TBI throttle body, restrictive or normal?

[mmigacz]

I have a stock 5.7 liter TBI (500 cfm) on my 351W (ford). I noticed while datalogging at WOT, that the Map values get as low as 92 kpa. When the engine is off, the map sensor reads 99 kpa. Suspecting that my air cleaner (spark arrestor on a boat) was the reason for the pressure drop, I took the air cleaner off and collected some more data at WOT. No difference.

Would it make sense to change the throttle body to a 454 throttle body (670 cfm)? This might be a waist of time, but I have a 454 throttle body to try. Any opinions?

[BLG355]

I have similar issues, mine dips down to 88kpa at 5600rpm. I too tried the air cleaner stuff but I am currently looking into doing a 454 TB to see if i can get that back up. I found a chart that converts kpa to vacuum, and mine is pulling over 4" of vacuum at that speed, pretty sure I need an upgrade, you are probably in need too.

[steveo]

definitely, bigger throttle body will be the winner. just keep a few things in mind.

just think that you're basically introducing an tps vs airflow multiplier by increasing the size of the throttle plates.

that's not a huge jump in size, but your throttle response can get a bit 'less linear', low to mid throttle will flow a crapload more air with the bigger throttle body.

you'll need to drive AE a bit harder, im not sure if it's a linear change that you can just calculate, but at least 15-20% change at low rpm?

also dont forget your transmission. if it's a cable driven automatic trans, you're going to be decreasing relative line pressure to throttle angle, which is bad. you can either get a shift kit to bump up line pressure or do some tv cable geometry. if it's an electronic trans, anything tps vs line pressure will need to be tuned.

[1BadAction]

Same deal here. 101kpa engine not running and mid 80s WOT. My problem is I'm already running a 454 TBI so I need to send mine away to have it bored. At that point I have a case of the "might as wells" and will want to do the intake, morel lifters and dizzy while the truck is down. I'm probably going to wait another month or so, too hot for that working on a truck shit right now, lol.

I was reading about vacuum at WOT and the rule of thumb with a carb is more than 2" is too much and you need to go bigger. I'd assume you can go bigger yet with a throttle body, since we don't require the same air speed as a carb for our part throttle driveability.

[Xfirez51]

I ran 2 x 2.13" TBIs on my 84 Xfire. MAP would drop 2-3 kPa at WOT. Figured I was flowing close to 750cfm. Power peak was at 4800rpm. Just giving you some kind of benchmark. The motor was a 5.7L w TFS 23d heads, 64cc chambers and 190cfm runners.

[mmigacz]

If I do try the 454 Throttle body, should I put the injectors from the 500cfm in the 670cfm throttle body? Currently, i'm running 18 PSI (72 lb/hr) of fuel pressure and the wide band shows no signs of running out 0f fuel (VE table is 80 at WOT, 12.8 AFR).

[Xfirez51]

If I do try the 454 Throttle body, should I put the injectors from the 500cfm in the 670cfm throttle body? Currently, i'm running 18 PSI (72 lb/hr) of fuel pressure and the wide band shows no signs of running out 0f fuel (VE table is 80 at WOT, 12.8 AFR).

Doesn't mean the injectors are not going static. Something that needs to be measured.