I think my viewpoint is clear, but in case the trees are lost in the forest... I don't see any purpose for pushing compression ratios to the point that race gas or additives should be used at the strip "for more power or in addition to more timing". The little you find is what you detuned out from running on the edge in the first place. If you want to add that for peace of mind...AOK. Unless your time is worth very little or you like playing with things then don't swap gaskets and try things "just to see" or slightly steer, It's counterproductive. Fine if correcting a problem though. Make it safe, don't guess. Pull back slightly from others' found line of tolerance. If you find a line of tolerance within your own examples, you can math out variances for different paradigms. You can make up the power with airflow, including cam. Make it do exactly what you want it to do. Eat cake. Good top end work tones down cams that act wild under crappy top ends, and extends the powerband down low as well as high. LSA's are quite relative, these tiny cams don't have enough overlap to cause any issues with venturi signal strength beyond idle. The duration is too small, the overlap pulls on the carb hard as soon as the throttles open up. Even with a raspy idle they wake right up. Using duration, wide LSA or any other way to "bleed off" compression is counterproductive to the powerband with a street bound air-starved engine. The only thing it does is starve the cylinders to make it easier to tune idle and help hang on longer after peak hp is reached. You'll pass by the torquey part of the powerband hoping for more rev and there's more time spent accelerating a mild car at the shift recovery rpm than the little blip up top. Dyno tests are valuable, fickle, relatable and unrelatable all at the same time. Sometimes they are useful and other times they are used to steer work or other types of marketing propaganda. You'll get little useful info with the typical parameters shared as the subtle differences make more impact than the face value of the combo. Example is that I can deliver two different top ends sharing similar CFM #'s that would have drastically different behaviors. Advice is as good as what you are paying for, take it or leave it. A dyno sheet would be a great confidence builder worthy of the price paid to produce it.
I am sure I will have to address some clearance issues. I have read that the Best gasket compresses to about .040.
Tom T. has posted that his pistons come with a 1.250" to 2.000" compression height. Still maintaining .300" top ring land.
I love this thread - I find that with very different specs I have a similar DCR results as dlmwtvl : 7.33 with .043 gaskets and cam straight up (current config), 7.58 with cam advanced 4 degrees and 8.1 with advance and .015 gaskets. Tommy and I have been talking about timing, and I'd be interest in what others have run with pump gas on a similar DCR.. Now I'm also thinking about total clearance with a steel gasket, and I might be tight at .020.. I believe the deck is pretty parallel (Tommy?) When we talked about it, he mentioned that optimal clearance is between .025-.050... Bore 4.215 Stroke 3.64 Piston 4.211 rod length 6.217 Upper ring land 0.2 piston in deck 0.005 deck height 10.0035 head cc 124 piston dome 39.2 head gasket bore 4.42 head gasket thickness 0.043 CID 406 static compression 9.6 with .043 head gaskets static compression 10.2 with .015 head gaskets TA-25 cam
David, You'll have to "Dimple" the pan for the front four rods. Make sure you have addiqite clearance on ALL points of the oil pump. In one case a longer oil pump shaft was needed.This requires other things also as now the oil pump is at an angle. In another case the center oil galley had to be removed & a piece of black pipe had to be used in it's place, but that was because he wanted to use 7/16ths. rod bolts Lots of things to think about when making changes and you don't know until you get there & have to solve the problem/problems. Interesting talk. Ring land was "supposed" to be .300" but it/they decided that .200" would be best as stock. With a 6.850" rod on one build we were NOT yet into the oil ring, close, but not into. Tom T.
Dave, best head gaskets compressed to .045 just took a set off. they were the black graphite. the blue fel-pro are about .043. yes Toms pistons will work great.
Rhett, when you do dcr are you using valve close at .050 or advertise duration.? i been using at .050 thats why the big difference in yours and mine dcr.
8ad - I mean to say that any specific point within that range is acceptable. my .020 falls out of that range. Joe - I used the calculator that downloads here: http://www.empirenet.com/pkelley2/DynamicCR.html. It prompts you to use advertised duration, and then calculates your ABDC close point, and how your intake centerline changes that depending on where you install the cam... took a while to get used to but feels like the best DCR calculator out there....
Gotta use the actual closing point, that's a strong benefit of degree'ing the cam. If you are in the planning stages, the actual cam card from the mfr will work. The topic of dynamic compression being a starting point with many variables around it is relatively inaccurate, so it seems conflicting to be so exact with something having a wide berth in real world effect. It accurately applies within your own exact paradigm and under specific limits you work around. Rhett - not busting your chops, I didn't think that's quite what was meant, but it begged some clarity given the thread topic. There's diminishing returns less than about .030" and nearly no effect approaching .060", and in fact the danger zone. One important thing overlooked is the residual heat in the exhaust valve and the effect of wide seat contact vs. narrow contact resulting from interference angle machining. People thoughtlessly toss specs around for valve jobs without considering it's effect on octane tolerance, or are unaware of what the shop left them.
8ad - no prob, I got what you were getting at. We did degree the cam, and it was pretty close to spec... On valve seat contact, how do you calculate what you think seat contact will be when you spec a valve job?
I don't calculate it. I see very little need for a narrow exhaust contact. Street bound engines get wide contact to pull more heat from the valve face, esp. when there are potential issues with pump gas. Shops get lazy and either use a "for sure it seals" interference or set the contact squarely in the middle of the valve face for wear reasons (?). Even new valves aren't always perfectly ground or angled. Some engine family's castings will distort when the head is bolted to a torque plate, but that's a different level of finicky. Set the seat OD near the valve's OD and use as small of a valve as you can get away with. The top angle should be no more than an edge break if possible, with preference on consistent heights. Even a cut seat benefits from a light touch with a well dressed stone, for surface finish and heat transfer. Vizard printed a study he conducted way back, something to the effect of "every .010"? wider is worth X* of heat from the valve face". I don't put much weight in exhaust flow bench numbers to any power equations, if I even flow them. I'm more concerned with overlap flow potential after blowdown, preceding the intake valve opening. I'm probably never dealing with anything restricted enough at the exhaust port to sacrifice seat area for throat area. If a person was to intentionally push the envelope with dynamic compression, why would there be little regard for hot spots in the chamber? The residual temperatures and load are more of a problem than pressure prior to combustion. You can fool the engine during the compression phase somewhat, but it's tougher with a small cam, high compression and a lower rpm range.
I have already got the rear sump aluminum pan and pick up from Russ. It is beautiful!! I hope it clears...
