1275 compression ratios and horsepower

[TulsaFred]

As I'm buttoning up my "cheap rebuild" 1275, I came across some information alluding to different compression ratios and resulting horsepower differences from year to year.
Seems to be the result of different pistons. My pistons have a pretty good dish to them, so I'm concerned they may have low compression.

So what are the different stock pistons, compression ratios and horsepower figures? Anyone know?

Fred

 

[jlaird]

You need a copy of Vizards book Tuneing the A series engine.

 

[vagt6]

Jack's correct. If you're serious about building a 1275/A Serices engine, Vizard's book is the Bible on the subject.

Unless, of course, you pay someone like Hap Waldrop to do it for you!

Used copies can be found on eBay, etc., or just get a new one. IF you're doing the work, you will be repaid for the cost of the book many times over.

Good luck, let us know . . .

 

[Spridget64SC]

The 1275's came with two compression ratio's and likely two differenct horsepower ratings. The 8.8/1 enginese were in the 1967 to 1971 range and the 8.0/1 engines were in the 1972 to 1974 range. Mas or menos.

Horsepower is kind of interesting. All my dyno runs have shown that HP is very dependent upon ignition timing. The usual things are obvious. Cams, porting, carbs, exhaust, displacemetn and compression boost power. But if you want to seen things change when you change something adjustable on the engine, then ignition timing is the one to adjust. Just be sure to adjust fuel along with it or you might burn a piston. We watch EGT and AFR closely to make sure we haven't gone too far.

For stock engines, mostly the 8.8/1 1275's, the most commonly advertised figure I've seen was 63 BHP at 6,300 rpm. Haven't seen a number on the 8.0/1's, but the number should be about 58-60 for it. The dish in the 8.8/1 engines is around 11.1cc and the dish in the 8.0/1 engines is upwards of 15cc's.

Remember when measuring things to compute compression ratio, to add in the area above the top ring. Many people forget this compression decreasing location.

HTH,
Mike

 

[TulsaFred]

Mike,

Those are the compression ratios I've seen as well. I'm going to measure the dish on my pistons, but I suspect it will end up being 15cc's/8.0:1 ratio.
I think the hp figure on this engine is 56 +/-.

Pretty low.

I'm not looking for a race engine or anything, but that is a big difference in compression ratio and horsepower for an otherwise identical and stock engine.

If I put in the earlier pistons with less dish, should I expect the compression to go to 8.8:1, and the horsepower increase from 56 to 63? That's a 12.5% bump in power.

The cost would be only the pistons and cost of a machine shop pressing in the pins. Maybe I should do this now while the engine is apart.

But, I didn't bore the cylinders, just honed them in my garage with a 3 stone hone. Would replacement std bore pistons cause any problems?

thanks,

Fred

 

[glemon]

If your bore is standard should be fine to get replacement pistons, maybe better if there was some wear on the pistons,I don't think you need a machine shop to press the pins, but could be wrong, if you are on the cheap you can have a machine shop skim the head, cheaper than new pistons.

Greg

 

[TulsaFred]

I found this on Wikipedia:

"The engine now grew to 1275 cc ... a de-tuned version of the 75bhp @ 5800 rpm Cooper 'S' engine, giving only 65 hp (48 kW) at 6000 rpm and 72 lbf·ft (98 Nm) at 3000 rpm. A reduced compression ratio of 8.8:1 was used instead of the 9.75:1 employed on the Cooper S engine."

"In August 1971 the compression ratio on North American engines was reduced to 8.0:1. Engine power output fell to 54.5 bhp @ 5500 RPM and 67 lb-ft of torque @ 3250 RPM."

Bummer, from 65 hp down to 54.5 hp!

I may have to bite the bullet and buy new pistons now... what's another few hundred dollars and few hours of disassembly time, and few more hours taking/picking up from the machine shop...?!

Fred

 

[Billm]

Why stop at 65 hp- 75 is listed as possible? Go for it! (you don't need that cylinder head to last anyway!!)
BillM

 

[Spridget64SC]

What you may be seeing HP number-wise with the different publications, is the difference between measurement devices.

Check your cc's, check your piston casting number. I believe they should be something like M2211. These are the smaller cc dish pistons. M2213 is the bigger dish. Doing this from memory, so I might be off a digit or two.

You can press the pins in and out with a hydraulic press. Need one that is about 12-15 tons. A support. I make this out of a 4x4 that I cut with a bandsaw to the curve of the piston and drill a hole for the pin to exit. I then make a tool on the lathe to fit the end of the pin and a "head" to push against. The shaft of the tool needs to be about 0.795" to slide through the pin bores in the piston and in the rod. When installing pins, use some anti-seize or good quality wheel bearing grease to help things press in the rod and lube the piston pin bores.

The easier thing to do is to cut the head.

About 0.012" off the head reduces head volumn by 1.0cc. Remember all tha high school math and wondering what it would ever be good for? Well, for one thing you can use it to calculate compression ratio. Volume swept in the cylinder bore plus the static volume above the piston (and the land area) divided by the static volume is the compression ratio.

For a desired compression ratio of 9.5 to 1, there needs to be a static volume of 37.5cc's. This is just reversing the math for finding out the compression ratio. The simple way to figure this out is to divide 1275 by 4, which = 318.75cc. Then to find out what static volume you need, subtract 1 from your desired compression ratio (9.5 - 1 = 8.5) and divide that number (8.5) into 318.75. This results in 37.5 cc's needed. From this number, subtract the piston dish, the negative deck volume, the land volume above the top ring, the gasket volume and this will tell you how much you need in the head. Remember all that HS math. Well, now you get to use Area, Volume and english to metric conversion.

If it was me, I would cut the head 0.015", stick a little cam in it (Isky MM55, Kent 276 or Piper HR270), new lifters and be happy. Adjust the timing for 34 - 36 degrees total advance, tune the carbs, do some plug checks, and have fun.

Make sure you have hard seats installed in the exhaust for the unleaded fuel.

Mike

 

[TulsaFred]

Well I measured the dish with a syringe today.
I only get about 11-12 cc's, so maybe they are 8.8:1.
I think I need another method to confirm. I really don't want 8.0:1 pistons and the resultant lack of power.
Since my research suggests that this is an early 1972 engine, and that US market 1275s had 8.0:1 after 1971, I'm concerned even thought my crude cc measurement seems to indicate 8.8:1.

Maybe pics will allow someone to ID :

Whaddya think?

Fred

 

[BlueMax]

Fred,
If you are wanting to know what the cc’s of your piston you have to remove the piston from your block, because the engine block will interfere with you testing. You then need a peace of Plexiglass large enough to go across the top of your piston that is perfectly level that has a 1'4" hole drilled in the center. Apply a very thin layer of grease to the outer edge of piston that will prevent the denature alcohol from leaking. I use alcohol because it evaporates quickly and doesn’t leave any residue. I like to use green denatured alcohol or I’ll dye it red so I can see when top of the piston or cylinder head is full. You only want to fill to the bottom of the fill hole, no more or otherwise you calculations will be inaccurate. The best tool to measure your volume is a graduated burette, but a high quality hypodermic will suffice.

There are other factors that have to be consider in calculating your compression ratio that Mike Miller has previous commented about. But I will say that the engine plan must be establish prier to the assumptions where by you know all the answers before you do your final assembly.

Best of luck.

 

[TulsaFred]

Thanks BlueMax,

sounds like a good technique, much better than my method of squirting measured quantities of water into the dish until it looks full :0
Still, as a rough approximation, I think my dish is 11-12ccs.

For me, I just need to differentiate between two stock pistons. The early pistons have "less dish" and are 8.8:1, the later pistons have "more dish" and are 8.0:1.

I don't have any experience calculating compression ratios directly, and the methods you and Mike describe may get me to a calculated ratio. However, without any experience it seems like the chance of an inaccurate calculation is pretty high. In my business we have an old saying "see one, do one, teach one". That's a pretty fast learning curve, but here I'd even be skipping the "see one" part!

Anyway, I thought if anyone has visually seen the two versions of the stock pistons they may well be able to just look at the pic and know which ones I have.

BTW, do you think the little bit of pitting seen on the tops of my pistons are of any consequence?

Fred

 

[jlaird]

Woops, think this is done with piston if you are going to trim the head. Need to take into consideraton the side of the piston down to the compression ring and such.

I know you just figuring the piston. So, no part number on the bottom?

 

[TulsaFred]

Hey Jack,
I have the pistons and rings installed, but I can't see a part number from underneath.
Fred

 

[jlaird]

Only thing I can think then is there is no part number. Always something. How about on the top of the piston, do you have one that is real clean? Looks like you do however I see nothing in the pic.

When in dought punt I guess.

 

[Spridget64SC]

Sometimes the Brits put the part numbers in the dish as well. Just depends upon the supplier of the pistons. There might be a single number in the dish (like the "2" shown in the top picture) as well which indicates factory adjustment to standard. There would be a number on the block face near the tall stud hole. Early BMC engines went up to "4". Never saw one that way. Biggest I've ever come across was a 3. BLMC later went to letters (A, B) and only two sizes.

From your pictures, I can tell you those are the 11.15cc (nominal) pistons. Hence they are for the 8.8/1 engine. The "dishier" 8.0's have a thinner ridge to the edge of the piston crown.

Now you need to turn the engine to TDC and measure the negative deck. Dial calipers will do it quickly. Measure at the ends of the piston where the pin is located. This will negate piston rock. Average the two readings. If you don't have a dial caliper then use stacked feeler gauge blades and your finger to rub across the gap until you feel like the stacked blades and block surface are equal. Finger tips can feel about 0.001 - 0.002" difference. That's accurate enough.

Then you have to know the cc's in the head. That is the next biggest influence on compression. A clear plastic plate sealed with grease like Alan suggested will tell you the volume of the head. Post the numbers for the deck, the gasket you are using and the cylinder head volume and I'll do the mental math and a pretty close compression number.

That pitting is of no consequence. Seen it many times and worse. The crown is plenty thick. Just make sure there are no sharp edges on the piston circumference to create hot spots and sensitivity to detonation.

BTW, #2 has a nice ring ridge on the thrust side of the cylinder.

Mike Miller

 

[TulsaFred]

Sometimes the Brits put the part numbers in the dish as well. Just depends upon the supplier of the pistons. There might be a single number in the dish (like the "2" shown in the top picture) as well which indicates factory adjustment to standard. There would be a number on the block face near the tall stud hole. Early BMC engines went up to "4". Never saw one that way. Biggest I've ever come across was a 3. BLMC later went to letters (A, B) and only two sizes.

From your pictures, I can tell you those are the 11.15cc (nominal) pistons. Hence they are for the 8.8/1 engine. The "dishier" 8.0's have a thinner ridge to the edge of the piston crown.

Now you need to turn the engine to TDC and measure the negative deck. Dial calipers will do it quickly. Measure at the ends of the piston where the pin is located. This will negate piston rock. Average the two readings. If you don't have a dial caliper then use stacked feeler gauge blades and your finger to rub across the gap until you feel like the stacked blades and block surface are equal. Finger tips can feel about 0.001 - 0.002" difference. That's accurate enough.

Then you have to know the cc's in the head. That is the next biggest influence on compression. A clear plastic plate sealed with grease like Alan suggested will tell you the volume of the head. Post the numbers for the deck, the gasket you are using and the cylinder head volume and I'll do the mental math and a pretty close compression number.

That pitting is of no consequence. Seen it many times and worse. The crown is plenty thick. Just make sure there are no sharp edges on the piston circumference to create hot spots and sensitivity to detonation.

BTW, #2 has a nice ring ridge on the thrust side of the cylinder.

Mike Miller

Thanks Mike!
Excellent info that can only come from experience. I was hoping someone could just look at the piston dish shape and know which pistons they were.

BTW, I think I wasn't clear. I'm not all that concerned with the specific compression ratio of the engine. It's not a high performance application. I just didn't want to hamstring it with 8.0:1 pistons and give up a lot for simple free power and torque.
I'll check the specifications and measurements and post when I get a chance.

Fred

 

[racingenglishcars]

I'm apparently a glutton for punishment, and beating a dead horse, but while I'm rechecking my 1.3 Marina motor for the Bugeye, I ran into exactly the same problem with compression ratio and which pistons do what.

After finding very little online help, though this thread is the absolute best I've found so far, I decided to make an excel sheet which calculates A-series compression ratio and includes enough functionality to help even the least experienced.

So here it is:
https://www.4shared.com/file/2QL1Obkj/A-Series_compression_ratio.html
Sorry about the trouble getting through this free file sharing site.

Please give me feedback.

 

[Talonaer]

That is an absolutely fantastic spreadsheet. I've got to start putting my engine back together at Easter and need to work all this stuff out so that will save me alot of time! Thank you so much.

I'm popping a 12g295 onto a 948 + 60 with flattop hepolites (18519's) and was worried it might drop the compression ratio too much. Looks like it'll drop it from about 9.8 to 8.9 which isn't too bad! Need to check the deck and if the heads been skimmed heavily or not but would be fairly happy with that kind of figure.

The car will be a fast road car so is there much reason I would want a compression ratio much higher? When I win the lottery and get an ickle shorrock should help as well .

Edit: I am not very experienced with this kind of thing (first project car) and have dropped myself seemingly in at the deep end. Still working my way through Vizzard so please excuse any absolutely atrocious wrongs that may be present in this post.

edit 2: I realise I need to work out what the frack the cam in my engine is first before can finally settle a figure. Tbh if its something ridiculously racey I'll probably switch it out for a swifttune SW-5 or something thats got half a chance of going forward at low RPMs.

 

[58Custom]

<snip>
After finding very little online help, though this thread is the absolute best I've found so far, I decided to make an excel sheet which calculates A-series compression ratio and includes enough functionality to help even the least experienced.

So here it is:
https://www.4shared.com/file/2QL1Obkj/A-Series_compression_ratio.html
Sorry about the trouble getting through this free file sharing site.

Please give me feedback.

Brilliant! Very nicely done and easy to use.