fitting larger inlet valves?
Turbo Mini Forums

hi, im modding my cooper at the moment. its an mpi, so has the smaller 33mm inlet valves. i have modded the head in the past, porting and polishing, having it skimmed to raise cr.

am going to fully rebuild engine. will be changing the valve guides for bronze alloy ones.

i was wondering how much work is involved in fitting larger inlet valves, 35mm (like mg metro, cooper s, 1300gt etc).

at a guess i realise that the old seats will have to be machined out, and new ones pressed into place.is this right? how much should my local machine shop charge for this?

i am happy to lap valves in etc, just need machining work done. any advice on what this should cost, and what parts i need (do they normally suply new seats)?

many thanks

You don't need any seats installed for the inlets, the existing seat in the head is simply cut larger to suit the valve. Obviously to gain any benefit, both the throat & port will need to bbe worked to suit.

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Phil's right, but to add. If you're using the standard cam, there is very little to be had changing for a bigger inlet. This is if the head is a good one.

Any pictures??

as above dan, a change of cam with 35.6mm inlet valves and my thoughts would go to a kent 266 cam would make a good torquey engine
with the right exhaust etc.........................

well the engine i have planned is as follows.

1360 overbore, reshelled crank, centre crank strap, lightened duplex timing chain, sw5i cam, new oil and water pumps, new fan belt

head is already ported and skimmed to decent level. the cr will be around 10.7:1
using double valve springs, bronze valve guides. 1.5 roller rockers.

exhaust is maniflow lcb, with decat pipe and 1.75 rc40

have a ported inlet manifold (matched to head, with 52mm alloy throttle body (modified to open to 90 degrees), K and N induction.

will also use a power boost valve. will be running this wthin the confinds set by the standard ecu. am currently running standard bottom end with 10.7 cr. it goes fine but smokes a little, hence the rebuild.

this will be going with a 1275gt close ratio gearbox, hardened diff pin, centre oil pickup, extended simp plug and something like a 3.2:1 final drive

With that spec, the 35.6 valves will likely give you a gain in top end, but rob you of midrange torque. The 1.5 rockers won't ghelp this either. What do you want from the engine?

With that spec I would argue at having a more breathable exhaust think about upping it to 2" and sourcing somthing other than an RC40 I ran much the same many moon's ago and the thing pulled like a train and would eat anything upto 1800 in an all out drag. Top end was around 125 indicated

Own the day

There's no need for a 2" exhaust on a Sw5 cam no matter what the other specs are.

what i want from the engine is as quick an mpi as possible, with as much power and torque that can be obtained with the constraints of the standard ecu. the ecu will be the limiting factor regarding power.i would hope for anywhere between 70-90bhp

am already running at 10.7:1 without a powerboost valve. it fuels ok, and goes really well. this is with a standard bottom end including the stupidly high 2.7 final drive..

i do a fair amount of motorway driving.

the exhaust i have will be fine and not restrict.

i know you have a huge wealth of knowelage vegard, but was wondering about what you said about the inlet valve.
from what i understand the 33mm inlet will restrict at this level of tune. as some models from standard used the larger 35mm inlet (at a less state of tune), i thought that this would be a good addition to to my engine?

cheers

For a road engine I don't need power above 5500rpm. All my driving is below this. Apart from the occasional burst. What I want from a road engine is as much torque I can get below in this rev band. For this I would think the 33mm inlets and 1.3:1 rockers would be my choice with a SW5. If you want the maximum amount of power the engine can deliver, 35.7mm valves and 1.5:1 WILL increase maximum power, but it will also rob you of midrange torque. It depends on what you want from the engine.

i always thought that 33mm inlets restricted even standard 1275 engines. i understand the smaller the valve the higher the velocity, ie more torque. though i thought that the 33 inlet restricted power and torque due to it being so small?

The head restricts power, not the valve size.