Better flow with 7mm valvestems vs. 8mm?

[127]

After blowing my engine i a big way last season, I'm getting around to preparing the head for next year and changing all 8 valves and probably valve guides as well, I was thinking if the flow into the cylinder would benefit if I opted for valves with a thinner stem?

The head is from a Fiat 127 1301cc GT Sport and is fitted with 40mm inlet valves and 32mm outlet. The ports has been cleaned up and polished, and the valve guides has been grinded so they don't stick into the ports.

Perhaps the exhaust valves should remain 8mm stems to get better heat transfer - Don't know if it matters?

Would like to hear if im onto something.

/Brian

[Guy Croft]

The difference Brian betw 8 and 7mm stems is barely noticeable on the flowbench which means the same in terms of power too.

The unfortunate fact is that it's more_often_than_not the bigger guides associated with bigger stems that is the flow problem.

GC

[127]

OK, I see. I admit that 8mm vs. 7mm stem might be too litle to gain any effect. The difference has to be greater. It was just a thought anuway.

Regarding the guides, I'll have them grind so that they are flush with the walls of the inlet.

Im struggleing to get more that 100Hp from my 1301cc, which (in my book) should happely put out 20Hp more. Guess the headwork I have had done isn't helping me as much as I hoped for.

[Guy Croft]

It might be difficult to get that engine to really 'fly' but your 100bhp target is by no means impossible. First problem is the ratio of ex to inlet valve area is only 64%, needs to be 70% or more. That small ex valve is going to hold oyu back a bit, and the more the inlet flows the worse it will be. I imagine the inlet port will need to be about 30mm diameter and the ex about 24-25mm.

Don't shave the guides, put them in lathe or drill and bullet-nose the bit that sticks into the port. The guide and seat contact face seal life will be drastically shortened if you shave them off and it will be impossible to do a precision seat grind (you won't be able to centre-up).

It's years since I did one of these so pics would help. I'm interested in port dimensions, inlet/fuel system, camshaft lift and type (lift at tdc is very useful). (Note: If the cam is a regrind, you're wasting your time) The CR needs to be as high as you can get it, not easy on a little engine, aim for 10/1 if you can, but that said it may only be possible with custom forged pistons. Precision seats and maybe alterations to the form of the back of the inlet valve will be crucial. The ex header and muffler will be important too.

Just a few notes for starters. Some excellent closeup photos of the ports and valves, seats please so I can give you more.

GC

[127]

I have some of the informations for you.

I have done some measuring and the valve sizes are..
In: 37mm
Out: 33mm

..which gives a ratio of 89%.

I have just taken pictures of the valves I have used until now.Regarding the cam, it's a new C&B cam, not a regrind. I also found a data sheet I received from C&B before I ordered the cam.

The old valves.

valves_1.jpg (26.44 KiB) Viewed 6141 times

Bottom view

valves_2.jpg (12.56 KiB) Viewed 6141 times

127_Sport_1300_corsa_max.pdf

C&B datasheet

(14.99 KiB) Downloaded 449 times

Pictures of the inlet etc. will have to wait a week or so. My workshop is at my parents house, 180km away. I'll see if I can get some pictures done next week..

For fuel system I have a pair of 40mm DCNF's. Don't know which size jets or venturi they are using.
Both inlet- ad outlet-manifolds are matched to the holes in the head.
I'm running the stock outlet manifold, a 4-2-1-type with about 25cm(32mm dia) primary, 80cm(40mm dia) secondary into a 2" pipe with a single pass-thru muffler at the end.
I have done some calculations on the manifold, and even tho the are stock, the dimensions are not too bad.
The best results I had was with a resonant-tuned exhaust that exited just in front of the rear wheel. It felt like the engine was more responsive with that setup, but also way too loud for competition (111db@4500 RPM).

[Guy Croft]

37/33 valves give E/I valve area ratio of 80% actually not 89% but nonetheless thanks for telling the right sizes and 80% is fine. Of course an evaluation of that particular E/I (valve areas) is meaningless as a 'yardsick' of inlet-ex behaviour if the valve throats and ports are not optimised as well.

The cam spec looks OK to me. What are the 4-2-1 lengths?

You really should buy a George Polley flat oval muffler. 18 x 8 x 5" from memory, maybe 4" deep, cannot remember. That is what I recommend on most of my units. GC race proven over and over, no back pressure, robust, low cost, I'd expect 103dB or less. I lot of people like to waste time looking for 'their own' brand or 'style', even having them specially made. My job here is to recommend the best. That is.

The part number is AX891 email them at

info@polleymotorsport.co.uk

If you publish more photos - which I hope you will - I need much higher definition than those valve shots, can't see the details clearly there.

GC