Block choices for road engine

[manoa matt]

Aloha Guy,

Over the past few years I've acquired the components needed to put together a fast road engine for my 1978 Fiat spider 1800cc, which is also my daily driver. Some components were acquired individually, while other parts came from parts cars. Ideally I'd like to do most of the work myself in line with your teardown, cleaning, inspection, preparation, and assembly procedures. Inevitably there will be procedures where the services of a good machinist will be required. My initial plan for the engine specifications is as follows:

1592cc head
Stock intake and exhaust valves,
Abarth "blue" valve springs,
Intake runners’ gasket matched to intake manifold.
Factory Fiat "waffle" manifold gasket matched and runner diameter consistent as far back as possible
Dual 40mm Solex P11-6 downdraft carburetors
40/80 Alquati intake camshaft
Stock Fiat exhaust camshaft
Adjustable cam pulleys
Fiat stock 79-83 electronic ignition distributor

Exhaust system will be either:
4-2-1 tubular steel header with 1 5/8" primary tubes leading to 1 3/4" secondary tubes leading to 2 1/2” exhaust pipe straight back with a single free flow muffler.

4-1 tubular steel "snake" header (offered by popular Fiat mechanic out of California) This is a stepped header design with the same diameter tubes as the aforementioned system just a different design. Same 2 1/2" exhaust pipe and single free flow muffler.

The problem I'm having now is which block do I use? 2 liter or 1.8 liter, both have their pro's and con's. I have three engine blocks to choose from:

2L from an 1980 Fuel injected Fiat Spider
1.8L from a 1976 carburated Fiat Spider
1.8L from my 1978 carburated daily driver Fiat Spider (currently in the car)

The 2 liter block will obviously have more low end torque, but will not rev as high as the 1.8 liter. If I go with the 2L block, I will just flex hone and re-ring, both of which I can do myself. The 2L block with 1592cc head should bump the compression from 8.1:1 up to 8.6:1, which will help with the cam and dual carbs, but will it be enough?

If I go with the 1.8L block I will need to buy higher compression pistons to take advantage of the cam and carb combination. The pistons available in my price range are:

84.4mm 8.9:1 flat top pistons
8mm dome top pistons with a compression ratio somewhere around 9.6:1, these come .4mm, .6mm, and .8mm oversize.

The 1.8L block being shorter will make the installation and hood clearances much easier especially with the electronic distributor. I like the torque curve and rev capabilities of my current 1.8L engine. Obviously the cam and carbs will change the torque curve, but I'd like to keep the broad torque range and high revving characteristics. I frequently take the engine up to 4000 rpm's prior to shifting. I have never taken my current engine above 6000 rpm's, and don't plan to do so with my "new" engine.

A friend of mine has a 1979 spider with a 2L block ,1.8L head, 1800 manifold, Weber 32/36 DFEV carb and a 4-1 "snake" header system. All valves, cams, and pistons are stock. Although the engine is strong and produces excellent torque, he's never had the engine above 3000 rpm’s, simply because there is no need. I believe the car can come off a dead stop in 3rd gear with no problems.

Based on the components stated above for the top end, what block and/or pistons would you choose to obtain the desired engine characteristics with a strong emphasis on budget? Inspection and checking at teardown may ultimately be the deciding factor on which block to use.

Also will the stock Valeo organic clutch be able to handle the demands of the new engine?

thanks,

Matt Scarton

[Adam124]

Hi Matt,

I realise that you've addressed this post to Guy, however thought I may be able to help in the meantime.

In my opinion and from the description of your driving style you may well find the 2 litre engine more suitable. 6000rpm is by no means a stretch for any Fiat Twin Cam motor so the added low-end torque will be beneficial - consider that a fast 1800 will still be building power at this stage. Using the 2 litre has the added benefit of allowing you to rebuild a motor whilst continuing to use your daily driver and also sounds the simplest for you to recondition. Admittedly I do not know if it will have clearance issues when used with the downdraughts and electronic ignition, however suspect this has been done before!

With your driving style in mind, 4-2-1 extractors/headers would sound most appropriate as 4-1 headers are more often used on high rpm engines.

The projected 8.6:1 is quite a low compression ratio for a naturally aspirated twin cam, ideally aim for the 9.6:1 you mentioned by using the dome-top pistons. As you are upgrading the performance of your motor you may want to look at installing an upgraded pressure plate and clutch and these are readily available from Valeo.

I'm not sure if you've decided upon that head, you might find the spare 1.8l one a better starting point.

Good luck with the build and forgive me for the intrusion.

[vandor]

Hi,

Just a few comments, since I have been down a similar road many times before.

First of all about the 2000 engine not revving as much: mine with two 40-80 cams revved
quite well to 7000 rpm. An 1800 could maybe do 8000, BUT will you REALLY be driving it
at those rpms? I bet 7000 is plenty high of a redline. The 2000 will have more torque
throughout the rev range, so it will feel more powerful.

As far as pistons go, be very careful and do not believe what someone who is trying to
sell you something says. I would think that if they are flat tops, then they are very
close to stock CR. I know they say the deck is higher, but I have yet to see such a set.
The 8mm domed pistons would give almost 11:1 CR, more with the 1592 head...
If I were you I'd buy the flattops, then cc each component and figure out actual CR.
Then have the block decked if you need to increase the CR.
In my 2000 I have "8.9:1 CR" pistons with 2mm domes, but the block is decked 1.25mm (0.050")
to give a CR of 9.8:1. You should shoot for ~9.5:1 CR with today's gasoline...
How much you can deck the block depends on how far the pistons stick up. On my engine
they were 0.030" below the deck originally, so now they stick up 0.020", which is fine, as the
headgasket is 0.060" thick.
On a recent engine I built the "9.8:1" pistons were even with the deck, so they are not all the same.
I put the piston CRs in quotes as that is what these pistons are called, but that is
not necessarily the CR they give...
bye,

Csaba
'71 124 Spider, much modified