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Discussion Starter #1 (Edited)
Hey guys.

I have never received a definitive answer about throttle body size.

Here are my LT1 mods:

cam: 219/227 .549 .565 111
heads: LE2 205cc runners shaved to 55 cc combustion chambers
Intake Manifold: LE ported LT1 intake
Injectors: Bosch III 28#
Headers: Custom built 1 7/8" primary to 1 1/2" to 2.5" collectors Tri-Y long tubes

So, should I stay with the stock throttle body or go to a bigger size?

I have heard that the larger throttle bodies will cause a drop in engine torque output.

Thanks to anyone who actually knows the answer here.

Best,

Tim
 

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I can't say I have experience deciding throttle body sizes, but I have read that a 52 is good for even a stock engine but going to a 58 without needing it will hurt your torque.

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Remember if you change your throttle body,you need to have your ECM
reprogrammed. Typically you will loose a little bottom end when you increase T.B or Carb. size because you slow up the velocity within your intake system. Still no answer to your T.B question though. ed
 

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GM uses a 48mm throttle body on the 502 RamJet engine making 500 HP and all the torques most folks would need. Back in the days of those "semi-regulated gas leaks" called carbs, too much carb would cause a loss of torque and could make the car fall flat on its face if floored from a standstill. This was because the airflow also "sucked" the gas from the float bowl.

We have injection and also have MAP and MAF sensors to let the ECM adjust for plenum vacuum. The MAF can flow around 600 or something like that so why would you need a 1000 CFM throttle body unless you're going to speed density tuning? With all that said, I do use a 58mm on most of my builds and have read that there is a small gain to be had on a modified engine.

Think of a carb. Take one that flows 750 CFM will support a lot more horsepower than you're making with your mods so why go too big on the TB?

Here are a couple of flow numbers:

Stock TPI/LT1 48mm Throttle Body w/o airfoil -- 783.0 cfm (500 HP and 500 TQ on a RamJet 502)
Stock TPI/LT1 48mm Throttle Body w/ airfoil -- 821.9 cfm
TPI/LT1 52mm Throttle Body w/o airfoil -- 848.9 cfm
TPI/LT1 52mm Throttle Body w/ airfoil -- 898.8 cfm


Really, I like the 58 mm TB's but they're not really necessary. They won't hurt anything and it'll feel faster because of the increased throttle response. And BTW, the airfoils do seem to make a small improvement. Wheather or not you need that improvement for horsepower will depend on your build and the thickness of your wallet. Have a great day and good luck with your decision.

 

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I would run a 52mm TB on your setup. .

You wouldn't have to reprogram the PCM/ECM either.

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yes you will need to reprogram the PCM if going with any TB larger than stock. It is the line pressure of the tranny mostly that needs adjusting or tranny failure can/will occur.

Tim.

If you go larger, IMHO, get the stock TB bored to 52. It will be more "plug & play" in terms of idle quality. A 58 mm TB will require you machine (bore) the intake manifold opening. Not needed if going up to a 52mm

Aftermarket TB's don't have the idle air inlet hole in them and often cause idle issues because of this. The "drill" mod can resolve this or the "slot the TBS screw holes" and crack the blades of TB open slightly. The later is not a method I subscribe to. You will need a scan tool to see wtf your IAC counts are with a aftermarket TB and make either of the TB mods from there to correct what will very likely be idle issues when going to a aftermarket/larger TB
 

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Ive got a 58mm on my setup and i like it i have a big cam new custom heads port and polished intake 32lb injectors and headers with no cats and a borla catbac and the car is very responsive its got alot of torge but i did everthing at once so its hard to say just hom much diff the tb made. And u shouldnt have to get it tunned the computer should automaticaly b able to adjust for the extra air comming in. Those computers are actually very smart. My car was tuned but it had to be tuned for the cam
 

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Ive got a 58mm on my setup and i like it i have a big cam new custom heads port and polished intake 32lb injectors and headers with no cats and a borla catbac and the car is very responsive its got alot of torge but i did everthing at once so its hard to say just hom much diff the tb made. And u shouldnt have to get it tunned the computer should automaticaly b able to adjust for the extra air comming in. Those computers are actually very smart. My car was tuned but it had to be tuned for the cam
You will need it tuned for correct IAC and blade position, because most if not all aftermarket castings are made to be for TPI cars. So the IAC is crappy and most end up with holes drilled in throttle blades to hold a decent idle.

Also, as I am sure detroitX will chime in, an untuned auto car with a large throttle body will not work well for you as far as line pressure to throttle position goes.

And LT1 PCMs are pretty dumb. :roflmao:



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GM uses a 48mm throttle body on the 502 RamJet engine making 500 HP and all the torques most folks would need.....
An engine shop I know of recently did a before/after engine dyno where a 502 ramjet picked-up 32HP just from going to a 52mm TB. Which basically validated everything I''ve said about GM and their 48mm TB on this engine.

Understand this.....the ONLY reason GM put the 48mm TB on the the 502 was because the 48mm TB is the only twin-bore TB that GM ever made or contracted to be made. C'mon.....you didn't REALLY think that GM would re-tool their machinery JUST to ensure they put the proper part on a limited production engine.....did you? They only used that TB on the ramjet because it's what they had on hand and they were looking to keep costs down.

And remember......this TB was originally designed for the 305ci TPI engine. So to state that it would also be sufficient for a 502 big block is to ignore the history behind this part.


IMO......a stock LT1 screams for more than the 48mm TB......and a heads/cam LT1? Definately a 52mm TB.

KW
 

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New 2 Chevy, Not sure what your plans are for your new throttle body,but
I would be careful with how you proceed. First off all PCMs have
the ability to make minute adjustments,they were basically designed to adjust for weather conditions,altitude and drivig habbits,but were not designed to make broad sweeping changes such as component changes and the like. If
you like surprises, then leave your PCM alone and see what you get, If you want to follow another school of thought
have a dialogue with a good tuner like Bryan Herter and see what he says. Just sayin, ed
 

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i would go with a 52mm and heres why

i think the whole "bigger TB = less torque" thing is associated with stock head/cam engines that wouldnt be able to use a bigger TB to its full potential. lets face it, people love bolting stuff on engines and i could see why someone would want to upgrade their TB as one of their first mods

but once you start upgrading internally, i cant see why a bigger TB would be a bad thing. your heads are opened up, and your cam is on point, why not feed it? especially with a ported intake
 

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I can tell you from experience at the track when I was running high 11's that a 58mm TB vs stock only picked up about one tenth in the 1/4 and tuning is highly recommended to accommodate for the higher volume of air at lower throttle position.
 

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An engine shop I know of recently did a before/after engine dyno where a 502 ramjet picked-up 32HP just from going to a 52mm TB. Which basically validated everything I''ve said about GM and their 48mm TB on this engine
I heartily agree with everything you said. I only mentioned the 502 to show that a stock 48mm can support 500 HP. On that size engine I'm certain more would be better. The 48mm still flows over 750 CFM and many, many old school carb engines make 550 HP+ with a 750 double pumper. Put a 1000 CFM 3 barrel carb (58mm=1000CFM also) on a street engine and it would be way over carbed. Heck, I put a 1000 CFM 3 bbl on my new stock 1969 440 Charger (a few years ago)and when I stood on the gas it just about died. I knew less then than I do now.

Now if he was going to be twisting that 383 to 7500-8000RPM's and making 500-600 HP then absolutely upgrade. I just don't think there's that much to be gained from his fairly mild 219/227, 28# injector build. But, as I said earlier, I do like the 58.........a lot. I'd certainly use one but he shouldn't need to.
 

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The throttle body on an LT1 is simply an air valve. Comparisons to carburetors have very little relevance. Carburetors that are too large for a particular application hurt performance for reasons having to do with diminished air velocities through the venturis. The fuel delivery in an LT1 is, obviously, different.

I like the 58mm TB's because of the crisp feel that you get at low throttle settings due to the larger throttle plates. A little bit of throttle opening lets in a large amount of air when compared to a stock TB. I, personally, like that. Does an LT1 need a 58mm TB...? Probably not...especially a stock LT1. Will a larger than necessary TB hurt anything...? Not at all...

My advice would be: if you're going to the expense of buying a TB of any kind, buy a 58, recalibrate your TPS maps to keep the tranny happy and enjoy it.

The big question I would ask the OP (who appears to be MIA in this discussion...) is: how did you arrive at that cam/heads combination? Those 205cc intake runners are awfully large for that cam...
 

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Also, as I am sure detroitX will chime in, an untuned auto car with a large throttle body will not work well for you as far as line pressure to throttle position goes.
I melted... one... two...three... Yes, three clutch packs in my trans last season. A lot contributed to the issue but a primary suspect was the trans tuning itself as it wasn't spot on. I have a 58mm TPiS TB and without close look into the trans tuning the line pressure can be messed up in a 4L60E which causes premature wear in your clutch packs. Now that the transmission tuning has been gone over and nailed down I have a 65E clutch pack (sub par compared to another clutch stack I wish I had) surviving a beating.

So yes, if you switch TBs and have a 4L60E you NEED a tune. 700R4 or T56 I'm not sure.
 

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Discussion Starter #15 (Edited)
Wow guys looks like I started a good thread here.

I'll be back on in a few hours.

My car is a 95 Bird. The Tri Y headers are being custom built with my car in the header shop.

Lloyd Elliott did my heads and had Lunati grind the cam for me and he also ported my intake and made the TB holes big enough for a 58mm TB.

My cam choice makes gobs of torque according to Lloyd.

Just having second thoughts about using such a large TB. I want my setup to be as close to perfect as possible.

Seems the factory TB bored to 52mm is a good choice. Who does this work?
 

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Discussion Starter #18 (Edited)
Guys I appreciate all your input. I really do! Thank you :)

Yep Lloyd recommended the 58mm, and I bought it already. I didn't mention this, but it's irrelevant to my original question.

I should just trust Lloyd. He knows his stuff, but I assumed he only knows about heads and camshafts.

So what's all this about the TPS needing to be swapped to an adjustable one?

This I don't understand. I know my IROC had an adjustable one, from the factory.

What's the deal with the IAC? These fixtures and sensors swapped right over to the Professional Products 58mm TB that I already have. And I have a good tuner who sends me tunes via email, since I have the software and cable and all that.

And I am thinking I will be plenty happy with my 219/227 cam. Lloyd said this cam makes great torque and has tons of usable power. Also, I used the Crane springs that they recommend. I'm not so sure these springs would work properly with the 226 cam Lloyd has. Anyway, bigger is not always better. Heck my car should feel a lot faster, even before my head and cam swap, with all the suspension stuff I am putting on it and with the new Eaton and 3.73's.

Has anyone here run a similar cam to mine?

I know most guys go with at least the 226, but whatever.
 

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Wow guys looks like I started a good thread here.

I'll be back on in a few hours.

My car is a 95 Bird. The Tri Y headers are being custom built with my car in the header shop.

Lloyd Elliott did my heads and had Lunati grind the cam for me and he also ported my intake and made the TB holes big enough for a 58mm TB.

My cam choice makes gobs of torque according to Lloyd.

Just having second thoughts about using such a large TB. I want my setup to be as close to perfect as possible.

Seems the factory TB bored to 52mm is a good choice. Who does this work?
Hey, there he is...! :lol:

Don't get me wrong... I have no problem at all with the cam choice. And, yes, it will make a ton of mid-range torque. I just think that it would work better with a 185-190cc intake runner. 205's seem a bit on the large side to me to be using with that cam grind.

Don't worry about it...! Grab a 58mm TB and don't look back. Does your engine need it...? Nope... Will it hurt anything...? Nope...! Not as long as the tune compensates for the change.
 

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Discussion Starter #20
Yep here I am. Was at work all day and crashed hard last night after a long day.

Anyway, yeah those heads could definitely use a larger cam.

I am tempted to go with a 226 but I am not sure if the Crane springs will work with it. I know they will handle the lift but not sure if they have enough pressure.



Hey, there he is...! :lol:

Don't get me wrong... I have no problem at all with the cam choice. And, yes, it will make a ton of mid-range torque. I just think that it would work better with a 185-190cc intake runner. 205's seem a bit on the large side to me to be using with that cam grind.

Don't worry about it...! Grab a 58mm TB and don't look back. Does your engine need it...? Nope... Will it hurt anything...? Nope...! Not as long as the tune compensates for the change.
 
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