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Thread: Larger exhaust?

  1. #1
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    Larger exhaust?

    So I'm getting to the point of test fitting the engine, already intending on building my own headers/exhaust, had a look at what space I have and it seems fairly limited.

    Has anyone ever been successful fitting a larger exhaust in the 33? Inside the frame, not exterior.

    It seems like if I remove one of the cross braces and weld in a straight I might be able to fit what i want (2x 2.5) but that seems like a major undertaking. Any other ideas? Oval maybe?

  2. #2
    Seasoned Citizen NAZ's Avatar
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    I too was faced with this challenge of not enough space for a proper exhaust. If all you're after is 2.5" you can go with oval tubing through the under floor section of the chassis. Since I'm using a modified chassis and needed 3.5" exhaust I made custom headers and mufflers that exit out the side of the engine compartment.
    IMG_0525.JPGIMG_0526.JPG

  3. #3
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    Hi sorry I should have been more specific. I was talking per side. A dual 3.25 (total, both sides) would also work but there's no way that's fitting, so I was planning on dual 2.5 per side (quad 2.5 total) probably into a dual inlet muffler with a 3.5 or 4 outlet. With 1.75 headers I'd be making a 4 into 2 (basically, just to get through the frame) into 1 (per side).

    You say you modified the chassis? That's what I was looking at. Seems like I'd just need to cut one of the braces. I'm not sure what that would do to the structural integrity though. Does FF answer or get involved in questions like those?

  4. #4
    Seasoned Citizen NAZ's Avatar
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    The design of the FFR chassis does not allow for an appropriate size exhaust system for even a moderate performance engine and is a serious shortcoming in my opinion. However, I would not recommend cutting the chassis unless you have the engineering skills to determine what effect that will have on the structural integrity. The chassis is a well designed truss type assembly that is very rigid and a bit overweight in my opinion. If you cutout some of the tubing you can significantly impact the rigidity if not carefully determining where to cut and how to properly reinforce it. If dead set on running an exhaust under the floor you should get an engineer (a structural or mechanical engineer with a PE) to design the chassis modifications needed. Or you may want to rethink running the exhaust on the outside similar to the roadsters or an early Corvette rather than modifying the chassis. I'm sure you can get a single 2.5" oval tubing each side through the FFR chassis but not sure you could get any bigger even in oval and almost certain you can't get two 2.5" runs on each side without modification. Yes it is possible to modify the FFR chassis and yes I modified mine but I have an engineering background and many years experience designing structures including race cars and am confident in my design. If you look at the photo albums in the "Album Gallery" section you'll find my chassis design under 33 Hot Rod Roll Cage. Of course I was not modifying the FFR chassis just to fit a bigger exhaust. And still I'm running the exhaust on the outside rather than under the car. Good luck on your build.

  5. #5
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    How much power are y'all putting through your engines that you feel the 2.5in dia is insufficient?

  6. #6
    Seasoned Citizen NAZ's Avatar
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    540HP on the engine and up to ~700HP on spray. I've run the flow calculations and built my exhaust system to meet the flow demands for the RPM range I'll be running on the track. I'm building a super pro drag racer that will also be street legal. If I was building a street rod and not worried about maximum performance I'd just use the parts FFR sent me and call it good enough. And my car would have been on the road months ago.

  7. #7
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    I was able to snake 2 1/2" stainless through the frame with 44 series flow masters. Caveat...at each place it passes through I wrapped it with one layer of thermal cloth and used the high temperature spray. At this point you now have the exhaust "locked" in the frame and it will echo through the frame a little. I used flex connectors at the engine. I went up over the rear with typical rubber hangers. Lots of heat shield under and over the floor. Some of the guys have made plates to weld on each side of the frame with through tubing so that 3" pipe can go through straight and then exit ahead of the rear wheels. This maintains frame integrity. Oval pipe is pretty expensive and you still have to cut and weld the frame. I do not know if the IRS makes it easier or worse for rear exit exhaust. I'll be under 500hp naturally aspirated so 2 1/2 " is plenty. At the time I put the exhaust in I thought I may put more motor in it, but I am not racing this one.

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    Hahaha, good god NAZ, that will be insane. I now understand your concerns about backpressure and flow rates on the 2.5" exhaust. I cant wait to see a video.

  9. #9

    Steve >> aka: GoDadGo
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    Quote Originally Posted by NAZ View Post
    I too was faced with this challenge of not enough space for a proper exhaust. If all you're after is 2.5" you can go with oval tubing through the under floor section of the chassis. Since I'm using a modified chassis and needed 3.5" exhaust I made custom headers and mufflers that exit out the side of the engine compartment.
    IMG_0525.JPGIMG_0526.JPG
    Those headers look quite familiar.

    Are those Shoenfeld IMCA Shorties flipped upside down?

    That's what is on my MK-4.

  10. #10
    Seasoned Citizen NAZ's Avatar
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    No Dad, these are custom made headers I fabricated from scratch based on flow calculations for my particular engine combination. You can see the welding fixture I made in the first photo. It's funny you can buy a set of ready made headers that fit a Camaro for several hundred dollars less than you'll spend for just the materials to make your own headers.

  11. #11

    Steve >> aka: GoDadGo
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    Quote Originally Posted by NAZ View Post
    No Dad, these are custom made headers I fabricated from scratch based on flow calculations for my particular engine combination. You can see the welding fixture I made in the first photo. It's funny you can buy a set of ready made headers that fit a Camaro for several hundred dollars less than you'll spend for just the materials to make your own headers.
    I Wish I Had Your Fabrication Skills!

    Had a tough enough time moving stuff around to make my combination fit in the MK-4.

    Thanks NAZ!

  12. #12
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    Quote Originally Posted by DarthTedious View Post
    How much power are y'all putting through your engines that you feel the 2.5in dia is insufficient?
    The engine is a Gen V LT4, which is rated around 650, I'm also intending on a cam/pulley so I'm hoping to build flow for 800. Using 2.2cfm/hp as a rule of thumb and figuring that the flow isn't going to be optimal throughout, I'm figuring I'll need either 2 3.5" (which just isn't going to happen) or 4 2.5 (which might, with frame mods?). I'm also playing with the idea of running 1.75 headers through the tiny opening so I don't have to mod frame at all. I _think_ I might be able to make that work. But all in all... damn. I really did not expect the frame to be a problem for exhaust size here, and I'm a little disappointed. I'm not a chassis engineer but it sure seems like they could have left a little more room there.

  13. #13
    Seasoned Citizen NAZ's Avatar
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    BlackSunshine, with that LT4 you'll be challenged to get an appropriate size exhaust through the chassis without modifications. Running 1.75" primaries is on the small side already and if you make them long enough to snake through the chassis you'll be killing your top end. You're right, it is disappointing that there was not sufficient room for an exhaust system to support even a moderately powered engine. We've had 500HP engines from the factory as far back as the sixties and now days you get more than 700HP from the factory and it comes with a warranty. It's common to see 1,000 plus HP on street machines today so anything around 500-600 HP can be considered "moderate" power. And a dual 2.5" exhaust system (about the max size that fits) is not an adequately sized exhaust for that moderate power level. So hey FFR -- add that to your lessons learned file for the next design. Some of your customers really want to build a "hot rod" not just a street cruiser.
    Last edited by NAZ; 02-17-2017 at 12:02 AM.

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