Predicting engine power_01

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Guy Croft
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Predicting engine power_01

Post by Guy Croft » Thu Aug 10, 2006 11:05 am

This is in the GC Q & A also:

I have Fiat Strada (Ritmo) 130TC circuit racecar. I rebuilt the engine and I attached 130TCs technical specs. I¢ž¢m going to test engine next week but before that I have few questions:

-Bore 85mm, 2049cc
-Forged pistons
-CR 11,5:1
-Light standard Abarth 130TC flywheel
-Flywheel, crank and rods are balanced independently and married together. Rods are polished like the Red Book says!
-Cylinder head blueprinted, std 43,5mm/36mmm valves
-130TC head casket
-GC triple springs
-GC IIID inlet cam (100 deg)
-GC IIIA exhaust cam (110 deg)
-Standard Digiplex electronic ignition system from 130TC
-Jetting: Solex 40 carbs and choke 41mm
-Lubrication: std sump and oil cooler plus extra oil cooler from Volvo
-3qt Accusump and electric valve kit set - from L.A-USA
-Cerametallic clutch plate and cover from Lancia Integrale

Questions:
1. What do you recommend on plugs?
2. What do you think about HP and torque on 130TC?



Plugs
These need to be race type NGK B9EGV, copper core, gold palladium fine wire centre electrode. The cylinder temperature will be far too high for any 'production' type plug. You should not have any cold start problems with these and they will never cause detonation/pre-ignition.

Predictions:

I have not bench-dynoed an engine to that particular spec on 40 carbs and so you'll have to try the 40s (maybe from 34-36 choke) to see what happens.
To make some estimations let¢ž¢s refer back to the GC engine 210 ¢‚¬Å“featured in Case History No1 in the GC manual. Let¢ž¢s build a ‹Å“virtual engine called ‹Å“X¢ž¢ and see if we can make sense of it all by comparative analysis.

TC 8v 2 liter engine No. 210 on bench dyno test (without silencer/muffler) had:

Peak power 182bhp at 7000 and held this power to about 7300, dropping to around 179 at 7500. Torque on 45s with modestly sized 38mm chokes (it was tuned for driveability) was about 148lbf ft at 5500.

Let¢ž¢s ‹Å“interpolate¢ž¢ (read between the lines and see what we can discover):

Capacity/CR:
210 had bigger a cubic capacity +50cc (86mm bore), 2091cc compared with your 2049, this is worth about 5bhp and 2 lbf ft torque. The difference in CR would have a negligible (GC ‹Å“un-guessable¢ž¢) effect on anything much.

1. Engine X would have:
177bhp at 7000
146 lbf ft at 5500


Cams:
210 had a pair of GCIIIA rally cams, about 10.4 true lift, while you have one IIIA on the ex and a GC IIID with about 10.9mm true lift on the inlet. The effect of this will be to generate more top end power and peak a bit higher say 8bhpat 7350rpm if we used a pair, but the net effect with one IIID and one IIIA will be less, say + 5bhp and peak power a bit lower. The torque will peak slightly higher because of the additional overlap (lift at tdc) from the IIID and be slightly stronger. I won't go into cam timing as well, far too involved.

2. Engine X would now have:
177bhp at 7200rpm
149lbf ft at 5600rpm



Fuel system:
Note: I¢ž¢m not clear about your carb/choke size.

210 ran 45 carbs with 38mm chokes. The torque and by default ¢‚¬Å“ power, is related to the choke size by the area of the choke. More air thru the choke means more into the engine, provided velocity is not reduced below a certain threshold.
The biggest choke you can run on 45s is 40mm diameter, on 40s 36mm diameter.
A 38mm choke in 45s has an area of 1134sq mm, the biggest choke you can use in 40s - 1018sq mm. The 38mm choke will flow 9% more air. I¢ž¢d also expect relatively better response from 45s with 38mm chokes than 40s on 36m chokes because, in terms of areas:

45/38 has a choke to barrel ratio of 71%
Even a 45/40 combo has a ratio of 79% (for example)
Whereas:
40/36 has a choke to barrel ratio of 81%

The 45/38 combo has a significantly smaller ratio than 40/36 and so the air velocity will increase more and generate a stronger fuel signal - the pressure drop generated in the choke region to pull the fuel out for the main jet. This is how chokes, or venturis work in carbs. Fuel signal quality is a major factor in developing torque/power. To get the same response from 40s you¢ž¢d have to go down to a 34mm choke.

Additionally, predisposing that we accept we want optimum signal too, we need to be aware that the change in section that the airstream sees going thru the carb is less intrusive with a big choke, so potential for loss or gain there too. Air does not follow your preferred contour ¢‚¬Å“ it goes where it wants to and does not like changes of section. I do a lot of flowbench dev these days and I see it all the time. Moreover the bigger throttle plate on the 45 will impede the airstream less at all throttle positions.

Do not, however, in spite of what I have written, assume that bigger is always better, running 48s on the 2 liter, which I have much experience of ¢‚¬Å“ may lead to a very power torque curve because the air velocity thru that very big carb is just too slow to fill the cylinder unless very aggressive cams are used. And even then I could not better the performance and driveability that 45s gave - on the same engine - by making the carb change. Frankly I think 48s don¢ž¢t work well on any of the 8v TCs.

Engine 210, I¢ž¢d say, based on numerous dyno tests with many different 2 liter engines, 12bhp less with 40s, even with the biggest chokes 40s would take. The rpm at which peak torque, power occur would be affected too, small chokes will reduce the peak attainment speeds, though, that said, they may - from the driver¢ž¢s point of view - improve the low speed driveability. This may be what you want.

So let¢ž¢s assume you have 40s with 36mm chokes:

3. Engine X running 40s on 36mm chokes would now have:
165bhp at 7000rpm
143lbf ft at 5450rpm


I think this is roughly what you will see.

The Digiplex may add on a bit of top end power and mid-range torque and will probably give an enhanced (less peaky) torque curve than an old fashioned centrifugal distributor system, because the advance curve, assuming you have hooked the vacuum sensor up to the inlet manifold, will give rpm/load-sensitive correction.


The type of exhaust manifold (header) and silencer (muffler) that you fit will have a huge bearing on the final output. I know you are very careful and study these things carefully, but worth noting for the general interest that if it¢ž¢s wildly wrong it won¢ž¢t get anywhere near those figures and may cause damage to the piston rings from back-pressure.

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