Which head gasket?

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Apr 1, 2010 | 11:44 AM

RK4ME

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From: west Michigan

Which head gasket?

The build I'm planning on doing on my '04 RK consists of 95" SE flat tops, '06 heads and possibly 26Ns w/.030 Cometic head gaskets. My question is this: If I go with 21Ns, would the .030 head gasket give me more static compression than I need? With 21s, would I be better off with a .040, or stock head gasket? I don't have a tach but I was out on the freeway last night fot the first ride of the year and remembed that my stock motor (PC5 & SE/AC) has nothing in fifth gear at 70 mph. The Big Boys drive ratio comparator puts me at about 3K RPM here. To accelerate, I have to go to fourth gear to make the engine "spin" (instead of pull). In third gear at 50 mph on a tree-lined back road, with little wind resistance,(also @ 3K RPM?) the engine pulls good for a stocker. This is what got me thinking about 21s, although the 26s w/.030 Cometics might do well in fifth gear at 70 mph too. I know the 21 vs 26 thing gets beaten to death here but I'm mainly wondering about the right head gasket for the 21s if I go that route. Thanks for any good advice.

Apr 1, 2010 | 12:14 PM

djl

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From: san antonio

if you are not milling the heads, it won't make any difference. One thing for certain, you should use the .030" head gasket. The thinner head gasket does provide a .3 point compression boost over the stock head gasket.

The other benefit of the thinner head gasket is the tightening up of the squish are which will improve the efficiency of combustion; a faster and more complete burn of the A/f mixture.

Your static CR will be the same with either cam, about 9.35 assuming your chambers are 85cc; YMMV. The corrected CR with the 21 will be about 8.9 and with the 26 about 8.7. You will simply move the torque curve for the 26 to the left but both cams will start to fade at about 4000rpms.

One thing you could consider with either cam is the use of the 1.725 ratio rockers in lieu of the 1.65 ratio stockers and inrease the lift of either cam. This will help on the upper end.

Apr 2, 2010 | 02:31 PM

mtclassic

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I have a little experience with both cams and with head gasket thickness. If you are going to run stock heads and do nothing use .030 with the 26's and if you use 21's use the .040. I ran the 21's initially with stock heads and stock gaskets in my 04 eg and it ran very strong made 94 ft/lbs of torq. I then decided to throw in a set of .030 head gaskets and I made 97 ft/lbs but it was prone to pinging now and then and it would kick back on hot starts. I then went with 26's and did not have near the issues of pinging or kick back on hot starts. Many people who have never used them will say they fade early but you can kiss the rev limiter in 4th and 5th gear and they still pull strongly at 100mph. Dyno charts may show them dropping off but real world seat of the pants show they still pull strong after 4K. When I was having my ping problems with the 21's and .030 hg I called AMS who specializes in bagger builds using 21's and 26's and he assured me that 21's were a better match with the .040 vs .030 because of the slightly lower corrected CR.

My real world seat of the pants is that from 50-100 in 5th gear the 21's easily out pull the 26's. I have approximately 25,000 miles on each cam in the same bike.

Apr 2, 2010 | 05:12 PM

djl

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From: san antonio

With the poor quaility of fuel that we have to deal with these days, incorporating "singh" or "quench" grooves on either side of the combustion chamber will help mitigate ping. I think you could groove the heads and run the .030" with either cam.

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Apr 3, 2010 | 10:23 AM

RK4ME

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From: west Michigan

Since I already have the .030 head gaskets, I'll probably go with the 26s and maybe the 1.725 rockers but if those rockers are going to give me more on top, then it's starting to look like I want the 37Ns instead. Too many choices. Thanks for the advice.

Apr 3, 2010 | 10:38 AM

mtclassic

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Quote:

Originally Posted by RK4ME

The 26's will still give you more down low vs the 37's based on the earlier closing intake. I bought a set of 1.725 non roller SE rockers but ended up selling them as I got so many conflicting stories about how they actually would perform. In the end I did not find anyone who actually did back to back dynos with just the rockers being the only varible. In fact the only person that I found that actually thought they may have done some good (he had a very nice dyno for 26's) did a lot of other work at the same time. I found as many who reported negative as reported positive (not many in either case). If you decide on 37's make sure you deck your heads to 83cc or less or you will be soft on the bottom end. I have experience with 37's as well as the 21's and 26's and the 37's need more compression in a heavier bike.

Apr 3, 2010 | 01:00 PM

djl

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From: san antonio

Quote:

Originally Posted by RK4ME

I dug through some flow comparisons on the '06 OEM heads and flow flattens somewhere between .45" and .500", depending on the comparison. I tried to get flow data from Bean the last time he ported a set of heads for me and he said he flows a set now and then to check but doesn't provide flow data with each set. He reckons his heads flow "comparable to the SE Performance" heads. At any rate, if the typical BigBoyz street port is done, unless you request otherwise, the stock intake valve will be retained, which is not necessarily a bad thing for a 95" build. Having said that, flow will improve and probably not go flat until about .550" lift.

The intake lift of the 26 is .490"; the 37 is .510"; however the TDC lift of the 26 is .138 vs .174 for the 37. With an 82cc chamber volume, you are looking at a difference of 8.9 corrected CR for the 37 vs 9.0 for the 26; the 26 intake closes at 35* and the 37 at 38*.

Sorry for taking so long to get the point but the 26 with (or without) the high ratio rockers is probably the better choice for a touring bike. JMHO.