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I originally said this: I aim for something around .040" myself, but if it ended up being .030", I wouldn't worry, and if it was closer to .050", I wouldn't go out of my way to spend money to make it tighter because it makes so little difference. and I stand by it.
The rawits has stated that he is on a budget, so I would have spent the money on machining the spring pockets for better springs or something else that is more important.
That [decking the block & reducing the quench] was absolutely the only change. Same pistons, same cam, injection untouched (it was a D-jet engine). Compression did come up slightly from decking, of course, but I don't think that accounts for much of the improvement.
If the reduction in clearance didn’t automatically force a change in the required amount of ignition timing, then the timing must have been wrong for the motor before you touched it. Also, if you made an insignificant change to the compression ratio, then you must have removed an insignificant amount from the deck. Additional compression ratio is always going to be a bigger factor in power development than altering quench. Perhaps a simple adjustment of D-Jet fuel + alter the ignition timing is all it needed in the first place.
I'm talking about factory engines with no prior rebuilds.
What brand of pistons? 6 or 8 bolts?
I've never dared go tighter than .030, and usually a bit larger than that, depending on the specific parts. But when I was first building the MPPE and trying to understand why this matters, the racing machine shop I was using decked the block for .037" clearance. Mike Aaro jumped all over me and I had to take it back and have another .005" taken off, so he, at least, thinks every little bit counts.
Do you actually believe that there is a worthwhile difference between .032” & .037”? Mike Aaro doesn’t impress me much, especially after you told me he opened up your exhaust ports to gasket size, that a very basic porting mistake. He really should have been able to tell you exactly how to jet your Webers since he provided the head & told you what cam & headers to use.
Notice what he says about what each lobe should be like too.
Yes, I hadn't heard that one before -- interesting. Why does a slower valve velocity work better for exhaust? I understand how a B20 might want longer duration on the exhaust side, but why not high velocity as well?
I’ve been saying that for years, my very first low budget engines I was building myself in the late 80’s when I was a lowly paid apprentice + the Australian dollar was terrible featured 4 dual springs on the intake to save money + a dual pattern cam with a fast & high lift intake lobe & gentle stock lift exhaust lobe. The reason is two fold, the exhaust valve has to open against seat pressure + exhaust gas pressure on the valve head, so it can’t be as tough as the inlet. Reason two is the exhaust valve is smaller, so it just doesn’t need the same lift.
....the Chevy heads he's dealing with have much better swirl than a B20, and the B20 could therefore benefit more (not that squish causes swirl, but it does help mix intake throughout the chamber).
I don’t agree with that at all, I feel the number one reason that you can make a decent engine out of a B20 is because the inlet port provides good swirl & it’s easy to modify it to make great swirl. I still prefer to call it quench rather than squish too as I believe the primary point of having it is to extinguish any pre-ignited pockets of air/fuel mixture at TDC. If the gap is too wide, you don’t get quench action & the combustion chamber then reverts to being like an open chamber design which then forces you to run a lower compression. The squish action stirring the mixture is a very minor thing IMHO.
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