Compression Ratio Calculations, Head shave, for the engine guys...

I am just about to put together the lower end of my new engine for my MGA. It's an early MGB 5-main block and the head is being done by Sean Brown. I am trying to conceptualize a reasonable target compression ratio. Here are some specs on the engine:

.060 over press-fit country brand pistons, zero clearance to the deck surface.
Payen head gasket @ .040" thick
Early cylinder head with 42cc stock volume (to be adjusted accordingly).
Assume 8cc's for piston dish

I calculate the oversize bore to be 81.78mm (80.26mm stock + .060")
The stroke is 88.90mm

The Swept area is 467.02 CC
The Unswept area is 53.21 CC. (42cc combustion chamber, 3.21cc head gasket, 8cc piston dish)

This yields a compression ratio just under 8.8:1 (8.78:1)

In order to achieve my desired target of 10:1, I will need to shave around .120" off the head--this is assuming .010 off the head is roughly worth .1 in compression ratio.

How does this sound? Is .120 off the head too much to be safe? Does general wisdom reveal that I will need to take more?

Paul,

Here's a handy on-line calculator for static compression ratio. You can probably experiment with different combustion chamber sizes to find out where you have to be to achieve your target ratio. However if Sean is doing your head work I would certainly defer to him for his specific recommendations...

Dick

http://www.gtsparkplugs.com/CompRatioCalc.html

This is a good topic for discussion here, but as pertains to your specific project, X2 to what Dick said about just talking to Sean about it.

Hi,

I can't quote a reference, but I thought that the limit of shaving a head is 40 thou.

Herb

I recall Hap (Speedracer) posting that he takes more than .120" off his race heads. Maybe he will chime in and confirm.

0.120 is a big cut!

Here's a link to a thread in which both Hap and Sean participate and talk about these big cuts.

http://www.mgexperience.net/phorum/read.php?1,1826261,1826261#msg-1826261

Years ago, I did this as well using long math formulas, not exactly my strong suit, but I always muttled my way thru it, now days, I use online CR caculators like mentioned by Dick, I compare many of them against each other and they always come up really close, so that how I do it this days.

Paul there is no possible way you would have to take .120" off a your cylinder head with a zero deck height to get to 10 to 1, and I have to wonder why you even went the zero deck height route with street engine, thats not even something I do anymore with high CR race engines, due to varying connecting rod lengths. My race engine has flat top pistons at .005" piston to block deck height and I had to cut .125" off my head and that got me 13.3 to 1 on the race engine.

This is something I do with every engine I build, is set the CR higher, and regulate a CR target of where I want to be. Feel free to call me, and we can talk about it. Once you get Sean the numbers from the bottom end on your side, he can deck the head where it needs to be to get to the target. I recently did my own personal street engine, had a deck height of about .027" after aa clean up block decking, had 7cc piston dishes and after porting my combustion chambers CCs, they were like 45cc on my 12G1326 early head (thats something most people never take into account, the combustion chamber volumes grows duing a port job, no matter how careful you are to not enlarge them) and I think I had to take about .060" off the head to sneak in just under 10.0 to 1, something is flawed in your caculations.

Now on getting to your final combustion chamber CC volume will be on Sean I guess, once you give him your bottom end data, here's how I do this. I would have my bottom end built and assembled, already have all that data before I go to the cylinder head, then I would CC the combustion chambers in the head, record what I have now, then use the CR cacualtor to see where I am, then start putting lower combustion chamber volumes on the caculator to see what they need to be to get to my target. Once I know the CC volume I need to be at to hit my target, I put that amount of fluid into the combustion chambers, and use a depth mic to see how far down I need to go to just reach the surface of the liquid, , double check myself, then cut the head that amount.

I am going to revive this old thread, with some developments:

My engine is back together. The engine guy put the pistons about .002" recessed to the deck surface--beautiful job. Sean did the head and I have that back as well. He said he took about .040 off the head which started life as the early deep combustion chamber height. According to Sean I should be between 9.5 and 10:1 compression. The head looks fantastic...real art work.

I had an MGA backing plate machined for the rear seal so I solved the problem matching up with the MGA gearbox.

At this point I can't decide about the cam shaft. I think I will be able to sell the 1500 I am removing since it is still a 100% good engine. This means I will not try to remove the crane cam in that engine.

Money is an issue but these look to be my choices:

Delta D9
Delta KB
APT VP11
APT VP12
others?

I am a little nervous about the D9 and the VP12 being a little too "rude" for general road work. The D9 and VP12 seem to be the best match for the rest of the engine though. I personally would like the D9 cam as the price is right and I think it would be super fun to drive. My father however probably would not enjoy a cam that has problems idling and stop-go traffic.

Is the D9 too radical for a car that is still a primarily a street car

Did you do anything to the lower end (balance the rods, ARP fasteners, etc.?) If you did then the VP12 is a great cam - provided you want to rev that high. On street motors that won't go past 5500 stock redline the 11 is a better match.

I don't find the VP12 fussy to drive around town, it likes a idle of about 1200 rpms to smooth out, the real question is I think what Alan asked, is he going to rev it, or not, if not, then no need for the VP12, the VP 11 miay be a better choice. The D9 grind is about the same as the VP 12, but with crappy specs for the end user, you kinda got to fiquire out yourself, I set them at 102 on cam timing, and you'll need a vernier cam gear or offset key to get there,and thats on any performacne cam, as they all require them to be advanced.

I'd go the VP12. Really not that radical. Mine has a choppy idle at 900RPM. Like Hap says, you want it smooth, turn the idle up. I like the sound myself. Definitely says "I'm not stock" ;) Pulls strong from 1500 on up. I have the rev limit at 6,300 for my own piece of mind but it would pull higher. I will admit that my carburetion and ignition is not the norm though.

I personally can't see going as far as you have with compression and head work to skimp with a mild cam grind. No mind reader here but If you are like me, I doubt you spent the time & money to get to this point just to putt around. I'm also a believer in matching compression and camshafts from my SBC days. There's some theory that more duration and overlap bleeds off compression pressure at lower RPM thus helping with detonation. At the other end, the raised static compression overcomes the loss from the duration and overlap. Rule has always been raise the compression and add duration in the cam. Low compression..use a mild cam.

Just my take on it. And I won't use anything but an APT cam. Good luck with the project. Sounds neat!!