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Thread: TBI throttle body, restrictive or normal?

  1. #1
    Fuel Injected! mmigacz's Avatar
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    TBI throttle body, restrictive or normal?

    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?
    Last edited by mmigacz; 09-10-2014 at 09:12 PM.
    1986 ski centurion boat, 351W, gt40P heads, edelbrock performer rpm, 1227747 ecm, 72 lb/hr injectors (@18 PSI fuel pressure), .490/.490 lift, intake 278 duration, exhuast 282 duration, 112 lobe separation

    1969 camaro, 355ci, performer RPM intake, camel hump heads, 1227747 ecm, 454 throttle body (@18 PSI fuel pressure),

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    Fuel Injected! BLG355's Avatar
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    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.
    My Build: 95 K1500, 355ci w/ OEM roller setup - zero decked, Eagle steel crank, Scat bushed H-beam rods, Speed Pro 2V flat top pistons - Rotating assembly balanced. Dart 165cc Iron Eagle S/S heads - 72cc chambers - 1.94 / 1.50 valves. Isky roller cam - 204/209 dur @ .050 - .480/.496 lift with Comp Magnum 1.6 rockers, Edelbrock 3704 intake - Bored to 52mm - 454 throttle body, Delco EP381 fuel pump @ 18 psi running through 80# 454 injectors. Hedman Headers into 3" Dynomax exhaust.

  3. #3
    LT1 specialist steveo's Avatar
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    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.

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    Fuel Injected! 1BadAction's Avatar
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    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.
    94 Blazer, Turbo'd 350 TBI - DD
    1991 2500 Suburban Adventure truck - 4wd conversion, 4-link F/R, 582ci CNP Big Block with Terminator X EFI backed by a 6L90 and twin stick'd NP205 t-case
    2012 Porsche Panamera Turbo - Date night car :)
    1979 16' Action Marine/"Johnny Cash" Merc Bridgeport Champ Motor - Metalflake Maniac

  5. #5
    Electronic Ignition!
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    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.

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    Fuel Injected! mmigacz's Avatar
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    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).
    1986 ski centurion boat, 351W, gt40P heads, edelbrock performer rpm, 1227747 ecm, 72 lb/hr injectors (@18 PSI fuel pressure), .490/.490 lift, intake 278 duration, exhuast 282 duration, 112 lobe separation

    1969 camaro, 355ci, performer RPM intake, camel hump heads, 1227747 ecm, 454 throttle body (@18 PSI fuel pressure),

    2016 Chevy Cruz

    2014 Chevy Silverado

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    Electronic Ignition!
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    Quote Originally Posted by mmigacz View Post
    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.

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    Personally I would take a couple of hours with a grinder, some carbides, and sanding rolls and perform the Ultimate TBI mods. Combine it with an injector spacer and if you are running a dual plane manifold a 1" open center throttle body spacer. A stock throttle body flows about 490 cfm with the IAC closed and about 500 with it fully open. With ultimate TBI mods its about 540 CFM iac closed and 550 cfm with the IAC fully open. The larger big block throttle body is 660 cfm with the IAC closed and about 685 cfm with it open.

  9. #9
    LT1 specialist steveo's Avatar
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    ultimate tbi mods help that much? well that might just make up that 8kpa of vacuum without too much dicking around with the results of going for bigger tb bores..

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    Fuel Injected! BLG355's Avatar
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    Quote Originally Posted by steveo View Post
    ultimate tbi mods help that much? well that might just make up that 8kpa of vacuum without too much dicking around with the results of going for bigger tb bores..
    That's what I was just thinking. I have done the mods and I am at 88kpa @ 5600rpm, I guess it would be a lot worse without them done.
    My Build: 95 K1500, 355ci w/ OEM roller setup - zero decked, Eagle steel crank, Scat bushed H-beam rods, Speed Pro 2V flat top pistons - Rotating assembly balanced. Dart 165cc Iron Eagle S/S heads - 72cc chambers - 1.94 / 1.50 valves. Isky roller cam - 204/209 dur @ .050 - .480/.496 lift with Comp Magnum 1.6 rockers, Edelbrock 3704 intake - Bored to 52mm - 454 throttle body, Delco EP381 fuel pump @ 18 psi running through 80# 454 injectors. Hedman Headers into 3" Dynomax exhaust.

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    Quote Originally Posted by BLG355 View Post
    That's what I was just thinking. I have done the mods and I am at 88kpa @ 5600rpm, I guess it would be a lot worse without them done.
    Try a 1" open center spacer, I bet that you end up increasing your MAP by 3-5 KPA alone. My old 83 G20 was making 245 RWHP with the stock throttle body. Removed the choker ring, ultimate TBI mods, injector spacer, and a 1" open center throttle body spacer and was making 265 rwhp.

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    Fuel Injected! mmigacz's Avatar
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    Quote Originally Posted by Fast355 View Post
    Personally I would take a couple of hours with a grinder, some carbides, and sanding rolls and perform the Ultimate TBI mods. Combine it with an injector spacer and if you are running a dual plane manifold a 1" open center throttle body spacer. A stock throttle body flows about 490 cfm with the IAC closed and about 500 with it fully open. With ultimate TBI mods its about 540 CFM iac closed and 550 cfm with the IAC fully open. The larger big block throttle body is 660 cfm with the IAC closed and about 685 cfm with it open.
    I didn't know about ultimate TBI Mods. I researched these modifications last week and completed all the grinding and porting changes this weekend on a spare throttle body I had. I'm waiting for the spacer and rebuild kit to be shipped to me. Thanks for the suggestion!

    I'm curious on something. I was looking on ebay under some throttle body's that come with the mods. The seller was advertising 650 cfm on a 46mm bore throttle body. Does this seem too high?
    1986 ski centurion boat, 351W, gt40P heads, edelbrock performer rpm, 1227747 ecm, 72 lb/hr injectors (@18 PSI fuel pressure), .490/.490 lift, intake 278 duration, exhuast 282 duration, 112 lobe separation

    1969 camaro, 355ci, performer RPM intake, camel hump heads, 1227747 ecm, 454 throttle body (@18 PSI fuel pressure),

    2016 Chevy Cruz

    2014 Chevy Silverado

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    Quote Originally Posted by mmigacz View Post
    I didn't know about ultimate TBI Mods. I researched these modifications last week and completed all the grinding and porting changes this weekend on a spare throttle body I had. I'm waiting for the spacer and rebuild kit to be shipped to me. Thanks for the suggestion!

    I'm curious on something. I was looking on ebay under some throttle body's that come with the mods. The seller was advertising 650 cfm on a 46mm bore throttle body. Does this seem too high?
    Seems a little high, but not impossible. Remember that the values I mentioned were actually flowed DRY at the 4bbl standard of 1.5 in/hg, rather than the traditional 3" of a normal 2bbl carb. The ~50mm 454 TBI unit is 660 @ 1.5 in/hg and it does not have the benifit of thinned throttle shafts and thinner blades like some aftermarket bored throttle bodies do. It would not suprise me if they advertise 650 cfm at some other (higher) pressure drop. 650 cfm @ 1.5 in/hg dry flow is more like 580-600 cfm wet.

    I will be interested to see your results, hopefully very positive results like I have seen.
    Last edited by Fast355; 09-15-2014 at 04:27 PM.

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    Fuel Injected! mmigacz's Avatar
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    Fast 355, you mentioned an extra 10 cfm with the IAC completely open (compared to closed). I noticed that my IAC is limited to 60 steps at WOT. Should I be increasing the max throttle follower steps to something higher? If so, how many steps?


    Also, I ordered a 1/4" plastic injector spacer. Do most people substitute the spacer with the stock gasket, or do they add the spacer and the gasket together?
    1986 ski centurion boat, 351W, gt40P heads, edelbrock performer rpm, 1227747 ecm, 72 lb/hr injectors (@18 PSI fuel pressure), .490/.490 lift, intake 278 duration, exhuast 282 duration, 112 lobe separation

    1969 camaro, 355ci, performer RPM intake, camel hump heads, 1227747 ecm, 454 throttle body (@18 PSI fuel pressure),

    2016 Chevy Cruz

    2014 Chevy Silverado

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    Quote Originally Posted by mmigacz View Post
    Fast 355, you mentioned an extra 10 cfm with the IAC completely open (compared to closed). I noticed that my IAC is limited to 60 steps at WOT. Should I be increasing the max throttle follower steps to something higher? If so, how many steps?


    Also, I ordered a 1/4" plastic injector spacer. Do most people substitute the spacer with the stock gasket, or do they add the spacer and the gasket together?
    I for the life of me cannot remember the maximum IAC opening of a TBI. Its either 160 or 255. I used to max out the value for throttle follower and increase the decay rate considerably.

    On a factory big block it is just substituted. I think some people add both for additional height. That being said you may have to grind on the back of the TBI unit for clearance for the fuel line fittings.

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