Intake manifold vacuum pressure?
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Intake manifold vacuum pressure?
I ride a '17 eg with a 124ci m8...just was wondering if anyone could tell me what the intake vacuum pressure is at idle? Does it change with intake size or any other variables? I think i could measure it from the 1/4" tube out the top of the throttle body? Thanks in advance...if I'm not mistaken you can get an idea of exhaust velocity or lack of by testing this way. Seems like a good alternative (instead of an exhaust backpressure guage) to setting up drag pipes to get the correct operation out of them. Thanks again yall...I appreciate your time!
Last edited by Lostintennessee; Mar 16, 2025 at 02:35 AM.
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hahaha
have fun with that one
for starters, look at the way the engine is fueled. at idle you will not get a stable reading unless you are running carb per cylinder and the cylinders are balanced.
yes intake volume makes a big dent and you cannot go wrong with smaller and more velocity than too big and lazy velocity unless you are WOT and the engine can take advantage of it.
you will never get drags for the street tuned without a dip, you can choose where to place it, hence the name DRAGS.
case in point:
ran a pontiac with high rise tunnel and trip-duce. if you ran syn carb on the street you would have drivability issues, not so much on the track. but if you run off the center and progressive link the other two, way more street friendly.
if you stomped the trip in syn on the street, serious lag because you will have more fuel on the floor than in the tunnels. today, port or direct injection solved a lot of issues around in the past. a trick back then was to groove the manifold floor to direct fuel drop out to the cylinders, usually the rear and front since they tended to run lean compared to the centers.
have fun with that one
for starters, look at the way the engine is fueled. at idle you will not get a stable reading unless you are running carb per cylinder and the cylinders are balanced.
yes intake volume makes a big dent and you cannot go wrong with smaller and more velocity than too big and lazy velocity unless you are WOT and the engine can take advantage of it.
you will never get drags for the street tuned without a dip, you can choose where to place it, hence the name DRAGS.
case in point:
ran a pontiac with high rise tunnel and trip-duce. if you ran syn carb on the street you would have drivability issues, not so much on the track. but if you run off the center and progressive link the other two, way more street friendly.
if you stomped the trip in syn on the street, serious lag because you will have more fuel on the floor than in the tunnels. today, port or direct injection solved a lot of issues around in the past. a trick back then was to groove the manifold floor to direct fuel drop out to the cylinders, usually the rear and front since they tended to run lean compared to the centers.
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From: Iowa
Thanks Bustert 🙏...
my idea was to put a solid core baffle in a 2-1 exhaust...I know I'll have to add disruption or restriction to bring the torque back up..an ill prolly lose some. This is more like a r&d project for myself till i can get the exhaust im aiming for. Something to play around with..for sound and performance. Where i live its much better to be heard than seen..these drivers are terrrrrrrrible! We are ranked 3rd in the country i think lol ...anyways..i believe there there are two ways to check for exhaust velocity...vacuum guage on the intake or exhuast backpressure guage. Wich one would you rely on more? TIA!
my idea was to put a solid core baffle in a 2-1 exhaust...I know I'll have to add disruption or restriction to bring the torque back up..an ill prolly lose some. This is more like a r&d project for myself till i can get the exhaust im aiming for. Something to play around with..for sound and performance. Where i live its much better to be heard than seen..these drivers are terrrrrrrrible! We are ranked 3rd in the country i think lol ...anyways..i believe there there are two ways to check for exhaust velocity...vacuum guage on the intake or exhuast backpressure guage. Wich one would you rely on more? TIA!
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have fun with that one. hardly anyone drives windows down and besides, the hd is in the lower frequency range and those frequencies are omnidirectional, meaning direction of travel is harder to determine.
there are many styles of baffles and i made a bunch, each with +/-'s.
for open baffle this works well and fights reversion
this one works really well if you run long pipes at least 40"
blind and open perfs require a lot of experimentation
there are many styles of baffles and i made a bunch, each with +/-'s.
for open baffle this works well and fights reversion
blind and open perfs require a lot of experimentation
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From: Iowa
Dang!! I'd a never thought of doing it that way ...that's a good idea...I was thinking I'd have to loli pop or torque cone ..something of that sort? Forcing the exhaust into a spin or tornado effect. I've seen louvered baffles but don't they make your engine run a little richer? I might be mistaken..just something I remember coming across. So the mfgr's baffle that's originally designed for it is perforated but only the last 2-3" before the outlet. Maybe for sound or reversion mitigation. It's an s&s sidewinder 2-1..you can see the baffle on their website. I'm basically eliminating the perfs. Thank you so much for your input!!..I'll post my results in the near future.
I ride a '17 eg with a 124ci m8...just was wondering if anyone could tell me what the intake vacuum pressure is at idle? Does it change with intake size or any other variables? I think i could measure it from the 1/4" tube out the top of the throttle body? Thanks in advance...if I'm not mistaken you can get an idea of exhaust velocity or lack of by testing this way. Seems like a good alternative (instead of an exhaust backpressure guage) to setting up drag pipes to get the correct operation out of them. Thanks again yall...I appreciate your time!
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From: Iowa
Thanks Mr. Ramberger for the reply...
so measure it with the exhaust backpressure guage?
Will that be accurate? It measures from the o2 sensor holes and is supposed to be 1 1/4 at idle...
thanks again for the response !!
so measure it with the exhaust backpressure guage?
Will that be accurate? It measures from the o2 sensor holes and is supposed to be 1 1/4 at idle...
thanks again for the response !!
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Joined: Sep 2021
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From: Iowa
Ok E. R.
I'll ask you kindly to explain? Aren't all i.c.e.s supposed to be no more than 1 1/4 psi back pressure at idle? If I can't measure either way I was proposing then how?
Thanks in advance for your response!!
I'll ask you kindly to explain? Aren't all i.c.e.s supposed to be no more than 1 1/4 psi back pressure at idle? If I can't measure either way I was proposing then how?
Thanks in advance for your response!!
Long story short a car is multi-cylinder with long intake runners exiting into common manifolds.
A Harley v-twin has an intake manifold that is extremely short and experiences a lot of pulses, dumping into single pipes (crossovers are not the same as a common manifold and measuring after it) and you will be running drag pipes. At Idle, the reversion in the pipe is a lot different that at 6000 rpm. Add to this a Harley had a 360 + 45 and 360 - 45 degree firing pattern.
So you can't tune your Harley with drag pipes like you would tune a V8 based on running intake manifold readings and exhaust back pressure. Drag pipes suck for tuning and you'll probably benefit from torque cones or a washer on a threaded rod that you can turn in the muffler to find the balance between TQ and HP.
A Harley v-twin has an intake manifold that is extremely short and experiences a lot of pulses, dumping into single pipes (crossovers are not the same as a common manifold and measuring after it) and you will be running drag pipes. At Idle, the reversion in the pipe is a lot different that at 6000 rpm. Add to this a Harley had a 360 + 45 and 360 - 45 degree firing pattern.
So you can't tune your Harley with drag pipes like you would tune a V8 based on running intake manifold readings and exhaust back pressure. Drag pipes suck for tuning and you'll probably benefit from torque cones or a washer on a threaded rod that you can turn in the muffler to find the balance between TQ and HP.