Low Compression 103" Build!?

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Nov 27, 2010 | 11:07 AM

#11

Sadthorn

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From: Minnesota

Thanks for the info guys, guess I didn't consider the cylinder fill issue that can come from having too large of an intake and not enough piston to really pull in the mixture. The only way I see these heads working well is for full on wide open throttle applications in drag racing. I may just keep these heads around for a future project.

Nov 28, 2010 | 06:45 AM

#12

Hillsidecycle.com

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Quote:

Originally Posted by rob71458

SE110 heads are better suited for 110's or larger. They have fairly large valves and ports. I think you would be disappointed with them on a 96". Besides they have a larger combustion chamber, which would drop your C/R, unless you went with a domed piston.

What Rob said.
The 204 is a decent cam, and combined with more cubes(103", or even bore to 107") and a good cylinder head, it'll run very well.

Scott

Jan 20, 2011 | 05:33 AM

#13

steve 55

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From: coralville, iowa

SE211 cams

Hillside, I'm doing the 103 big bore, with 211 cams. i,y,o. can I use my stock 96 heads with a.030 gaskit with no problems? And will I need c/r's on the heads. Thanks in advance for any info.

Jan 20, 2011 | 10:37 AM

#14

djl

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From: san antonio

Quote:

Originally Posted by Faast Ed

Depends what his heads cc out to. My stock heads cc'd to 84 cc's. Would have put him at 9.6:1.

Check your calcs again. The OP is talking 103" and with 84cc chambers, .030" head gasket and flat top pistons static will be 10.2. He will need 90cc chambers to hit 9.6.

Jan 21, 2011 | 08:40 AM

#15

DeweysHeads

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Here is an example, assumptions 85cc piston .005 down

Attached Files

103_85cc.pdf (12.7 KB, 1156 views)

Jan 21, 2011 | 10:20 AM

#16

SURFOR Chop

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From: Seattle, WA

There's a similar compression ratio calculator to the attachment Don is showing on BigBoyz website ... you enter the parameters of your build, select the cam and it will show static and corrected CR ... allows you to do some 'what ifs' ... like decking the stock head, 0.030 gasket, etc.

Jan 22, 2011 | 05:59 AM

#17

DeweysHeads

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The thread title is "low compression 103", well a 103 stock is not low compression any more. The MOCO added ACR compression releases to avoid issues for the factory 103 in 2008 and retrofitted 07 Police bikes! I have changed my mind on using these heads on a 103 motor as one of my colleagues did the swap with sucess. As I rethought the concept I understand how it works. Cam is critical to sucess.

Plan A
If you wanted to go the 110 head route..
21928-08 is the part number of a piston that will work if you use the 110 heads and put you at 10.1/1 with a .030 hg. Not "low compression" build however. Premium only good tune required and ACR valves functioning.
Plan B
The heads milled to 90CC and flatop pistons would have you at 185ccp and 9.5/1. This is conservative and would run on midgrade if the tune was right. You will be opening up both valve reliefs to get the big valves to work. If you used KB pistons the reliefs may clear without any rework. YMMV be sure to check

The 110 heads need the guides changed and it would be wise to derate the springs a little say 150# seat with some beehives. Staying with the 204 you may be pleasantly surprised that it is a good runner. The advanced nature of the cam helps on both sides coupled to conservative overlap, low lift and bingo those big heads might surprise you. That little cam delivers more when those ports and valves start working and the motor will act like it has more lift and duration than it does.

Taking a look at the Chevy L92 SUV motors heads and you willl get a glimpse at what can be accomplished with a good design and less than huge cam profiles. This motor makes 403hp SAE net. Valves are 2.156 and 1.59 and cam is204/211 @ .050 .525 lift. Just another way to skin the cat.

Jan 22, 2011 | 07:45 AM

#18

Sadthorn

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From: Minnesota

Good Morning! I actually just finished rebuilding the heads. Just lapped the valves last weekend for a clean seal on the seats. The ACRS are working great and I am in the process of setting up a small system of my own design to activate these ACR's manually and be able to use them on a motor I will build for drag racing. I am looking into boring a set of cylinders I bought off ebay to fit my new wiseco pistons. Still investigating crank and rods, cases, etc. to complete building the lower end but that's a whole other topic unrelated to this thread.

Very helpful information from this post on possibilities for a healthy street motor and not letting a guy try something that will produce a dog on the road. Saves the cash lost to trial and error, thanks everyone!

Last edited by Sadthorn; Jan 23, 2011 at 06:30 PM.