Large exhaust valves
Turbo Mini Forums

I just wondering what are the advantages and disadvantages of installing larger exshaust valves. Ie offset 32mm or just 30/31mmn. Are they benifical on all types of engine ie mild low boost engines? Or just for big power 200bhp+. Do you get Better fuel economy, due to less pumping losses and back pressure? Less lag?

Edited by alaskanow0 on 5th Sep, 2012.

Class C 3rd Place Avon 2011 14.18 @101mph

Lots of factors to add into the mix. but you can only really go so far with the exhaust valves before you run the risk of cracking the head between the valve seats. do a search and i think 29.5 is the recommended max but i could be wrong on that.

Own the day

Edited by alaskanow0 on 5th Sep, 2012.

Class C 3rd Place Avon 2011 14.18 @101mph

I run 31mm exhaust/35mm inlet BUT no inserts,I would rather reset tappets/regrind seats than risk head cracking.I only do about 500 miles pa but some of them are quite hard!

'Where does the engine go?'

im on 36/31mm with inserts no probs so far .

if your revving it above 6k then id go with the 31mm.

if pk power is less than 6k i'd stay on the 29mm.

Edited by robert on 6th Sep, 2012.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

I remember MED did some 33/33 heads a couple of years ago. They've stopped though.

I seriously doubt it!

This is true dave and we all know whos engine that one and only head of that kind was fitted to,

what you don't know is i had said head in my possesion and did some extensive flow tests of which the results are somewhere safe, so safe i can't find them,lol lest to say it was a lot of work and money for not a lot of flow per say

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Stu Gurr was telling me he did a head with the same size inlent and exhausts.

I had serious issues years ago when a company tried to do heads with 32mm exhaust valves.

The first head they did with offset holes in the guides did survive, but the casing cracked over time between the seats. That was originally an Avonbar ported head and it worked great for me, although it was another person who did the larger valves.

I had this other person make me a replacement head as the cracks in the old one were getting pretty big, even if it didnt have any noticeable effect on performance....they drilled the head and fitted inserts for an offset normal guide.
These got ripped out of the head within a handful of miles. They then drilled/plugged the inserts into the head. The guides then got ripped out of the head.
Eventually they did another head with offset drilled guides as before. But with all the heads modified by this person, the car would never rev properly above 6k. I thought it was because of the guide issues on the first head they did...seems not
Although that wasnt a valve issue...but it was head related.

Refitting my old cracked head and the car ran perfect again. I spent weeks with the other head trying to get it to work to no avail.

Eventually went to Swiftune and they made me a new head with std size exhaust valves and a superb porting job and it worked perfectly again, revving cleanly to over 7k in all gears with ease.

I really dont see there being much need for larger exhaust valves.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

it was soo log ago i cant actually remember what he said, thing it was 2006, and he was talking about some time before that.