MadUsa1

What is a safe rev limit for a forged 2.5L running standard 2006 UK STI AVCS heads?

I'm looking for a safe 8000 rpm limit and trying to work out what I need to get there.

MartynJ

A big enough turbo to flow up there !
Other than that the stock AVCS heads aren't bad, but they can be improved with decent cams.

Martyn

MadUsa1

Well something like an MD321T or an SC46 should do it no?

The heads themselves though, are they ok at such high rpm?

MartynJ

Too small, a 46 with a 10cm housing might flow up there, but I would look at an SC54 or a rotated Billet if you really want it to pull that high.
Why do you want to rev to 8000rpm ?
If you are much more likely to make peak power at 6000rpm and fall over the cliff in torque terms, why rev to 8000rpm ?
In my experience of the 2.5 engine, to really get it to pull to an 8000rpm limit you need a big turbo and decent heads and cams.
Like so !



35/42R on race gas with Cosworth heads and cams.

MadUsa1

It's not about maximum power - I know max power will be delivered earlier in the rev range.

But I do a lot of track work with my car so figured it'd be nice to have the extra revs to give a wider powerband.

Also, I'd like to know that my engine won't blow up if I miss a gear...

MadUsa1

Besides, all the JDM cars rev to 8k...

matt-c

iTrader: (5)

JDM are 2.0 though.

MadUsa1

Yeah I know that, but there's no reason a properly built 2.5 couldn't rev to 8k and beyond.

The cosworth 2.5 used in the Type 25 has an 8k limit for example.

Alan Jeffery

The solid buckets fitted to the AVCS heads will easily allow the RPM as there aren't any shims to flip out. The trick is to keep the cam running on the valve. Dual springs and titanium retainers would help. I'd also fit a Cosworth belt guide and kevlar belt.
Assuming the engine is balanced well, and as Martyn says, you have the flow to match, you should be able to hit 8500 perfectly well.

Just one afterthought, with the stock heads, you're likely to "pinch" flow wise before you get to that kind of RPM. In that case, although you could hit the revs, you may find it's actually quicker to short shift and use the torque instead.

Safe? if you want really safe, don't go there!

MadUsa1

Thanks for this, very helpful.

So I'd need the following:

http://www.rogerclarkmotorsport.co.u...productID=2076
http://www.rogerclarkmotorsport.co.u...productID=2369
http://www.rogerclarkmotorsport.co.u...productID=2367

What about the crank - does it need to be a nitrided item or will the standard EJ257 crank from a 2006 STI be ok?

New_scooby_04

iTrader: (4)

Know of one forged 2.5 on standard heads that had to be taken home in a hoover bag after the owner repeatedly hit a 7400 hard cut out.

If you're running standard heads and cams, isnt it just better to accept that the engine wants to do it's work lower down the rev range and set the hard limit at the stock 7k rpm and aim to change up at 6.5k (when the good work has been done anyway).

When Bob was mapping mine he commented that the 2.5 really wanted more air at the top end than even the rather lovely SC46 can produce!

All IMHO I'm far from an expert!

MadUsa1

NS04, thanks for the feedback, good to know.

No question, high revs put a lot of additional strain on an engine and I don't intend to hold every gear to 8000 rpm.

However, sometimes it's nice to have the extra revs on the track - for example if you want to hang onto a gear on the way into a corner.

And of course, if you miss a gear it's nice to know the engine isn't going to self-destruct.

MadUsa1

I got some info from Duncan (Dynamax). He's been using UK AVCS heads on his 700+ bhp monster for a while now and has the rev limit set at 8500rpm.

He runs cosworth high rev dual valve springs and titanium retainers, and he runs some more aggressive cams, but the heads are otherwise standard.

Billet

iTrader: (60)

Once you've changed the above, isn't it only the valves that are still standard?

MadUsa1

Well not really, the heads are still standard!

The dual valve springs and titanium retainers cost 500 quid.

The alternative would be £3k's worth of Cosworth CNC heads...

From that point of view the uprated valve gear is pretty damn good value.