New engine wants too much initial timing
Racer
Joined: Jan 2014
Posts: 339
Likes: 9
From: Goodyear AZ
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.
Melting Slicks
Joined: Jun 2016
Posts: 2,979
Likes: 332
From: Woodinville WA
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; May 20, 2018 at 11:35 PM.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
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!
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
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; May 21, 2018 at 09:34 AM.
Melting Slicks
Joined: Oct 2008
Posts: 2,154
Likes: 69
From: Regina Sk Canada
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.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
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
