Increasing compression with an Andrews 37
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Increasing compression with an Andrews 37
So I've come to a crossroads with my engine. I have an 06 Dyna Low Rider that I have put on some Vance and Hines Pipes, the SE forward cone intake, and an Andrews 37H cam. I am happy with the high end response on the cam and have some pretty good air flow that I have tuned the bike to compensate for, but my compression is too low with the stock jugs, pistons, and heads. Eventually I would like to get both the head work done and put in some big bores, but the question is which first. The second one may have to wait a year or two. I am starting at 8.65 static compression, 8.16 corrected compression.
I have done some calculating on the Big Boyz compression calculator and have figured out I can get to 9.45 static, 8.9 corrected with my 4 degree cam offset, a .030 head gasket, and while getting the heads ported and flowed can get them milled to a 79CC cylinder head volume. When I do the jugs I could then go to the .040 gasket, flat top pistons, 95 ci jugs, and take out the 4 degree cam offset to get 9.7 static and 8.99 corrected compression which is what I have heard is the sweet spot for the cam.
Otherwise I can do the jugs first to 95 ci, domed pistons, and take out the 4 degree offset to get 9.76 static, 9.06 corrected then get the heads ported and flowed later without milling.
I guess the third option would be to do the jugs to 95 ci, flat top pistons, use the 0.030 gasket, and leave in the 4 degree offset to get 9.32 static, 8.78 corrected. Later do the heads with milling to 81 cc, 0.30 gaskets, and take out the 4 degree offset to get 9.7 static and 8.99 corrected.
I have done some calculating on the Big Boyz compression calculator and have figured out I can get to 9.45 static, 8.9 corrected with my 4 degree cam offset, a .030 head gasket, and while getting the heads ported and flowed can get them milled to a 79CC cylinder head volume. When I do the jugs I could then go to the .040 gasket, flat top pistons, 95 ci jugs, and take out the 4 degree cam offset to get 9.7 static and 8.99 corrected compression which is what I have heard is the sweet spot for the cam.
Otherwise I can do the jugs first to 95 ci, domed pistons, and take out the 4 degree offset to get 9.76 static, 9.06 corrected then get the heads ported and flowed later without milling.
I guess the third option would be to do the jugs to 95 ci, flat top pistons, use the 0.030 gasket, and leave in the 4 degree offset to get 9.32 static, 8.78 corrected. Later do the heads with milling to 81 cc, 0.30 gaskets, and take out the 4 degree offset to get 9.7 static and 8.99 corrected.
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Just a couple of thoughts. Forget about domed pistons and 79cc chambers; could have valve to piston clearance issues. Obviously, the best option would be to bore to 95"/98", have heads worked and set static CR at 9.8 which is what the 37 cam like. Actually, if the bike is EFI and you have access to a good tuner and good fuel, I think you could bump static up to 10:1. In either case, compression releases will come in handy.
Considering the options you outlined and limited by the fact that you can't do everything at once, the third option would be my choice.
Another option would be head work and a cam change; no big bore. Heads ported, decked and compression releases installed; set static at 9.4 (78cc chambers) and replace the 37 cams with a set of S&S 551s. The 37 is a popular cam and you could recover some of the upgrade cost by selling them on Ebay.
Nothing against the 37 cams; always a good choice but too many options to achieve the desired results IMHO. A cam replacement accompanied by the appropriate head work will get you the low end you want in one step and without the expense of boring cylinders and buying pistons; a cam replacement would cost about half of going bigger bore. However, if/when you decide to go bigger bore, you would need to replace the 551 cams with something like the S&S 570, or the Andrews 57 which is identical to the 37 but with more lift. Food for thought??
Considering the options you outlined and limited by the fact that you can't do everything at once, the third option would be my choice.
Another option would be head work and a cam change; no big bore. Heads ported, decked and compression releases installed; set static at 9.4 (78cc chambers) and replace the 37 cams with a set of S&S 551s. The 37 is a popular cam and you could recover some of the upgrade cost by selling them on Ebay.
Nothing against the 37 cams; always a good choice but too many options to achieve the desired results IMHO. A cam replacement accompanied by the appropriate head work will get you the low end you want in one step and without the expense of boring cylinders and buying pistons; a cam replacement would cost about half of going bigger bore. However, if/when you decide to go bigger bore, you would need to replace the 551 cams with something like the S&S 570, or the Andrews 57 which is identical to the 37 but with more lift. Food for thought??
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Interesting, dj. I guess the compression releases would be needed for the 551s and they do have higher lift than the A37 that the stock 88 can take. I don't know about the cost being half though. Sure I can sell my current cam and only lose maybe $50 on that part swap, the head work alone is around $500-$550 so around $600 and that is if I do the wrenching myself. I can get the Wiseco pistons for around $270, top end gaskets for $70, and the cylinder bore for $120 and the cost is only $460 if I wrench it myself. Seems to me the head work would be more expensive. I think I will take your advice on the options I gave though and do the 95" first. How about doing the 95", go with the 9.5:1 domed pistons, the .040 gasket, and take out the cam offset to get 9.66 static and 8.96 corrected then later do the heads?
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From: san antonio
Interesting, dj. I guess the compression releases would be needed for the 551s and they do have higher lift than the A37 that the stock 88 can take. I don't know about the cost being half though. Sure I can sell my current cam and only lose maybe $50 on that part swap, the head work alone is around $500-$550 so around $600 and that is if I do the wrenching myself. I can get the Wiseco pistons for around $270, top end gaskets for $70, and the cylinder bore for $120 and the cost is only $460 if I wrench it myself. Seems to me the head work would be more expensive. I think I will take your advice on the options I gave though and do the 95" first. How about doing the 95", go with the 9.5:1 domed pistons, the .040 gasket, and take out the cam offset to get 9.66 static and 8.96 corrected then later do the heads?
First question I have for you is about piston choice. If you have a 9.5:1 piston, that would indicate that static CR would be 9.5:1. Actually, to accurately compute compression, you will need to know the dome volume of the piston you are considering. However, assuming that the selected piston does produce static CR of 9.5, according to my calculations, leaving the 4* advance offset in place will result in corrected CR of about 9:1 and CCP of 185psi which is a bit on the conservative side but a very comfortable place to be.
So, you need to verify the dome volume of the 9.5:1 piston you are considering, run the calcs and see the results before pulling the trigger. You also need to consider that while the heads are off, it would be foolish to reinstall heads with 40K mile without disassembly, clean up, good multi-angle (SERDI) valve job and new guide seals. Once you see how much the head service will cost, you will see that spending a bit more gets you a set of decked and ported heads and you are back to considering whether or not to stick with the 37 cams or go for more cam. It's a conundrum.:icon-wink:
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No disagreement there for sure...none takin...
With out a doubt cubic inch is where it's at but you still need to make it work...
With out a doubt cubic inch is where it's at but you still need to make it work...
Last edited by 98hotrodfatboy; Mar 17, 2016 at 03:04 PM.
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Grand HDF Member
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From: Iowa
First question I have for you is about piston choice. If you have a 9.5:1 piston, that would indicate that static CR would be 9.5:1. Actually, to accurately compute compression, you will need to know the dome volume of the piston you are considering. However, assuming that the selected piston does produce static CR of 9.5, according to my calculations, leaving the 4* advance offset in place will result in corrected CR of about 9:1 and CCP of 185psi which is a bit on the conservative side but a very comfortable place to be.