Lets talk throttle bodies

[futurevr4man]

Hey fellas,

I am planning to upgrade my intake/throttle body. I built my intake piping at 3.5" in order to prepare for a 90mm throttle body in the future. My maf and all are sized for it. All that's left now is getting my plenum and throttle body sized to match, which is no problem either.

My question is, is it worth it? Will I get any noticeable gain? I did some research and found a couple of websites with various equations, and I have had one website convince me it's a great move, and the other convince me that I am wasting my money. I am hoping some will chime in with some first hand experience, and some people chime in with equations and theory. Sometimes what doesn't work on paper actually does make a difference in the real world, and vise versa.

Pertinent info:
Going from a 70mm throttle body to a 90mm throttle body
3.75L
90mm stroke
94mm bore
Td05 20g's (640cfm each according to stealth316)
Stage 3 heads (idk what they flow)
Peak power, aiming for 8000rpm's
Anything else just assume I have the mods to support it

I would love a technical discussion about this, so please chime in!

[sergechronos]

Can't speak for the 3/S, but on an 00 Mustang GT upsizing the TB to a 90mm made a difference in top end performance at the expense of low end throttle response. Given the GT was NA and the 3/S is FI, likely different. The FI will cover up a lot of weaknesses, however I still have to believe it would help at the top end to remove the restriction (and also a larger plenum for the intake)

[J. Fast]

Our engines are 3.0 liters or 183 cubic inches or 0.105903 cubic feet. Using Heywood's equation for calculating engine airflow rate: CFM = (CID x RPM x VE)/(3456). At 5000rpms with peak VE around say 85% (15% due to pumping losses and etc.) engine airflow is around 22500CFM or 168340gpm or 10.6 meters per second.

Using the above data and inputting a manifold pressure of 200kpa (roughly 30psi) into Mathmatica's Pipe Friction System Pressure Loss Calculator I determined the pressure loss from the bottleneck at the throttle body resulted in following:

A 2.25" stock diameter throttle body has an effective pressure drop of 3.62psi with the engine pumping 10.6 cubic meters per second and a MAP value of 29.6psi (200kpa).



Upgrading to a 3.5" diameter throttle body the effective pressure drop is 2.16psi with the engine pumping 10.6 cubic meters per second and a MAP value of 29.6psi (200kpa)



The difference between the two is 1.46psi. This means with the restriction removed the manifold will see roughly 1.5psi more pressure.

If I recall correctly on my setup on 20G's (w/3.1L bottom end) on the dyno I produce around 20whp for every pound of boost. If the TB diameter increase frees up 1.5psi then theoretically the upgrade alone should net around 30whp.

Also, I did some toying with a couple different situations and increased absolute manifold pressure to 250kpa in the simulator. The theoretical differential between the stock TB and a 90mm one at that MAP pressure is about 3psi. At first glance the improvement fetch's around 60whp with a higher boost pressure. Of course this is all theoretical but if temperature, cfm, and pressure is constant, given the system is closed the numbers should fly.

...When photobucket.com finishes with their site update and allows me to upload I will insert screenshots of the calculation data for both the 2.25" stock throttle body and a 3.5" throttle body.

EDIT... FIXED!

[futurevr4man]

I see the direction you took it. That's not at all what I was thinking but its a helpful way to look at it as well

I was concerned about air velocities, but I can't convince myself that matters as much with a FI engine. I feel like the air is being forced in either way.

Also, your 5000rpm peak makes more sense than the 8000 I posted. I will see peak load around 5000rpms give or take 500.

I expect to lose some resolution in lower rpm throttle control, but I am hoping for a large gain in sustainable power in the upper rpm's. where did you get the cfm that my engine will flow? I don't know if that's realistic or way out there, but I am curious as to the number.

Lastly, 200kpa (absolute) you're only looking around 16 psi gauge. Realistically I plan on running 20+ on the street at all times... so a pressure gain of 2+ psi in the manifold would be excellent. I hope it translates to the real world that well.

I would love to hear more input from anyone!

[J. Fast]

I see the direction you took it. That's not at all what I was thinking but its a helpful way to look at it as well

I was concerned about air velocities, but I can't convince myself that matters as much with a FI engine. I feel like the air is being forced in either way.

Also, your 5000rpm peak makes more sense than the 8000 I posted. I will see peak load around 5000rpms give or take 500.

I expect to lose some resolution in lower rpm throttle control, but I am hoping for a large gain in sustainable power in the upper rpm's. where did you get the cfm that my engine will flow? I don't know if that's realistic or way out there, but I am curious as to the number.

Lastly, 200kpa (absolute) you're only looking around 16 psi gauge. Realistically I plan on running 20+ on the street at all times... so a pressure gain of 2+ psi in the manifold would be excellent. I hope it translates to the real world that well.

I would love to hear more input from anyone!

The equation to determine engine CFM is from Internal Combustion Engine Fundamentals: John Heywood: 9780070286375: Amazon.com: Books. I posted the equation of how to calculate in my last post.

CFM = (CID x RPM x VE)/(3456)

The numbers aren't far off. That's just how they're evaluated in Internal Combustion Engine Engineering Fundamentals. The only number you can really play with is Volumetric Efficiency. But V.E. can never exceed 100% or you'll have yourself a perpetual motion machine.

Heywood says a 3.7 liter engine with your bore and stroke operating at 5000rpms and 85% V.E. is evaluated by:

cubic inch displacement or CID = Number of Cylinders x 0.7854 x Bore^2 x Stroke

CID = 6 x 0.7854 x 94mm^2 x 90 = 3.748 liters or 228.686 cubic inches or 0.132 cubic feet

Next insert the answer to CID in the CFM equation.

CFM = (228.686 x 5000 x 85%)/(3456) = 28123 cubic feet per minute (13.27 cubic meters per second or 468.71 cubic feet per second)

Boost pressure has almost 0 effect on pressure drop. Going from 200kpa to 300kpa to 400kpa the pressure drop is virtually negligible. What this next example proves is we're free to increase or decrease boost pressure and have no ill effect of pressure drop from the throttle body restriction. The only pressure effects are related to the losses from intercooler and backpressure.

Here's the same test as above on a 3.5" throttle body but with 100kpa more pressure. The pressure loss is the same as it was when the manifold pressure was 200kpa.




Doubling the manifold pressure from the first sample, now 400kpa at the intake manifold with velocity constant at 10.6m/s you can see the pressure loss is negligible again.




Running more simulations, when I increased the fluid velocity at the throttle body by either 1) changing displacement, or 2) increasing RPM, or 3) changing V.E. the pressure drop increased. When I simulated an increase in velocity which is a direct result of a change in RPM, or increase displacement, or improvement in Volumetric Efficiency (by port matching or working on the heads, cams, and etc) the resultant was and increase in CFM (see above CFM equation to evaluate at varying RPM's and V.E.). When the engine CFM / velocity increased the pressure drop at the throttle body increased as well.

If you take the 2nd example I posted earlier with a 3.5" TB passing air at 10.6m/s at 200kpa the pressure drop was 2.16psi. When I increase the engine RPM to 6200rpm from 5000rpm on a 3.0litre engine, OR increased the engine displacement to 3.74litres both result in a piping velocity increase from 10.6m/s to 13.2m/s.



I think that should give you the answer you were looking for from the scientific/engineering side.

From the paper side the restrictions at the stock TB are 1) 2.25" inlet diameter, and 2) increase with engine CFM above 5000rpms. Not boost pressure. Until I ran the simulation today I did not know that.

[fastfalcon94]

Have you picked out a brand of TB yet? I'm curious what you are using. Have you found an 80 or 90mm one that works with the stock TPS?

[futurevr4man]

Interesting thoughts there jfast. I'll have to chew on that sometime when I'm not drinking hahaha

Fast falcon, I am going to be using a ford style tps... I have flashable, and I'm going to FORCE Greg to help me figure it out > but I am hoping it doesn't give me too many fits.

I am getting my setup from Morgan's Motorsports FYI. Von showed me the intake and TB at NG and it was well made, and a nice looking design.

[GCtech Industries]

Not bad, but for some reason ,why everyone always forget about the lower plenum? and runner size? just making a Surge tank does not increases runner volume or velocity....

[J. Fast]

After a bit of experimental time wasting today I think I'm going to build a cfm potential chart for various displacement engines and velocities based on different throttle body combinations. I think if we can tie in some dyno testing for confirmation of theoretic assumptions it will be great.

I also hope a few tps integration ideas are discussed and implemented for Chrome users.

To answer your question from earlier falcon, I have a boost leak fix CX Racing billet TB and a Q45. Both will be tested this spring on the dyno. I also plan on testing a custom intake manifold.

[NOMIEZVR4]

Interesting thoughts there jfast. I'll have to chew on that sometime when I'm not drinking hahaha

Fast falcon, I am going to be using a ford style tps... I have flashable, and I'm going to FORCE Greg to help me figure it out > but I am hoping it doesn't give me too many fits.

I am getting my setup from Morgan's Motorsports FYI. Von showed me the intake and TB at NG and it was well made, and a nice looking design.

Post some pics yeah?