[RAFO9A]

Originally Posted by chiphead

Timing and idle mixture affects manifold vacuum. If the carb really is lean at idle, then fattening it up will smooth the idle and possibly require closing the throttle plates down to reduce engine rpm. At that point you can reduce the amount of base timing required (and maybe add some throttle plate angle back) to provide a stable idle in/out of gear and decent manifold vacuum.

With that said, I try to run as much base timing as possible without starter kickback. In my experience tuning 350s and 400s at 9:1 compression, that's usually 12-17* BTDC.

I think you've got too much base timing, the total comes in too fast, and you should have 34-36* at 2800 RPM, but that's my keyboard diagnosis. Real-world testing tells the real story. Do you have any kickback when starting, or pinging at 2500 RPM? Did they try more timing and/or a later RPM for timing to come all-in?

What is vacuum at idle?

There's no kickback when starting, nor pinging at 2500 rpm. But say I decrease the base timing (it's at 22* now), what should i look for? As I go from 22* towards 17*, how do I know if the engine likes one over the other?

[chiphead]

Typically as you reduce base timing, you'll see engine vacuum drop off, and idle quality degrade. It'll idle rougher and struggle to stay running, especially when cold. Tuning the idle and off-idle is a balancing act between ignition timing and carb settings. If you dont have enough base timing, it'll make low vacuum and idle like crap. On the other hand, if the timing is right and the carb is too lean or rich, it'll also idle like crap. The temptation is to add more ignition timing to compensate for carb problems or low vacuum caused by low compression/big cam combinations.

I tune by experience and trial and error. Typically I'll set an unknown engine for 14* base timing and turn the carb idle down to 800 RPM. I'll already have the carb dialed in on the bench to get it in the ballpark.

Then I'll warm up the engine and see how the idle quality and vacuum is. If it's weak or stalls, I'll bump timing up to about 16-18* and see how it responds. Then I'll cover the carb with my hand to richen up the mixture. If the engine RPM increases a bunch, I know the carb is lean. (if it bogs/stalls it may be too rich). Then I modify the carb to increase idle fuel and re-try. Usually the engine will idle smoother and require closing the throttle plates to reduce idle back to 800. Sometimes with big cams I have to add idle bypass air along with extra idle fuel to improve idle. Then I put my hand over the carb and see if the RPMs increase a bunch again. When it doesn't show a huge increase then the idle mixture is pretty close.

When I'm starting and stopping the engine, I look for signs of starter kickback. The engine base timing is close when you can "bump" the key on a warm engine and she'll light right off. At least on my engine, it likes less than 23* at idle. I've had the vacuum advance pull in at idle (ported with too much throttle angle) and give 23*. The engine *stutters* and idles choppy from the too much timing. It's pretty obvious once you see an engine do that from too much base timing.

Then I check my total timing at 3000, and adjust the advance to get 36* while keeping whatever base timing I have. Then I road test and look for problems in the cruise, transition and WOT of the carb. I also watch engine temps and listen for the "valve rattle" sound of detonation/pinging. Then it's basically sneaking up on the optimum ignition and fuel settings for a given combo.....

[71 Ventura II]

Recurve the distributor with total timing "All In" between 2200-2400 rpm. You will want a mechanical advance of 20 degrees w/limiter to guarantee. Your looking for an initial timing of 14-16 and a total timing of 34-36 as a base. For sure 33 total without a limiter is way off from optimal. Good Luck tuning there's a lot more in it. JD

[Cruiser]

Back in 1974 My first car was a 1970 T-37 tempest/lemans, it was Granada gold, gold interior.It had a 350P 3spd manual/w a factory hurst shifter.I installed a dual exhaust ,4brl.quadrajet other than those mods it was stock.
One of my friends had a 1970 SS Nova 350 Chev 4spd.We street raced numerous times and it was usually a draw. Now I'm not talking stoplight to stoplight I'm talking about racing several miles on curvy roads.He would get SO MAD because I could stay with him ,he would about have a brain hemmorage.
My dad had a 1970 Yenko Deuce at the time built to the hilt(GOBI BEIGE BLACK STRIPES,turbo 400,wild cam, headers,... .Now THIS NOVA was different animal.I shifted it at 8000 RPM it was a beast.
With my T-37 I raced,a 1970 torino 429 Cobra jet(Yellow Shaker hood),1967 fairlane 500(289 4brl.),1969 428 Cobra jet,1971 Cutlass 455 & others these were all my friends cars,My Pontiac could hang with most of them & beat some of them.
I wish I had that car now (T-37) the engine was indestructable I drove the balls off of that car & it never did die( the drum brakes were horrible).
I also had several 305 chevys & they were pigs with asthma.I had a 1979 monza spider 305 4 spd it was a dog,a 1978 buick regal 305 it was a dog,1995 chevy 1/2 ton pickup305 5spd. superdog.Come to think of it why did I end up with so many vehicles with this engine?
I like both Pontiacs & Chevys Ive got a 1970 355 SBC 12:1 full roller motor in my garage right now that I dropped in to a 1928 model A street rod & made one pass in the 10s (no Roll Bar the track officials weren't to happy).So I know 350 SBC can be beasts.But Most 305 chevys were SLOW.
But I still like my Pontiacs the most. Although I would Love to build a clone of that Yenko Deuce someday ( Iv'e still got a few of the parts from it).
Thank's Mark/Cruiser

[PontGuy]

Except for the very few HO versions all P350's came with 2bbl carbs and single exhaust which gave them a bad rap on the street "back in the day" compared to the Chevy 350's. But given equivalent intake and exhaust setups I'd take the Pontiac. I built several P350's over the years and even ran one very successfully in a dirt track car for a few seasons. I still have the last P350 I built on a stand in my garage- it is more valuable in parts than as a complete engine, but I haven't have the heart to tear it all down.

The dyno chart shows good gains compared to stock, but agree with others that RPM at peak HP seems oddly low. My experience that it is pretty easy to get a P350 to spin up to 6000 RPM and make a bunch of power up there, but it comes at the cost of bottom end torque which isn't that great on a 350 to start with. So it is a bit of a balancing act for a street car application. Using "seat of the pants" experience your curve looks something like what I would expect with the factory 066 cam after upgrading to a 4 bbl and dual exhaust. I never tried an 068 style cam in a 350 but I would expect it to be enough to get the engine pulling hard up to at least 5000 RPM. Agree with what others are saying about timing and carb setup. You need more than 33 deg total to get it to pull up there. The chart doesn't show the engine leaning out but will throw it out anyway- it takes a good Qjet setup with small rods on the secondaries for it all to work. I was never a fan of the Performer manifold and never tried one on a 350 so don't know if that is a factor. I doubt the exhaust system is holding you back much- a good dual setup with logs ought to work fine.

Not sure why the 305's came up in this thread. I only owned one- in a very nice '90 Camaro. The car was an absolute dog compared to my old Pontiacs and I sold it after only a year.

[F ROCK]

my 350 lemans would SMOKE my 85 305 powered trans am. not even close.

[HO Kenny]

I had a 68 350ho firebird convertable. The #18 heads were cleaned up but not ported. The tops of the cylinders were champherd from the factory to unshroud the valves. The BEST cam I had in it was the comp cams 268 he. Factory intake and a holley 650.
3.36:1 gear ratio and a m20 4 sp. 3 tube headers and good exhaust. That combo. was in the car from 1980 -1993? I never had it on a dyno or a real track, but I did a lot of street racing. The tq. curve was very wide and worked well on the street. I was fighting detonation at about 3500 rpm till I got it tuned right. It also seemed to run hot and it did take a lot of tinkering to keep it in tune.
No 305, factory or many built 350 chevys could beat it. I raced and beat a 69 400 fb, 68-69 gs 400, a few 340 darts, demons, and a 69 baracuda s when I had 3 of my dodge friends with me. It beat one of those AMC rebel machines as well.
I would shift at about 5400 rpm. If I shifted earlyier the tires would spin to much, later didn't help any and may have put it out of the tq. curve. I lost to a built 69 396 chevelle and a very very fast 67 nova. I seldomly raced againsed the big dogs but I lost to a 68 427 4sp vett. with 4.11s and felt good about it.
Once it pulled 6000 rpm in top gear. That is about 140 mph. with the tall 15" tires It pulled very well till about 110 on the speedo and took a while after that. Not bat for a heavy brick.
There was a marked out 1/4 mile on the highway and it did low 14's on a stop watch. That was at about 3000ft. I had to slip the clutch just right to make up for the tall 1st gear and rear gear.
Mabye I'm braggin but it's all true.
The mild 455 with 71 96 heads that it has now is way easier to drive but I doubt it would beat the 350 when it was dialed in. It still has the holley 650 on it. (my rev limiter). My former 68 RA 4sp firebird would have beat it and my 71 ta that has been tinkered with would beat it as well.

The moral of the story the 350p can built to be very potent. My brother had a tpi 350 GTA and my car was quicker. A small block 350 had to be built very well with low gears to compete. Those big bore short stroke 302 fords were not that fast either. I never met a old boss 302 though.
Sorry for the long rant
Ken