austinsailor Posted January 19, 2011 Report Posted January 19, 2011 As I try to figure out how to fix up the 251 truck motor into a sort of hot rod motor, I have lots of strange thoughts. I also have a 51 Fargo with what I've identified as a 228 ci log block. Compression on both are similar, so I figured that the combustion chamber must be smaller to keep it the same final ratio. It stands to reason, then, that if I took the fargo head and put on the 251 I'd gain compression with nothing else being done. Then, if I shave that head then deck the block, I might get some serious compression increase. I went out and pulled the fargo head this morning - the snowstorm won't get here for a few more hours, the sun is shining on the truck, and it wasn't too bad at 15 degrees. But I'm getting off track. I laid the two heads next to each other. Same gasket, same depth along the edge where the valves are, but where it narrows between the head bolts in the middle, the Fargo is 1/8" narrower. I guess I need to do a serious measurement of the volume of both, but it looks like my theory is on track. So, 2 questions. 1. Is there something serious I'm missing in the plan that makes it silly? 2. How high can the compression go without having problems? I think I've read that taking .080 off the head and .010 off the block gets to about 8 to 1. If my math says using this head will take me to, say, 8.5 or 9 to 1 will I have problems or just more power? Quote
Don Coatney Posted January 19, 2011 Report Posted January 19, 2011 Go here. http://www.jrbranson.com/HondaRacer/honda/How-to-CC-a-head.htm Quote
ron schmitt Posted January 20, 2011 Report Posted January 20, 2011 Milling or decking a flathead motor is tricky. Yes you can raise compression , but you can reach a point that air flow in and out of the cylinder is reduced enough to cause a power loss. And if you port or relieve the block from the valves to the cylinder the compression is reduced again. I had good luck my 217 plymouth with a fenton aluminum head, intake and headers. I used 2 stock carbs and the power increase was good. I hope this helps Quote
greg g Posted January 20, 2011 Report Posted January 20, 2011 (edited) George Asche has metioned to several folks here that any thing over a combination of milling and decking more than .090 requires moving up to premium gasoline. If remember correctly back before the days of knock sensors and or variable cam timing and computor controlled igition and fuel mapping, this generally was in the 9.0/10.0 to one area. I took .010 off my block and .030 off my head. If I remember correctly I was getting 150 to 155 psi cold cranking (dry) compression. From other information I have read, and from what my machinist calculated, that those changes along with .030 over pistons gave a compression ratio of 8.5/8.7 to 1. I have what I consider to be a very flexible and responsive engine, with the 2 carter's and single exhaust. And it runs fine on regular. Take a look on the car nut specs page for Dodges and Plymouth L 6's over the years payig attention to the HP ratings as over the years the compressio ratio steadily increased. I believe that for a street engine between .030 to .070 is pretty much the way to go. more gets you into that area where engines become finicky, don't idle well, and get less tractable for normal driving. The only way to tell would be to measure the capacity of the combustion area. I did mine simply and a bit more shad tree, but by a process that gave me a visual comparison of the difference between 2 heads. Set up the head on a level surface, Used my work mate bench/vise, I screwed a spark plug in, then filled the area to capacity with some pink RV plumbing anti freeze I had laying about. I then removed the plug from the head and recovered the fuid in a container. the plug was accessable through the open slot of the work mate. Then transfered it to a graduated measuring cup and noted the capacity in milliliters, did the same with the other head and noted the difference. Less volumn = more squeeze. The final capacity was not as much a concern as noting the difference between them. Edited January 20, 2011 by greg g Quote
austinsailor Posted January 21, 2011 Author Report Posted January 21, 2011 I measured the combustion chambers today. Original 251 truck head is 105 ml. Fargo 228 head is 98 ml. Quote
ggdad1951 Posted January 21, 2011 Report Posted January 21, 2011 Smaller is better. you said it not me! Quote
greg g Posted January 21, 2011 Report Posted January 21, 2011 Don't know how much difference 7 mil made orriginally, but if you mill the smaller head the increase in CR will be proportionally greater. Still say increased CR is the best and most economical upgrade you can do for more grunt. Quote
Plymouthy Adams Posted January 21, 2011 Report Posted January 21, 2011 squeeze is good but ultimately the abiltiy to breath, both intake and exhaust is the magic and only a better profile cam will give you the total benefits of that...any step can be made a little at a time as funds or time allow..but the combination of the 5 biggies (cam, carburation, exhaust, CR and ignition) is the ultimate goal if looking for some punch..however..there are drawbacks to even this..the final factor, drivability will be the results of the final gear ratio and what mode of driving you intend to partake..if you looking for the best of both worlds..one will need consider the OD/T5 or similar for the cruise and the performace afforded by the original differential..I have seen too many cars built that does not include all these and while nice in one aspect, limits yourself on the other... Quote
austinsailor Posted January 21, 2011 Author Report Posted January 21, 2011 Good thoughts Tim. Cam is on the way to Edgy, I'll probably add George Asche's tripple manifold (when I find a grand laying around), looking into Langdon's cast headers. Head grind and decking the block is a given. I have thought about the T5, which seems to be the #1 choice. I know nothing about them except what I've read on this board. So, I did a cragslist search to see what was available. Lots is available, and I figured out I need to know a lot more just to start. There were T5's in many cars, little 4 bangers, more powerful V-8's and everything in between. How do you know what to get? How do you know if it's strong enough, or are they all the same inside regardless of the original application? Gear ratios? Case configurations? Are input and outputs the same regardless of original application? Quote
Frank Elder Posted January 22, 2011 Report Posted January 22, 2011 You could keep it mopar by using the dakota 5SP, AX-15 I think it's called. There are kits for that also. Quote
David Muma Posted January 22, 2011 Report Posted January 22, 2011 The total amount removed usually includes about 10 thou off the block to true it up. The rest comes off the head. Quote
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