Let’s diagnose a hard brake pedal

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honeybadger

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Elaborate a little for those of us that are not as mechanically inclined.
mim assuming there’s not enough vacuum.
what would be normal?
Exactly. You typically want ~18in of vacuum or more. The part I don't know is how this translates to track use. Since I never measured the stock motor, I don't know what the motor is pulling for vacuum while engine braking at 7k for example.





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ShatterPoints

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Exactly. You typically want ~18in of vacuum or more. The part I don't know is how this translates to track use. Since I never measured the stock motor, I don't know what the motor is pulling for vacuum while engine braking at 7k for example.
Feel free to use my car as a baseline if you like. DM me if it will help.
 

K4fxd

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You have big cams and no VVT. That idle vacuum is correct for a large cam.

Engine braking at 7K RPM is going to be above 21 inches of vacuum but 7K WOT will be around zero. Borrow a vac can or better yet buy a vacuum pump and pull a vacuum in your crankcase along with your brake booster.

It looks like you are semi-pro or at least serious about racing

Talk with your engine builder.
 

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You have big cams and no VVT. That idle vacuum is correct for a large cam....
+1. Hearing how nasty (in a good way!) your current engine sounded at idle, I'm not surprised at all it's only pulling 16 inches at idle, actually thought it might be less than that.
 

K4fxd

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This is a reason big cam carbureted engines have to idle at 1250 or above.
 

svttim

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Nice to have confirmation. Now to fix it! :)
 
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You have big cams and no VVT. That idle vacuum is correct for a large cam.

Engine braking at 7K RPM is going to be above 21 inches of vacuum but 7K WOT will be around zero. Borrow a vac can or better yet buy a vacuum pump and pull a vacuum in your crankcase along with your brake booster.

It looks like you are semi-pro or at least serious about racing

Talk with your engine builder.
I think the part I am still unclear on is a situation like the below. From 0:20-0:54, I never experience the hard pedal lap after lap no matter how hard I tried. Yet, after the big straightaway, I could get it to fail every lap if I timed it right (right from throttle to brake with no delay). Assume this is because there is more engine braking in the esses.


Obviously this is from the engine being WOT for so long and not generating enough vacuum. The part I'm confused about is what specifically is different compared to a stock motor (which NEVER had this issue for me no matter how quick I jammed the brakes). If I understand correctly, the stock motor doesn't generate vacuum at WOT either. So am still a bit confused what the difference is outside of this engine obviously generating less vacuum at idle.

+1. Hearing how nasty (in a good way!) your current engine sounded at idle, I'm not surprised at all it's only pulling 16 inches at idle, actually thought it might be less than that.
Depending on the idle, it can pull less than that. Sending some logs to the tuner to see if they can affect it on their end with idle rpm, etc.


Nice to have confirmation. Now to fix it! :)
Working on it!
 

K4fxd

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I watched the segment and you are off the throttle most of the time. The engine is making vacuum.

You are over thinking it, on that particular straight you do not have enough vacuum reserve capacity. Run that straight without hitting the brakes until the end and you will have brakes.
 
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I watched the segment and you are off the throttle most of the time. The engine is making vacuum.

You are over thinking it, on that particular straight you do not have enough vacuum reserve capacity. Run that straight without hitting the brakes until the end and you will have brakes.
I get the fundamentals, was just trying to work through what specific details are cuasing the difference between two different engines. Sounds like you're thinking it's related to vacuum capacity left in the booster?

Wondering if a tweak to the tune to pull more vacuum + a canister will fix it? Would really love to avoid an electric pump if possible since I'd be ultra screwed if that failed.
 

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K4fxd

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None I can link to. It is something we did to my Tiger when we installed power brakes.

Just a T and a one way valve.
 

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I didnt. Gravel trap, tire wall and a body shop. Fortunately I was in a full competition car with legal cage. Cosmetic damage. Not bad for an off at 143 mph.. Walked away
Ouch. Glad you walked away. Was it a GT350?
 

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