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MG MGB Technical - Piston Crown to Deck Height
|Anyone know or can measure accurately the stock piston crown (top) to deck height on an undecked 18 V B-series engine when the piston is at the top of its throw? I think it's about 0.030," but I would like to know as accurately as possible from an unmodified stock engine configuration. My engine is currently in pieces and the parts of interest to this query are at various machine shops. |
|Close, It has been known to vary, little point in guessing. If you care, then you need to assemble it and measure it correctly, otherwise, just put it together. I like them flush for general use.|
|Mine were 13thou (I measured them a month or so ago when changing the cam). Peter Burgess recommendation is 12 thou. Height makes a difference to CR.|
|What I've always read is that stock style pistons are about .040" below the deck. For max squish/turbulence and compression a .010" clearance or even a little less is desired. I'm running about -.010 deck height, flat top pistions .040" oversize, with a 5 port alloy head and a mild modified cam. It works real well. I need to use mid grade fuel or better. The alloy head has the early large style combustion chamber shape and has been opened a little further to get more clearance around the valves. (i.e. the combustion chamber is a little larger than stock) the ports have been cleaned up a bit also.|
I'm thinking of going to a .080" forged flat top piston at deck height less than 010" below block deck height.
That, and a 280 range camshaft.
Right now with a 270 cam I have drieability at 2000 rpm and a real big torque peak about 3000 rpm, with good power on up past 6,000.
I'm expecting the extra cubes to make up somewhat for the hotter cam at the low end.
PS I've also lost about 50 lbs over the past year and have noticed that my car is faster than it used to be.
The Original Equipment specification is a clearance of .040". However, it would seem that the most practical clearance dimension is .012” with a 3/8th inch (.375”) to 10 mm (.401”) squish band around the periphery of the piston seems to be adequate for producing the best squish characteristics. However, it should be noted that this in turn may force a compromise when selecting a high-lift camshaft as some of them produce so much lift that it becomes necessary to relieve the deck of the block to a valve clearance depth that is greater than that of the piston/cylinder head clearance. The edge of the compression ring may be directly exposed to the heat of combustion, in turn leading to premature ring failure and piston land breakage.
|Thanks for the replies. The engine is being built for a supercharger, so the cam will be on the order of a fast road cam (e.g., 270) with reasonable, not radical, lift. I'm trying to compute the combusion volumes to have a set of JE's machined with the right dish to yield the CR I need. I do not want, or need, to deck the block for optimum squish under these conditions. After all is said and done, I will probably wind up pre-building the engine with the old pistons and new rods and making the measurement, but if the information is out there and enough to be statistically sound, then I'll know what ballpark to swing for.|
|Hi Steve- After instigating the thread debate between Peter and Sean on optimum squish, having noted Burgess's book recommendation of between 12-17 thou, I dry built my purpose built SC engine for 17 thou across the board, and it took 4 rebuild tries to get it right, and the eyebrow was respiffed after the final set. Initial pistop to deck showed 32-37 thou. I've always suspected this squish area is more important than normally considered for flame burn/propagation, although by the end of that thread, Peter guessed that up to 40 thou "... may be just fine "!?! Anyway, my SC build, 60 over, 8.5 measured CR, P270, Jackson 1.55:1 rollers, Burgess SC flowed head, stock exhaust, stock Moss needle, 4 inch ducted intake from the valence showed a best of 123 RWHP and 133 # torgue on the SAE dyno. Even master Dave Munroe pumping the HiFlow screw at 12-14 # has not produced those levels of measured power. I got a full 10 RWHP increase on the initial dyno runs by taking off the cone K&N filter(!!!!), and now have plumbed a "Y" into the induct ductwork to try a couple of 27,500 RPM fans in series at WOT to precharge the intake. And yes, we are careful to keep the fans off till WOT soas to not over lean the mix at runup. I have since installed the Moss smaller pulley and heat shield. This all brought to you by the cutting edge guys at On The Road Again restore and research using today's technology on yesterday's classics,and we are now ramping up final designs for bolton EDIS for the MGB/MGA and the EFI for the cross flow MGB. See the SU HIF needle thread above for more new tech answer to needle selection if you like. Cheerios, Vic|
Your car sounds very well set up. Keep us posted on the EFI and EDIS for MGB. I purchased Matt Kimel's EDIS out of the UK and it looks very well built. I have actually installed yet.
Quick question, I am traveling and do not have access to my car right now. With the 1.55 rollers and 270 cam, which valve springs did you use. I am changing from stock to 270, and will measure for coil bind. I would just like to gather all the parts before I start.
|Hi Shareef- Peter did the "Big Valve, Fast Road" flow on the head with double springs. I do not believe you need worry about bind till you get around the P-300 ( i.e. lobe lift .00300 x 1.55=.00465). He did however drill out the push rod column holes in the head a smidge for the increased rocking angle. FWIW, the P-285 I have on the other mgb roadster idles without lump at 800 and starts without lag, but goes like a scalded cat from 3k to 6k!|
I believe this bboard needs to start considering new topic headings for the popular application of these new techs, particularly the EDIS, which can transform most LBCs with 100k plus miles. It's amazing to me how precise this new timing approach is, with no detrimental meddling from worn dizzy spindles and the such! Cheers, Vic
This thread was discussed between 22/05/2007 and 01/06/2007
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