Stock throttle body size

[fast924S]

Does anyone know what the stock throttle body size is. I was looking at a add that says they can make the porsche throttle body 3mm larger and I just wanted to know what size the stock unit is. If you can get it in MM it will make it easier

[fast924S]

ANYBODY??? SOCAL???? TCMDOCS944?? COME ON, WHERE ARE THE PRO'S WHEN I NEED YOU GUYS?

[idontknow]

My 83 is roughly 55mm.

To be honest, you really don't need any larger of a TB. Unless of course your car lives at WOT. There was a lot of insight about CFm ratings and TB sizes and a whole lot of info dealing with volumetric efficiency on a Neon site, lemme see if i can find it.

Found it, here it is, hope it helps.

Gary Howell wrote:

"49mm TB flows 252 cfm
52mm TB flows 283 cfm
55mm TB flows 317 cfm
60mm TB flows 377 cfm

At 8200 RPM a 122 cubic inch engine will need 290 cfm at 100% volumetric effiency, using the formula ((Max RPM/2)*Displacement)/1728. Rule of thumb is to go 10% over because a naturally aspirated engine can go above 100% volumetric efficiency because of cam overlap, header design, etc. Go above that and you kill low end because of reduced velocity, go below that you starve the engine for air at top end. 110% is volumetric efficiency is 319 cfm."

[bryanthompson]

fast, you YELL TOO MUCH!

[fast924S]

So about 58mm would be around 319cfm or so

[idontknow]

no, I'll do the math for you

max RPM is 6400
cu inch displacement is 151 on a 2.5 ltr motor
so ((6400/2)*151)/1728 = 279.63
since VE can be 110% you get 307.59

besides that, 58mm will give you about 350cfm

Hope that helps.

[fast924S]

So enlarging the throttle body 3mm more wouldnt be a good idea even with the mods Im running?

P.S I suck at math

[idontknow]

No, you haven't increased the displacement (bore * stroke) or increased the max RPM so the engine won't use any more CFMs. If you do increase the TB size, you may get more top end but you sacrifice ALOT in acceleration ie stumbling and hesitation.

[fast924S]

well I wanna run FRwilks chip and have a 7,000rpm red line, also I have incresed my CR, other mods That I will soon do are a CAM and some light head work (Light porting, Full polish, 3 angle valve job) SO I was thinking maybe a larger throttle body would help a little

[TCMdocs944]

They make a bored and polished throttle body....like $250 with core.

It widens the body AND changes the flap to a larger size. Also appears that they change out the springs etc. Havent heard any numbers though.


As far as me being an expert..........Nope....Just read alot.
I am still Padawan.

[idontknow]

Ok, 7000 rev limit does change things. New value is 336cfm. You may want to go ahead and get the TB oversized since the stock size would be too small.

The VE formula was already designed for optimal intake and exhaust flow.

CR increase does not equal displacement increase. You could have a 2.5 motor with a 6.3 CR and another 2.5 with a 13.5 CR and still have the same displacement. ie stack 3" worth of head gaskets on and you too can have a 6.3 CR engine!

[fast924S]

With the proper top end, (racing valve springs, titanuim retainers, Lightened valves. Do you think the crank and connecting rods could hold a 7,000rpm redline??

[ae1969]

You have a few issues....

1. Lifters are one problem - Solid would be ideal for this application but since adjusting lash is impossible... lifespan would not be very good.

2. Rods. The NA rods would not hold very well..... but upgrades would be good. For occasional burst sure.... sustained .... NO.

3. The NA head has its limitations as well.... torque tends to drop off fairly sharply at the higher rpms. The efficiency of the head at the higher rpms is an issue. You will have more torque going up a gear than trying to redline this engine....

So a 7000 rpm redline may gain you at the track if you can avoid an upshift...... but on the street you probably will not gain any real benefit..

[fast924S]

I dont plain of running at High R's all the time, just a burst here and there. With some light head work it should breath a little better, maybe that would help the head problem. Please explain more on the Lifter problems??? Will the stock ones calaps at that hight of a rpm??

Another problem I was thinking about is, With stiffer valve springs the cam will be harder to turn and it might cause the timing belt to wear faster

[ae1969]

Looking at the lifters............ (7000 rpm MAX for our lifter setup)

1. Valve float..... unfortunately since they are hydraulic at the higher RPM the valves inertia will want to make the valve 'hang out'.....and whammo meet Mr.Piston. You can use stronger springs but the hydraulic lifters are really not designed for a stiff setup.

2. Lifter Collapse. At high sustained rpms the lifter can actually collapse....so the cam is only compressing the lifter and the valve is not even moving.......really messes things up.

....add a cam with a modified high lift and you will undoubtedly cause one or both of these problems.....

So...in the end.... solid lifter, manual adjustements, strong springs, wild cam etc...... and you will get the benefits you are hoping to achieve............. but I am still not sure how much more real power you will gain from revving her up that high.....

[fast924S]

Like I said I dont plane on runing that high of a RPM, I just want to be able to handle some small bursts above the redline. Im not trying to built a honda S2000 or anything so I dont really feel the need to change much of the stock top end, I think I will be change the Cam, Springs and retainers and having some valve work done, I think doing this will help the car out in mid to high R's and maybe free up approx 10-15 more HP

[MrPants]

Originally posted by idontknow
using the formula ((Max RPM/2)*Displacement)/1728.

where does this come from?

[Wellwood]

Originally Posted by MrPants

where does this come from?

Dredging up an old thread topic as I continue my quest for knowledge on the topic of throttle bodies...

It seems to be a volume constant based on inducting 1/2 of the displacement (representing air flow into the engine) with a conversion factor that yields cubic feet from cubic inches to yield intake air flow needs.

This is the first formula that I've seen that can be used to derive what TB is needed - but question its application. It seems as though that this works well for a normally aspirated engine system - and that using a turbocharger may not be an apples to apples comparison (maybe tangerines to oranges?).

Is there anyone who has additional insight to this topic after nearly 20 years?

[v2rocket_aka944]

Originally Posted by Wellwood

Is there anyone who has additional insight to this topic after nearly 20 years?

That formula is an ancient engine-building guideline to figure engine airflow CFM for head/exhaust/intake manifold flow. It doesn't really tell you anything useful for throttle bodies since most TBs aren't rated in CFM.

I spent a lot of time looking into throttle body sizing over the last few years and the short answer is, there isn't really any formula or concrete guidelines. You just need enough, and more than that doesn't do a lot

Turbo doesn't change the TB size needs of the engine relative to non-turbo.

One thing to keep in mind is cable throttle sizing vs DBW sizing.
Cable throttle engines tended to have exterior idle control valves, maybe a hole in the throttle blade (GM typically), and the 944 series even has a second idle bleed orifice tied to the brake venturi. What this contributes to is the cable type blades being a little on the small side to keep all these "leaks" in check relative to a decent idle speed.

DBW don't use an external ICV or anything, just crack the blade more as needed. So DBW tend to be larger on an equivalent size engine.


Misc reference notes, showing that power isn't directly related to TB diameter/cross section:

-- 944 stock - 2.5 or 2.7/143-162hp/55mm (I put a 60mm on my 2.5 NA years back and it made 0 difference on the dyno at any RPM with a stock cam, but a tuned MAF kit and some other stuff)
-- 951 stock 217-250hp/55mm
-- 944S 2.5 16v - 188hp/55mm
-- 944S2 3.0 16v - 208hp/60mm, I think
-- 968 3.0 16v - 240hp/63mm I think

-- Ford 4.6L 2v engine was 260hp/65mm (guys 8-10 years ago were putting these onto 951s at my suggestion, refresh951 on RL was one of the early adopters)
(Interestingly, when Ford came out with the 3.5 V6 around 2008 it also made about 260HP so Ford used the same DBW 65mm TB on the 3.5 and the 4.6 2v. So HP seemed more important than engine size)

-- Ford 5.4 2v variant was 230hp/70mm.
-- Ford 4.6L 3v was 300hp/twin 55mm, about equal to a single 78mm)
-- Ford 6.8 V10 was 300-340hp/twin 57mm - equivalent to about a single 80mm. This same exact TB was also used on the 4.6 32v Cobra engines, 99-01 N/A about 320hp and then 2003-04 Supercharged 390hp.
-- Ford Coyote 5.0 2011-2023 - 410-450hp/80mm (but look at this FACTORY 2024+ manifold - they added a whole second 80mm TB, among some other tweaks, and it picked up 20hp over the 2023 single TB https://performanceparts.ford.com/part/M-9424-M50D )
-------- note that the area of a single 80mm was sufficient to feed a 300hp 6.8L V10 all the way up to a 450hp 5.0 V8


-- GM made some FWD V6 engines 20 years ago, a 3.5L 215hp engine used a 65mm TB stock. A 3.4 (different engine family, 185hp) used 57mm. A 175hp 3.1 used a 52mm.
-- Chevy 350 Vortec (96-02) used 75mm.
-- LS series - 4.8/5.3/5.7/6.0 cable variants all used the same 75mm TB, from 255-405hp in that range (1999 4.8 versus 2004 LS6)
-- GM 8.1 big block in early 2000s - 340hp/75mm TB
-- Northstar V8, 4.0 or 4.6L variants (250-300hp) both used a 75mm
-- LS3 - 430hp/90mm


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IN summary if you're doing a 3.0 turbo or something like that, a 60-65mm will probably be sufficient and will work well with 2.5" IC piping. a 70mm won't hurt and might work to match up to 3" OD intercooler tubing well. a 75mm might be bigger than needed, but still won't hurt.

I use a LS 75mm TB on the n/a 250hp 3.9L V6 in my 944 and it works well. I get ambient pressure in the manifold a little before WOT so it might be a little big but it was $28

[Wellwood]

Thanks for the detailed information Spencer.

I think of the engine as an air pump used to convert chemical energy to kinetic energy. Anything one can do to make it easier to move air while extracting the maximum amount of energy from the fuel is the primary goal. The previous formula is a good rule of thumb I think for NA engines - turbocharged not so much as there is an external force cramming more air into the combustion chamber.

That said, in that light, making the entire system from the filter to the TB the same diameter I think minimizes the likely pressure drop that robs the engine of air. The intercooler is a nice effect as lowers temperature thereby increasing the density of the air being which further aids in packing more oxygen into the combustion chamber.

At this point, the intake valve cross-sectional area for flow is likely the best determinate - but again, with the compression forces of the turbocharger - the intake valve area is just an orifice with the 4V clearly able to flow more air than the 2V heads on this Porsche series.

It would be interesting to layout all of the previous examples you listed with displacement, valve arrangement and cross sectional area for flow, boosted pressure, CR, and rated power output to see if there is a trend of one particular sizing ratio combination that is the most ideal.

For my application, making the entire intake flowpath up to the TB will be as matched in diameter as possible and let the TB be slightly larger at 60mm (planning to incorporate a DBW TB) with the 4V 2.5L. In the end, it should prove to be driveable and have a respectable power-to-weight ratio in the 968 Cab.