Removing pre-cats
Discussion
Hi Alun, As the name suggests they help with scavenging by helping maintain exhaust gas speed as when gasses enter a big empty chamber [precat housing] they slow creating turbulence, which can build back pressure [bad]. So the theory goes.
Now I'm not sure how much effect it has if I'm honest will be negligible but in a total package of other mod's it can only help. It made sense to have them as they were being cut open anyway
Now I'm not sure how much effect it has if I'm honest will be negligible but in a total package of other mod's it can only help. It made sense to have them as they were being cut open anyway
Edited by macdeb on Sunday 6th December 12:41
Edited by macdeb on Sunday 6th December 12:45
A bit of safety too Mac? Yeah, good idea by the sound of it,,,
Sorry can't help with the cones but they are positioned in each manifold where the four pipes come together and before the bend going towards the Y piece connection.
I can also confirm that removing the pre cat cones reduced heat in the engine bay considerably on my car and the engine spins up with little restriction,,it's not so much more power as freeing the power already there,, I don't regret it at all,
Sorry can't help with the cones but they are positioned in each manifold where the four pipes come together and before the bend going towards the Y piece connection.
I can also confirm that removing the pre cat cones reduced heat in the engine bay considerably on my car and the engine spins up with little restriction,,it's not so much more power as freeing the power already there,, I don't regret it at all,
Similar to previous post; Pre-cats cats out last week whilst manifold gaskets were being replaced. Not noticably louder (thankfully - already loud enough with straight through exhaust), no changes in amount of pops and bangs as far as i can tell, but the engine definitely feels more lively. Revs more freely I think. No regrets.
ClassiChimi said:
It's interesting to note, I've just had a new aftermarket Ecu and bespoke mapping,,,
My car still does the same as Bobfather suggests,,
Shut off from any revs, cuts fuel and no pops until down below 2000 revs,,
If I squeeze the right foot a tad,, then pops introduced,,,
Does exactly as it says on the tin
The only real difference in pops and bangs going from CUX to Mbe Ecu etc is they are more pronounced, crack more,, but their still there which is the important thing,,
If you have no pops and bangs maybe your AFR is a bit lean maybe?
Its not a rich mixture you need- that burns just fine in the combustion chamber- you need a weak one- typically > 15.5:1 AFR. This wont burn properly in the combustion chamber, so the fuel that remains ignites in part as it hits the hot exhaust. You can add a bit of late timing to ensure you still have some burn in the exhaust if you want to be sure. You can tune the 14CUX to do this very easily on the non cat map, but its harder with the catalyst map as the ECU keeps pulling the mixture back to 14.7:1 that wont pop. Dead easy with after market if you dont hold it at 14.7:1 and dont cut the fuel on overrun to harshly.My car still does the same as Bobfather suggests,,
Shut off from any revs, cuts fuel and no pops until down below 2000 revs,,
If I squeeze the right foot a tad,, then pops introduced,,,
Does exactly as it says on the tin
The only real difference in pops and bangs going from CUX to Mbe Ecu etc is they are more pronounced, crack more,, but their still there which is the important thing,,
If you have no pops and bangs maybe your AFR is a bit lean maybe?
blitzracing said:
Its not a rich mixture you need- that burns just fine in the combustion chamber- you need a weak one- typically > 15.5:1 AFR. This wont burn properly in the combustion chamber, so the fuel that remains ignites in part as it hits the hot exhaust. You can add a bit of late timing to ensure you still have some burn in the exhaust if you want to be sure. You can tune the 14CUX to do this very easily on the non cat map, but its harder with the catalyst map as the ECU keeps pulling the mixture back to 14.7:1 that wont pop. Dead easy with after market if you dont hold it at 14.7:1 and dont cut the fuel on overrun to harshly.
Cheers Mark,,more air so that makes sence,, I've got it now,, that's got me thinking correctly about AFR at last 
You could do it in situ but when I removed mine one was rather stubborn, came out in bits so I'm glad I removed manifolds so could remove loose bits,, some people have done it in situ then fired the engine up before fitting Y piece to blow out any debris but I didn't feel comfortable with the idea,, ?
The risks are probably low to suck bits back up and into engine though !
The risks are probably low to suck bits back up and into engine though !
Dan, I did mine insitu and whilst it is a PITA job it certainly can be done. It took me about 2 hours each side once I'd removed the Y piece and the lambdas, it's really just brute force and patience that's required, get a selection of hammers, screw drivers, chisels, scraping tools etc. If you do adopt this approach I'd strongly advise sealing an old pillow case or similar around the open manifolds when starting it up (before you refit the Y piece) to catch all the crap that'll be left in them when you've finished. Tip: be careful when bashing around the outside of the bend as you could potentially hole the manifold if too aggressive with a hammer & screwdriver etc.
For all reading this thread:
By far the best time to de-pre-cat is when your TVR expert announces that reason for the tappety noise you have heard is that your manifolds are leaking from the 4 to 1 union.
While the welder has them off the car and cut open in order to weld up the holes from the inside, just bung him an extra tenner to lift the precats out.
Simples!
By far the best time to de-pre-cat is when your TVR expert announces that reason for the tappety noise you have heard is that your manifolds are leaking from the 4 to 1 union.
While the welder has them off the car and cut open in order to weld up the holes from the inside, just bung him an extra tenner to lift the precats out.
Simples!
Danblez said:
Thanks for the tips, I will be giving it a go over the hols.
Just double check before you start that you have no leaks from the 4>1 pipe unions in your exhaust manifolds - you would curse if, three months after you have sweated bashing the pre cats out, you had to have the manifolds taken off the car, cut open in the area of the precat, the hole welded, and the manifolds then welded up again.To check for exhaust gas leaks, find a thin piece of rubber or plastic tube a couple of feet long (or attach a feather to a stick), start the engine, then stick one end of the tube in your ear (or hold the stick) and move the other end of the tube (or the feather) around the manifold gaskets and the 4>1 union. If you suddenly hear a louder engine noise (or the feather starts to flutter) you have a leak that will require the manifolds coming off. Precat removal is easiest with the manifolds cut open, second easiest with the manifolds in a vice on the workbench.
I did mine at my TVR guy's workshops. I used a long masonry bit in a power drill (carefully!), the type you use to drill through house walls, and a jemmy. You can thump pretty hard. I was bashing away, the TVR guy came to see how I was getting on, took over for a couple of minutes and thumped about 4 times harder than I had been doing.
Take the lambda sensors out first (or be very careful around them).
So the only thing to watch out for is as mentioned earlier in this thread,,,be careful with your selection of drivers/ chisels as it's easy to go through the honeycone and knock holes in your pipes,, I didn't manage to breach mine but I do have a number of indent marks where I didn't realise I'd gone so far,,,?
I also used a long reach 300mm 10 mm masonry drill bit on hammer,, it removed one easily but the other took a good few hours to slowly break it up mainly by using a hammer and driver?
Good luck
I also used a long reach 300mm 10 mm masonry drill bit on hammer,, it removed one easily but the other took a good few hours to slowly break it up mainly by using a hammer and driver?
Good luck
As said before, easy enough to do in situ but we careful, use a masonry bit and just drill several holes round the cat, then try to join the holes with the drill or hammer and chisel. pull the middle out (eventually) and knock the remaining bits out, You will find the first one will take ages and the 2nd will be a lot quicker, dig all the loose bits out then hoover, then a pillow case over the ends, fire it up and give a quick rev, recheck for any missed bits and refit 'y' piece.
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