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Author: Subject: Obscure reasons why an engine might be down on power?
MikeR

posted on 22/11/23 at 01:23 PM Reply With Quote
Do you actually know what HP the engine came with from the factory? Or are you trusting the published HP and speed figures?
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cliftyhanger

posted on 23/11/23 at 09:27 AM Reply With Quote
I think most engines from teh factory were not as good as teh published figures. And depending on year, teh engines were tested without ancillaries (water pump, dynamo etc) making a sizeable difference.
And on top of that, you have not checked teh cam timing. It may not be a tooth out, but the chain may have stretched etc, and the cam lobes may have wear
The only thing to do if you really want to find out is an engine strip and a fresh build. Probably replacing a lot of stuff in the process, with teh risks associated with new/remanufactured stuff.

Plan b is to not worry too much!

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smart51

posted on 23/11/23 at 10:28 AM Reply With Quote
quote:
Originally posted by Prof_Cook
I am lacking technical skills in BHP to MPH correlation and grateful for any clarification on what the theoretical relationship is.

But assuming 22.6 BHP gives a 67 MPH max speed. The most you are getting is 62 MPH (this is 92.5% of 67MPH), then assuming a linear relationship this (92.5% of max power) suggests about 20.9 BHP so 1.7 BHP is what is missing.

[Edited on 21-11-23 by Prof_Cook]


The power needed by the car is a little bit involved, but not impossible to estimate. Given my engineering background, I've made a spreadsheet model of it.

RW horsepower is the sum of 3 parts:

a constant * speed cubed (in m/s) * coefficient of drag * frontal area. All the numbers are on the internet.
weight of the car * g * speed in m/s * coefficient of friction of the tyres (always hard to get the right number).
an amount of power to keep the engine running, and pump fuel and oil etc. A total guess, but a small number.

Flywheel horsepower is RW horsepower * efficiency of the transmisssion

My model is easily calibrated by the quoted power and top speed of various engine sizes in the car. The 17.5 BHP car from the early 60s would do 59 MPH. The 18.0 BHP car from the late 60s would do 60 MPH.

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smart51

posted on 23/11/23 at 10:31 AM Reply With Quote
quote:
Originally posted by MikeR
Do you actually know what HP the engine came with from the factory? Or are you trusting the published HP and speed figures?


My car was deliberately throttled from the factory by fitting a tiny carburettor. The quoted figure was 18 BHP. The same engine was used on the Fiat 126 with the bigger carburettor with a quoted 22.6 BHP. My car had the bigger carb fitted.

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smart51

posted on 23/11/23 at 10:36 AM Reply With Quote
quote:
Originally posted by cliftyhanger
I think most engines from teh factory were not as good as teh published figures. And depending on year, teh engines were tested without ancillaries (water pump, dynamo etc) making a sizeable difference.
And on top of that, you have not checked teh cam timing. It may not be a tooth out, but the chain may have stretched etc, and the cam lobes may have wear
The only thing to do if you really want to find out is an engine strip and a fresh build. Probably replacing a lot of stuff in the process, with teh risks associated with new/remanufactured stuff.

Plan b is to not worry too much!
You have come to the same three conclusions as me, which is reassuring. Plan b is where my head is. I'm worryingly tempted to go down the rabbit hole though.

I'm trying to think of a halfway house. Currently that's in the form of getting a DTI and measuring the vale opening and closing points WRT crank angle to see if the cam is advanced or retarded. I only need to remove the rocker cover for that.

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cliftyhanger

posted on 23/11/23 at 10:54 AM Reply With Quote
quote:
Originally posted by smart51
You have come to the same three conclusions as me, which is reassuring. Plan b is where my head is. I'm worryingly tempted to go down the rabbit hole though.

I'm trying to think of a halfway house. Currently that's in the form of getting a DTI and measuring the vale opening and closing points WRT crank angle to see if the cam is advanced or retarded. I only need to remove the rocker cover for that.


Ah. Are you 100% the engines are the same head/compression ratio and cam?

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smart51

posted on 23/11/23 at 11:37 AM Reply With Quote
quote:
Originally posted by cliftyhanger
quote:
Originally posted by smart51
You have come to the same three conclusions as me, which is reassuring. Plan b is where my head is. I'm worryingly tempted to go down the rabbit hole though.

I'm trying to think of a halfway house. Currently that's in the form of getting a DTI and measuring the vale opening and closing points WRT crank angle to see if the cam is advanced or retarded. I only need to remove the rocker cover for that.


Ah. Are you 100% the engines are the same head/compression ratio and cam?



I'm pretty sure the head is the same. There were only 4, the 500cc head, the 600cc head, the 650cc head and the Panda 30 head.

Nobody can tell me about the camshaft though. The early cars are well documented. My car was the runout model, which was built along side the Fiat 126. They increased the size of the engine for the 126, didn't want to make 2 engines, so fitted the big engine to the 500 as well. But they restricted the engine on the 500 so that it wasn't faster than the new car. So far as anyone knows, the only change was fitting the tiny carb from the 1950s 500.

But, they may have changed the cam too. Nobody seems to know. The 500 had 3 cams in its life, though one was only used for the first few months of production. The 126 had a slightly different cam. It is possible that Fiat used one of the older cams in my car, just to hold it back a bit.

19/50 50/19 1957
09/70 50/19 1957/8 and 1960 - 65
25/51 64/12 Sport and 1965 - 71
25/56 66/16 126

There's not a lot to choose between them. The late 60s cars made 0.5 BHP more by using the "sport" cam.

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smart51

posted on 23/11/23 at 11:43 AM Reply With Quote
A small correction. The compression test on my engine (when done hot and I remembered to use WOT) gave 120 PSI on cylinder 1 and 130 PSI on cylinder 2. In theory it should be 110 PSI but no-one seems concerned about slightly high numbers.
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Mr Whippy

posted on 23/11/23 at 12:39 PM Reply With Quote
Add a go faster stripe, that will give you at least an extra 10bhp
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