[Z OH 6]

Originally Posted by Checkmate1

Someone in TX said he saw the rear bumper sag on a ZLE that consistently hit over 150mph at COTA. He strongly recommended leaving the stock suitcase muffler intact to prevent the parachuting effect.

I'd like to see a video. I'm just not buying it.

[KingLT1]

I have seen several reports from those that drag race the 6th gen platform make a cover for that area or remove the lower bumper cover if the stock suit case is removed because they say it acts like a parachute. Most of those guys don't post on this forum...they are over at FB or IG. My opinion is, as long as you have some sort of mufflers back there I don't think it's a big issue. It's mainly for those with straight pipes or dumped exhaust.

[Z OH 6]

Originally Posted by KingLT1

I have seen several reports from those that drag race the 6th gen platform make a cover for that area or remove the lower bumper cover if the stock suit case is removed because they say it acts like a parachute. Most of those guys don't post on this forum...they all over at FB or IG.

A parachute in regards to slowing the car down or actually pulling the rear fascia off? I can understand there might be SOME drag there if its removed which might impact 1/2 mile trap speeds, that's basic physics, but I just don't believe the rear fascia is going to fall off or start flopping around uncontrollably. Like I said, I'd like to see a video if its exists which it sounds like it should be if its common.

[KingLT1]

Slowing the car down. I don't think it's going to come off but I have seen where the bumper cover has become distorted between the tips from where the air force has pulled the bumper lip down.

[TheRealJA105]

Originally Posted by JT Camaro

pics?

Took these pics awhile ago for this post and forgot until now. Like I said it's not pretty, but it is for sale if anyone wants to make an offer. My car is sold.

[JT Camaro]

Originally Posted by TheRealJA105

Took these pics awhile ago for this post and forgot until now. Like I said it's not pretty, but it is for sale if anyone wants to make an offer. My car is sold.

Given the size and distance between us, ill likely make my own, but thank you very much for the pics as reference. It might just be my eyes, but a lot of it looks like felt

[TheRealJA105]

Definitely not felt haha. Its actually 2 of the universal ebay diffusers that are textured plastic and some aluminum house flashing riveted to the top side to fill the air gap from the diffuser to the underbody of the car. Mounts to the bumper supports in the back and 3 self tappers into the rear cross member at the front.

[RHD ZL1]

Originally Posted by RabbitZL1

Up to a certain point. I myself tested this. Gained exactly 2mph at the track. Didnt totally eliminate the muffler, only added cutouts before the muffler. The muffler itself I would say up to 900 there has no major effect on hp. I was told by Justin Keith that he lost 3mph from replacing the oem muffler with aftermarket mufflers. (air caught under the rear bumper). He goes 9.50s.

Thanks Rabbit, certainly proves that there is some truth to it all.

For anyone interested, i used some cardboard as a template and cut out a profile to “fill in” the cavity and in effect make the rear portion from the diff to bar as flat as possible. Once happy with the profile simply traced it onto some Marine grade alloy sheet 1/16” (1.6mm) thick (Grade 5052 much better corrosion resistance than normal alloy).

Few pics of before / after plate fitted.

Pic 1 taken from under the diff looking towards the rear bar.

Pic 2 taken from under the diff looking towards the rear bar after plate fitted.

Pic 3 pic from rear of car.

It certainly was a lot easier than i thought as the plate is almost flat (no curvature required). I'm surprised the high end Exh companies don't offer this as part of their system. They could easily have the plates water jet / laser cut to suit their particular systems.

[JT Camaro]

Originally Posted by RHD ZL1

Thanks Rabbit, certainly proves that there is some truth to it all.

For anyone interested, i used some cardboard as a template and cut out a profile to “fill in” the cavity and in effect make the rear portion from the diff to bar as flat as possible. Once happy with the profile simply traced it onto some Marine grade alloy sheet 1/16” (1.6mm) thick (Grade 5052 much better corrosion resistance than normal alloy).

Few pics of before / after plate fitted.

Pic 1 taken from under the diff looking towards the rear bar.

Pic 2 taken from under the diff looking towards the rear bar after plate fitted.

Pic 3 pic from rear of car.

It certainly was a lot easier than i thought as the plate is almost flat (no curvature required). I'm surprised the high end Exh companies don't offer this as part of their system. They could easily have the plates water jet / laser cut to suit their particular systems.

Looks Great! I need to do the same

[TheRealJA105]

Originally Posted by RHD ZL1

Thanks Rabbit, certainly proves that there is some truth to it all.

For anyone interested, i used some cardboard as a template and cut out a profile to “fill in” the cavity and in effect make the rear portion from the diff to bar as flat as possible. Once happy with the profile simply traced it onto some Marine grade alloy sheet 1/16” (1.6mm) thick (Grade 5052 much better corrosion resistance than normal alloy).

Few pics of before / after plate fitted.

Pic 1 taken from under the diff looking towards the rear bar.

Pic 2 taken from under the diff looking towards the rear bar after plate fitted.

Pic 3 pic from rear of car.

It certainly was a lot easier than i thought as the plate is almost flat (no curvature required). I'm surprised the high end Exh companies don't offer this as part of their system. They could easily have the plates water jet / laser cut to suit their particular systems.

Now that's a proper job! Makes my half ass solution look even worse I also notice your rear bumper has a 3rd support added in the middle. I wonder when Chevy did that? My car was a 16, what year is yours?

[RHD ZL1]

Thanks guys.

@JT, If you are planning on doing this, here is a little more info that may help you or anyone that wishes to do the same on their car. On my exh system just behind the diff, there are 2 slotted plates welded to each pipe, the slotted plates over lay one another and once the desired pipe locations are in place an M10 bolt holds them together.The bolt is longer than required so i used that as the first mounting point for the alloy plate and then used a second M10 nut to hold the alloy plate down (very difficult to see in my previous post/pic however if you zoom in you can see the M10 bolt going vertical up just behind the diffs oil drain plug). I mention this as i don’t know if you would have something just behind the diff as the first securing point for the plate. I then used the 4 studs that hold the heat shield in place. I removed the nuts and had 4 extensions made with M6 threads on both sides (see pic) so these double as a nut to hold the heat shield in place and as additionally support for the alloy plate and then used the 3 bolts that screw into the rear bar so there is a total of 8 locations holding the plate in place. There is nil deflection in the plate. I made the plate and had it fitted a week or so ago and have been driving the car around to make sure there were no issues prior to my post as i didn’t want to post this up and then someone copies and they have issues. Another bonus to all this that i have noticed is that my car had the “rear bar sagging” problem where the panel gap between the bar and the cars body near the rear tail lights increases a little. When i fitted the alloy plate i did put load on the bar (to reduce the gap) to get it into place and then marked the last 3 holes in the alloy plate to line up with the bar. I’ve now noticed that the gap stays the same as factory and doesn’t increase. I can only assume that the 3 bolts that screw into the bar are now preventing the bar from moving / sagging as the alloy plate is held in place very securely. That said, the increase in gap was very small (I’d imagine similar to what you guys see on your cars) however now there is no gap issues. Win Win.


@JA105, mines a 2019. Its the only MY ZL1 we received here in Aus so i have no idea how many support bolts the previous models had in the rear bar.

[SFV1LE]

Why make your own diffuser when these from ZL1 Add-ons, for muffler-delete, are only $250? Stock muffler design is $180.

[RHD ZL1]

Originally Posted by SFV1LE

Why make your own diffuser when these from ZL1 Add-ons, for muffler-delete, are only $250? Stock muffler design is $180.

There are multiple reasons.

A) I’m on the other side of the world. Sometimes shipping costs from the US to Australia is double if not triple what the parts are actually worth.

B) My exh system was designed here in Aus. I don’t know how the muffler positioning / layout compares to the US based systems. There is no guarantee that it would fit my layout.

C) Some of us actually have the skill set to make our own stuff.

D) It’s fun ! Most car guys “get it”. Very satisfying working on your own car and making something that is to suit your particular car that fits, works and looks good. Way more satisfying than clicking “add to cart”.

[KingLT1]

Originally Posted by SFV1LE

Why make your own diffuser when these from ZL1 Add-ons, for muffler-delete, are only $250? Stock muffler design is $180.

Perhaps there are people still in this world that like to DIY? Yes I know a trait that is slowly going by the way side...Pretty sad actually. Not saying there is anything wrong with buying one pre-made either but there is no need to give a guy shit for wanting to make his own...especially when it turned out as good or better then what you can buy.