Engine Build Math & Notes

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cgranju
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Engine Build Math & Notes

Post by cgranju » Mon Mar 04, 2019 1:48 pm

HI everyone, I'm seriously enjoying finally (because my work space for complete cars is nil & I have a garage under construction...well, rain mostly) digging into a 1756 "hot" rebuilt engine I picked up about a year ago. So, I'm brushing off some old measuring skills & trying to develop some new ones. I've also spent an embarrassing amount of time researching here, FS & GCRE on threads where engine building/CR math was discussed. In light of that, I thought a thread about good engine builds and the math & measurements that made them would be a nice thing here. Obviously, if this gets into someone's "secret sauce" recipe, some may not want to go there. Of note (and not "secret") I found threads on the use of factory 1800 pistons in decked 2L blocks w/ 1800 head to get 9.8:1 CR quite interesting. Unfortunately, I don't have any great build specs to share, but here is what I'm taking apart & measuring (with hopes to be able to simply put it back together & run it):

The engine was built maybe 10 +/- years ago by a guy who was a mechanic & had a machine shop as part of his business (that may be significant later). It was never run, just put together 98% complete. It is an early 1756 (smaller flywheel bolts, offset conrod oiling holes, had in-head thermostat, etc...ie, '74 model, I presume). It has "hot" (quotes, because I don't have the specs yet) cams and had IDFs bolted on, so the guy who built it intended it to have a higher-than-US-spec compression ratio. What I found was interesting & at first glance seemed like it was not anything special. Clearly I have all new bearings (a new oil pump..yay, at least that's of value!), new flat top pistons, and a freshly decked head with new valves (though one exhaust doesn't match the others...hmmm). Cursory measurements may indicate the guy was using machine work instead of go-fast parts to build the CR. What I've found so far follows.

84.4mm bore w/ pistons marked "84.36"
combustion chamber: 44cc
Pistons at TDC 0.53mm below block deck
piston valve reliefs: 1.5cc
IMG_7442.jpeg
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Using some basic static compression estimation math it looks like this should be around 8.8:1 (if my basic math is correct). This is based on a nice, clear way of stating the calculation that I lifted from elsewhere on the 'net (I used the deck height relative to piston top TDC as a dish, so technically, deck height is another variable).

Compression ratio is V1:V2

Where V1 is;
a1) The volume in the cylinder when the piston is at BDC
b) Volume of valve flycuts in the piston (+)
c) Volume of any dome (-) or dish (+)
d) Volume in the head gasket (+)
e) Volume of the combustion chamber (+)

Where V2 is:
a2) The volume in the cylinder when the piston is at TDC
b) Volume of valve flycuts in the piston (+)
c) Volume of any dome (-) or dish (+)
d) Volume in the head gasket (+)
e) Volume of the combustion chamber (+)

So, there it is. Starting from the inside out on this engine, the next step is to inspect & measure everything inside. Does the collective knowledge base here have actual geometry/specs or even volumes for the domed pistons available?

the other side. Note the sloppy paint. That's easily corrected, but not trust-inspiring in terms of attention to detail (or the fact this is a Fiat not a Ford, but I'll go with the blue for now).
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Chris Granju
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Re: Engine Build Math & Notes

Post by fp55scca » Mon Mar 04, 2019 5:56 pm

--Chris, did you really mean "44cc" combustion chambers? I've really never seen a TC motor with volumes as small as 44cc? Which head are you using? Original '74 head to go with the '74 block? 54cc's would be on the large side, but not necessarily if ported and relieved around the valves, and then not milled.
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cgranju
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Re: Engine Build Math & Notes

Post by cgranju » Mon Mar 04, 2019 7:00 pm

Yes, I really did mean 44. That is with a spark plug & of course valves. I will Admit that measurements are cursory at this point. I quickly realized my Econo graduated syringe is not ideal and I’m getting more proper lab equipment...but, Really if anything it would seem like the type of error I would induce would be measuring too large. From what I read, 49cc is a stock 1800 head combustion chamber.
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Re: Engine Build Math & Notes

Post by vandor » Mon Mar 04, 2019 11:14 pm

It looks like he put the main bearing caps on in the right order :-)

Stock 1800 head has 47cc combustion chambers. If he wanted to build compression he should have decked the block so the Pistons stick out a little, say 0.5mm (.020"). That would have been a better way to build compression than milling the head. The only drawback is that there would be less piston to valve clearance on high lift cams. Most street cams would be fine.

I decked my block 1.25mm and am running 40/80 cams with no clearance problems. My pistons have only 2mm domes but the CR is ~9.8:1.
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Re: Engine Build Math & Notes

Post by Cobra427 » Mon Mar 04, 2019 11:54 pm

Vandor,

What is the difference between taking material off the block rather than the head? Either way the piston is going to be closer to the valves. (me personally would do a little of both the ensure both are milled true)

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Re: Engine Build Math & Notes

Post by cgranju » Tue Mar 05, 2019 7:56 am

Csaba - Did you (or would you) re-camfer the piston bores? The fire ring bore of a head gasket still has ample space, right?

Originally I thought this had the head & block decked, but it has no appearance of the block being decked. Piston height would come into play here and I don't know how these pistons compare in height to stock ones.

Main caps are in order, rods not so much :( (but another thread here mentions that & perhaps there was reason)

Looks like I will try some small dome pistons on this. I do hope to be able to capitalize on the recent machine work on the lower end, but if the head turns out to be problematic (for this block or even in general), it's really not a big deal to use another one that's been worked less.

Thanks for the feedback. I'd still like to hear about the math on other builds to have some "field data" to compare with.
Chris Granju
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Re: Engine Build Math & Notes

Post by beek » Sat Mar 16, 2019 9:25 am

taking material off the block increases the quench area. No matter how much you remove from the head that wont increase. The quench area has a lot to do with power
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