LS2/7 ETC TB startup

[Bruce Melton]

Assuming a 402 (C5 ETC) moving from 78mm/LS6 to a 90mm LS2TB/L76 setup what should be changed and maybe a starting value going in?

B4349 ETC throttle area conversion>?
B4403 IAC effective area>?
B4307 Desired airflow>?
Other>?

I have seen various attempts to calculate B4349 and they are all over the map from leave it alone @.0255 to .0157 to .0330.
Can someone point me in the right direction on the above?

Did this recently with a LS7 and am not sure my approach was from the proper starting point re. above values.
RAFIG scan values we half ~7 vs~3 in B4307.
IAC steps were maxed.

Desired airflow is really the air required to provide a AFR that works @ idle? Thus if you AutoVE the idle to run @14.7 then the Desired airflow is used to lean beyond the ~14.7 due to cam overlap?
When you enter the B4307 values will it go there absolutely or will other factors contribute to a commanded blend shown in RAFIG?

(engine specs in sig below)

Thanks,
Bruce

[Redline Motorsports]

Bruce,

Not for nothing but I have had a couple C5 ETC cars that we installed 90/90's on in which we changed B4349 from .0255 to .0157 and nothing else and the cars ran and acted like stockers!

This is a great topic which seems to constantly be a debate. My thought is that this parameter is there to tell the PCM the actual area of throttle bore it is working with. We know at .0255 that the car runs fine. If you increase the throttle bore the effective area is increased and if the PCM doesn't know it, it is seeing more air then it is expecting.

I also look at this table kind of like the IFR table. If all you did was install 30lb/hr injectors in a car that had 26lb/hr, the change in the IFR table will keep all other tables in check and the car should be where it was before the change. Now granted we know there are other tables like injector timing etc... that sometimes have to be trimmed but you see my point?

Somewhere I did a whole math runout that got the .0255 to .0157, I'll have to see where it is.

Howard

[Bruce Melton]

Howard,
I have put Fast 90s with LS2 TBs on Vettes with no value changes, but when you change displacement and all it does not sort out as well (even when you change cyl vol).
We need a proper starting point.
The LS2, and probably LS7 stock tunes do not seem to have a TB area (b4349) yet it seems the lower than .0255 values inhibit the ability to "find idle".
I Explored this with Marcin and he does not have personal experience in this area but he was told to increase the effective area value by 48%! Logically it would seem that 90/78= ~15% = ~.0293 (.0255 stock value for 78mm) ? Anyway, larger rather than smaller value.

[redhardsupra]

it's not 90/78, it's area of a circle we're comparing here, and areas of circles have r^2 not r in them, so it's 90^2/78^2

[Redline Motorsports]

Regarding the effective area when going from a stock to 90mm on an ETC car;

I was under the assumption that the effective area is directly related to the area of the throttle blade. Based upon this I have done this math;

Pi*radius (squared)

90mm/75mm

(Pi*45*45)/(Pi*37.5*37.5)
(6361.725)/(4417.864)
1.44 or 44% change to the effective area

I don't have my 'black book" of notes but I think I ended up at a value of .0157 for effective area with the 90mm.

I have experimented with several cars with 90's and with just changing that parameter alone took car of 90% of the idle issues with a 90. The classic high idle touchy throttle goes away and most of the other idle parameters need either none or little adjustment. It would make some sense that as long as the PCM knows the new effective area that it would rescale all other tables so they wouldn't need to be stretched dealing with the larger area. I guess kind of like changing the axle ratio on an automatic and its effects to the entire shift schedule..

From a previous post I had out there.

[Bruce Melton]

it's not 90/78, it's area of a circle we're comparing here, and areas of circles have r^2 not r in them, so it's 90^2/78^2

I knew that just did not apply it

So:
78= 4776
90= 6361
133%(.0255)=~.0334

[redhardsupra]

Redline:
you can drop the PI and /2 from both terms, since they're gonna cancel each other out anyway, then you're just left with d1^2/d2^2

how do you go from the ratio to the 0.0157 number and what units is it?

[Redline Motorsports]

RHS,

I found some old notes from a while back and this is what I had;

Cross sectional area of a 78mm= 4776 sq.mm
Cross sectional area of a 90mm= 6359 sq.mm

Units for B4349 are %/sq.mm

100%/4776 sq.mm= .0209 %/sq.mm for a 78mm TB (many factory tunes are at .02077)

100%/6359 sq.mm= .0157 %/sq.mm for a 90mm TB

It is assumed that the engines airflow requirements haven't changed if the TB was the only thing changed. By changing B4349 it should put all the other parameters in perspective. I'm sure theres a bigger formula inside these PCMS which use B4349 to scale the other parameters. This again is just my thoughts and is open for abuse!

Howard

[Bruce Melton]

From the repository it looks like most stock Camaros and Corvettes are .0255 and trucks are .0208.

[Redline Motorsports]

From the repository it looks like most stock Camaros and Corvettes are .0255 and trucks are .0208.

Like I said......kind of a made up theory that seemed to have worked! Still looking for more views on it. We need a GM engineer to chime in!

Howard