Compression and Starters : V-Twin Forum: Harley Davidson Forums

Bluescb · 01-07-2006, 09:15 AM

Finally took my heads off and sent them to SBC for scottsmans. Can't wait to get them back! I have an '04 RKC with 95" and 37G's. This setup should give me the Hiway Kick I need to stay safe on I95 thru Raleigh in rush hour!

I was told by many V-twinners to use a Cometic .030 gasket and set the CR for 9.7-9.8 for TW37G's.

Three Questions
1) Will this be too hard on a stock starter---need compression releases?

2) Does the 9.8 CR effect reliability to any great degree?

3) Can I reuse the Top End gaskets?

thanks

LittleBear · 01-07-2006, 09:35 AM

No on all three counts.

SYCLONE · 01-07-2006, 09:43 AM

Quote:

Originally Posted by LittleBear

No on all three counts.

LittleBear....quit with the long winded replys !!!

LAF · 01-07-2006, 09:43 AM

Quote:

Originally Posted by LittleBear

No on all three counts.

And on the 3rd one the cost is not worth the grief.

AzFAL · 01-07-2006, 09:45 AM

I've got the same set-up in my FXDL w/ a 44CV carb. I would recommend getting compression releases since you've got the heads off & getting worked over. I live here in the Phoenix area, SBC is about 15 mins. from me. I've had some problems with starter kick-back.

It gets up around 115 here in the summer & when stopping for gas & then trying to restart the bike is when I most often have the kick-back. It still does it when it's in the high 70's sometimes too. No problems when the bike is cold.

A lot of people say you don't need them becauase it's not a real high compression build, but I think it's good insurance that will save your starter especially since you've got the heads off already.

If I had to do it again based on my experience I'd definitely do the comp. releases. BTW I have had to replace my starter once.

csoday · 01-07-2006, 11:16 PM

Comp releases are not needed om ratios less than 10.5-1

genoway · 01-08-2006, 12:47 AM

On 2, only if you can't get gas over 90 octane in hot weather.
May ping in hot weather with regular gas.

No problem with premium.

And as Little Bear said, no on 1 and 3.

MegaGlide · 01-08-2006, 07:52 AM

Quote:

Originally Posted by csoday

Comp releases are not needed om ratios less than 10.5-1

A 37 is an early closing cam and cranking pressures are high w/ it. A judgement based on static compression alone isn't fair to the guy who has to pay to relpace the starters...

floyd78 · 01-08-2006, 08:18 AM

For the minimum cost involved in getting the releases it is nice to have in case you run into problems. My build is a 10:3.1 compression and I didn't give it a second thought. I've heard to many stories of people having kickback in hot weather starts. Even with lower compression builds!

Bluescb · 01-08-2006, 09:23 AM

I already do get some kickback on real hot days after filling her up. I'll talk to charlie. Thanks everyone.

Great quote MegaGlide!

JRS · 01-08-2006, 09:33 AM

I run Redshift 577 cams and pop-up Wiseco pistons in my FXDX, and I get hot start kickbacks all the time.

I'd definitley recommend getting releases; they're worth the peace of mind. My starter is living on borrowed time.

John

floyd78 · 01-08-2006, 11:55 AM

Hey on a side note when you go to get your releses TP Engeneering makes a cool release that has threads on the pushing end for valve stem caps. Nothing to do with the functionality but makes them look cool. Not to bad of a price eather 25 bucks apiece. Good Luck!

AAL · 01-08-2006, 03:02 PM

On the Harley Tech Talk site, there's a section entitled Documents. Under this, there's a section for twin cams, and under that there's a spread sheet showing compression ratio, that can be used to calculate the static and other ratio for various builds. The thing I find interesting is that the stock heads are, as I understand it, supposed to be 83cc, but in fact rarely are, and are usually 85 to 86 or so. That really reduces the compression ratio.

LAF · 01-08-2006, 03:41 PM

Interesting that people at 9.8 are needing releases.

I had always heard that until you hit 10.5 they were not needed. I saw someone said they had 10.3, and I would have had them in that close of a range.

I guess if your heads are off and you think you want them then put them in while the porter has them, even if he says you don't need them, ask him to humor you.

I have them and it only took me a time or two to forget to use them as that sound is unforgettable, and I reach for them before anything else on the bike, and may check them again before I crank it.

AzFAL · 01-08-2006, 04:21 PM

Quote:

Originally Posted by LAF

Interesting that people at 9.8 are needing releases.

I had always heard that until you hit 10.5 they were not needed. I saw someone said they had 10.3, and I would have had them in that close of a range.

Yeah that's what I'd always heard too which is why I didn't get them. 91 octane is best gas you can get around here. MegaGlide has a very good & important point on cranking pressure also.