70mm Throttle Bodies Bad?

[ViperTony]

I recently came across a set of 70mm TB's from SVS through their great sale (Thanks Ron!). The price was right and I figured it would be another excuse to get under the hood of my Viper and possibly do a write up of the install. I knew that by installing these TB's on my stock engine that I wouldn't see much gain in HP or TQ. I'm finding out through past threads here and abroad that installing 70mm TB's on a stock GenII engine will actually hurt performance and cause a loss of torque.

What I haven't been able to find out is why would 70mm TB's cause a loss of power on a relatively stock engine. I have installed Belanger headers and catback, hi-flow cats, K&N's, Smooth tubes, T&D rockers, Trend pushrods as well as a Vec3. My most recent dyno run (without the T&D's) produced 480rwhp and 525rwtq on a Mustang Dyno. There was power left on the table and I need to update my Vec3 to take advantage of it. I don't know what the T&D's have done to the numbers yet but I certainly feel it in the SOTP.

My assumption is that the TB's would compliment these mods. After all, if the engine has the mods to move more air and breathe better then adding TB's would be a good thing. Right? After all, the engine is a great big air pump. My second reason for experimenting with the TB's is that my Vec3 logs have shown that my engine wants to breathe better and that, besides the valve train, perhaps the intake has become a restriction at this point.

If 70mm TB's are a bad thing on my setup, why? Is it because the intake plenum needs to be matched in size to the TB's 70mm openings? Are the stock heads a restriction and perhaps a call to Greg Good for some ported heads is in order? But if the stock engine is getting all of the air it needs (supposedly) through the stock TB's, why would larger TB's cause performance to suffer? It's not as if the TB's themselves are pushing more air into the engine, they're simply acting as a gatekeeper of air, so to speak. Sure, the gates are bigger but only as much air as the engine can handle will get through.

Sorry for the rant...but I would like to know, from those in the know, the rationale behind 70mm TB's hurting performance on a stock GenII engine.

Thanks,
- Tony

 

[JonB]

One opinion:

Larger TBs are a later part of a series of mods, done in progression. If you just fit them, they simply tilt your power curve HIGHER and you lose a lot of low-end power. Ie without other flow-mods FIRST these TBs dont work till you get above 70-80 mph. You will fall flat, off the line with a STOCK motor. Fact.

Intake-Headers-Exhaust-Roller-Rockers as you have at a minimum to go 70mm. Better at 66-68mm. Port-matching the intake manifold, or a Honed intake manifold a plus. So I suspect with your mods you will see some gains higher up the power band, and not much down low, if anything..... If you do JM or GOOD heads etc, you are THEN ABSOLUTELY READY for 70mm.

You ask why:
All the intake and exhaust 'plumbing' in your motor basically functions like an air (or water) pump.
Lets say you have a garden hose, and you want more FLOW out the business end.
Your buddy is a firefighter and gives you a 3" diameter length of fire hose.
You splice it in a couple feet away from your garden hose nozzle. What happens?
PTHBBBB. Nothing out the nozzle, til the larger section fills, and THEN you still have no more flow! IE you need a larger "hose" [intake, manifold, heads, exhaust] all over to make a difference. And the Roller-Rockers make the plumbing PUMP better.

 

[dave6666]

An increase in diameter of a flow path can actually be a flow restriction by changing the characteristics of the flow.

In the chemical industry I'm in we deal more so with liquids than gasses, although there are plenty of chemical plants that have all gases and no liquids, but different flow rates through the same diameter pipe change to different flow characteristics, which may or may not matter. These are all things based on friction, viscosity etc.

The same thing can be said of a change in diameter, whether making it smaller or larger. Same flow, but now you changed the math equation.

Complicate it even more so by making this change right in front of a distribution plenum. Flow restrictions or changes are best kept away from areas of distribution, or manifolding.

In the chemical industry, a rule of thumb is no flow obstructions or change in direction within 20 pipe diameters upstream of an instrument such as a flow measuring device, or 10 pipe diameters downstream. Not enough room to do that under most car hoods, but compare the flow instrument in that world to the distribution plenum of a car's intake.

You could spend a lot of time doing the math here, but the quicker route is the empirical test. Put it on a dyno and see what happens. Personally, I doubt the loss of power would be much, but I also wouldn't doubt that there is a loss for an N/A engine with stock heads.

Yeah Tony, call Greg Good and make it happen!

 

[ViperTony]

Thanks Jon, Dave. So...its not so much a loss of power but the power moves up further into the RPM range. I'm not sure I would like that... Right now, I'm very happy with the power band I have with my setup. While the Belanger headers make good power it got moved up into the mid-higher RPMs. My old 1 5/8" stepped headers made power in the low-mid RPM range. I felt it and saw it on the dyno.

Dave: I'd love to send my heads to Greg but it will have to wait a few more months...if anything it will be a winter project. But I will install the TB's and compare dyno results here.

 

[plumcrazy]

talk to GG and see what he says

 

[ViperTony]

talk to GG and see what he says

I will as soon as he frees up from the cat fight on the other site.

 

[JonB]

Thanks Jon...............So..its not so much a loss of power but the power moves up further into the RPM range. I'm not sure I would like that...

Correct. Larger TBS swing the power bell curve upward. Most owners want low-end grunt. And our motors wont turn 6500-9000 RPM anyway!!!

Which is why road racers who are at 40-100% throttle 99% of the time benefit.
And why ALMS-SCCA-SPEED-GT all penalize VIPER with restrictors...and NASCAR has restrictors at some high-speed tracks.

 

[Viper X]

Not an expert at this, but I can relay some personal experience.

I tried the 70 mm throttle bodies on my JM heads, 708 cam, headers, 3-inch exhaust, roller rockers, tuned, etc. 2001 GTS ACR.

No power increase anywhere and lost some torque when compared with OE throttle bodies. This may just be my engine, but I doubt it.

Spoke to Kevin at Exotic Engine and he recommends against using them unless you go up over 700+ fwhp. You will likely need to spin the engine a bit higher to achieve this, so most of the guys posting above are correct. The two OE throttle bodies tend to allow plenty or air for most applications.

I also found them to be a bit unfriendly on a road course. If I moved the throttle just a little bit, I sometimes got a bit more power than than I wanted, especially in high speed turns.

My 2 cents,

Dan

 

[ViperTony]

Not an expert at this, but I can relay some personal experience.

I tried the 70 mm throttle bodies on my JM heads, 708 cam, headers, 3-inch exhaust, roller rockers, tuned, etc. 2001 GTS ACR.

No power increase anywhere and lost some torque when compared with OE throttle bodies. This may just be my engine, but I doubt it.

Spoke to Kevin at Exotic Engine and he recommends against using them unless you go up over 700+ fwhp. You will likely need to spin the engine a bit higher to achieve this, so most of the guys posting above are correct. The two OE throttle bodies tend to allow plenty or air for most applications.

I also found them to be a bit unfriendly on a road course. If I moved the throttle just a little bit, I sometimes got a bit more power than than I wanted, especially in high speed turns.

My 2 cents,

Dan

Dan, was this done on your track car? Did you use a Vec or get a PCM flash to re-tune for the TB's? I assume the tune probably did nothing.

 

[Camfab]

I've got JM Strikers as well with STOCK throttle bodies.

 

[plumcrazy]

i know on JD's paxton setup a longtime ago, he swapped just the TB's and it lost power. you can ask joseph for more info but im pretty sure thats what he told me.

and DLM told me to stick with my OEM TB's very recently and im over the 700rwhp mark...

 

[ViperTony]

i know on JD's paxton setup a longtime ago, he swapped just the TB's and it lost power. you can ask joseph for more info but im pretty sure thats what he told me.

and DLM told me to stick with my OEM TB's very recently and im over the 700rwhp mark...

I saw JD's post about it on the other site. I'm more curious what causes these engines to lose power, or move power into higher RPMs, by using these TB's. I've seen a few "TB's are Bad" posts but no logic or scientiifc reasoning behind the warning until now. Then again, I've read a few posts where people did see gains (supposedly) but provide no rationale for it.

I'm one of those guys who has to go through the experience before it sinks in or not. I don't know why, but I feel the need to dissect this one for myself.

 

[Camfab]

At the most basic level you have a larger opening at any given throttle position. Velocity drops off and you lose the ramming effect to the cylinders. The reality can be much greater than that as Dave pointed out, but at the most basic level that's a simple explanation. Intake porting and port matching without concrete testing will diminsh power as well. Unlike what most people will tell you the stock Gen II intake as a whole will produce big numbers without mods. The stock heads are the problem point.

 

[99 R/T 10]

One of the factors is that when you replace the stock TBs withthe 700MM, you will have a restriction or lip on the manifold. If the hole is not enlarged to accept the 70MM TB's it will not flow smoothly and you will loose power. So just bolting on 70MM TBs will hurt power with out any other mods(1.7 RR, headers). The opening must be opened to the size of the 70MM TBs.

 

[ViperTony]

One of the factors is that when you replace the stock TBs withthe 700MM, you will have a restriction or lip on the manifold. If the hole is not enlarged to accept the 70MM TB's it will not flow smoothly and you will loose power. So just bolting on 70MM TBs will hurt power with out any other mods(1.7 RR, headers). The opening must be opened to the size of the 70MM TBs.

Thanks. I'd imagine it would make more sense to get the intake opened up if I have headwork done, then again, it seems that the stock intake can handle pretty much anything you throw at it. What about extrude honing the intake? Is that a process that simply opens the intake ports to match the TB's?

BTW: There is some sanity behind my mods. At some point I'm either going to get some head-work done and/or slap on a Roe S.C. Hence, the TB's were in mind for that end-game not so much my NA setup.

Thanks for all of the replies!

 

[Camfab]

Extrude Hone = lots of money for very small gain. I've heard the larger throttle bodies are effective on Roe type applications. I have no practical experience in this area, other than the fact that my old roots blower loved the bigger Holley Double pumper I threw on it. Different motor, different time.

 

[ViperTony]

Found an auction for a set of 70mm Accufab TB's on ebay, ad states:

"This auction is for a used set of polished Accufab 70MM Throttle Bodies. They are in great condition and can be used on Vipers from 1996-2002(not incuding the 96 RT/10). They function perfectly and can also be used for upgrading the Viper to increase HP. When combined with exhaust mods and K$N filters, you should see about 40HP increase. These normally will cost over $700, but get them here for less then half. Shipping will be a flat $25. If you have any question, feel free to ask. "

eBay Motors: Dodge Viper 96-02 Accufab 70mm Throttle Bodies (item 160279808294 end time Sep-11-08 13:49:45 PDT)

 

[FE 065]

I know a guy who put larger TBs and Edelbrock headers on his '95 RT/10 over a Winter, and didn't pick up anything at the strip the next Spring..not even .01 sec.

 

[PhoenixGTS]

At the most basic level you have a larger opening at any given throttle position. Velocity drops off and you lose the ramming effect to the cylinders.

Off the cuff with no science behind it at all, this does not make a lot of sense when you look at the parts. If you stick your finger past the lip on the intake manifold that would be ground out to make the hole 70mm you can feel that the plenum is much larger than even the 70mm hole would be. This means that the stock set up has the air going from the constricted throttle body to the larger plenum, which as you state does not sound like it would maintain velocity, yet anecdotes say this stock configuration makes more torque than having the larger throttle body and modified intake where the size increase of the air pathway is less.

For the record, my car came with 70mm throttle bodies when I bought it used and I have never driven a car with stockers to "feel" any difference. But that said, I like the idea of giving bottom end for more top end. I am already traction limited with sticky tires so more bottom end would just make the car harder to drive. I don't know how in the heck the Roe guys drive those cars making 500 ft lbs @ 1,500 rpm and 600 ft lbs @ 2,000.

 

[99 R/T 10]

Found an auction for a set of 70mm Accufab TB's on ebay, ad states:

"This auction is for a used set of polished Accufab 70MM Throttle Bodies. They are in great condition and can be used on Vipers from 1996-2002(not incuding the 96 RT/10). They function perfectly and can also be used for upgrading the Viper to increase HP. When combined with exhaust mods and K$N filters, you should see about 40HP increase. These normally will cost over $700, but get them here for less then half. Shipping will be a flat $25. If you have any question, feel free to ask. "

eBay Motors: Dodge Viper 96-02 Accufab 70mm Throttle Bodies (item 160279808294 end time Sep-11-08 13:49:45 PDT)

Looks like the link is bad, is this the ones you were looking at:

eBay Motors: Dodge Viper 96-02 Accufab 70mm Throttle Bodies (item 160281701876 end time Sep-17-08 09:04:39 PDT)

 

[FE 065]

Off the cuff with no science behind it at all, this does not make a lot of sense when you look at the parts. If you stick your finger past the lip on the intake manifold that would be ground out to make the hole 70mm you can feel that the plenum is much larger than even the 70mm hole would be. This means that the stock set up has the air going from the constricted throttle body to the larger plenum, which as you state does not sound like it would maintain velocity, yet anecdotes say this stock configuration makes more torque than having the larger throttle body and modified intake where the size increase of the air pathway is less.

I don't like that larger area behind the TB flange either, but I think I've have read something about wanting the velocity to slow as it enters a plenum.



At the most basic level you have a larger opening at any given throttle position. Velocity drops off and you lose the ramming effect to the cylinders.

While the intake tract does extend up to at least to the back of the air box, I'd say the ramming effect to the cylinders is done by the entirely by the long intake runners in this set up-especially since they're perpendicular to the plenum..

 

[ViperTony]

Looks like the link is bad, is this the ones you were looking at:
eBay Motors: Dodge Viper 96-02 Accufab 70mm Throttle Bodies (item 160281701876 end time Sep-17-08 09:04:39 PDT)

I think that's it. There's a few Accufab TB auctions going on but the price seems pretty good on that one.

While the intake tract does extend up to at least the back of the air box, I'd say the ramming effect to the cylinders is done by the entirely by the long intake runners in this set up-especially since they're perpendicular to the plenum..

I doubt there's any ramming effect to the cylinders with the stock airbox, tb's and intake unless, of course, we're talking about a S.C. setup. The engine is a big air pump and is literally sucking air in through a pair of straws, i.e. the TB's. To me...it would seem that if we made the air pump bigger (head work, etc) then the straws (TB's) would need to be made bigger to help the bigger air pump work efficiently.

 

[FE 065]

Ram Theory

Ram Induction

 

[ViperTony]

Ram Theory

Ram Induction

Wow...I learned something today. Those articles explain a lot. Is it possible that the larger TB's can throw off the synchronization of the pressure wave to the intake valve opening?

 

[PhoenixGTS]

Ram Theory

Ram Induction

1) note that the old "ram" manifolds discussed there have NO PLENUMS. They are all tube.
2) why would throttle body size on an intake with plenums change resonance? The distance between the intake valves and throttle bodies are still sxactly the same.

 

[Grant]

What I haven't been able to find out is why would 70mm TB's cause a loss of power on a relatively stock engine.

Well, most technical information posted on Internet forums is just very incorrect, and you can't really measure differences of a few HP with dynos Have you seen before/after tests? You don't need velocity before the intake runners after all, and the resonant frequency isn't dependent on the diameter of the pipe.

My assumption is that the TB's would compliment these mods. After all, if the engine has the mods to move more air and breathe better then adding TB's would be a good thing. Right?

Easy test: Get a sensitive vacuum gauge (say, 0-30 inches of water) and hook it up to the intake manifold. Note what it reads when the car is off, and go WOT at a low RPM in 2nd or 3rd gear up to redline. The gauge will probably indicate more vacuum as the RPMs rise, representing a restriction in the intake (throttle body, air filter, etc) system. The amount of vacuum is roughly proportional to the power lost.

The amount a throttle body (or any pipe) can flow at a given pressure drop is roughly proportional to its diameter squared. I know stock LS1 TBs (75mm) start to become a measurable restriction around the 400whp level. Two 52s would probably flow about the same, though its hard to say (since flow is more complicated than simply measuring the diameter).

Also note that FI setups don't need as large of intake piping for a given power level than a NA setup. This is because for a given mass flow rate, denser, slower-moving air has less of a pressure drop than less dense, faster-moving air. The pressure drop is roughly proportional to the velocity squared and the density (not squared).

 

[Grant]

Wow...I learned something today. Those articles explain a lot. Is it possible that the larger TB's can throw off the synchronization of the pressure wave to the intake valve opening?

Typically, that pressure wave travels between the valve and the plenum. A wave traveling in a cylinder gets reflected when it encounters "open air", which in this case is the plenum. Sometimes the intake/TB system is designed to resonate as well (sometimes called a Helmholtz resonator) in an attempt to match up moments of higher pressure in the plenum to intake valve events (or in the case of production cars, to control noise). I've never messed with anything like this before, but I'm told this really only works on engines with only a few (2 or maybe 3) cylinders per intake manifold.

 

[ViperTony]

Well, most technical information posted on Internet forums is just very incorrect, and you can't really measure differences of a few HP with dynos Have you seen before/after tests? You don't need velocity before the intake runners after all, and the resonant frequency isn't dependent on the diameter of the pipe.

Easy test: Get a sensitive vacuum gauge (say, 0-30 inches of water) and hook it up to the intake manifold. Note what it reads when the car is off, and go WOT at a low RPM in 2nd or 3rd gear up to redline. The gauge will probably indicate more vacuum as the RPMs rise, representing a restriction in the intake (throttle body, air filter, etc) system. The amount of vacuum is roughly proportional to the power lost.

Grant, do you think that the MAP sensor or the internal Vec3 MAP sensor would be sufficient to test with? I can easily log both sensor readings through the Vec3. I can do a before and after log of the TB swap.

 

[Grant]

Grant, do you think that the MAP sensor or the internal Vec3 MAP sensor would be sufficient to test with? I can easily log both sensor readings through the Vec3. I can do a before and after log of the TB swap.

Tony,

Thats how many people (including myself) usually test throttle bodies and intakes on LS1s, so I'd say yes assuming the resolution isn't terrible. Any time you see significant vacuum in the intake manifold there is room for power to be gained, though not necessarily from the throttle bodies.

 

[Camfab]

Off the cuff with no science behind it at all, this does not make a lot of sense when you look at the parts. If you stick your finger past the lip on the intake manifold that would be ground out to make the hole 70mm you can feel that the plenum is much larger than even the 70mm hole would be. This means that the stock set up has the air going from the constricted throttle body to the larger plenum, which as you state does not sound like it would maintain velocity, yet anecdotes say this stock configuration makes more torque than having the larger throttle body and modified intake where the size increase of the air pathway is less.

For the record, my car came with 70mm throttle bodies when I bought it used and I have never driven a car with stockers to "feel" any difference. But that said, I like the idea of giving bottom end for more top end. I am already traction limited with sticky tires so more bottom end would just make the car harder to drive. I don't know how in the heck the Roe guys drive those cars making 500 ft lbs @ 1,500 rpm and 600 ft lbs @ 2,000.

Phoenix, honestly before you shoot down my simple explanation you should try stock throttle bodies. I think you might be surprised.