Poor Servo Assiatance With Throttle Bodies

[mos 1]

Hi,

 

i fitted gti6 brakes to my 2.1 s16 running throttle bodies today as well as a 23mm master cylinder

everything works and is bled correctly but the servo assistance isn't great.

now up to today i had always known that this particular car did not have great brakes when compared to my other 205 which is a 1.9s with twin carbs, but i always put this down to the 1.9s already having gti6 brakes and a 23mm master cylinder

i also though the extra weight of the iron block 2.1 was contributing to this as well as the smaller standard 1.9 brakes.

 

but now both cars have identical brakes it appears its actually a lack of servo assistance, so i put this to the test.

at first i suspected a faulty servo on my s16 but after putting the two cars side by side and using a length of pipe to try running each cars engine when attached to the other cars servo (hope that makes sense) i realised that both servos seem fine and the problem is the amount of vacuum my s16 engine seems to produce.

this is doubly annoying as my s16 already has two takeoffs for the brake servo from the inlet manifold where as my 1.9s only has one but still manges to generate more servo assistance.

 

anyone else experienced this with a 16v engine over an 8v or is it even something to do with the fact one car has carbs and the other bodies, i cant get my head around why one would produce such poor vacuum compared to the other so i know i am clutching at straws there.

i am hoping to put a vacuum gauge on both cars to get some proper readings to back up my findings but has anyone got any solutions to this poor servo vauum, i would really like to improve the servo assistance on my s16 to match my 1.9s as the brakes on that are great

 

thanks

 

mark

[welshpug 1,678]

being an S16 it would have originally had a vacuum pump on the end of the inlet cam, is this no longer fitted?

 

may be useful to refit and see if you get a satisfactory pedal feel/braking force.

 

the fact that the S16 and GTi6 both use vac pumps suggest that there may not be enough vacuum.

[Bally 4]

not just the servo, when you add that big mc it gets harder, or appears harder cause the travel is less, I put on my big mc for five minutes when I had on TB's, took it off and put back on the 1.9 mc

[mos 1]

not just the servo, when you add that big mc it gets harder, or appears harder cause the travel is less, I put on my big mc for five minutes when I had on TB's, took it off and put back on the 1.9 mc

 

yes thats what its like on my 1.9s which has the same setup gti6 fronts and 23.8mm master cylinder.

but the brakes on my s16 are like if i was driving down the street in my 1.9s and then switched off the engine so there no servo assist

[16v205 9]

I know youve probley checked already but I guess your pipe isnt split or maybe leaking at the joiner? How does it run with just 1 vacuum pipe attached and the second take off blocked up?

 

Ages ago I played with motorbike throttle bodies on my mi16. I had to find a servo take off from the manifold somewhere, so I popped out one of the motorbike injectors and used a thin tube to supply the vacuum.

 

I found you'd get a good pedal if you gave the servo time to depressurise but if you had to press the pedal a few times i succession then the vacuum wasnt able to build up quick enough resulting in me braking without any servo assistance. Give it 5-6 seconds between pressing the pedal and full servo assistance was available. Once Id joined another injector hole/servo take off into the rail servo assistance was fine.

This was the result I was finding from not being able to generate a vacuum fast enough for the servo, if its constantly poor and you never get any assistance on your s16 id be more likely to check elsewhere first.

 

Rich

 

Edited to say it should produce more than enough vacuum on tickover to easily run a servo.

Edited May 4, 2007 by 16v205

[B1ack_Mi16 67]

When I was using both vacum takeoffs mine seemed to generate a bad vacum, as I think the cylinder were drawing air from each other through the take-offs.

 

Now I just have vacum-takeoff on one cylinder and it works very well.

 

23.8mm cylinder with Brembo calipers front, whuch may have a little bigger total pistonarea than 306 calipers anyway, so that'll probably help too.

 

Closing the throttle butterflies more at idle will generate more vacum too, and try to keep it idling by upping ignition advance a little, downside is that it get's harder to cold-start.

[mos 1]

thanks for the reply guys

 

i do get some servo assistance, just not much and both servos work fine on the 1.9s 8 valve and both work poorly when attached the 16v, so it definately the engine and not one of the servos

 

i did try a fresh length of servo hose just to rule that out and it made no diiference

 

black mi16 and 16v205, its interesting what you said about using one takeoff instead of two, it never occurred to me that they may be effecting each other, i will try one and see what happens.

blackmi16 when you switched back to one you didnt get any running rough problems or the car stalling under heavy braking did you? i think these potential issues are the reasons mattsav gave me for why he attached two outlets to the inlet manifold in the first place.

if that doesn't do the trick i may even try incorporating a couple of one way valves in the lines to try and prevent the two outlets effecting each other.

what do you guys think, could that do the trick?

 

prior to reading blackmi16's reply i had actually been thinking that i might need to connect all four cylinders, but even with my limited knowledge i am pretty sure that any petrol engine creates enough vacuum for decent servo assistance that its unnecessary to go to this extreme.

 

welshpug good point about why they had vacuum pumps, but i thought this was due to the variable vacuum operate inlet manifold and not as a result of the engine producing poor vacuum.

 

unfortunately even though i described my car engine as an S16 its actually a 2.0ltr 405 mi16 but i though if i wrote mi16 everyone automatically assumes its the alloy 1.9.

with the engine being out of a 405 and not a 306 i believe from searching on here that they did not come with the vacuum pump fitted to the end of the camshaft. mine certainly didn't and it had an ordinary thermostat housing. in fact i dont have a vaccum pump at all as i bought the engine in pieces from Dazza on here. anyway i have since replaced this with one from a 1.9mi16 that mattsav gave me as it allowed me to fit the kms coil pack on the side of the head.

 

i am reluctant to move the coil pack if i can avoid it but that said i do want good brakes so if it has to go elsewhere so be it. but can a vacuum pump be fitted else where on these engines, such as drive off the fan belt etc.

or i imagine you must be able to buy an electric vacuum pump somewhere

[Rippthrough 98]

So long as you keep the take-off lines both small in diameter and a fairly long in length before they converge, you shouldn't get the scavenging problems.

[B1ack_Mi16 67]

So long as you keep the take-off lines both small in diameter and a fairly long in length before they converge, you shouldn't get the scavenging problems.

 

I haven't had any problems with stalling when braking so I doubt you'll have that problem either.

just try and connect one and see what happends, I would guess it'll be fne, but it also depends on that your TB's are closing as much as possible at idle, which will create a bigger vacum.

[16v205 9]

If youve got a one way valve to hand its worth a go.

 

I used to have a single take off with a one way ball bearing valve from a rover turbo on my jenvey/longmanns manifold. This was also with a 23.8mm mc and never suffered with lack of servo assistance.

 

Rich