compression ratio
 

Hello :)

I have a few quick questions regarding the possibility of raising the compression ratio on the VTR. Has anyone tried to accomplish this via the old head milling technique? If so, how much did you take off and what were your results?

I realize that J.E. and Moriwaki make pistons too that will up the comp ratio. Can an engine equipped with these run on (R+M/2) 91 octane (at sea level)? What kind of performance increase can one expect from installing these?

thanks again :)

Mikstr

if you up the compression I bet it could run on 91 but I'd run 93... I run 93 right now when 91 would prob work just as well...

The accepted method is to mill the heads .010". With the heads milled and JE pistons, the compression ratio is 12-12.5:1. Moriwaki pistons give a slightly lower compression ratio. But still plenty for a big, slow-turning twin. It's really a matter of how fast the mixture burns and at what crank angle the maximum cylinder pressure occurs.

I made the mistake of advancing the ignition timing 4 degrees after decking the heads and JE pistons. The bike would run fine on pump gas most of the time, but had detonation problems when riding really hard at the track. Race gas made it ok. Putting the timing back to the stock advance got rid of the problem. An HRC race ignition worked perfectly. No detonation and noticably more HP (also with a full Stage1 exhaust system) with no detonation.

You'll probably get to ~115 HP with compression. Stage 1 cams and oversize valves, with an exhaust system gets close to 120HP.

I have a full stage 1 engine with one short season on it (1 track day and two road trips). A lot less expensive than if you try to do it all to your motor. I have full blueprint specs available.

Let me know if you're interested.
rand.c@earthlink.net

Re: compression ratio
 

Thanks RC :)

I am looking at both the head milling or new piston options. I would think that the head milling would be the cheapest option. I would also like to have the ports massaged at the same time. In the end, I want to still be able to run it on 91 octane as I like to tour with the bike. I would just like a bit more punch (don't we all? lol).

If milling the head by .010", how much will the comp ratio go up (still have stock cams and all), and, perhaps more importantly, hp and torque?

I would LOVE to buy your engine but that is outside my budget for now (sorry)

cheers
Mikstr

I ran 91 octane in my engine without a problem.
I haven't measured the CC volume. I planned to but the weather was good and I wanted to ride. My guess is that the comp. ratio would go from 9.5:1 to 10:1. Maybe 5-6 HP at 8500 RPM with ported heads. You should see a slightly noticable improvement in torque across the rev range. That's the conundrum. Everything you do adds a little bit. If you do a lot you get a big improvement. But it comes in small increments.

Pump gas would be fine. You could still get away with some more timing advance.

Make sure you slot the cam sprockets and degree the cams. Cam timing will change significantly when you deck the heads.

Re: compression ratio
 

On thing to consider when deciding which option to take is that milling the head will add more slack to your cam chains, and as we know the CCT's are a problem already. It was because of this that I went the piston option (and I needed new pistons anyway). I wanted to tun standard cams, valves and ports so I decided on 10:1 compression, to avoid any issues with detonation. I have heard of detonation problems with VTR's in Aus with our fuel when running increased compression.

If you wish to go this route you will most probably need to get pistons hand made, which as it happens I got done in th USA at a firm called CP. Genuine Honda pistons were $450 AUD and the hand made forgies were $650 AUD. Good value I think given that you can pick what comp ratio you want.

If you are going to go higher than this you really need to do other mods as well such as camshafts, otherwise you will not really get full value from your compression ratio. In the VTR full benefit is obtained when done as a pacakge. It is possible too that without more cam overlap, at low revs too high comp may cause detonation. Lumpy cams will bleed off a bit of cylinder pressure at low revs to avoid this.

I have got a lot of this info from an Aussie VTR guru, and this seems to agree with what RCVTR is saying too, so I reckon you need to follow this line of thinking.

Cheers,
Shayne

Don't forget that it won't add more 'slack' to the cam chains, the tensioners will take that up. What will result is cams that are out of time with the crank. Usually when people mill the heads you also have to slot the cam gears. This allows you to degree-in the cams to spec. That's a pain in the butt to do. Most guys would be better off with pistons alone.

Boycott JE!

Jamie Daugherty
http://members.iquest.net/~daugherj

wait so you recommend moriwaki pistons instead?? I'd really like to do cams and pistons at some point and was figuring je pistons but not sure what cams.... got a link to what you guys got for pistons and cams and where I can find them? Gotta keep these pesky 600 super sports at bay :P

Re: compression ratio
 

That is what I meant Jamie, I should have been more clear. Tensioner will take up extra slack, however I did not want to cause that. Just being picky, and I had to get new pistons anyway.

EngineNo09, one thing to consider when buying your pistons is their weight. The VTR likes a light assembly, hence 520 chain conversions and lightened flywheels. The Moriwaki pistons are light, not sure about others.

I've had really bad experiences with JE and will never use them again. Arias will make pistons, as will Wiseco and others. I didn't think the JE's were high enough compression anyway, so I'm going with a custom set from one of the above mentioned places.

Jamie Daugherty
http://members.iquest.net/~daugherj

Re: compression ratio
 

Check out CP, can't remember which state of yours they are in. They were reccommended to me as a light weight high silicone piston, and they price was good too I thought for a hand made set.

Re: compression ratio
 

What kind of "bad experiences" did you have with JE?
Ciao

I had a set of VTR pistons that I tried to return. This was after I found that the cases I was using needed bored. First problem: no oversizes. What kind of crap is that, everyone's got a standard bore huh? Second problem: since they had been taken out of the box, but not installed or anything, they would not allow me to return them for credit. I had measured them when they arrived, just like anyone building an engine should. The mic's I used left small marks, more like smudges, on the skirts. JE said that any marks would cause them to reject the parts. Garbage! I ended up selling them on eBay for a huge loss. Never again, I'm using Arias or Wiesco from now on because they would not have done that to me.

That was like 4 or 5 years ago. As you can tell I'm still pretty ticked about it.

Jamie Daugherty
http://members.iquest.net/~daugherj

Re: compression ratio
 

Ah...OK....simple "customer care" problems.
No technical problems!
Ciao

Poor customer service IS a technical problem! The worst kind of problem in fact.

Jamie Daugherty
http://members.iquest.net/~daugherj

Re: compression ratio
 

Well....I can't agree 100%.
OK, customer service is an important thing, but I prefere a poor customer service for good quality stuffs than a good customer service for poor stuffs.

Obviously, it would be the best to have good customer service for good material! :wink:
Ciao

I just realized I made a mistake on HP estimate for increased compression. I don't think you can get to 115 HP with compression alone. Probably closer to 110 HP.

Don't worry too much about bore size and out-of-round condition. The VTR and RC51 engines tend to run near the maximum bore size and being unsupported at the top they tend to be slightly out of round. Torquing the head down tends to make them come back to near-round. JE pistons tend to run on the small side to fit the minimum spec.

They run great even when the bores are near the upper service limit, with JE pistons. (That's what I was told by a builder of many of these engines, and matches my own experience).

Re: compression ratio
 

Another advantage of the custom piston is that you can get an oversize, which as Jamie mentioned, not everyone makes.

These newer engines with Nikasil plated bores never actually need re-bored . I don't know why someone would tell you different . Millenium Technologies just replates to spec . Thats just the norm of things unless there is severe cylinder wall damage .

As far as why JE only offers std bore sizing it is most likely is because of road racing /classes that don't allow but X amount of cc's for a V-Twin along with probable low volume of sales leads them to carry one size only .

Re: compression ratio
 

I was led to beleive the VTR had iron bores (whereas the RC runs Nikasil)

Mikstr

Re: compression ratio
 

Nope on the iron bores . VTR1000F Superhawks uses a coated bore .

Re: compression ratio
 

Thanks for the info. I think that is a definite good thing :)

cheers
Mikstr

Re: compression ratio
 

My 98 model doesn't. Well not since I bored it oversize anyway......

First I have heard of the coated bores too, news to me.