Is the stock 1500 engine compression ratio known?

Dr.Jeff

Dr.Jeff

True Classic
In another thread we discussed what the actual, true compression ratio is for the stock US spec 1500 SOHC engine. Here is that thread: https://xwebforums.com/forum/index.php?threads/milling-head-on-1500cc-engine.36162/

There seems to be some discrepancy about it. The factory manual I believe states it is 8.5:1. But some basic calculations indicate it is actually less than that. Steve C has outlined the method to actually measure and calculate it, which he has done for the Euro spec engines in Australia. Has anyone done this for the US spec 1500 engine? To do it properly requires a bit of lab type equipment to get some of the needed measurements. Considering all of the engine rebuilding, modifications, head milling, race prepping, and other work done to these engines, I'm sure someone on Xweb has determined the actual (true) compression ratio?

Furthermore, are there any differences in compression ratio between the various 1500 US spec engines? 10 bolt vs 14 bolt. EFI vs carb. Smog spec vs others. Early "Fiat" vs late "Bertone". Changes in pistons, gaskets, valves, whatever. Etc, etc.

Can anyone please add some light to this question? Granted it might not matter that much in most cases if the number is 8.0 or 8.5 or whatever. But for some specific applications it does matter. And this would be excellent data for Xweb to have. So we are hoping to find a verified value for this.
Thanks
 
I finally assembled a simple "CC" (volume) measuring rig using some old lab gear I had and a little shade tree engineering. It is good enough to get readings of the cylinder head chamber volume, which was my primary intent.

This is on a stock 1500 FI SOHC, US spec head. It has had a valve job so the valves may be a slight bit deeper than a new head. Although the head was not warped or damaged in any way, I skimmed it about 1 or 2 thou just to clean the face up and get the right surface finish needed for a MLS gasket. There was no evidence of the head having been worked on previously. Perhaps the valve job offsets the light skimming and these measurements are very close to a new stock head?

The combustion chamber volumes measured at 37 to 38 cc. For some reason I seem to recall that they were thought to be around 32 cc? But that was what I found.

I believe all of the other measurements needed to compute the compression ratio have been performed previously. I remember SteveC posting the information, but he did not have access to a US spec head to measure the chamber volume. Therefore the actual compression ratio could only be estimated (please correct me if I'm remembering this wrong). I searched for Steve's post with that information but I believe it was one of those things that is buried under a different thread topic/title. I also looked in his "ultimate SOHC' write up but did not find the figures for the 1500 assessment (maybe I missed it). So I do not have everything together to finish the CR calculations with my chamber measurements.

Does anyone recall the prior discussion that has the rest of the data? Or for that matter does anyone have the data?
The formula used for this is easy to find, but I believe we already have all of the actual data to plug into it - with this chamber size assessment completing the puzzle. It would be nice to finally determine the actual CR for the stock US spec 1500, instead of what's claimed in the factory manual or the estimated values we keep tossing around (see the first post in this thread). Thanks
 
I found one thread with some calculations by SteveC. Not exactly what I was looking for though. In the posts he offered here he includes several estimated or assumed values. None the less it is a start:
https://xwebforums.com/forum/index.php?threads/milling-head-on-1500cc-engine.36162/#post-314777

For these calculations he starts with the values known for a European spec engine and adjusts the head chamber volume that was estimated to be a US spec head. If I follow his logic correctly, it appears he overestimated the size of the US chambers. My measurements found 37cc and he used 40 cc. However I think his number includes .5cc correction for the area around the piston's top ring, which means he estimates the chamber as 38.5cc...only 1.5cc over. The discussion does not give all of his calculations so I'm not certain just how much difference this makes. I'm also not sure that some of the other figures from the Euro spec engine can be used this way. For example aren't the pistons different (e.g. valve recesses are larger on US pistons)? Again, not certain how much difference we are talking about. But if he estimates the US CR as 8.0:1, then perhaps it is closer to 8.2:1 or possibly 8.3:1 ???

Hopefully someone can still offer better numbers to help complete the correct calculations.
 
Since the prior post I went out and CC'd another head. Same description as the first; stock FI 1500 US spec. The chambers on this one were all closer to 38cc (compared to 37 on the other). So very similar to the first, only slightly larger. This puts the heads even closer to what SteveC estimated in the previously referenced thread (40cc). With the added practice of doing another head the technique is a bit more refined and the results are a bit more consistent. I'd say there is no more than =/- 1cc of error overall. But it is impossible to get any closer, just to may small variables at play with each measurement. So the actual CR might be around 8.1:1 or 8.2:1? I'd still like to get all the other numbers and plug everything into the formula to confirm things.
 
A fellow forum member also pro Italian / French / British mechanic who personally owns 3 X's did that CC measurement some years ago on a US 1500, bone stock.

If I recall correctly that yielded 7.9:1

Yours Truly,

Jim
 
Thanks Jim. I had not come across any info for actual measurement of the US spec 1500 CR. All prior conversations that I saw only gave speculation. Without putting my measurements together with the additional data needed and doing the calculations, I'm not sure exactly what number it will give. But your info is very helpful, thanks.
 
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