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I've posted a screen shot of the modifications (compare changes) I did to tune a L31 Vortec with a stock roller cam using stock TBI Heads and a stock TBI Intake. The original $42 .bin was the Hypertech 128192 chip to begin with. For a budget performance engine, I think the L31 / TBI combination makes for a good dollars and cents build! The Hypertech .bin needed lots of help with the timing table:yikes: to work with the L31 Vortec roller camshaft.
dave w
not really supprised. I had to give mine alot more timing to run the vortec heads and not be a slug on wheels.
But this one is TBI heads. Dave is the man when it comes to finding max timing advance... Vortec would have been more.
ah i misread that. I thought it was a vortec with a tbi. I also looked at the bin and screen shot. man did he add some timing.
The 1988 K2500 is using 87 Octane fuel (Washington State Law Requires 10% Ethanol) with the timing table changes I posted. I think Ethanol helps run more advance?
The Vortec short block assembly was built using cast dish pistons, to save on the budget and keep the compression ratio close to 1988 TBI engine specs.
Fact or Fiction ... Timing is more HP than Compression?
dave w
I would expect less, not more, advance with L31 heads. Swirl port heads do have better combustion properties than older versions but the L31's are a generation advanced again.
Compression increases provide diminishing returns and the percentages are not high above 9.5:1. As compression increases the practical spark advance limit decreases within a given octane rating. Excess spark advance is no better than excess compression. Forcing a piston toward TDC as combustion begins creates power loss and at some point that loss can be great enough to offset gains.
In theory we would have instantaneous combustion slightly prior to 90 deg ATDC. Ideally, if we could get all combustion to occur from 0 deg to 90 ATDC we could extract the most work. As a practical matter due to the speed of combustion we have to start ignition BTDC with the goal of attaining peak cylinder pressure at 12 - 15 deg ATDC. Typically this provides the maximum average pressure and maximum power. Fuel composition, combustion chamber design, rod:stroke ratio, piston dome shape, engine temp, fuel temp, air temp, spark plug type are all factors in combustion speed. But more advance = more power has not been a good strategy for many years.
Ethanol tends to require more advance. The oxygen molecules in the ethanol can create leaner mixtures, the flame rate is different, and the higher latent heat of vaporization requires more energy to convert liquid fuel into gaseous form for burning. People tend to think that ethanol allows more advance but again, this infers that more advance is a universal goal. In truth replacing gasoline with appropriate mass of ethanol in a properly tuned engine will result in a power loss without increasing advance as well. If ethanol allowed more timing then it would seem this fuel swap wouldn't require any change in spark to produce the same power and adding advance would create additional power. But it's not to be so.
I wasn't sure if the ethanol was a factor with the timing table I developed or not. I thought board members should know the fuel used to develop the timing table had 10% ethanol.
dave w
Dave, I was responding to "Fact or fiction" question.
I spent a bunch of time getting my Sunbird to do what it does on a mostly stock configuration. I was using 89 octane fuel pre ethanol days along with a couple of ounces of acetone and MEK per 10 gallons of gas. Then along comes E10 and I have to back off boost and timing slightly because my injectors are maxed out. I would say it is important to mention fuel type.
With my tuning experience, I've had to "Chip" several poorly planned SBC engine builds including compression ratio's at or slightly above 9.5:1. I've tried several different ways to get enough timing in a 9.5:1+ engine, and never really got excellent results. DataMaster TTS has a chassis dynometer function when data logging. It seems to me, or maybe because the way I tune, I can get better overall performance with a compression ratio of about 9:1 or slightly less and lots of timing ( 38 ~ 42 degree is most cases in the 20 ~ 40 Kpa portion of the timing table). Typically I seem to avoid spark knock with timing between 12 ~ 18 degrees in the 80 ~ 100 Kpa portion of the timing table.Quote:
Originally Posted by 1project2many
dave w
More compression equals less timing on same fuel.
12 to 18 in 80 to 100 MAP but 30 (safely) all in by 3500 RPM to 6000 then start to pull some on todays 93 pump gas.
Knock sensor is lieing... :innocent2:
SCR above 9.5 can require some thinking. Fast burn heads, proper quench, no hot spots in chambers, flat top pistons all make high SCR more likely to work. Older cams with more overlap can self-egr to some extent and will have higher spark advance numbers, esp. if exhaust is restrictive. Open exhaust, good scavenging, low overlap can require less timing. I tend to assume there's knock when the KS says so unless I've got a very good reason not to. But if the engine provides expected power, returns good mileage, and stays together, then you've got the timing right, regardless of what you expect the numbers should be.
Ummm...the 3/4 ton TBI 350's were 8.5:1.....did that change with the pistons?
The engine math came out to 8.63:1 with the 423NP pistons and stock 65cc TBI heads.Quote:
Originally Posted by jameslleary
dave w
AFAIK the L05 was ~8.5:1, I believe they were referring to 9.5:1 as being sort of a point at which a lot of things change regarding ignition advance and power increases when running on pump gas. As far as the significance of the fuel used for tuning being E10 for this particular setup it does matter a bit and is definitely worth mentioning. 1project probably summed things up better than I could have, but I still felt compelled to pitch in my 2 cents. As long as the engine being tuned isn't highly modified and/or running right at the limit in one way or another then 0-10% ethanol in the fuel isn't going to have a significant impact on things. With the vast differences in the formulation of gas from one refinery to the next you are going to see small differences in the fuel trims and spark advance limits. Even the gas that is sold as 100% gas is going to have some type of "combustion enhancing" additive in it. The difference is that the additive in the other types of gas is a petroleum distillate or refining byproduct of some sort.