Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Seconary rod suggestion?
55 Tony:
The cam was advanced 2 degrees total.
I run 92 or 93 octane, no knocks or pings although it doesn't like lower octane at all. Actually using a compression calculator guessing at the head cc's and guessing how much it's been decked, it said somewhere close to 10.25:1. But I *think* the two guesses where on the conservative side, that's why I showed the heads pictures. I thought someone who has seen a lot of 781's could guess mostly be how close to the valve the surface is. I'd love to see the same shot of a new 781 head. The second pic is really hard to see what I'm trying to show, but I'm trying to show how close the head surface is to the adjacent rough casting that isn't milled. If the head got shaved much more, that entire rough cast area would be milled too.
I found this in my notes.
Piston volume -25.70cc
head gasket .039 ... I think?
Was told it was milled considerably. From what I found on the net, they were 118cc new.
bored .030 over.
Decked, but I don't have any numbers on that.
I have checked the compression with another gauge that was the rubber push and hold type. It read about 180psi and then I couldn't hold it tight enough and the pressure would push the gauge out of the hole.
It's quite a difference from the old flat top pistons with a chamfered edge, then it had about 140 psi. And that is with the same gauge I'm using now.
I don't have a measurement for what ever it's called, the piston down in the cylinder, but I can say that with clay on the piston to check for valve clearance, it was getting sort of close to the minimum, whatever that is/was.
I don't see the numbers for the rockers I used, but I recall that they lift a bit higher than stock rockers, whatever that ratio is.
Pistons are speed pro L2465F30 https://www.summitracing.com/parts/slp-l2465f30
I don't know why, but even though I checked "notify me of replies", I don't get an email. I just happened to check in here and saw the new replies.
68rs/ss:
In a 454CID engine at 10 to 1 or a tad higher compression I'd expect around 170-180psi cranking pressure based on the engine builds I've done here with similar parts......Cliff
Cliff. Can you share some of your secrets on the 454 builds you have done? I will be replacing my 350 with a 454 with stock 781 heads in the future. Cam choice, pistons used, head mods, intake and of course, Q-jet needed. Street car.
Phil
Cliff Ruggles:
For all engines built here we take steps to establish very tight quench in them. The target is .035" and never over .040" for any reason.
The parts used to build them would depend on the goals for the project and power level needed.
For a truck 454 we'd keep the compression ratio a bit lower and smaller cam on a wide LSA.
I typically don't use aftermarket intakes on most builds, the factory iron ones are fine, even the "flat" intakes that came on a lot of 454's. From idle to 5000rpm's it difficult to outrun one of those intakes with anything in the aftermarket, plus they keep things low for plenty of hood clearance in many applications that need it.
The factory Q-jets that came on the Big Block Chevy engines are excellent, all years aside the pre-1969 models. Those have the early hinge pin location and large float, so if the engine is going to be making big power a later carb would be a better choice.
Big Block Chevy engines in all configurations have tremendous potential to make power, and excellent parts selection for them.
For a "max-effort" pump gas street build we will usually start with the passenger car oval port heads, install 2.19/1.88" valves in them, a little clean up in the bowls, under the valves and short turns. I'll target the compression ratio around 10 to 10.5 to 1, then use a cam with pretty long seat timing, and 112 or 114LSA. Crower makes a cam (can't remember the part number at the moment) around 234/244 @.050" on a 112LSA that really works well in those builds. Decent idle, plenty of vacuum for power brakes, and strong power from idle to near 6000rpm's.
I avoid any of the "modern" lobe profiles for engines built here, or tight LSA. They require too much spring pressure to keep things happy above the lifters and aside from a "ratty" idle they just don't deliver in terms of power production.
Of course if you are into "bling" and want a really lumpy idle, low vacuum at idle, stinky exhaust and poor street manners there are PLENTY of cams out there that fit that bill........Cliff
68rs/ss:
Thanks for the advice on the BB build. Sometimes we do not realize how good some of the factory parts are. Phil
55 Tony:
--- Quote from: 68rs/ss on May 03, 2018, 03:28:22 PM ---In a 454CID engine at 10 to 1 or a tad higher compression I'd expect around 170-180psi cranking pressure based on the engine builds I've done here with similar parts......Cliff
Phil
--- End quote ---
I'm not sure if you don't believe the compression I said I read or if you think the gauges are faulty ... or something else? I had hoped to loan a gauge from one of the local auto parts stores to see what it measures but I ran out of time and steam today. Will it make much difference if the engine is cold?
No one seems to be able to say from the pictures I posted, if the valve relief area is cut into from milling and shaving the head a lot over the years or if that is common from when they were new.
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