Engine breathing

[sonscar]

So the carbs suck air/fumes out of the camcover,but where does the fresh air go into the engine?or does it not need fresh air?On other cars I have had there is some form of controlled air supply to the crankcase but I cannot readily see such on the TR.Thanks in advance,Steve..

[saabfast]

The connection from the cam cover to the inlet manifold relieves the pressure in the crankcase caused by gasses getting down past the piston rings on compression. Without it the crankcase would pressurise forcing oil past the seals etc.

[sonscar]

I realise that but my question was where does the engine breathe in?On my MGB the oil filler cap has a hole in it,on my classic Mini the cap has a hole init,on my Rover V8 there is a air breather on the opposite cam cover to the flame trap,so where is the equivalent on the TR7?Or is there not one?Steve...

[john 215]

Hi.

Breathes through all the leaking gaskets and oil seals

Not unusual for a engine not to draw fresh air in, no modern engine I know does so.

Cheers John

[saabfast]

As John says, 'modern' engines do not breathe fumes to atmosphere, the 'inlet' is air/products of combustion leakage in the engine and it is fed back to the inlet manifold to be burnt off. Older engines (probably like the MGB) have a vent in the oil filler cap, generally to relieve pressure rather than let air in, with a gauze mesh in the cap to coalesce the oil in the vapours. Pressure from the crankcase (from piston ring bypass) passes up the pushrod holes to the rocker cover to vent out of the cap as they do not generally have a piped system to the inlet manifold to burn the fumes off. Some, like old Jags, have the vent piped to an oil catch tank.

[sonscar]

No it does not.The air enters the cap and is sucked into the inlet via the tappetchest as is the MiniThe Rover 8 efi idles very badly when the rocker cover air breather is blocked.I guess the TR7 remains a mystery.Steve

[FI Spyder]

Not sure about English but old engines used to just have a pipe that went down and vented by the bell housing with road speed providing a bit of suction. The oil filler cap was also open so air could enter if needed (or exit). When pollution control came into being, the valve cover area was vented to the intake to be burnt by engine combustion. This took care of any blow by, oil filler cap was sealed so the system was under a slight vacuum. When the engine was not running and hence no vacuum, the gases are fed to a charcoal canister where they are absorbed until next engine run where they are then sucked into intake to be burnt.

[Rubberbiscuit-1]

My 1970 TR6 had the type of system you describe. It also had no apparent way for fresh air to purge the crankcase. The rocker cover had a largish (3/8" maybe larger) ID hose "Y" 'ing off to each Stromberg carb down stream of the throttle plates. On the carbs' nipple connections were plastic restrictors with a much smaller hole in them, maybe 4mm or so. I'm guessing your 7 is exactly like this including the utilization of a charcoal canister that also "T'd into it. The drilling size in those two plastic reducers is important to the correct scavenging of the crankcase because it determines the amount of suction needed in the crankcase, and along with suitably calibrated metering needles, correct carburetion.
However, all this is really dependent on an engine that is right up to snuff with not too much extra age wear/age related blow-by. Too much blow-by can increase pressure in the crankcase because the plastic carb reducers are now not capable of scavenging all the increased blow-by from the crank case so leaks may develop.
Here is some good info on PCV. I used it to plumb an American style PVC system into my TR8 when I converted it from FI with an above similar crank case vent system to a Holley carburetor.

http://www.britishv8.org/Articles/Posit ... on-PCV.htm