Timing differences between small blocks, 350 vs 400
Thread Starter
Melting Slicks
Joined: Oct 2000
Posts: 2,268
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From: Roanoke Virginia
Timing differences between small blocks, 350 vs 400
I found this response in my MS Outlook Inbox, forgot I had it, want to share with you guys. This is from Mike Soldano, he makes guitar amps out west but is also an avid sports car-a-holic. Helps explain why the 400's and 383's like a little less total advance.
"Hi Jeff,
I'm somewhat surprised to be getting a hot rod question forwarded to me from Musician's Hotline. It's true that I have a fondness for the small block
Chevy. I'm also into the old 409 Chevys and early 394 Olds motors as well.
CARS ARE FUN!!
Let's see if I can be of any help to you on your timing question. I haven't
messed that much with the 400 inch small block, most of my experience has
been with the 283, 327, and 350s. But here's what I've observed. With all
things being equal (compression, cam, carburetion, etc.), it seems that the
shorter the stroke, the more timing the motor can tolerate and use. Back
when gasoline was still gasoline ‹ and actually had some octane rating ‹ I
knew a guy with a 301 (.120 over 283) that had an honest 11:1 and was
running around 40 degrees total lead (16° initial), no problem. It helped,
of course that it was in a pretty light car, a '38 Chevy coupe. One winter
he cracked the block (five bucks worth of anti-freeze could have averted
that stupid disaster), and I bought the car from him. I completely rebuilt
the car and stuck a 355 in it with '70 smog heads (don't remember the
casting #s) and TRW flat tops. The pistons were about .030 down the hole,
but I was running factory steel shim head gaskets and I'd milled the heads a
bit, so it ended up with around 9.2:1 compression. The cam was a Comp Cams 280 hydraulic with 236° @ .050 and .480" lift. I found that I could only
run about 38° total, even though it had such low compression. And rather
than running a lot of initial advance like I was used to (which really
helps for low end grunt), I had to cut back to 12° and put more in the
distributor ‹ 13° (26° crank) all in by 2500 rpm. Needless to say, I was
amazed that a motor with such low compression could ever rattle AT ALL!
Now, on the other hand, my daily driver '66 Nova wagon has a pretty much
stock 283 in it ‹ .030 over TRWs, milled stock heads, factory iron 4bbl
manifold and 600 cfm Holley carb. The cam is real mild and the compression
is probably around 9:1. However, the short cam duration actually raises the
dynamic compression, and the Nova has real high rearend gears (2.73:1) and a Powerglide (stock converter, of course). All these things should make for
problems involving timing and octane, right? Well dig this ‹ I run the
cheapest gas there is, it's got 14° (28°) in the distributor, 12° initial,
and I can't make it rattle no matter what I do! And it's jetted "fried
valve lean" (for better mileage) to boot, which should also encourage
detonation.
This puzzled me for some time, until I read an article about some motor
build up in Hot Rod magazine a few years ago. They did a build up on a long
rod, short stroke 350 Chevy (327 crank in a 400 block). This thing had 11:1
compression, ran fine on 87 octane pump crud, and made over 400 horses!! I was blown away! But what I learned was that the long rod/short stroke combo made the motor less detonation prone because the piston dwelled longer at top dead center during combustion. That explains why my 350 was touchier than my 283. I found that the 394 Olds in my Model A can also tolerate a lot of timing, so I did the math and found it had the same rod ratio as the 283, which I thought was interesting.
So what does all my rambling me to you?? The 400 SBC has a long stroke and short rods, and the factory open chamber heads have a lot of area for
secondary flame fronts to occur and cause detonation ‹ that's probably been
what you're up against. With today's gas, I think that your 9.7:1
compression is about as far as you can go with iron heads, so you were smart
in going that route. And you're right, you need to have some decent initial
timing with a hot cam, or the car's going to be a slug. So, I would say you
should run as much advance as you can without detonation, and if you have to cut it back, take it out of the distributor so that your initial timing can
stay at 12-14°. You might also be able to get away with a little more total
lead if you bring it in a little later. I like everything to be in by 2500
rpm, but you might try holding back until 2800 and see if that helps. Also,
fattening up the carb might control the rattle and pump up the torque a bit
at the same time. If when all is said and done, you end up with 32° total,
that's fine. As with all tuning matters, it doesn't really matter what the
final number is, as long as it isn't rattling. Once a motor starts
detonating, it's not only melting the pistons, it's also not making power
anymore. So push it as far as it will go, but stop when it wants you to,
and that's where you're making max power. Good luck, Jeff, and let me know
what you finally end up with!!
Later,
The Amp Man"
"Hi Jeff,
I'm somewhat surprised to be getting a hot rod question forwarded to me from Musician's Hotline. It's true that I have a fondness for the small block
Chevy. I'm also into the old 409 Chevys and early 394 Olds motors as well.
CARS ARE FUN!!
Let's see if I can be of any help to you on your timing question. I haven't
messed that much with the 400 inch small block, most of my experience has
been with the 283, 327, and 350s. But here's what I've observed. With all
things being equal (compression, cam, carburetion, etc.), it seems that the
shorter the stroke, the more timing the motor can tolerate and use. Back
when gasoline was still gasoline ‹ and actually had some octane rating ‹ I
knew a guy with a 301 (.120 over 283) that had an honest 11:1 and was
running around 40 degrees total lead (16° initial), no problem. It helped,
of course that it was in a pretty light car, a '38 Chevy coupe. One winter
he cracked the block (five bucks worth of anti-freeze could have averted
that stupid disaster), and I bought the car from him. I completely rebuilt
the car and stuck a 355 in it with '70 smog heads (don't remember the
casting #s) and TRW flat tops. The pistons were about .030 down the hole,
but I was running factory steel shim head gaskets and I'd milled the heads a
bit, so it ended up with around 9.2:1 compression. The cam was a Comp Cams 280 hydraulic with 236° @ .050 and .480" lift. I found that I could only
run about 38° total, even though it had such low compression. And rather
than running a lot of initial advance like I was used to (which really
helps for low end grunt), I had to cut back to 12° and put more in the
distributor ‹ 13° (26° crank) all in by 2500 rpm. Needless to say, I was
amazed that a motor with such low compression could ever rattle AT ALL!
Now, on the other hand, my daily driver '66 Nova wagon has a pretty much
stock 283 in it ‹ .030 over TRWs, milled stock heads, factory iron 4bbl
manifold and 600 cfm Holley carb. The cam is real mild and the compression
is probably around 9:1. However, the short cam duration actually raises the
dynamic compression, and the Nova has real high rearend gears (2.73:1) and a Powerglide (stock converter, of course). All these things should make for
problems involving timing and octane, right? Well dig this ‹ I run the
cheapest gas there is, it's got 14° (28°) in the distributor, 12° initial,
and I can't make it rattle no matter what I do! And it's jetted "fried
valve lean" (for better mileage) to boot, which should also encourage
detonation.
This puzzled me for some time, until I read an article about some motor
build up in Hot Rod magazine a few years ago. They did a build up on a long
rod, short stroke 350 Chevy (327 crank in a 400 block). This thing had 11:1
compression, ran fine on 87 octane pump crud, and made over 400 horses!! I was blown away! But what I learned was that the long rod/short stroke combo made the motor less detonation prone because the piston dwelled longer at top dead center during combustion. That explains why my 350 was touchier than my 283. I found that the 394 Olds in my Model A can also tolerate a lot of timing, so I did the math and found it had the same rod ratio as the 283, which I thought was interesting.
So what does all my rambling me to you?? The 400 SBC has a long stroke and short rods, and the factory open chamber heads have a lot of area for
secondary flame fronts to occur and cause detonation ‹ that's probably been
what you're up against. With today's gas, I think that your 9.7:1
compression is about as far as you can go with iron heads, so you were smart
in going that route. And you're right, you need to have some decent initial
timing with a hot cam, or the car's going to be a slug. So, I would say you
should run as much advance as you can without detonation, and if you have to cut it back, take it out of the distributor so that your initial timing can
stay at 12-14°. You might also be able to get away with a little more total
lead if you bring it in a little later. I like everything to be in by 2500
rpm, but you might try holding back until 2800 and see if that helps. Also,
fattening up the carb might control the rattle and pump up the torque a bit
at the same time. If when all is said and done, you end up with 32° total,
that's fine. As with all tuning matters, it doesn't really matter what the
final number is, as long as it isn't rattling. Once a motor starts
detonating, it's not only melting the pistons, it's also not making power
anymore. So push it as far as it will go, but stop when it wants you to,
and that's where you're making max power. Good luck, Jeff, and let me know
what you finally end up with!!
Later,
The Amp Man"
