SOHC engine tuning option

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SOHC engine tuning option

Postby James Bowen » Tue Mar 13, 2007 9:09 pm

Guy, others,

I'm looking into starting another SOHC engine to build for sprints and himclimbs and trackdays.

I'm going to try to employ all I've learned (mostly on this site) since the last build. I'll again be trying to do most of the work myself.

The question is : I'm looking at using a late model Tipo 1580cc engine. Due mainly because the head has 39.5mm inlet valves as standard. And the pistons have an oil spray to the undersides, and of course the slightly increased capacity.

Is there any reason to not use this engine, as opposed to the "standard" 1498cc unit that the X1/9 comes with?

Many thanks, James
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Postby Guy Croft » Thu Mar 15, 2007 2:41 pm

James, hi

no reason at all I can think of. I have just posted a 1640cc Fiat SOHC article that may interest you by the way, see GC Virtual Workshop:

http://guy-croft.com/viewtopic.php?t=816

I have feedback from a very reputable engine builder who does his own top-level dynotests and who does a lot of very competitive National level race engine development, I do the occasional XE/Zetec head for him. I quote: Power loss from oil sprays on normally aspirated units.

GC
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Postby James Bowen » Thu Mar 15, 2007 8:56 pm

Guy,

Many thanks. Interesting article. I imagined that a 40mm inlet valve would be maximum, due to achievable port size, (31 - 32mm as mentioned) and shrouding.

Also exhaust flows look very good, beyond 65% - 75% ? With a 13:1 compression ratio, could the ex valve be smaller than the 34.5mm?

Looking again at the Tipo head. It runs 39mm inlets and 31mm exhaust valves. I was thinking of staying with these sizes, unless you think that a 33mm exhaust valve would be of benefit. (assume 10.5:1 - 10.8:1 compression ratio, and quite a high overlap cam - circa 3.25mm at TDC)

Re: the quote, significant compared with possible gain in cooling and reliability?

Regards James
James Bowen
 
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Location: Brighton, UK.

Postby Guy Croft » Fri Mar 16, 2007 9:32 am

james, hi

I imagined that a 40mm inlet valve would be maximum, due to achievable port size, (31 - 32mm as mentioned) and shrouding.

There isn't really any limit to how big you make the inlet valve and it will generally tend to have a throat diameter smaller than the narrowest section of the port. We were concerned about shrouding but the cc work seems to have taken care of it.

Also exhaust flows look very good, beyond 65% - 75% ? With a 13:1 compression ratio, could the ex valve be smaller than the 34.5mm?

Potentially yes, in fact the valve size on that unit is not 34.5mm it is 33.8mm, I had forgotten that the valves started as 34.5mm and I ground them down; I have updated the post accordingly, and added some notes re this E/I ratio issue. Ultimately although you can guess at the ex/in flow ratio from square of valve diameters as shown in that post you cannot really make a judgement as to whether the valves are 'optimum' size until you examine their flowtest results. The higher inlet flow your generate the lower the E/I is going to be so you have to be a bit careful.

Looking again at the Tipo head. It runs 39mm inlets and 31mm exhaust valves. I was thinking of staying with these sizes, unless you think that a 33mm exhaust valve would be of benefit. (assume 10.5:1 - 10.8:1 compression ratio, and quite a high overlap cam - circa 3.25mm at TDC)
Judging by my own results with 42/33.8 valves you should be fine. Net area ratio of 31/39mm valves suggest an E/I ratio of 63% so much will depend on actual flowrates achieved.

Re: the quote, significant compared with possible gain in cooling and reliability?
Well, here is the rule of thumb with tuning. If you have a mod or component like that in an engine and it develops better power without it, that is a sure sign the engine is running fine and doesn't need it. Except perhaps in terms of long-term (in-car) reliability. Now, with respect to oil sprays, what they do is two jobs in one - 'cool' the underside of the piston and provide lubrication to the bores.

As far as bore oiling is concerned in my exp you can isolate them and even the holes in the early rods and still have enough splash and vapour to do the latter job, I ran my 124 CSA like that for years, and from a metallurgical point of view the peak temperatures the piston sees (be it cast or forged) are nowhere near any dangerous threshold on a normally aspirated or low boost engine that's operating well and without any hint of knock (detonation).

Manufacturers might say both jobs are equally critical but the primary role - cooling the piston with engine oil (piston underside will be at a mean temp of 280 deg C or so and the oil at say 1/3rd of that) is to prevent overheating of any 'end-gas' in the combustion chamber that might lead to detonation itself. But as that particular test shows even at very high CR and on low octane fuel there isn't a hint of detonation - the power would be way down if it was knocking.

So, knowing what I know now I'd be tempted to plug them off, maybe put some oil dispersion slots in the rods cheeks just to help with bore lubrication, like my race rods below.

GC
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Last edited by Guy Croft on Sat Mar 17, 2007 8:19 am, edited 1 time in total.
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Postby James Bowen » Fri Mar 16, 2007 3:25 pm

Superb Guy, thanks again.

Regards, James
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