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Compression ratio

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Morris

Morris

Yoda
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I did some CR calculations on my car since I have the head off. I was shooting for something in the 9:1 to 9.5:1 range... it looks like I am a lot closer to the 11:1 range. Oops. I guess I shaved a bit too much off on my last rebuild. Well I guess that explains the pinging and broken rings.

Any thoughts on how to combat pinging on pump gas?
 
Back off <span style="text-decoration: underline">max</span> igition timing to around 25 degrees BTDC (check this over 3500 rpm).

And run carbs rich. MAybe in the 12 to 13 range (you have an O2 sensor....right?)
 
I assume you shaved the head. How about a new head?

Did you double check you numbers?
 
Usually the BLMC competition manuals gave a figure for how many thousandths shaved off the head reduced the combustion chamber by 1.0cc. This nominal figure could then be used to calculate the compression ratio. With slanted walls on the combustion chamber, it takes less cutting to stay at a 1.0cc figure.

One could mark the head with "Dykem" and locate the boundary of the head gasket fire ring. Then open the combustion chamber up to reverse the situation. Clean up aroung the valves near the sides of the combustion chamber and possibly lay the combustion chamber wall back and pick up a couple cc's. This should drop the CR and get you back into "Sporty" range.

Check the workshop manual for a cranking cylinder pressure and compare to your compression gauge. One can test the engine and see about where it falls to verify if indeed it is up around 11/1. My compression gauge says approx. 151 psig is 11/1. This is ball park for street engines with low overlap cams. Out the window for higher overlap racing camshafts.

HTH,
Mike
 
Looked at my MG1500 Competition Manual last night. Guess what?, there is not a mention of how many thousandths reduce the head volume by 1.0 cc. The 1275 A-Series equivalent has it. The BLMC MG1500 Comp. Manual dances all around it. They say shave the head 0.200", flush the deck to the TR-6 pistons turned around backwards, use a certain head gasket and what to do to the head to end up with about 30.0cc's. But, they never state a compression ratio. Well, at least that is what the 1st Edition said and I've never seen a second. Maybe the Triumph Spitfire 1500 preparation information has something?????

Sorry,
Mike Miller
 
Spridget64SC said:
Looked at my MG1500 Competition Manual last night. Guess what?, there is not a mention of how many thousandths reduce the head volume by 1.0 cc. The 1275 A-Series equivalent has it. The BLMC MG1500 Comp. Manual dances all around it. They say shave the head 0.200", flush the deck to the TR-6 pistons turned around backwards, use a certain head gasket and what to do to the head to end up with about 30.0cc's. But, they never state a compression ratio. Well, at least that is what the 1st Edition said and I've never seen a second. Maybe the Triumph Spitfire 1500 preparation information has something?????

Sorry,
Mike Miller
Click to expand...

Mike you may not remember me doing a couple of SCCA limited prep 1500 race engines, I used a Rick Cline formula, TR6 flat top standard pistons with a zero deck height, and cut .100" off the head, that produced just tick under 11.0 to 1.


On side note, here's the deal on using TR6 pistons flat tops in 1500 a standard TR6 piston is .040" over in a 1500, but because the TR6 has a longer stroke you have deck alot off the block (like almost .300"), and it comes dangerously close to the freeze plugs on the rear of the block, add to that you either need to shim the rocker pedestals or use custom shorter pushrods to get the valve geomentry back in check, Then you would use a 1300 head gasket as the recessed steps around the cylinder bore would be long gone.

Ok, here's another issue you have to deal with when raising the compression ratio on a 1500 with 1500 pistons, you can only do very minor block decking, and I mean very minor, nothing more than a clean up due to the recessed steps around the cyinder bores, if you cut it too much the 1500 head gasket will no longer seal and the 1300 head gakset will not work ethier due to the fact it was designed for flat block deck and not the recessed steps around the bore on a 1500. If you are raising the compresion ratio on a 1500 with 1500 pistons you need to get it on the head side (and there's is plenty to work with), not the block side due to the 1500 head gasket.
 
Here's how I fiqure how much to cut off a head for a desired compression ratio. See what your combustion chamber volume you have now, you'll also need the following data, bore, stroke, compressed head gasket thickness, piston to block deck height, piston dome/dish volume (for a flat top piston use 0 ), There are plenty of good (and free to use) compression caculators online, enter your data with you current combustion chamber volume to see where you are at now, then reduce your combustion chamber volume on the caculator until you see the CR you want, then put that amount of liquid in the combustion chamber and measure with a depth mic (you can get them cheaper than you think) until the depth mic just touch the surface of the liquid in the combustion chamber, double check a few times to make sure you are right, and that will be the amount the head needs to be resurfaced to get to your desired CR.

Now I do this with fancy 100 CC glass burret and stand, because I do this job quite often, but you don't need to invest in such a tool for a DIYer one time deal, go to your local drug store and buy a feeding tube syringe, it will cost you a couple of bucks at the most, you need a small sqaure of plexiglass that will fit over the combustion chamber will a couple of holes drilled into the plexiglass sqaure for inserting the liquid, seal around the chamber with common grease for a seal. You can even for-go buying the depth ic, if you have dial caliper, use something straight to bridge the combustion chamber and then measure down to liquid until you just touch the surface of liquid with the bottom end of the dail caliper, and then back out the thickness fo whatever you bridged thecahmber with, again double chack your findings several times to check yourself. Also you need to level the head on a bench for doing this, oh and remember to install a spark plug inthe chamber you are testing.

Here's the main issue of using head casting thickness or even manual suggestions, valve height can easily vary your findings a couple of CCs, so doing the math is necessary, and add to that you never know if the head has been decked, and how much before you got it.
 
Hap:
That's a lot of good information. I was hoping that when Mike Barrett (BLMC Tech Director) wrote the MG1500 Cometition Bulletin back in the 70's, he would have put in some information about how much cutting the head changes the volume. Like what is in the same bulletin for the 1275 engines. But it was not there.

I agree with what you have stated above. It is all good practical information. We both do this kind of thing enough that we have the nice burrettes, cover plates, stands and such. I was hoping the MG1500 manual would have given enough information to get a good estimate without measuring too many things.

Mike
 
Would a different cam profile have any effect on what CR I can safely run pump gas. I have read a few confusing posts on other forums where posters claim that a longer cam duration or a later opening intake valve will have a positive effect on your engines ability to run high CR on low octane.
 
cam profile does effect actual compression, and therefore affects point at which pre-detonation occurs.

My understanding: for all practical purposes a streetable cam is not going to vary the theoretical compression much from the actual compression.
 
Morris said:
Would a different cam profile have any effect on what CR I can safely run pump gas. I have read a few confusing posts on other forums where posters claim that a longer cam duration or a later opening intake valve will have a positive effect on your engines ability to run high CR on low octane.
Click to expand...


What we are discussing here is static compression ratio, and yes dynamic compressed ratio can be lowered somewhat with higher lift and longer duration cam, but at the end of the day gains are still gotten using a higher lift and duration cam.

Alot of things in performance engine building ends up being compromises, for example when you use a higher lift cam you ask the rocker arm to go down further in it's arc, which lower the rocker arm ratio, but at the end of the day a gain is stil made.

Ironicly enough we did a R&D test yesterday on a SCCA limited prep 1275 race engine, on those engines we have to use stock rocker arms. What we did was make a plate the is held on the side of the head by the 5 head studs on the spark plug side of the head, so I could we could torque everything down like it would normally be, and using magnetic base and dial indicator, go one by one to each valve retainer and measure actual operating valve lift, this would tell us a couple of things, were we under the max allowed .450" ( SCCA rules for this engine)and what was our rocker arm ratio, and did the stock rocker arms vary much in ratio. Well my first 4 I check were within a couple of thousands, but few of them at the other end varied a bit, we had a low .414" lift and high of .435", the .435" is where we would like all of them to be, because SCCA check max valve lift with zero valve lash, so with .015" valve lash, that was perfect. Now in the end we maybe could play with swapping other rocker arms on for the couple that resd a little low, but these heads don't gain much flow at maximum valve lift, so it would probably make little to no difference.

Now I have not had time to sit and do the math yet, but we're using one of Mike's Comptune race cams and I know first hand the advertised lobe lifts are all within a thousands or two (this is the norm on most cams), so any valve lift varience we get is in the rocker arms. If the valve lift number were real low, we could have palyed with custom lnegth pushord or rocker arm pedestal shims, but being our findings are what they are we are close enough.

I know all this is getting a bit gear-heady, but Mike and I have a habit of doing such things we you get us started :smile:
 
W/R/T Cam profiles and compression, any street type cam will not appreciably change the final cylinder pressures prior to ignition. Durations and lobe centers are just not radical enough to make much difference. There are engine designs which use "adjustable" cam lobes to change the operating characteristics of the engine, but that is not the case with anything ever imported into the USA from BLMC as a mass produced product offering, at least that I am aware of.

Physical machining of engine parts or a change of pistons is required to get compression ratios up to something in the performance realm.

W/R/T rocker ratios, I have effectively done the same kind of operation that Hap describes for a number of 1275 LPHP builds. I have a couple of coffee cans full of #'d rocker arms with the ratio noted/coded on a piece of paper in the can. Sets of 8 rockers with closely equivalent ratios are grouped in plastic bags awaiting application to an LPHP engine. I have developed 3 LPHP cams, each with lifts varying by about 0.004 - 6" and having very close to the same durations. This way, I can build an LPHP engine using one of the cams, a particular set of 8 rockers and end up with 0.450" or just under lift. Each engine, although having a different cam, yields virtually the same performance. Differences are pretty much attributible to the air/fuel flow paths (Carbs, heads, exhausts).

Some rockers aren't readily usable because the ratio doesn't match up with existing cam patterns in my cam grinders inventory. With 3 really good cam patterns to choose from, and some others that are usable, almost all rocker ratios can be used. So far, I've found stamped rocker ratios from approximately 1.26 up to 1.34. Reversing the math from a max lift of 0.450', one can calculate what the cam lift can be. Now find a good racing duration range and build the engine, dyno and see what the engines performance actually is.

Yep, Gear-Heady stuff for sure :crazy:
Mike
 
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