with the 4.11 gears in the back.

What was responsible for the 25 hp increase in 91? Was it just the dual exhaust? If so might be a good place to start...

I have a friend with one of these...best thing he did was put a rather large turbo about mid way back under the cab. Megasquirt EFI system
then it would get out of it's own way finally

Why did you go with that large of split between the intake and exhaust? Was it for a stock exh system? I would think it would have made more torque with a single pattern cam.

pdq67,
cam specs and price please