I have a 98 FXD stock other than stage 1 bolt ons. During a top end gasket swap I found that both my exhaust valves were leaking so I brought the heads to a shop for a valve job. I spent about an hour there talking to the owner about what I wanted to do and what his opinions were. I was woefully unprepared having not done enough research before going there. This shop is highly regarded in my area so I feel like I can believe him and trust his experience. I wish I had written it all down because my memory is like a pasta strainer.

As it stands, he is going to blend the bowls during the valve job. He's going to replace my springs with Twinkie springs and use the vitron seals. He's also going to resurface the heads. As long as the guides are good he wont touch them.

I remember him talking about shaving the heads for more compression. He said the cam I'm using would be happy at 9-1 compression. I am installing an EVL-3010 cam-specs in link below.

http://www.vthunder.com/catalog/cams.pdf

He also mentioned instead of shaving the heads I could bore out the cylinders and put in oversized pistons (I think he said domed) to get to the 9-1 CR.

I'm not sure which way to go, or to do anything at all and just have the valve job done and call it a day. I'm over budget as it is. You know how it works though, for just a few more dollars...

What's the better way to get more CR to get more seat of the pants power? Bore and piston change or shaving the heads?

Downsides to either? Is the extra expense of doing either worth it?

Thanks!

EDIT: I actually found a thread with lots of good info and gave me a better understanding of things.

https://www.hdforums.com/forum/touri...pistons-3.html

Sounds like shaving the head down is not as preferred as larger pistons. I suppose I could keep the bore stock and run some domed pistons, but then I am reading that the domed pistons have disadvantages.

Opinions welcome.

 

Nothing wrong by raising the mechanical compression ratio, via surfacing the heads.
A great combo we've used here for waaaaaaaay over 15 years is to remove .065", with a .030" head gasket, and install a Wood 6.
Must've built 80-100 Evo's over the years with that set-up.
It'll hover at 80 hp, and 90 ft/lbs.
Scott

 

Thanks Scott.

Now that's with stock bore and pistons? Would I run into issues with things not lining up right like intake manifold or the head bolts bottoming out before hitting their torque value?

I'm learning as I go here and appreciate everyone's advice.

 

Only info I can add here terrapin is Scott is doing my machine work and really helped me out because I didn't know what the heck to do!! I'll be at 9.5 compression with new pistons and rings , stage 1 port and polished heads and I used the EVL-3020 cam in mine

 

The intake may need some slight touching up with a hand file but nothing serious, 030 on the head gasket has no effect on head bolts bottoming out. It would be hard to screw up a intake as I ran a Twinkie intake will work on a Evo and it has a small gap all the way around on the mating surfaces and seals so the your Evo intake doesn't have to be perfect.

If tight on money then shaving the heads it is, if some spare coin comes up then Joes compression ratio of 9.5-1 is better, your 3010 cam will also work with that and should be a torque monster compared to stock. Evo's respond to compression very well, even better than a Twinkie

 

IMHO the best place to raise compression is with pistons. The reason for using oversize ones is simply to remove wear to date from the cylinders and to match pistons and cylinders to each other. However Scott does have rather more experience in these matters!

 

Thanks for all the opinions guys. I am going to think on this and go back monday with a notebook and talk to the machinist again. I like the sounds of "torque monster"

 

Best return on cash for waking up an EVO is a good cam and Head work.
That has been well know in the EVO world for a long time.

 

Smitty, when you say headwork, just exactly what are you talking about? A competition valve job, porting, blending, or do you mean head resurfacing too?

 

Piston replacement advantages:

Evo's @ stock 8.5-1 are slugs, any compression bump helps but 9.5 to 10.0-1 is a target with a torque style cam which is intake valve closing @ 38-40 and 10.0-1 being more touchy with the 40 intake closing but rewarding. The 3010 is at 39 intake closing so it can run up in compression with 9.5-1 being safe for all conditions and won't have to worry about fine tuned timing and carb settings as it will more forgiving than @ 10-1 which is looking for all out giddy up but nice when dialed in.

If done right bores will be round and perfect ring seal for optimal performance and no oil useage

9.5-1 pistons have a small dome and not as much of a problem for hot spots to cause detonation and a .030 head gasket can finish off calculated compression to 9.8-1 range if the builder desires more compresson

Newfound power that can cause erections during throttle roll-on and eliminates any need for Viagra


Disadvantages:

Larger domes can cause hot spots to speed up the detonation process

Extra rotating weight but more of a disadvantage to race and high rpm engines

Too much compression, wrong cam selection can open up octane issues and detonation

Added costs


If you want to learn about numbers, cam selection, compression ratio and how it is affected, attached the best calculater going. You can see how cam intake closing affects compression, head gasket thickness and just plain out neat to play with

http://bigboyzcycles.com/EVOComp.htm

You can put the biggest baddest set of flowing heads on a 8.5-1 stock compression Evo and will have a slug with a set of flowing heads, has to have compression to ignite gases pulled into the cylinders. Ev 27's, Woods W6 works in stock engines because they build compression by their designed intake closing and can ignite the extra fuel being pulled in by the larger valve lift but let them work with more compression starting out and efficiency comes in. Either way you go, bump the compression.