Hi all,
Tried and tried to fix a high idle problem with my new HSR combo. Turns out the much bigger TB was screwing everything up. Perhaps its just too much air at idle on my stockish motor.
Stuck with the stocker TB, runs like a champ. I'll try the bigger TB if i stroke it and put on better heads maybe.

 

well yea the 58mm is wayyyy to big for your 355. unless it had a big cam and highflow heads... but it wouldnt run the idle up really high unless u had it set that way. maybe u messed the IAC up installing it. or the idle air asjustment screw was dialed in wrong. thats the only way... it still woulda idled fine. how much of a diff did the HSR make?

the 48mm tb isnt a restriction up until about 350hp. 52mm is good from there to about 450-500. then u need the 58mm for anything above and in a 383 with high RPM reving.

 

It's funny ford uses a twin 55mm throttle body on there 4.6L mustangs stock and people get worried about using a 58mm on a 5.7L

Sounds like you had a problem with your idle air adjustment screw or with the IAC. Could also be the thing was machined wrong to.

 

yea well most who put on 58mm tbs on a stock 350TPI will lose throttle response and low end tq thats why they dont put them on until they start stroking or adding HP. i didnt kno they had 55mm TBs on stock 4.6L... arent they like the LS1's and use single blade one opening TB? the only one that uses a double blade is the new mustangs but theyre pushing over 300hp off the showroom floor not 225hp like the TPI 350

 

Got any dyno charts to back this up? Cause I have yet to see any real proof besides people repeating the same BS talk that's been going on here for years. And yeah I used a 58mm TB when my car was fairly stock with no loss in throttle response and low end tq.

 

Yeah the 58mm came with a deal i got second hand off another TGO member who had it on a 383 w/trickflow heads. I new the 58mm would be too much, but wanted to try it coz it looked sweet! Anyhow i didnt think it would affect the idle so much even when i didnt touch the iac or idle or anything like that. I checked it all is case i had but nothing came up. The only thing i can think of is that is the iac cavity might have been machined a little large..?

I am yet to see a gain with the HSR at ther strip but can feel the effect of the torque curve being moved up though. Two reasons for me not seeing a gain (i think) would be my lack of prom tuning ability (it currently runs like a pooch - coz i fiddle all the time lol), and the stock convertor/2.77 gear combo. I seriously gotta get some 3.73's/3000rpm stall!

 

I had a 58mm BBK throttle body on my stock 305 briefly. My idle was way off and the car kept stalling until I adjusted the idle screw and follow the other idle adjustment procedures.

On the completely stock motor with only the throttle body changed I didn't notice any significant loss in throttle response. When I left the car in drive and planted my foot the car used to change at 4500rpm every time but after I changed my throttle body it changed at 5000rpm.

I was just testing my car by seat of the pants feel so I have no numbers to show weather it made the car faster or slower but I had no major negative effects like everyone will say you'll have. I say try adjusting the idle on the 58mm and try it for yourself and see how it feels.

 

I tried and tried to do just that. Did everything over and over. Dunno. Might give it another shot later on. Al i know is that when i swapped back to the stock TB, she was right straight away. It's a mystery...

 

hmmm.... wat about the TPS... should be in the .52-.56 range at idle. if its low its gona mess wit the idle and same thing if its higher...

as far as proof... TPIS did tests in there book "TPIS insider hints" they tested stock TB's and aftermarket TB's and theres no reason to ever go higher than a 48mm unless ur pushing around 400hp bc then it becomes a restriction. its a simple thing to figure out. a larger TB will decrease velocity since u will be sucking in air throught a larger hole... its like sucking water through 2 diff size straws, and the TPI relies on velocity since its TUNED PORT INJECTION!!

altho sum wont see a change in seat of the pants hp from too large a TB im sure it would show sum differences on a dyno. most ppl cant feel a 5-10hp diff at the motor

 

Funny you mention the TPS as the 58mm TB is a LT1 model. So i spoke to TPIParts.net and was assured the TPS sent the same voltage signal, got an adaptor harness/TPS and all was good. Until i tested the TPS voltage at idle. Too high. So i drilled and tapped the TB and rotated the TPS. Made no difference, but its like Holley put the TPS mount holes in the incorrect orientation. The TB must have been made late on a Fri arvo!!

 

so the drilling didnt help? was the reading in the .5 range? i think the TBs are made for a few diff styles besides TPI and LT1... i would think u had gotten one for a diff style like u think... thats prob why the idle isnt correct.

my 58mm professional products is a TPI/LT1 model and inorder to swap it to TPI i just had to swap the cable linkage

 

funny, i cant get my holley 58 to idle high enough for my 383

you have to reset IAC and adjust TPS for this throttle body. For my 89 my TPS wouldnt fit the TB without binding the TB blade shaft screw. I had to put a few small washers under the TPS bolts to move it away from the TB so when snug, it didnt bind up the TB shaft. else the TB would open and never close back

tune will help the low end thats for sure. I found 15lbft at the wheels through the rpm band from 3000-4000 rpms and about 8-10lb ft from there on up, with just simple timing adjustments in the 3000-4800 rpm range in the spark table. And i was very lean, more was to be had. STill ran 12.9's in the winter with that setup...just bolt on L98. Gotta love HSR!
bring in full timing of roughly 35-36 degrees by 3000 rpms and your gonna pick up. once you get 3000 stall and 3.73 gears, that car will be an animal

 

"a larger TB will decrease velocity since u will be sucking in air throught a larger hole... its like sucking water through 2 diff size straws, and the TPI relies on velocity since its TUNED PORT INJECTION!!"

The "TUNED PORT INJECTION" begins with the runners at the plenum. Has nothing to do with the plenum, throttle body or cold air intake.

You want the velocity to be slower going through the throttle body into the plenum. With the air moving slower it is easier for the air to make 90 degree turns into the first sets of runners. The air mass wants to go straight ahead and to the back of the plenum.

So with faster air the rear sets of runners will have a tendency to run lean and the front sets of runners will have a tendency to run rich. A bigger cold air intake and throttle body helps to slow the air down and even things out somewhat in the plenum.

 

CUSTOMBLACKBIRD: Yeah the drilling did help bring it just into the limit with TPS voltage. It worked fine mechanically so i dont know whats up. Perhaps the previous owner did a mod to it. I wondered why he purchased an LT1 TB with the HSR, and didn't tell me bout it.... It came in an LT1 box and i had to use and LT1 TPS.
ORR89ROCZ: I tried to reset IAC na all that stuff, to no avail. Might try in the future once the motor mods warrant it. I agree with what you said about tuning. It runs rough as down low/mid at the moment, but cleaner up top. Thats what i want to sort out. I have an Innovate wideband gauge so that helps a lot. It constantly wanders with the AFR though, but gives a pretty good indication. Hopefully my tuning skills will increase and get it running like stock - smooth as, easy starting, no off-idle hiccups, etc.
Thanks for the spark advice, i need it lol!

CHEERS!