What rods/jets needed to dial this in?

[EspriTA350]

I have a 72 Pontiac Firebird 350 with a 17056274 qjet on it from 1976. This carb has already had the idle air bypass mods done to it and is freshly rebuilt from everydayperformance. I'm soon putting in a cam and higher cr iron heads on the car and want to make sure I don't: (1) kill the motor running it too lean or too rich, (2) maintain and/or further improve it's daily driving manners. Currently it nets 16.5 mpg on the highway with the ac on, and a 2.73 rear end + the usual TH350.

Specs on cam are:
hydraulic roller
LSA: 114
valve lift int: .478, exh: .498
duration @ .050 int: 210, exh: 218


Specs on heads:
Pontiac #16 small valve (1.96 int, 1.66 exh) with pocket porting and multi angle valve job. Stock 1.50 rocker ratio. Slightly milled combustion chambers down from 75cc to 72cc.

Car also has dual exhaust and full length tri-y headers. I'm aiming for a compression ratio in the low 9's

What size rods and jets should I try to dial this combo in?

[tayto]

ask the builder of your carb?

[EspriTA350]

Edit: Realized I forgot to mention it also has a 4 barrel iron intake, which is gasket-matched and lightly ported to suit the heads and cam. If any other info is needed please ask.

ask the builder of your carb?

I prefer Cliff's experience, thanks.

[Cliff Ruggles]

It would be best to ask the folks who built it.

When I build them here they are set-up exactly for the application starting with air bleeds, idle tubes, DCR's, jets, metering rods and PP spring.  I also use a high flow N/S assembly and small float set to custom specs.

The folks who did the build may have modified the carb in areas differently than I would have and used different rebuild components, float height, etc.   So it's best to ask them how to proceed........Cliff

[EspriTA350]

It would be best to ask the folks who built it.

When I build them here they are set-up exactly for the application starting with air bleeds, idle tubes, DCR's, jets, metering rods and PP spring.  I also use a high flow N/S assembly and small float set to custom specs.

The folks who did the build may have modified the carb in areas differently than I would have and used different rebuild components, float height, etc.   So it's best to ask them how to proceed........Cliff

Ah I understand. I will go from there, then. Thank you.

[Cliff Ruggles]

I will say that I've ran my 17057274 on 5 different engines now and haven't touched the jetting. 

I did have to step up the idle system once when I installed my first 455 engine with a pretty "hefty" cam in it.  I've used the APT system to fine tune part throttle A/F for each engine, but the main jets, primary and secondary metering rods are still the same and have been for all 5 engines........Cliff

[EspriTA350]

I will say that I've ran my 17057274 on 5 different engines now and haven't touched the jetting. 

I did have to step up the idle system once when I installed my first 455 engine with a pretty "hefty" cam in it.  I've used the APT system to fine tune part throttle A/F for each engine, but the main jets, primary and secondary metering rods are still the same and have been for all 5 engines........Cliff

Only idle circuit and APT adjustments for a 455 with a cam huh... that's great!!

Edit: missed the part where you said all jets and rods were the same for 5 different engines.. wow! Guess there's one less thing I have to worry about.

[Cliff Ruggles]

Keep in mind that I don't have to re-jet or add fuel because all of my engines are high compression, tight quench, and very well chosen camshafts.  They also have excellent flowing heads on them. 

This actually improves VE and BSFC so there isn't any need to throw more fuel at them even though each one I've done has more power than the one before it.

Also consider that all N/A engines operate in similar parameters as far as A/F ratio requirements are concerned.

Of course IF you lower compression and start throwing in camshafts with really tight LSA you will need a LOT more fuel at idle, off idle and lower RPM's to make them happy.  It's been that way for the near 50 years I've been in this hobby and unlikely to change anytime soon.......Cliff

[EspriTA350]

Keep in mind that I don't have to re-jet or add fuel because all of my engines are high compression, tight quench, and very well chosen camshafts.  They also have excellent flowing heads on them. 

This actually improves VE and BSFC so there isn't any need to throw more fuel at them even though each one I've done has more power than the one before it.

Also consider that all N/A engines operate in similar parameters as far as A/F ratio requirements are concerned.

Of course IF you lower compression and start throwing in camshafts with really tight LSA you will need a LOT more fuel at idle, off idle and lower RPM's to make them happy.  It's been that way for the near 50 years I've been in this hobby and unlikely to change anytime soon.......Cliff

Interesting. Well since I will be increasing compression a decent bit to 9:1 (stock is supposed to be just a lil below 8:1. Yow.) and the cam + heads were matched to each other for an all-round improvement everywhere, from what you say I expect it won't need much more fuel at all. I have not dialed in the quench though.

[Cliff Ruggles]

Quench needs to be tight, especially in a Pontiac build with iron heads.  I would NOT build one with more than .040" quench distance for any reason........FWIW....

[EspriTA350]

Quench needs to be tight, especially in a Pontiac build with iron heads.  I would NOT build one with more than .040" quench distance for any reason........FWIW....

That's one thing I'm not sure about. I know the heads have at least .020 milled so that should improve the quench some, correct? I am using run of the mill fel-pro 8518pt head gaskets and cannot find anything other than speculation on what the actual compressed-thickness of these gaskets are. I would prefer cometics with their tighter bore and all, but getting my engine machined for the proper RA finish is just not an option for me (and won't be for awhile).

[tayto]

bring piston to TDC (use dial indicator), measure from top of deck to top of piston. then add your compressed headgasket thickness to this and you have your quench measurement.

You can adjust quench with headgasket thickness or getting deck milled. the popular thing in the SBC circles is to "zero deck". machinist will mill deck so it is exactly @ top of piston, then the headgasket strictly sets your quench.

[Cliff Ruggles]

The Felpro head gaskets compress at .039-.040".  This works out well if the block is zero decked or pistons sticking out a few thou at TDC.

You'd be surprised how much better that deal works out than having the pistons .030-.035" in the holes at TDC and using a .050-.060" "rebuilder" head gasket on them.

I see the latter all the time, and it's rare to even talk to a customer wanting parts or carb work that has the first clue as to what the quench distance or true static compression ratio is on his new engine build........FWIW.....