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Discussion Starter · #1 ·
The stock throttle body for a Zetec is 55mm. Would a larger throttle body improve HP on a bolt on modded zetec? I would imagine it would hurt low end torque since a larger TB would decrease velocity. Would installing a smaller diameter TB improve low end torque but hurt HP? Wondering if a 48mm TB will move the torque curve lower into the RPM's?

Looking to port match the stock intake to the cylinder head. I'll be deburring and smoothing the cylinder head intake path and unshrouding the valves too. Valves will stay stock but may moderately cam it. Is the 55mm sufficient for the increase in CFM? I have an Escort S/R ECM. No issues?

John
 

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Discussion Starter · #5 ·
Nova,

Do NOT port-match the intake to the head unless you have looked at both the intake port overall shape/size and the head port size/shape. Introducing areas of velocity drop/pressure increase in the intake tract is serious folly. If there are any restrictions, remove them as long as it doesn't change the overall port shape/velocity profile. Since we have such small engines, we need as much port velocity as possible at lower/mid RPMs as possible. Look at port intel square inch versus overall square inch and find the areas of restriction. Remove the restriction without FUBARing overall flow (PM me, brother, for more info).
You da man! PM will follow.

John
 

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Discussion Starter · #7 ·
I just sent Deathride a PM but you're right. It would be a good idea to share this info on the open forum. This way you and I can ask whatever relevant questions we may have along with brainstorming with Deathride to figure out what would be the best way to achieve our goals.

Death ride if you're cool with sharing on the open forum I'm good with it. If not, no worries.

John
 

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Discussion Starter · #23 · (Edited)
And yeah, open info is good info.

If you are porting, pay attention to the short-side floor area. Be gentle and smoothly reshape it to take the harsh turn out of it (this applies to both intake and exhaust sides). Teardrop the intake port splitter (think leading edge of an airfoil) to improve transition from the throat to the valve. As to opening the bowl area directly above the valve seat, DO NOT go more than 90%, as you want meat there for a good (minimum 3-angle, but for all-out, 5-angle) valve job. Believe it or not, the most positive difference in flow will be made by just that valve job. On the exhaust side, raise the port roof slightly (to match whatever header/manifold you use) and fill the floor the same amount (air flow wants to stick to the roof, and has been known to stall/revert in the lower right corner). This, in effect, is raising the port for a more direct/smoother line out for the airflow to follow. I have done this on 4.6L DOHC heads with success.
As to unshrouding the valves; very good idea but you have to keep the cylinder bore (not the head gasket bore diameter) in mind. That will dictate how much you can potentially remove to unshroud. Pay attention to the shape of your cut, as it shouldn't be harsh. Any removed material in the chamber will drop compression (so keep in mind how many CC's you remove). Some can be regained by shaving the head or using higher compression pistons.

Again, unless you really want to slow incoming air, DO NOT port-match the intake unless you have opened up the upper intake and this will merely align/keep the port velocity moving. Unless your engine has been modded for more low-end torque, losing any velocity down low will make the car very sluggish (even with a larger throttle body) as port velocity isn't aiding in cylinder-filling. The trick is to experiment with throttle body size until you find the one that doesn't kill low-end and hangs on further into the rpm band (just like carburation, there is too big).

What are your overall plans/goals for the engine? Will it see boost? Will you be revving the piss out of it all the time? Auto or stick car? Final drive ratio and tire size? Believe it or not, this all needs to be considered when modifying the engine.

TQ x RPM /5252 = HP
Any change to torque has a greater effect oh HP than RPM does. Higher RPM moves the HP band up.
Missed the last part of your post.

Overall goals are a torquey street engine. I like some neck snap in my cars.

No boost plans at this time although I've contemplated it seriously and, knowing me, will eventually get there but for now I'm trying to keep it N/A. Don't need 5-0 flipping a bitch because they heard the blow off valve go off as they passed me going the other way!o_O:LOL:

Street motor so no need to high rev outside of normal OEM spec but if I'm going to do some head work a set of higher revving springs/valve train upgrades may not be a bad idea.

Manual stock G5M although I have an '03 trans I pulled from the yard waiting for an Mfactory LSD that I want to drop into the wagon. Current trans grinds 3rd gear telling me 3/4 fork is probably bent at the shift tab.. I need to get up to speed on some of the other things I can do to improve performance/durability of the trans. From what I've read their good up to around 200hp in stock mode. Right now my butt dyno is telling me with the bolt ons I have I'm probably pushing around 150 hp at the crank. Final drive ratio I'm assuming is 4.10 since the trans in the car was out of a 2000 ZX2. Running a 205/45/17 tire.

John
 
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