Race Engine Forum UK

Hi fellow Fiat folk

I'd appreciate some advice from those who know better than me.

I’m building a Fiat 127 1301cc engine (76x71) for fast road and sprint and hillclimb use and have some queries I hope you will be able to help me with on squish, compression ratio and inlet length.

I am targeting 110 bhp at 6000-6500 rpm and a rev limit of 7500 rpm.Spec follows.

Camshaft
After a lot of discussion with Kent Cams they have designed and re-ground a cam for me which is a mild competition type to meet the above needs. The info they have given me is below (with their measured figures of the standard cam in brackets):-
Profile reference D213
Angle between lobes 108 (109.8)
Base circle reduction 0.152
Max lift 0.398 (0.360)
Durations at 0.040 lift IN 290 (223) EX 248 (224)
Increase in lift at TDC 0.030
Timing - Inlet fully open 105ABDC

Head
I am having the head ported with standard valves.

Induction
I am fitting a pair of 40DCOEs on a bespoke manifold.

Exhaust
I will have a 4 into 1 made by Maniflow 1.25in bore and 30in headers and use my existing Jetex straight thro' oval silencer or upgrade to a Simpson stock rod silencer from George Polley.

Bottom end
All standard components, fully balanced, with new rings and big end shells on used but very good bores and journals.

Ignition
Distributor to be modified on rolling road (probably at Aldon Automotive), fitted with Lumenition.

My questions – Squish, CR and total inlet tract length
After some research I am thinking of machining the block to give 0.040in clearance between the flat top pistons and the head at TDC, at which they would be about 0.025in proud of the block. Could anyone suggest whether this is appropriate and if not what I should use for this state of tune and mechanical spec.
I intend to use Super Unleaded and a CR of 10.5 :1. Again views on whether this is appropriate to suit the cam and other items of the spec appreciated.
I am proposing to design the manifold, carb, trumpet combination to give a total length from valve to bell mouth of 13 inches, am I right about this given the cam and revs envisaged.

I look forward to advice from those who know these engines beter than I do (not difficult!)

Chris

Hi
Sorry my post had a typo. Cam is timed inlet fully open 105 deg ATDC not ABDC. Obvious really.
Chris

Hi Chris

can't see anything wrong with your proposals really.
Just a couple of points:

1. A 30" 4-1 header is OK but increasingly I'm learning of results that demonstrate 4-2-1 gives a much better torque curve. I would use 28" py 12" sy 4-2-1. Yes, long py, short sy, completely at variance with contemporary practice. Cylinder-pipe linking is of course 1&4, 2&3.

2. In running the piston 'up' make sure you dry-build and have 100 thou vertical piston to valve clearance and 60 thou radial. Dry build is featured extensively in GC V/W.

Utter waste of time and money messing with the distributor! The best way to get a good result from an old-fashioned CB type distr is strobe it up and plant the gas pedal.

GC

Hi Guy
Thanks for reviewing my spec and making your suggestions on exhaust and ignition. I have found some +0.40mm pistons so will rebore now instead of reringing. I now have to finalise squish and CR as I now know my chamber volumes and can't get to 40thou squish and keep my CR at 10.5. My chamber volumes are 25cc at present though I could get another 0.5-0.75 cc I'm told. I could also run the pistons lower.

The way it works out is:
Squish .040, pistons .030 up, CR12.01
Squish .070, pistons flush, CR10.98
Squish .080, piston .010 down, CR10.57 (factory set-up)

If I get the chambers opened up to the max of say 25.75cc we have:
Squish .040, pistons .030 up, CR11.80
Squish .070, pistons flush, CR10.76
Squish .080, piston .010 down, CR10.37

I'm going to run an unleaded/octane booster additive giving 4* equivalence but it seems I need to avoid static CR above 10.5 anyway. I'm inclined to run the pistons flush and open up the chambers by 0.5-0.75ccs but would I be wasting my money for no benefit compared with running them 10thou down? I'd appreciate your views as I see your guidance elsewhere suggests chasing tight squish figures is pointless but I'm unsure where the benefits cease.

Chris

Chris, hi

I try to discourage folks from getting technical with the subject of squish.

Squish is all about burn rate. Burn rate is faster if the homogeneity of the gasoline-air is optimised at ignition and during the flame event. Inherently - and in the first instance - we are aided by powerful vortices that form as the piston travels up the bore on the compr stroke. They are generated by the scraping of the top ring and the vortices curl inward to the centre of the cylinder - giving very good mixing. The faster the engine runs the better they perform.
There are a number of other ways of achieving good mixing, given that the fuel system calibration actually delivers the right A/F ratio.

1. Swirl. Turbulence from offset or shaped ports that encourage the mixture to rotate as it enters the chamber
2. Injecting in the right place in the inlet tract - ideally outboard of the throttle plate on FI engines; that in itself encourages turbulence in the fuel stream.
3. Achieving very high velocity in the port and - by default - across the valve region. I am increasingly of the view that - up to Mach numbers 0.5-0.6 (and probably no higher) power is likely to be higher with a smaller - rather than larger - port. The Mach number is predominantly a function of bore, stroke, rod length, engine speed and valve area.
4. Squish. The proximity of the piston to the head develops further vortices - a form of radially-inward swirl.

Now as far as the latter is concerned, it can only be effectively modelled by advanced computational method (meshing). The air is being 'worked' ie: compressed and fired across the piston crown. The energy loss due to squish has to be matched closely against the burn rate to get a clear idea of any gain in combustion speed. Anything in the way of the expanding flame-front upsets the burn rate. High compression hemi or pent-roof head 2 valves per cylinder engines are the worst in this regard although of course their layouts usually score highly on cylinder filling, so in the end when you see enough engines (as I have) you realise that most of them has some merits in some areas but not in others but the thing that really governs power output is cubic capacity!

Thus you can see there are 3 parameters that I (we?) have a sporting chance of 'tinkering with' to get increased power, the 4th is quite beyond the my scope and that of this website. The reality is that the manufacturer will have modelled (either analytically or by test and dev) the squish on the OE unit and you'd be best sticking with that. High speed engines, due to effect (1) don't need tons of squish anyway. As a rule of thumb you must keep the piston at least 1mm from the head in any event to allow for expansion, deflection and stretch. If you get a great result from 'tight' squish clearance, good luck, but you are unlikely ever to be able to pin the result on squish without a definitive back-to-back test at the same CR.

There are some very interesting visualisations here:

http://www.vrvis.at/scivis/swirl-tumble/


Try putting your engine parameters into the GC table attached, check your CR data. It has 8v Fiat data in the yellow rows but you can enter your own.
All said and done, it's not easy to get a high CR on a little engine. 10/1 is great for pretty-well anything. Higher may be more powerful and certainly will be more thermally efficient. Less may not be a disaster. If you keep the underhood and engine cool (75 deg C) and run ducted ambient intake air, race plugs and are running twin carbs or throttle bodies (rather than single carb) then you should not have any detonation problems at 97 octane.

Thanks Guy for your advice on CR and squish. I'm going to take it and run Squish .080, piston .010 down, CR10.57 (v close to factory set-up). I've a couple of follow up queries but I'll post on them as they get further up the list.

Thanks for sharing your experience.

Chris