cheap power mods
Discussion
whats the cheapest way of improving the power on my 94 4.0 chim, its had the sports mod done to the exhaust and the precats removed by a dealer, its got the top notch nitrons fitted but it feels abit wooly compared to the 430 that i last had ,and accel is slow when in top gear at 3000rpm, and it takes a while to get past 130mph, its just had a big service and runs really well and goes ok in the lower gears ,just want a little bit more without having to get a 5.0 ...
Can't be done really .. the precats make no difference on the 4 litres accepted wisdom tells us, and only 5-8 bhp on a 5 litre it seems.
Have you thought about shedding weight? .. carbon seats? .. wheels and tyres on TVRs are very heavy and every kg you shed here is worth 3kg anywhere else on the car. Driver training?
If you're still wanting more power then ACT carbon inlets, a pair of properly ported heads (there's some howlers out there, be careful) and a few hours with Mark Adams are about as good as you'll get, not sure they qualify as budget mods, but they're probably the best in terms of what you get for your money.
Have you thought about shedding weight? .. carbon seats? .. wheels and tyres on TVRs are very heavy and every kg you shed here is worth 3kg anywhere else on the car. Driver training?
If you're still wanting more power then ACT carbon inlets, a pair of properly ported heads (there's some howlers out there, be careful) and a few hours with Mark Adams are about as good as you'll get, not sure they qualify as budget mods, but they're probably the best in terms of what you get for your money.
ok fair enough , but i went to TVR power in coventry today to see about getting my exhaust manifold fixed as it is leaking right in the middle where all the pipes come together ( common prob ? ) and they pointed out that i had the pre-cats removed ,which was done by a previous owner as shown by the paperwork, but TVR power state that this acually cuts engine power as the rover engine likes a bit of back pressure, can any engine tuneing experts shed any light on this and if needed can i refit the precats in some way .......
bit of a red herring that one, no engine likes back pressure, but you can use it to tune the torque characteristic into something more favourable .. if having the precats moves the peak torque so the spread is more useful than the car will be faster, but backpressure limits exhaust gas flow, always has always will.
joospeed said:
bit of a red herring that one, no engine likes back pressure, but you can use it to tune the torque characteristic into something more favourable .. if having the precats moves the peak torque so the spread is more useful than the car will be faster, but backpressure limits exhaust gas flow, always has always will.
I'm not so sure it is that red a herring. I do have some interesting power graphs that show drops in power when the exhaust is "improved". Interestingly enough adding additional silencing to the 520 improved the power and response!
Rover engines do like some back pressure, but a good intact system should produce this without cats, the only reason removing cats can reduce power (and they don't normally)is because you are left with a huge empty vessel where the cat used to be, to do the job properly an internal reducer should be fitted or better still a longer primary pipe manifold, this is even worse with the main cat, however power reductions would be small.
The Cat system is good for sub 300 bhp/ lb ft engines, above this and it restricts, we bench tested a 330bhp / 350 lb ft engine with and without cats, with the cats it lost 20 bhp ! and about 10 lb ft.
Removing the pre-cats will improve the noise and thats what most people are interested in, done properly can also improve response.
Tim
The Cat system is good for sub 300 bhp/ lb ft engines, above this and it restricts, we bench tested a 330bhp / 350 lb ft engine with and without cats, with the cats it lost 20 bhp ! and about 10 lb ft.
Removing the pre-cats will improve the noise and thats what most people are interested in, done properly can also improve response.
Tim
There is no such thing as a cheap power mod or everyone would do it.
Make sure the car is set up right (on a rolling road if necessary), make sure the oil and filters are clean, plugs OK etc.
The only cheap way to make a car go faster is to lighten it. My chimeara felt dramatically slower with a passenger and a full tank of fuel.
Make sure the car is set up right (on a rolling road if necessary), make sure the oil and filters are clean, plugs OK etc.
The only cheap way to make a car go faster is to lighten it. My chimeara felt dramatically slower with a passenger and a full tank of fuel.
the reason the precats change the response is due to the sudden change in volume that the gas sees .. it's the same as when the gas reaches the end of the pipe, if the volume change is sufficient then that causes the reverse pulse to happen at that point, in effect you'll get a negative pulse returning to the cylinder rather than the positive pulse from the face of the catalyst honeycomb. You would fully expect this to compromise low speed trapping efficiancy of the cylinder, but that's not a back pressure issue it's a wave reflection issue.
As Tim says, long primary pipes are the way to go for torque through the rev range.
Incidently, ever wondered why nitrous drag cars have no exhaust or very short manifold pipes direct to atmosphere? .. because there's so much exhaust gas that *any* length of exhaust sacrifices power .. it's far more important to get rid of the gases as quickly as possible than it is to have a tuned length of pipe. With nitrous you can make as much torque as you like (engine permitting) so tuning exhaust lengths is somewhat irrelevant.
As Tim says, long primary pipes are the way to go for torque through the rev range.
Incidently, ever wondered why nitrous drag cars have no exhaust or very short manifold pipes direct to atmosphere? .. because there's so much exhaust gas that *any* length of exhaust sacrifices power .. it's far more important to get rid of the gases as quickly as possible than it is to have a tuned length of pipe. With nitrous you can make as much torque as you like (engine permitting) so tuning exhaust lengths is somewhat irrelevant.
so after all that and i must say usefull information ,do i need to refit these things to my 4.0 non hc chim, i relise that there is no such thing as a cheap power mod , but then i dont want anything on the car that will reduce power at all, yes the exhaust is a bit noisier and the main exhaust is also supposed to be a sports one ( whats the difference please) ,i like mucking about on engines and anyway that i can improve things would be great.
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