Throttle Body size

[Stag76]

I have a Triumph Dolomite Sprint 2 Litre engine and am using a 65mm Ford Throttle Body, which is probably too big for this engine. Is it possible/advisable to use a restrictor in the inlet to compensate for a too-large throttle body? If so, where should it be positioned...before or after the Throttle Body Plate?

[cygnus x-1]

65mm might not be all that much too big. What is the expected HP output of this engine?


C|

[Stag76]

Probably 140-150 HP.

[Matt Cramer]

It's a little too big, but over-TB'ing isn't nearly as bad as overcarbureting. The worst that might happen is that 70% throttle opening isn't functionally different from 100%. I doubt a restrictor would solve that, but then again, the problem you're trying to solve isn't a very big one anyway.

[Stag76]

Thanks for that...I'll persevere with it for a while to see if I can get a better tune. What you said about 70% throttle opening equating to 100% is correct, a rapid throttle opening to about 40% TPS causes the vacuum to raise to 100kpa for about 2-3 seconds, and this is making it hard to get the ignition advance and AE correct.

[Bernard Fife]

Stag76,

A too large throttle body isn't like a too large carb (where the vacuum signal gets weak if the carb is too large). Too large a throttle won't hurt the power. Instead, the problem is at the other end - near idle and cruise throttle positions a very small change in the throttle position results in a large change in MAP, and the car can be hard to drive smoothly when going slowly.

It is completely normal for a decently built engine with a properly sized throttle body to have 100 kPa at 2500 rpm and 25% throttle on an engine that will go to 6000 rpm. This is because the engine is only going 1/3 as fast (so even is the VE is the same you will only draw 2000/6000 = 1/3 as much air), and the VE is likely lower than at peak torque too. However, the larger the throttle body, the more pronounced this effect is. But no reasonably sized restrictor will help.

Lance.

[Stag76]

I've progressed some with this, but am having trouble getting a good throttle response. There is a .5 second delay after opening the throttle until the revs rise, then the engine continues to rev for 2-3 seconds (depending on the size of the throttle opening) after the throttle is closed, then it is very unsteady with the revs rising and falling until it recovers (or stalls). I've done the normal checks for air leaks, but can't find any, and the manifold vacuum is OK at 37kpa at 900rpm idle.
I'm using a modified manifold with injector cups fitted, a home made plenum and the Ford 65mm throttle body. The volume of the manifold and plenum is 6.5 litres. Is it possible that this is too big for a 2-litre engine, and is contributing to the throttle response?

I've attached a DataLog.

[drmiller100]

absolutely throttle response can be caused by the manifold being too large.

Is the 6.5 liters including the runners, or just the main body of the manifold?

Is there any other restrictions between the atmosphere and the manifold besides the throttle body?

Basically, when you open the throttle body, air has to rush in and "fill" the manifold before any air goes past the intake valves.

BTW, going to a smaller throttle body will probably make this problem worse.