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454 low end torque is weak

B

black dawg

1/2 ton status
Joined
Aug 25, 2011
Posts
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Location
montana
stock 76 short block
vortec cam
stock 781 heads
exhaust manifolds
performer intake with stock 454tbi adapter
th400 with low stall towing converter
90 3/4 ton suburban with 3.73 gears

power below 3k feels soft, but right at 3k it comes to life and runs pretty dang good.

I really dont run the thing much over 4k, are there heads that would work better for me. My other thought is the tbi454 adapter is open in the center, and I wonder if closing it up would help gain some low end.

Is anybody else running a low stall converter on a stockish 454. I wonder if this is contributing to the low output. This thing WILL NOT squeak a tire from a dead stop WOT.

You guys have any thoughts on this? It all in all runs good, just feels a little soft on the low end, and at 3k starts running hard.
 
Did you happen to make sure the intake and head ports are the same design and not different? Have you checked the base timing? Compression? Hows the spark?
 
that performer intake needs a lot of work to match the 781 heads...

HPIM1331.jpg
 
454Sub said:
Did you happen to make sure the intake and head ports are the same design and not different? Have you checked the base timing? Compression? Hows the spark?
Click to expand...


Head and intake ports are same design. The intake ports were slightly smaller ont the man. vs. the head.

spark is good, and have tried several different timing settings. Responds good to added timing.

I have not checked the compression I will do that here after a bit.

I just remembered that the timing set I used looked like it had the cam retarded some. I wonder if all the vortec sets were like that? I wish I would have degreed the cam now, cause thinking about how it looked it is probably atleast 4deg retarded. I bet that is why it is soft down low.
 
Also remember the stock bottom end plus open chamber heads will make LESS than 8:1 compression, not going to be a beast regardless.
 
nvrenuf said:
Also remember the stock bottom end plus open chamber heads will make LESS than 8:1 compression, not going to be a beast regardless.
Click to expand...

just checked psi on 3 holes, and came up with 110psi.

my book shows 76ish 454 should blow 150. That seems high to me for an under 8:1 turd.
 
Compression? If it has flat tops it will be 8 or less. If this is the case it will need alot more int timing. Like 20 will make a HUGE difference. Big blocks like timing and with low compression they need more. I in fact have welded up the slots on my dist to give me 26 int and 38 total.
 
black dawg said:
just checked psi on 3 holes, and came up with 110psi.

my book shows 76ish 454 should blow 150. That seems high to me for an under 8:1 turd.
Click to expand...

Cam will throw off the PSI, if it is even don't worry unless it is uneven.
 
I dont even think you can match the small circle ports to the ovals, I think it will grind into a water jacket or intake runner. I had to do quite a bit of grinding to my intake to match my newer style roller block heads.

Degreeing the cam can help, I just set mine a 0 but my timing set came with marks for both ways. That was too far into what I wanted out of it, I dont need all that extra power, mine has no problem chirping the tires with 40s.
 
80' 427 said:
Cam will throw off the PSI, if it is even don't worry unless it is uneven.
Click to expand...

I wonder where I could find the specs of the stock 76 cam vs the 97 vortec cam. I am betting the intake closing is pretty close between them... maybe not?
 
black dawg said:
I wonder where I could find the specs of the stock 76 cam vs the 97 vortec cam. I am betting the intake closing is pretty close between them... maybe not?
Click to expand...

Pretty sure it is way different. One is a roller and has .450 life. The duration on a 76 is about 198/210 the 97 is 202/210 or so. Either way I wouldn't worry unless it is different across the cylinders. Timing needs work and will make a big difference. My k30 has a similar motor and will smoke the 39.5s.
 
do you guys think the small port heads would make any more power in the 1000-4500 range than these 781 heads? I built this thing (the burb) for towing, and I do run it pretty hard, but rarely over 4500.
 
I think those stock peanuts will poop out at 3500-4000, the big ovals are just as good as the peanuts and will go up to 5500 untouched, and bang away hard up to 6500 with some port and bowl work.
They will make plenty of low end torque...open that performer up to match the 781's and let it fly. It can be done without hitting water jackets, my head guy did that same thing for my 781's. If your keeping the stock short block, maybe shave the heads .010 and get a thinner head gasket to bump up the c.r. some.

here is a caluculator to figure that out...the 781's should be 119cc uncut.
http://www.kb-silvolite.com/calc.php?action=comp
 
thanks, I was hoping to hear that the 781 heads would work fine. I think I will go after cam timing first, because my damper is in rough shape and I plan on replacing it soon.
 
my torque curve starts at 2000rpm and is over 500, peaks at 550 and 3500rpm, still over 500 @ 4500rpm.
your using a roller cam though right?
Mine is a solid flat tappet.
 
Mine is over at 4500 with my merc marine cam but I am running a 460cfm carb. Waiting till christmas break for my projects.
 
My 406 sbc was put together with the cam advanced 4 degrees to help the bottom end torque. It is built between the "406 Impersonator I and II" engines that CHP did. With a little bigger carb, mine should make 525tq, but it still isn't easy to spin the tires on dry pavement. I would not call it a tire burner. It is also a C20 with th400, gear vendors overdrive unit, 14bff, and 33-12.50x16.5" tires. It makes far more power than my 383 did and the truck performs better than a modified BB TBI 454 that I drove. I don't know what the real numbers are though.

My edelbrock intake needed a lot of work. The head guy got it to flow great numbers and matched it to the elelbrock heads (they also needed a lot of work). My cam is 224/224 at 50 and 470 lift. Engine is 10:1 CR. Cranking pressure should be above 200 psi, but I have not checked it. You can really feel it start to pull at 2k rpm and it eases up a little at 4800rpm but it still has power to 5500rpm.

It certainly sounds like your combo might be a bit miss matched for what you want to do.
 
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