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General question about HP gains with a cut-out.....
So, I bought the TSP 2" LT headers with 3.5" collectors. Boosted Supercharged set-up.
If I run the 3.5" collectors back to the "Y" merge and then it tapers down to my current single 3" pipe to run back to my Magnaflow muffler and then out my tailpipes...........how much RWHP gains would I expect to pick up if I installed a 3.5" cut-out at the "Y" before it tapers down to the 3" pipe....?
edit:.....we expect to make about 1,100 RWHP with this set up.
I'm wondering if I should bother with a cut-out. How much RWHP would I be giving up by leaving it as a full exhaust, no cut-out.
I know the following is not going to be a straight correlation to your set up at all, but it might give you some food for thought. I saw a dynograph of a heads/cam car with a cutout making 430rwhp closed/436rwhp open.
I know the following is not going to be a straight correlation to your set up at all, but it might give you some food for thought. I saw a dynograph of a heads/cam car with a cutout making 430rwhp closed/436rwhp open.
6/430 ~ 1.4%.
0.014 * 1100 ~ 15rwhp.
I see what you mean....interesting. Absolutely not worth it in that cars case.
The better your full exhaust is, the less a cutout will gain..
Well, with 2" headers and 3.5" collectors thats about as good as it gets in that department......I guess maybe my two 3.5" collectors merging to a 4" single exhaust back to my magnaflow muffler....will pretty much be as free flowing as it gets.
I mean unless I'm gonna see 40-50 RWHP gain...I will not go with a cut-out. Just keep it full exhaust.
We've seen no gains and sometime losses on big HP cars with good exhaust with the cutouts open.
A 410 sprint car will lose 15-20hp by just running a very short high flow muffler.
With a supercharged engine exhaust flow is a weak link so I would guess he should see some gains, I don't think it will be 40-50hp but that also depends on the restriction a single 4" pipe and muffler is and header primary/collector size. Some lose torque with open headers but a simple 12"-18" collector extension will usually gain that back.
Last edited by LLLosingit; 06-16-2018 at 02:46 AM.
On my 99 TA w / heads & cam, Hooker LT's and a good design 3 inch collector dual 3 inch into 4 inch Mufflex Y pipe the cut out was worth 10 hp over the SLP Dual /Dual. Cam was a single pattern. Friend in the know said with a few more degrees exhaust it would have been less than 10 hp gain more like 5-7hp. Saw others gain about the same with cut outs with good Y pipe and cat back set ups.
On my 383 LS1 91 RS with a bad Y pipe that's notched, twin cut outs off the headers are worth 20 whp. If a big mufflex Y pipe had fit I think it would have been ~ 10 hp or so. This car also has single pattern cam. Gains for cut out would likely have been less with a couple more degrees of exhaust duration.
With a good exhaust set up I think in most cases the cut will be worth 0-10hp. With your big hp 800 hp car it might be more than the 400-550 whp NA/NOS cars I've seen dyno'd.
My guess is maybe 15-25 hp for 800 hp car with a good exhaust set up and optimized cam with correct exhaust duration.
Often the best part of cut outs especially remote electric cut out / cut outs is the gadget / intimation factor.
On my personal car, 00' Firehawk. Made 430rwhp with the cutout closed and 435 with it open. 1 3/4 LT, 3inch ORYP into the flomaster merge, 4inch to the axle then down to 3 inch into the SLP dual/dual muff. Cutout was in the 4 inch pipe under the back seat.
If a car has a VERY good full exhaust, a cutout is all about noise, and nothing more.
This is why I always thought they're kind of douchey (no offense to anyone who wants a few extra HP at the expense of all the noise). It's way cooler, imo, to make the car sound the way you want it to sound with a good exhaust system and skip the cutout. The only place I would use one is behind a turbo.
It all depends on how your exhaust is setup. I gained 24 rwhp with my cutouts open on a back to back pull, but I have 3 mufflers and a cat after my cutouts. If you want as quiet as you can be while having good clearance with a shorty torque arm but the ability to free up the power, then this is the way to do it but prepare to open the wallet.
I agree with the above! We can't always have the VERY good full exhaust I mention above. So you get by with whatever works and use a cutout when the need arises.
My LSA supercharged S10 has 4 mufflers, It's setup this way, Headers then about 2' of pipe to a pair of straight thru Magnaflows, Then another 1' or so of pipe and then cutouts, Then pipe over the rearend to a larger set of mufflers and turn downs just before the rear bumper. With cutouts open it's loud as hell but does have a decent tone to it. With the cutouts closed it has a fairly quite idle and cruise but still hear it coming when you step on it.
Here is a picture before it was complete.
If you’re losing hp with a freer flowing exhaust, it’s not because back pressure is good, it’s because your setup as a whole is no longer ideal. You would need to rework pretty much everything right down to your cam timing to extract the extra power you should have seen.
In pneumatics, the speed of a piston is controlled by the exhaust port - not the inlet - the less the restriction, the faster it moves. Back pressure is that very restriction in an engine. If you’re seeing losses with less restriction, it’s because your cylinder filling has changed and your cam timing events need to be reengineered. Those little things called “overlap” and “LSA” come into play here.
My LSA supercharged S10 has 4 mufflers, It's setup this way, Headers then about 2' of pipe to a pair of straight thru Magnaflows, Then another 1' or so of pipe and then cutouts, Then pipe over the rearend to a larger set of mufflers and turn downs just before the rear bumper. With cutouts open it's loud as hell but does have a decent tone to it. With the cutouts closed it has a fairly quite idle and cruise but still hear it coming when you step on it.
Here is a picture before it was complete.
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Its hard to beat that set up for a free flowing exhaust.......
I guess when it gets dynoed, we can always disconnect the "Y" from the collectors, or disconnect immediately after the "Y" merges into the straight 4", and do another dyno pull to see what it makes. If it happens to be 40-50 RWHP.........then I can add a cut-out.
06-15-2018, 07:24 AM
General question about HP gains with a cut-out..... 
