Lost 28 bhp after changing exhaust :-(

[lestershaw]

I was not completely happy with my exhaust sound for titt7 so I thought I would tweak it. I put on a straight through exhaust and kept the original y piece sounds fantastic.
However I pulled out to overtake and the chim did not seem as fast as usual .
Just got back from the rolling road to find my power max had dropped from 287.2. To 259.3, gutted. I need and want the power so the old exhaust setup is going back on.
Lester

[dbv8]

[clive f]

so you were using the standard catted y piece, with a straight through exhaust? but you lost 28bhp,
what set up were you using originally, and did you use the same rr as last time?

you`d normally expect a 10hp increase when removing the main cat, assuming pre cats have already been removed anyway, so if you are saying you lost 28 then I`d wonder if something was not quite right, if, it was on the same dyno.

[lestershaw]

I am surprised also, there seems to be a conflict of opinions whether the loss of back pressure would have any effect, these figures seem to suggest it does.
same rolling road used and same rolling road tester ( my son in law!)
before and after figures shown. i will have another rolling road test after the original exhaust is back on to confirm

Edited by lestershaw on Tuesday 11th October 19:17

[Goaty Bill]

Isn't that usually as 'simple' as;
1) not enough air coming in now that more can get out
or (more likely)
2) not enough fuel for the additional air coming in?

It's certainly how I remember it from my days with BB Chevys and Holley carbs.

Ahh the joys of having a box full of expensive jets and accelerator pump springs and constantly ripping a pair of four barrels apart between runs down the 1320... sigh

[lestershaw]

Clive
i took off what i believed to be an original tvr chimaera silencer with a catted y piece
i replaced it with a straight through exhaust and the same catted y piece.

i am now in a quandary because i want to put on your deactted y piece that i recently bought but i am worried that i will be down on power, i dont have time, skill or premises to do it myself so each time i do it it me a couple of hours tvr labour!

[lestershaw]

Goaty Bill said:

Isn't that usually as 'simple' as;
1) not enough air coming in now that more can get out
or (more likely)
2) not enough fuel for the additional air coming in?

It's certainly how I remember it from my days with BB Chevys and Holley carbs.

Ahh the joys of having a box full of expensive jets and accelerator pump springs and constantly ripping a pair of four barrels apart between runs down the 1320... sigh

2) not enough fuel? i am not sure how to read the read out accurately but my rr tester said it is running a bit lean, 28 bhp lean?

[lestershaw]

here is the previous read out taken from same rr

[Goaty Bill]

lestershaw said:

2) not enough fuel? i am not sure how to read the read out accurately but my rr tester said it is running a bit lean, 28 bhp lean?

I removed the almost open exhaust off my 454 first time at the quarter mile, it had been running beautifully on the street for a couple weeks.
Sounded like it was going to die about half track it was so lean. The popping and mis-firing was unbelievable.
I probably lost more like 150hp even before I could hear it going wrong.

The problem only gets worse as you increase speed / load of course - not absolutely directly linked to revs.

[lestershaw]

i found out on an overtaking manoeuvre that i thought i had covered, i knew i was down on power, but that 28 bhp is 10% , too much, especially when i am hoping to achieve 300 bhp plus with my winter project

[Goaty Bill]

It's a pretty depressing discovery when it happens to you.
I expect one of the other 500 boys will have been through exactly the same thing and be able to point you in the right direction.

Mine was delivered new with 'sports exhaust' i.e. no silencers, so I was lucky.
It's the reason I have hesitated previously to do more to free up the exhaust in case it all goes the wrong way.

[lestershaw]

thats what i was thinking too, the most depressing thing is that i bought a nice new shiny decat y piece from clive and i was going to get it fitted this weekend, now i am not sure what to do
even though yours was delivered with a sports exhaust, how were you sure it was achieving the best bhp ?

[Goaty Bill]

lestershaw said:

even though yours was delivered with a sports exhaust, how were you sure it was achieving the best bhp ?

I took TVR's word for it.
I got mine in the days when they were still making them, and it was a relatively common thing for the dealers to do (as far as I knew).
That, and I collected a lot of scalps, I mean a lot of scalps, back then. I never met anything quicker/faster for at least 4-5 years, and by then I could tell already the engine was no longer at it's best.

Most of the Ferrari, Porsche etc. drivers were used to having a good run against a 400 (they were everywhere at the time), but usually holding their own or even winning. A 5 litre Chimaera was uncommon, and they never seemed to notice the boot badge until it was too late


I will be following this thread with interest, out of self-interest

Edited by Goaty Bill on Tuesday 11th October 19:53

[lestershaw]

this is what it sounds like at the moment with straight through exhaust and ypiece with cats in, and precats still in, but with the power loss

http://www.youtube.com/watch?v=9dhA4qmK0V0

[Alexdaredevilz]

Some things not quite right here

I decatted my car and it felt better and combined with 45mm trumpets and self ported manifold I gained 30bhp

But the last dyno I had I was down in power,

Turns out my valves were shot and cam was worn, all in the space of 6months

How long between each dyno?

Have you got a air fuel before and after graph?

I done a leak down test to find my problem, as the compression test didn't show much, and I spent months! Trying to find my power loss

[Goaty Bill]

Alexdaredevilz said:

Turns out my valves were shot and cam was worn, all in the space of 6months

According to most experienced TVR mechanics I have talked to, 50k is your expected lifetime for a cam and lifters.
A flat cam can/does usually happen quite fast. Once the 'finish' is worn from the cam, it all 'goes south' pretty quickly.

Added;
I found the flat cam 'permanent' fix. I wasn't sure they were available for Rover V8s in this country, feast your eyes on this;
Roller Cam

I don't know the figures for a Rover V8, but on BB Chevys we could pretty much guarantee an extra 40hp just from the reduced drag of not having lifters bumping over cam metal-to-metal. That said we were also often using lifts near/over 600 and durations approaching 300.

Edited by Goaty Bill on Tuesday 11th October 20:08

[lestershaw]

Alexdaredevilz said:

Some things not quite right here

I decatted my car and it felt better and combined with 45mm trumpets and self ported manifold I gained 30bhp

But the last dyno I had I was down in power,

Turns out my valves were shot and cam was worn, all in the space of 6months

How long between each dyno?

Have you got a air fuel before and after graph?

I done a leak down test to find my problem, as the compression test didn't show much, and I spent months! Trying to find my power loss

Hi Alex
4 months between dynos
i was on 262 bhp power and then i took it to hilton and moss who found some air leaks and it was then up to the 287 of the last read out!!
i am not sure if i have a fuel after graph but i will dig out the original one where it was running much too lean before the air leaks were sorted.
the thing is, i actually noticed the difference in power output after the change of exhaust so i dont think its something that has happened since the last dyno, but hopefull all should be revealed saturday when i have it dynoed with original set up

[lestershaw]

the mileage on my car is a 30,000 confirmed by old mots

[Alexdaredevilz]

Goaty Bill said:

According to most experienced TVR mechanics I have talked to, 50k is your expected lifetime for a cam and lifters.
A flat cam can/does usually happen quite fast. Once the 'finish' is worn from the cam, it all 'goes south' pretty quickly.

Added;
I found the flat cam 'permanent' fix. I wasn't sure they were available for Rover V8s in this country, feast your eyes on this;
Roller Cam

I don't know the figures for a Rover V8, but on BB Chevys we could pretty much guarantee an extra 40hp just from the reduced drag of not having lifters bumping over cam metal-to-metal. That said we were also often using lifts near/over 600 and durations approaching 300.

I tried to go down this route myself, even bought a roller lifter to check fittament from a sbc kit

It did fit and a small amount of lathe work to enlarge the oil way would be needed

But

A one off billet cam would of have to be made

Putting the total to about £1200

And that's twice the price of a nos kit ;-)

[v8 racing]

there is definetly something not right apart from the exhaust, back pressure is a pure myth, the last thing you want is back pressure in an engine, back pressure is related to (pumping losses) a pressure working against the piston on the exhaust stroke, gasses doing there best to escape and a ste silencer restricting that no engine wants that, it happens though as you need to silence a car to decent noise limit.
I am not syaing your car has BUT i have seen really shoddy straight throughs, where the pipe fitted has been smaller than the original and where it has been fitted has caused reversal, (big back pressure), however saying all that looking at the graph you have a lot more torque than hp around 50ftlb at peak, but at 2000 revs you have still lost shed loads, either a plug lead wasnt firing or a fault with the rollers comes to my mind.