Tuned L31 Vortec with TBI Heads / Intake

[dave w]

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 to work with the L31 Vortec roller camshaft.

dave w

[PJG1173]

not really supprised. I had to give mine alot more timing to run the vortec heads and not be a slug on wheels.

[EagleMark]

But this one is TBI heads. Dave is the man when it comes to finding max timing advance... Vortec would have been more.

[PJG1173]

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.

[dave w]

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

[1project2many]

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.