Compression, Octane, 351W
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
Compression, Octane, 351W
Hello all, for some context I have a 351w that I rebuilt, its got the following mods done to it: Long tube headers, custom exhaust, 21lbs injectors, a comp cam 35-255-5, and ported and polished E7TE heads. On the heads for some reason the exhaust valves were 1.47 instead of the stock valves, I ordered stock valves and then they didn't fit, lol. When I rebuilt it I had to have the block bored to .030 over, and the heads and deck were both taken down by .010. It has the .030 dished pistons, .105 recess for the dish.
I crunched some numbers and my static compression ratio should be around 10.1:1 as opposed to the 8.8:1, I am currently dealing with an ignition problem, so my timing has never been right since the rebuild. but once I get that fixed I am worried that the motor will start pinging on the low octane fuel. I really don't want to run premium since its a gas guzzler, from various sources around the web I found that I might need to run 91 or higher but it isn't based of off the static compression ratio but the dynamic compression ratio. This is my first time diving into this so I imagine I am wrong about this, just wanted some opinions.
I crunched some numbers and my static compression ratio should be around 10.1:1 as opposed to the 8.8:1, I am currently dealing with an ignition problem, so my timing has never been right since the rebuild. but once I get that fixed I am worried that the motor will start pinging on the low octane fuel. I really don't want to run premium since its a gas guzzler, from various sources around the web I found that I might need to run 91 or higher but it isn't based of off the static compression ratio but the dynamic compression ratio. This is my first time diving into this so I imagine I am wrong about this, just wanted some opinions.
Cargo Master
Joined: Oct 2020
Posts: 2,687
Likes: 789
That cam is ground on 114 lobe sep and in on 110 so that will tend to reduce cylinder pressures at low speeds where detonation is most likely. With 210 @ .050 I don't think 10:1 compression will get by on 87 octane but 89 might be ok. You'll just have to give it a try.
A cam with more duration say 218 at .050 would be more forgiving of fuel quality as would the cam you have now if it was installed straight up with no advance or even retarded instead of +4 as it is ground.
A cam with more duration say 218 at .050 would be more forgiving of fuel quality as would the cam you have now if it was installed straight up with no advance or even retarded instead of +4 as it is ground.
Lead Driver
Joined: Jan 2019
Posts: 5,081
Likes: 865
From: N.W. Alabama
You have a tune, or a tunable PCM, those 21# injectors may be a problem on A/F ratio's, get you a gauge and sensor to keep a check on A/F ratio's, E7's had 1.78(in) and 1.44(ex) stock, you may be at the edge of stock PCM, working correctly, especially, if you didnt keep O2 for PCM, to adjust fuel.
Thread Starter
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
That cam is ground on 114 lobe sep and in on 110 so that will tend to reduce cylinder pressures at low speeds where detonation is most likely. With 210 @ .050 I don't think 10:1 compression will get by on 87 octane but 89 might be ok. You'll just have to give it a try.
A cam with more duration say 218 at .050 would be more forgiving of fuel quality as would the cam you have now if it was installed straight up with no advance or even retarded instead of +4 as it is ground.
A cam with more duration say 218 at .050 would be more forgiving of fuel quality as would the cam you have now if it was installed straight up with no advance or even retarded instead of +4 as it is ground.
Thread Starter
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
When I bought the truck it came with them on, I am planning on getting a twEECer tune in the foreseeable future, so this shouldn't be an issue from what I've read but right now I do agree it is messing things up.
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Thread Starter
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Tuned
Joined: Apr 2023
Posts: 468
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From: Near SLC Utah
You have a tune, or a tunable PCM, those 21# injectors may be a problem on A/F ratio's, get you a gauge and sensor to keep a check on A/F ratio's, E7's had 1.78(in) and 1.44(ex) stock, you may be at the edge of stock PCM, working correctly, especially, if you didnt keep O2 for PCM, to adjust fuel.
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Thread Starter
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
I do agree, but unfortunately the injectors came with the truck when I bought it, the PO didn't know squat about these trucks. I am planning on getting a tune soon so hopefully it should be fine in the long run
Moderator
Joined: Jun 2003
Posts: 3,549
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From: Calhoun GA
I would try it out with the injectors you have, they're not that much bigger than stock squirters (19lb'er). The ECU will likely be able to correct the fueling in closed loop with the 21lb injectors. However, in open loop it will be a little rich because it will be firing the 21's from calculated tables, as if they're 19's.
As for the timing, if you are set on running 87, then try it and see how it behaves under heavy load conditions. My guess is that you will end of having to reduce the base timing a degree or two to keep it from pinging (which will make it run a little more sluggish), or move up to a higher octane.
As for the timing, if you are set on running 87, then try it and see how it behaves under heavy load conditions. My guess is that you will end of having to reduce the base timing a degree or two to keep it from pinging (which will make it run a little more sluggish), or move up to a higher octane.
Cargo Master
Joined: Mar 2007
Posts: 2,848
Likes: 1,034
I purchased a Tweecer a couple years ago. Although I believe it is the best answer for me, it is a huge learning curve. Most of the stuff I learned was with reading, researching, and trial and error. The learning path is still on going. I'm not looking for the time I swap my stock 351W to a built 408.
If you are not good with finding your own answers and like to have your hand held, you might want to go a different route.
If you are not good with finding your own answers and like to have your hand held, you might want to go a different route.
Lead Driver
Joined: Jan 2019
Posts: 5,081
Likes: 865
From: N.W. Alabama
Any idea what fuel pressure your running, I kept, stock cam, used 1.7's,rollers, and ported intake, stock 19#'s, and raised fuel pressure to 36# at idle, was 32#, and runs rich, 10.5-11.3, but it can, roast the goodyear mud tires at any moment. 183hp @ 3,900rpm, 293tq @ 2,900rpm,
Thread Starter
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
I would try it out with the injectors you have, they're not that much bigger than stock squirters (19lb'er). The ECU will likely be able to correct the fueling in closed loop with the 21lb injectors. However, in open loop it will be a little rich because it will be firing the 21's from calculated tables, as if they're 19's.
As for the timing, if you are set on running 87, then try it and see how it behaves under heavy load conditions. My guess is that you will end of having to reduce the base timing a degree or two to keep it from pinging (which will make it run a little more sluggish), or move up to a higher octane.
As for the timing, if you are set on running 87, then try it and see how it behaves under heavy load conditions. My guess is that you will end of having to reduce the base timing a degree or two to keep it from pinging (which will make it run a little more sluggish), or move up to a higher octane.