11 replies to this topic

[minisilverbullet]

the car is an spi but i thought this is a relevant question for all engines. 

 

looking at buying modified cylinder head, as you know there are lots of choices. what is the ideal valve sizes? most seem to have 35.6mm inlet valves but the exhaust valves differ between 29mm and 31mm, which is optimal. what are the consequences of choosing one over the other? 

 

The engine will be a part from a set of high lift rockers be standard. 

[IainStallard]

I'd personally go for 35.6 and 29mm valves as its less likely to crack the valve seat with more metal between the valves. That's what I've got on my spi

[minisilverbullet]

I'd personally go for 35.6 and 29mm valves as its less likely to crack the valve seat with more metal between the valves. That's what I've got on my spi

 good to know, did you notice much performance increase with your modified head on your spi?

[IainStallard]

I think I posted on a previous topic of yours, but I made 10bhp extra and 6lbs of torque at similar points in the rev range

[6joshh6]

The performance difference isn't huge; the figures in Vizard give a 2-3 HP improvement from around 4500 RPM onwards. The larger valve should also net around 2% improvement in steady speed fuel economy. However, all that is dependent porting being done to accommodate the larger valve. If you don't modify the head with the larger valve in mind, it is quite possible you would actually lose power.

 

I would also be careful about putting high lift rockers on a standard engine. They tend to deliver most of the power higher up the rev range; on some engines they can actually lose bottom end as well (I believe cooperman suffered from this with 1.5 ratio rockers, albeit on a highly developed engine.). For a similar expenditure to high lift rockers, a camshaft change to something like an SW5 would give you a smooth responsive engine with more power all over the RPM range compared to just putting high lift rockers on.

 

It would probably be worthwhile investing in a copy of Vizard's 'Tuning the A Series engine' as for £20 it really is a comprehensive guide to understanding modifications, as well as any possible drawbacks. It's in plain english and would be a very worthwhile book to read if you're planning to do big changes to an engine. http://www.amazon.co...a series engine

 

One last thing, just out of curiosity, where are you thinking of purchasing this head from?

 

Josh

[minisilverbullet]

The performance difference isn't huge; the figures in Vizard give a 2-3 HP improvement from around 4500 RPM onwards. The larger valve should also net around 2% improvement in steady speed fuel economy. However, all that is dependent porting being done to accommodate the larger valve. If you don't modify the head with the larger valve in mind, it is quite possible you would actually lose power.

 

I would also be careful about putting high lift rockers on a standard engine. They tend to deliver most of the power higher up the rev range; on some engines they can actually lose bottom end as well (I believe cooperman suffered from this with 1.5 ratio rockers, albeit on a highly developed engine.). For a similar expenditure to high lift rockers, a camshaft change to something like an SW5 would give you a smooth responsive engine with more power all over the RPM range compared to just putting high lift rockers on.

 

It would probably be worthwhile investing in a copy of Vizard's 'Tuning the A Series engine' as for £20 it really is a comprehensive guide to understanding modifications, as well as any possible drawbacks. It's in plain english and would be a very worthwhile book to read if you're planning to do big changes to an engine. http://www.amazon.co...a series engine

 

One last thing, just out of curiosity, where are you thinking of purchasing this head from?

 

Josh

mmm lot sot think about! i wouldn't be happy spending the best part of 800 pounds for a 2-3 bhp gain. I don't think emissions rules will allow me a camshaft change. 

 

I have been looking all over, but turbophil seems to have good heads at good prices, but i have also found a local supplier with some longman gt7 heads for the same price. 

[6joshh6]

mmm lot sot think about! i wouldn't be happy spending the best part of 800 pounds for a 2-3 bhp gain. I don't think emissions rules will allow me a camshaft change. 

 

 

I have been looking all over, but turbophil seems to have good heads at good prices, but i have also found a local supplier with some longman gt7 heads for the same price. 

 

 

The overall gain on the head should be much more than 2-3 hp but if you're paying extra for the larger valves, it's probably worth weighing up whether they are really worth it! An SW5 is a high lift/short duration cam so emissions shouldn't really be much of an issue; the overall effect is similar to high ratio rockers except the timing is improved compared to the standard cam so you actually end up with a much more optimised cam profile which works all over the rpm range; whereas high lift rockers merely add lift to the standard cam profile which isn't exactly ideal for performance to begin with. That said, to install a new camshaft you would have to remove the engine and split the gearbox etc to do it, which might be a lot of hassle if you're just after bolt on mods. The best compromise would be if you go for 1.3 ratio rockers; you should get similar gains to 1.5 rockers until about 5500 RPM, however, low end torque should improve too.

 

turbophil's work looks very good, and longman have a very good reputation (although you may pay slightly more for the name). I would also have a look at slark race engineering, their work is very good quality and their prices seem to be competitive too.  

[Carlos W]

I bought a head from phill, he's a top bloke. I would've talked to him for hours if the wife hadn't been sat in the car waiting

[ACDodd]

35.6 x 29.4mm valve for road use. This reduces the chances of cracking as already mentioned.

AC

[minisilverbullet]

Decisions with minis is never easy!

I have narrowed it down to two heads turbophils with 35.7 and 29mm valves
Or the long an gt7 with 35.7 and 31mm valves ( 180 pounds cheaper, from the local supplier)

[gazza82]

35.7 & 31 puts seats very close together and more chance of cracking ...

[Cooperman]

To improve the overall engine performance requires:

 

A gas-flowed head with optimised valve sizes and higher compression ratio.

A camshaft to get more fuel/air mixture in by having a different higher-lift/longer duration, and accurately timed in.

A distributor matched to give optimum advance.

An induction system to get the mixture in.

An optimum exhaust system which, for a 1275 to 1380 is 1.75" bore size.

 

1.5:1 rockers give very little by way of improvement.