General Harry's Garage 594 Engine Rebuild

[fiat500]

It will be worth following this professional rebuild of a 594 engine to a mildly upgraded specification. Harry never lets financial constraints hold him back, and is always guided by the best and most experienced operators for the type of car he is having restored.

On this occasion, Harry is at Proetti's in London; there's probably nothing here that experienced 500 fanatics don't already know, but it's good to see and hear it directly from the expert.

 

[smart51]

The mechanic said there's little point in a fast road cam without also increasing the compression. Why would that be?

 

[Toshi 975]

It will be worth following this professional rebuild of a 594 engine to a mildly upgraded specification. Harry never lets financial constraints hold him back, and is always guided by the best and most experienced operators for the type of car he is having restored.

On this occasion, Harry is at Proetti's in London; there's probably nothing here that experienced 500 fanatics don't already know, but it's good to see and hear it directly from the expert.

Interesting video and to see the inside of Proietti’s, I did once visit on a Sunday when they were closed. My last 500L was a frequent visitor with the previous owner as the paperwork I inherited with the car bore testament to. The invoices generally began with the same sentence “Car towed in as non runner” then generally followed by “new plugs and points fitted” or points adjusted.
The previous owner was an art dealer who lived on the Portobello Road in London and had a reputation for spending the minimum on the car to the extent that internal handles were missing as was jack and spare wheel, quarter lights flapped free and the front seat covers consisted of brown parking tape. The wheel rims were rusted through in places and you could touch the inner tubes through the rust holes. For one MOT test he had Proietti fit a second hand tyre to get it through.
The exact history was a little vague about the period when the car was imported from Rome in 1999 except a note to say that Proietti had rebuilt the engine.
When I bought the car it came with an MOT that had two full pages of advisories, each of which should have been a fail in my opinion. However I kept the car as a running restoration with plenty to sort out.
When I developed an engine running problem I felt I needed to know what work Proietti had done on the engine build so gave them a ring. Attempts to access any records they may have of the car were falling flat as a fair few years had passed and the registration number threw up nothing. So I resorted to describing the car as it was when I bought it then bingo! I mentioned the brown packing tape seat covers and the guy said “oh the bloke who never wanted to spend any money” and found the records immediately.
I should add that the engine build was done before the art dealer bought the car.

 

[fiat500]

The mechanic said there's little point in a fast road cam without also increasing the compression. Why would that be?

It looks like there isn't much online that can explain this in language that I can understand, and I'm not a "soope-upper" of engines in any case. But it seems that the principle may partly be that because you have changed the opening and closing times of the valves, the inlet valve may still be open for more of the time whilst the piston should be on compression. I'm thinking that by reducing the volume available for combustion it may compensate for the side-effect of pressure that may be lost in this way.

Maybe the experts haven't notice this question yet, and a thread of its own would elicit a proper answer rather than my uneducated, speculative guff.

 

[Goldnrust]

As far as I understand it, thats the basics of it @fiat500.

Short answer: Increasing the compression ratio will increase power across the whole rev range, and help negate any losses in the mid range caused by the higher performance cam. You can safely incase the compression ratio further with a performance cam than with a standard cam.

Long answer:

The longer valve open time, does mean that there is more time when the intake valve is open after the piston starts it's compression stoke. It also means that there is a longer period of 'overlap' when the exhaust valve stays open whilst the inlet valve starts to open early.

At high RPM two elements means that this extended valve open time helps the cylinders fill better. Firstly the higher the RPM the more limited that time for air to flow in, and if you closed the intake valve when the piston reached the bottom of the the intake stroke there would still be a vacuum inside the cylinder. With the extended opening time, even though the piston is starting to rise there is still vacuum inside the engine and so keeping the valve open allows more air to flow in. Secondly, running at high RPM the air in the intake port has higher speed and has some momentum, which can mean it will still flow into the cylinder even a little after the pressure inside becomes equal to the atmosphere.

Neither of these things help at lower RPM though. At low RPM though, where the air speed is slower and there's more time for the pressure to equalise, then there's a chance that with a long overlap som of the fresh air coming in will go straight out of the exhaust before the valve closes, and then also possible that as the piston rises on the compression stroke it will start to push air back out of the inlet valve before it closes.

This will mean that if you put a performance cam on a standard compression engine that it may gain some peak power, but loose across the mid range possibly even to the point of feeling slower in the real world.

We measure how efficiently the cylinders are filling with air using 'volumentic efficiency' or VE. This is the percentage of the cylinder capacity that is filled with fresh air and fuel. The VE curve roughly matches the torque curve (and is essentially directly related). The performance cam will allow for better VE at high RPM, which is how it makes more power, but will have a lower peak VE in the mid and low range due to the air flow issues described above.

Now this is where it get's complicated....

There are two types of compression ratio, static compression ratio and dynamic compression ratio. The static compression ratio is what we quote most of the time, 7.1:1 in a standard 500. We calculate this by comparing the cylinder volume at BDC, in this case 250cc per cylinder, to the volume in the cylinder when the piston is at TDC, ~35cc. 250:35 = 7.1:1.

The dynamic compression rate is an estimate of what the 'real' world compression is taking into account the fact that the cylinders do not always complete fill with fresh air and fuel. A good naturally aspirated engine will work at 85% VE at peak torque, so in the case of our example 250cc cylinder, that means we put 213cc of air and fuel in per stroke. If you do the same calculation on this air volume we will see the dynamic compression is ~6.1:1.

Why does this matter? Higher compression ratio = more power, but it also increases the chance of uncontrolled burning/explosion of the fuel, 'detonation' 'knocking' 'pinging' etc. The limit where detonation is a risk is based upon the dynamic compression, not the static compression.

So with a performance cam we will have a lower peak VE, which means a lower peak dynamic compression, so we can safely increase the static compression ratio. This will bring our dynamic compression back up to a similar level as it was on the standard cam and so in turn bring our mid range back to where it was originally. It has the added bonus of making the engine even more powerful at the high RPM than the cam alone would do.

As a real world reference as to how cam timing allows for different static compression ratios safely, most car engines would consider a compression ratio of 9.5:1 or less 'safe', where their cam timing is based upon revving to 6-7,000 rpm. But a high revving super bike engine will safely run a 13:1 static compression ratio, as its very aggressive cam timing to allow it to rev to 15,000rpm makes it so inefficient at low RPM that the dynamic compression is much lower than even our old little 500 engine.

As an aside, the other limiting factor for safe compression ratio is fuel octane, the amount it resists detonation. The 500 was designed for a time of poor quality fuel, and so even a stock engine will take a reasonable compression ratio bump above the stock 7.1 / 7.5:1 without any problems when run on modern 95+ octane fuel, and with that comes some performance increase.

 

[smart51]

Hmm, so I should have had the head skimmed when I changed my cam. Too late now.

 

[Goldnrust]

That is the theory... Personally I doubt the difference between stock and a mild sports cam is really enough to make much difference? Maybe if you'd got one of the more aggressive 'race' type cams there would be more difference to feel, which would make raising the compression more of a necessity.

That said, the stock 500 compression ratio is very low by modern standards, and I'm sure good gains are made generally but pushing it up.

Having the head skimmed isn't especially expensive, and now you've had it off once you know how simple that job is. Maybe you should run it as is for a bit, then have it skimmed and tell us whether it feels any faster! haha.

I did warn you in one of the other threads that performance mods are a slippery slope... If you're having the head worked on then lots of people rate putting large intake valves in as a good idea, and porting the intake. Then you might want a bigger carb? There's always something to change to make it even faster!

 

[KurtVerbose]

Wonder what's happened with this? Nothing for 5 months.

 

[fiat500]

I think Harry's 500 engine has probably been re-built and fitted by now. He's really all about supercars, and the 500 is a novelty act. Although low priority, I'm sure he realises that a 500 will always get "clicks", and it will be uploaded soon.

He did a piece a few months ago where he analysed the views on his channel, and said that whilst his "restoration" videos had a dedicated band of viewers, they were not as popular (read "lucrative") as the supercar road tests.

 

[124BC1]

There was an episode of the tv series "Salvage Hunters - Classic Cars" a few years ago, that covered the restoration of a 500 and included a visit to Proetti Brothers workshop in London, where a brief demonstration of porting and polishing on a 500 cyl. head was given. (@ 2 min to 3min 45 sec.)

 

[bugsymike]

Very interesting thread.
Regarding high compression and detonation "pinking", as I understand it good head design is an important factor, as what you want is rapid burning of the fuel mix and not an explosion/detonation.
Hence the hemispherical type design as was used in the older Jaguars was so much more efficient than that used by many other car manufacturers of the day.
It allowed a more complete and even burn of the fuel mix.
A sphere would be an ideal for rapid burning, but not practical for many reasons.
I was always interested in Mazda engines in the 70s, the heads didn't look anything fancy but compression tests usually showed around 180psi, much higher than many English cars at the time, yet they were able to run quite lean mixture which can encourage detonation and run on 2 star fuel of the day, at the same time their OHC engines had a fair performance, whilst economical and clean emissions.

 

[Toshi 975]

Maybe I should have made a video
Me working away with a dremel on a cylinder head on my lap covered in aluminium filings. Middle picture shows the exhaust port with an old gasket offered up, last picture after porting.
The. Standard 500 inlet port is 26mm diameter and can easily be increased to 30mm in line with the 126 which makes the fitting of the larger 28IMB carb an easy job.

 

[Goldnrust]

Hence the hemispherical type design as was used in the older Jaguars was so much more efficient than that used by many other car manufacturers of the day.
It allowed a more complete and even burn of the fuel mix.
A sphere would be an ideal for rapid burning, but not practical for many reasons.

As you say the hemispherical heads do allow for nice fuel burning, with no corners or masked areas for the flame front to have to weave round. On the downside, compression is limited. The main way to rise compression in a hemispherical head is to use a domed top piston, but this creates it's own issues as the piston dome blocks flow around the cylinder and masks parts of the combustion charge from the flame front. The modern 'pent-roof' design seems to be the best balance, a sort of flattened hemisphere.

OHC engines allow for the valves to be angled to move in away from the cylinder walls which helps with both flow and denotation resistance.

Not that any of this helps us with our push rod 'wedge' cylinder head designs on the 500 unless someone wants to start making an OHC conversion!!

Tidy porting @Toshi 975. I would check that the exhaust flange is still a larger diameter than the port, not a smooth join. A sharp step up in diameter where the exhaust meets forms an anti-reversion step which helps stop any waves of back pressure in the exhaust from pushing exhaust back into the cylinder. Especially helpful with big cams with long overlap... or so the books say!

 

[bugsymike]

As you say the hemispherical heads do allow for nice fuel burning, with no corners or masked areas for the flame front to have to weave round. On the downside, compression is limited. The main way to rise compression in a hemispherical head is to use a domed top piston, but this creates it's own issues as the piston dome blocks flow around the cylinder and masks parts of the combustion charge from the flame front. The modern 'pent-roof' design seems to be the best balance, a sort of flattened hemisphere.

OHC engines allow for the valves to be angled to move in away from the cylinder walls which helps with both flow and denotation resistance.

Not that any of this helps us with our push rod 'wedge' cylinder head designs on the 500 unless someone wants to start making an OHC conversion!!

Tidy porting @Toshi 975. I would check that the exhaust flange is still a larger diameter than the port, not a smooth join. A sharp step up in diameter where the exhaust meets forms an anti-reversion step which helps stop any waves of back pressure in the exhaust from pushing exhaust back into the cylinder. Especially helpful with big cams with long overlap... or so the books say!

I agree not practical for may reasons, although the Jag option was good for it's day.
I suspect a decent inlet tract for each cylinder would be an interesting improvement.
I once spent shed loads on a Rotax 500 air cooled single with the ideas of making a Supermoto in an Armstrong monoshock frame, but the remains are still in the donor Jawa road frame with it's limited handling lurking in my garage since around 1997 I recall after it ripped the cush drive from the back wheel. I also hadn't got it's full potential due to a restricted exhaust at the time, it only started to really sound happier after it blew the threads on the exhaust front pipes.;(
I always liked the idea of this head conversion, something similar on a radical Fiat 500 in twin form would have been interesting.

 

[the hobbler]

I agree not practical for may reasons, although the Jag option was good for it's day.
I suspect a decent inlet tract for each cylinder would be an interesting improvement.
I once spent shed loads on a Rotax 500 air cooled single with the ideas of making a Supermoto in an Armstrong monoshock frame, but the remains are still in the donor Jawa road frame with it's limited handling lurking in my garage since around 1997 I recall after it ripped the cush drive from the back wheel. I also hadn't got it's full potential due to a restricted exhaust at the time, it only started to really sound happier after it blew the threads on the exhaust front pipes.;(
I always liked the idea of this head conversion, something similar on a radical Fiat 500 in twin form would have been interesting.

You do not need to go to OHC to obtain a 'Hemi head'----BMW achieved it with their superb 6-cylinder engine (later used by Fraser-Nash and Bristol) and Abarth achieved it with their (incorrectly called) "Radiale" engines. They built 1,000cc engines fitted into the (highly modified!) Fiat 600 based racers, and about 3 x 850 engines, using the 1,000cc block and a very short-stroke crankshaft. Officially, these 850TCR engines don't exist, but I can personally assure you---they DO! Finally, Abarth made some "Radiale" 595 engines, but never put them into production---they were VERY expensive to make, so Abarth (literally) binned them. Luckily, somebody picked the bits out of the bin---one was up for sale not so long ago, but it was expensive.

I agree not practical for may reasons, although the Jag option was good for it's day.
I suspect a decent inlet tract for each cylinder would be an interesting improvement.
I once spent shed loads on a Rotax 500 air cooled single with the ideas of making a Supermoto in an Armstrong monoshock frame, but the remains are still in the donor Jawa road frame with it's limited handling lurking in my garage since around 1997 I recall after it ripped the cush drive from the back wheel. I also hadn't got it's full potential due to a restricted exhaust at the time, it only started to really sound happier after it blew the threads on the exhaust front pipes.;(
I always liked the idea of this head conversion, something similar on a radical Fiat 500 in twin form would have been interesting.

One does not have to go to the expense of an OHC engine in order to obtain a "Hemispherical" head---in fact the BMW 6-cylinder engine (later used by Fraser-Nash and Bristol) was redesigned as a "push-rod hemi" as a cheap way of upgrading it! In our own field of interest, Abarth designed and built a 'hemi' head for the 110 (500) engine---the Fiat-Abarth 595 Radiale. The valves were turned around 90 degrees (to sit ACROSS the cylinder-head) with the exhaust coming out opposite the carburettor, rather than at the ends of the head (as per the standard 500/126 heads). Unfortunately, they proved so expensive to make that Abarth (literally!) binned all the parts. Luckily, somebody realised what they were managed to rescue all the bits----one of the heads was for sale about a couple of years ago, and a "695 Radiale" came up for auction in America recently.
Abarth also produced a "Radiale" head for the 1,000cc engine that went in the back of the Fiat 600 based variants---about 110bhp back in the '60s! Unoffically (it doesn't exist in any of the Abarth booksthat I have in my library) they built either 2 or 3 "850TCR"engines---using the 1,000cc block, with a very short throw crankshaft to bring the engine capacity back to a fraction under 850cc. I can personally confirm that this engine capacity/design exists

 

[bugsymike]

Humber Super Snipe push rod engine?
Probably the nearest I am likely to get to a 57 Chevy.
I had two of these cars in the past. Nether was this one.

 

[the hobbler]

Humber Super Snipe push rod engine?
Probably the nearest I am likely to get to a 57 Chevy.
I had two of these cars in the past. Nether was this one.

They were good cars----cylinder head looks like it had quite a bit of tuning potential. Having said that, I expect that the usual "Snipe" owner wasn't into the "engine performance enhancement" brigade!

 

[bugsymike]

They were good cars----cylinder head looks like it had quite a bit of tuning potential. Having said that, I expect that the usual "Snipe" owner wasn't into the "engine performance enhancement" brigade!

Funny you should say that. As an apprentice around 1969/70 we had a customer who owned the local cement works and he had an all white Humber Super Snipe ( well it was until he wanted to emulate the fad of vinyl roofs and got us to spray the roof black with that spray underseal, don't ask why) any way it was claimed he regularly came up Telegraph Hill clearing the top at around 100Mph on his return home to Torbay and this was before they smoothed it out, even if allowing for wildly inaccurate speedos in those days and the fact it would have been on crossply tyres he wasn't hanging around.
We used to have some fairly fast, for their days six cylinder cars to work on apart from the Humber, there was Mk 3 Ford Zodiacs, PA Vauxhall Crestas, a lady with an MGC Roadster, plus a few Austin Westminsters and a Vandem Plas 4 litre R, although they were a bit more sedate, so they all got a fast road test after servicing on the A road right outside garage.
The trick was hanging on for dear life at around 70mph with the tyres screaming on the sweeping bend and then you could nudge up to a speedo reading of around three figures by the end of the straight, allegedly of course.
In the early 70s we had a petrol customer with a lime green Ford Anglia with a Lotus Twin Cam engine which sounded pretty good.
Just to keep Forum interest we had several customers with Fiat 850 coupes and saloons plus a few Seat variants.
Also one of these Fiats in six cylinder 2300cc version, although even then I must admit it was a bit of a shed.

 

[fiat500]

Here you go; turns out to have been a 695 rebuild.

 

[the hobbler]

Here you go; turns out to have been a 695 rebuild.

I have just watched that video Peter---not that impressed! (a) I think that you know that I have a "pet hate" with regard to putting a fuel-filter right on top of the alternator and (b) Steff Proietti explained about all the (expensive!) work that they had to do to re-align the engine shrouding--WHY? The new barrels must have been 80mm barrels (tall---the 'factory' length for a 500 barrel is 90mm), so why not use a spacer between the top of the crankcase and the bottom of the barrels? Among a lot of tumers on the continent, this is the accepted thing to do---con-rods of the new required length are also available. The '695' kit that I have on the engine that I am building supplied a spacer-plate with the kit; you had built the engine 'Dry' (i.e. no rings on the pistons) with the supplied plate in place to ascertain the 'swish' hieght, and then had the plate skimmed to bring the barrels down to give you the correct 'swish' height. The top of the crankcase also had to be machined flat (in order that the spacer-plate sat absolutely flat on the top of the crankcase)---both those jobs most probably cost me a LOT less than the labour cost that Steff incurred modifying the engine shrouding.