New engine wants too much initial timing
05-20-2018, 10:47 PM
#41
Racer
Okay. Got this worked out. Lars was spot on. I trashed the stock vacuum can and went with the "B28" vacumm can from dura last, part number DV1810. It provides 16 degree advance coming in around 8" of vacuum. With this I was able to get my initial down to 18 and still have the car crank nice and easy and run well at partial throttle cruising. With the new can I'm around 34* at idle with vacuum connected on FULL manifold vacuum, not ported.
I read 33-34 degrees at 3000 and I plan on playing with my new distributor advance spring kit to fine tune this even more. Thanks to all those that chimed in.
I read 33-34 degrees at 3000 and I plan on playing with my new distributor advance spring kit to fine tune this even more. Thanks to all those that chimed in.
05-20-2018, 11:22 PM
#42
Melting Slicks
WCC Josh, look at the diagram. (Yea I know it's a Pontiac, but the effect is more pronounced with the jump from 1.5 to 1.72 ratio.)
Because the seat-to-seat duration remains the same but the valve opens further it means the valve also opens faster, which increases the duration slightly at all the points in between closed and max lift. -This is why switching to higher ratio rockers usually moves the torque and HP peaks upwards ever so slightly. (Watch that Engine Masters Season 2 Rocker Ratio episode again and pay attention to the RPM peaks; if duration didn't change, the peaks wouldn't change; only the torque and HP would change if duration wasn't affected and it was ONLY lift that was increasing with increasing ratios.)
My 227/228 @ 0.050" cam becomes much closer to 230/232 @ 0.050" with 1.6 ratio RRs.
Adam
Last edited by NewbVetteGuy; 05-20-2018 at 11:35 PM.
05-21-2018, 08:44 AM
#43
Had a 1976 L-82, 4-sp
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Royal Canadian Navy
If the engine runs well at 20* initial, why worry about it? You guys are WAYYYYY over thinking this stuff...
Give it the initial it wants, modify your mechanical to match the initial and yield the desired total... (Aka 36* at 3000), add your vacuum advance but limit its travel to say 8-10* , thus giving you a cruise advance of 44-46*....or less if it pings. Call it good!
Give it the initial it wants, modify your mechanical to match the initial and yield the desired total... (Aka 36* at 3000), add your vacuum advance but limit its travel to say 8-10* , thus giving you a cruise advance of 44-46*....or less if it pings. Call it good!
05-21-2018, 09:28 AM
#44
Had a 1976 L-82, 4-sp
Member Since: Mar 2011
Location: Some days your the dog and some days your the hydrant.
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Royal Canadian Navy
Yep!
WCC Josh, look at the diagram. (Yea I know it's a Pontiac, but the effect is more pronounced with the jump from 1.5 to 1.72 ratio.)
Because the seat-to-seat duration remains the same but the valve opens further it means the valve also opens faster, which increases the duration slightly at all the points in between closed and max lift. -This is why switching to higher ratio rockers usually moves the torque and HP peaks upwards ever so slightly. (Watch that Engine Masters Season 2 Rocker Ratio episode again and pay attention to the RPM peaks; if duration didn't change, the peaks wouldn't change; only the torque and HP would change if duration wasn't affected and it was ONLY lift that was increasing with increasing ratios.)
My 227/228 @ 0.050" cam becomes much closer to 230/232 @ 0.050" with 1.6 ratio RRs.
Adam
WCC Josh, look at the diagram. (Yea I know it's a Pontiac, but the effect is more pronounced with the jump from 1.5 to 1.72 ratio.)
Because the seat-to-seat duration remains the same but the valve opens further it means the valve also opens faster, which increases the duration slightly at all the points in between closed and max lift. -This is why switching to higher ratio rockers usually moves the torque and HP peaks upwards ever so slightly. (Watch that Engine Masters Season 2 Rocker Ratio episode again and pay attention to the RPM peaks; if duration didn't change, the peaks wouldn't change; only the torque and HP would change if duration wasn't affected and it was ONLY lift that was increasing with increasing ratios.)
My 227/228 @ 0.050" cam becomes much closer to 230/232 @ 0.050" with 1.6 ratio RRs.
Adam
Last edited by resdoggie; 05-21-2018 at 09:34 AM.
05-21-2018, 12:06 PM
#45
Le Mans Master
I am running about 25 degrees advance at idle so 27 is not that far off. I have the static timing set at 10-12 BTDC. 36 degrees total by 2500rpm.
05-22-2018, 04:00 PM
#46
Melting Slicks
My 355 SBC makes 350 HP and 390 TQ. It has 25 initial and 35 total. The more initial timing you can give it the better. Mine runs great, starts and shuts off fine no run on etc. Give the motor what it wants. Mine was dynoed this way.
05-23-2018, 10:48 PM
#47
Team Owner
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St. Jude Donor '05
good that you are taking the time to get it right- as ajorythm said no 2 motors are alike. Curve i have is the last one Id had guessed, an old timer with a Sun machine nailed it.
One guy will say "sbc like xx" when in fact it may do best on 30.
Or worse yet, lock it out then it pings like crazy & have an unhappy starter.
Sounds like you got it figured out
One guy will say "sbc like xx" when in fact it may do best on 30.
Or worse yet, lock it out then it pings like crazy & have an unhappy starter.
Sounds like you got it figured out