13 replies to this topic

[automalt]

Hi all.

 

I am in the process of building my GT highly tuned engine and would really like to know which is the

biggest exhaust valve that can be installed in a 5 port head.

 

My priority in this head will be the biggest exhaust valve that I can fit into the chamber.

 

Your help will be much appreciated.

 

Thanks alot.

[kyle9832]

not sure, but unless you are running boost the biggest isn't always best!

[OzOAP]

What is the biggest you can fit, or what is the biggest you need? What cc engine? What cam?
Are you offsetting?
What size inlet are you running?

Working on that the biggest combination in a head is probably 37x31 = a combined size of 68mm, the smallest inlet in a 940 head is 33, so you should be able to get a 35mm exhaust in. Im not sure why you would want to run a 33x35 but then I do wonder why a lot of people do some things.

[mini13]

32mm has been done for turbo applications, 36x32 IIRC biut your looking at offseting the exhaust guide to get it in.

[whistler]

I have 36x31mm and there's not much room left. Not offset. Not unleaded either as there's no room for a seat insert.

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[nicklouse]

to be honest you want to have the head and cam sorted before going further on the engine.

Easier to adjust the CR by machining the block or choosing the Pistons than having to redo the head.


But you are not giving much info.

[automalt]

Hello again my friends.

 

Thanks a lot for your feed back.

 

So, where am I getting @?

My wish is to build a 1550cc turbo charged 5 porter ( don't know if is better to go for the alloy head, yet ) I do know that there are some who are not in agreement

with me over this ' biggest exhaust valve ' matter but I wonder if anyone has ever tried and tested this yet. This is besides the fact that I do not wish to install a big cam but a small to medium cam with high ratio rockers. So my bet/quest is to assist the exhaust in better scavenging and turn the turbine - high ratio turbine as strong as possible.

 

I know that my theory may  be somewhat ' illusive' but unless you try you will always remain with your big question in your mind. Having said that, I am considering all your helpful inputs.

 

Thanks alot, again.

 

 

Automalt

[LSC]

wouldnt go more than 31mm on a turbo motor as they have a tendancy to crack inbetween the valves

36x31 with a decent job porting will see you over 200bhp depending on cam/turbo setup

depends what you want to do

im sure youve read everything over at turbo-mini.com and introduced yourself to turbominis.co.uk?

[mini13]

have you thoght about a different head?

 

K1100, fiat 1.2?

[automalt]

Hi guys.

 

Thank you all for your tips.

 

I am not yet a ' Turbominis ' member which is a good comment.

 

It's not an easy decision this valve size matter. If the biggest exhaust valve does not give more

edge in power output, it should be worse on an aspirated engine and so the head will be

useless then.

 

Well thanks again all.

 

Nice week end.

 

Automalt

[Ethel]

I wouldn't do any extreme N/A type mods to a turbo motor. A turbo should already give you enough extra power & heat to break things, so why make the engine more fragile? Extra swept capacity also needs extra combustion chamber volume to keep the compression in a sensible range.

[Sir Yun]

I'm going to be arsey.  

 

IMHO there is zero reason to build a 1550 cc turbo engine (how are you gong to do that anyway?) other than pub bragging purposes.

 

If you want more power concentrate on adding more boost reliably (well feed it more mass actually) without blowing it up.

 

 

here are a few more reasons why it is a bad idea

 

the bores will be paper thin and the power gains will be lost by bore flex. you need a very fat decompression plate to get the CR anywhere near sane for a TC engine, Your wallet is not nearly fat enough for the things you want,

 

You spend a fortune on a stroker crank, fancy rods etc and you will melt it at one point (because that is what high strung turbo engines are good at) taking out all the fancy parts and then you will be fed up. The gearbox will **** itself repeatedly,Somebody who does know how to do stuff will show up with an engine build from scrap parts that will eat yours.

 

 

these pipe dream projects generally work like this.

 

you have a ( in your mind great) idea, you don't have a clear plan but you buy some very nice parts, you start building the engine, after a long time you get it to run sort of, **** breaks, you run out of money, you sell the remaining parts at a great loss.

 

 

May I suggest this:

 

http://www.strojar.c....B.Heywood.pdf.

 

the question you have to ask yourself is the following .

 

given a limited space going bigger on the exhaust will have to come out of the intake. Ergo is the loss due to pumping losses greater than the loss do to compressor inefficiency due to the smaller intake valve. Also remember that on a turbo the compressors are coupled. you increased the exhaust valve but the intake has gotten worse. So you need more effort to get the air in and that is making it harder for the exhaust turbine to turn. the intake charge will heat up more, the exhaust will be hotter, the density and thus mass will be lower and it makes less power, it might produce more boost though, but not move more mass.

 

Coupled to this :by default the exhaust valve is way more efficient than the intake valve. so it looks wrong but actually it balances the intake and exhaust out a bit.

[mini13]

I'm going to disagree slightly with the above, and say 73.5mm bores are fine for turbo use,

 

1460 is about as far as you can go sensibly and keep the compresion ratio ok, it would still be pretty much off the shelf parts,

[Sir Yun]

73.5mm would be okey and have been used (but I still doubt that it is worth it compared to a standard overbore or a 73mm bore that will give a fair bit more bore thickness) I guess but 1550cc would either require  a 73.5 bore x 91.2 stroke or a 74 x 90 isch. while a 'normal' long stroke crank is 84mm. and the longest stroke MED sell is 86mm. and that would require a 75,75 bore to get to 1550.

 

anyway, go to turbominis and search.

Edited by Sir Yun, 31 May 2016 - 11:50 PM.