I'm just days away from building an 80" Evo.... I have the parts to build a stock engine, a 9.5:1 engine or a 10.5:1 engine. I have stock heads and "Head-Quarters" ported+milled heads. What I would like to know is what are some pro/cons of a high compression EVO?????

Hard starting?
Idle rough?
can I run 93 gas?
will it run hot?

Thanks for any info!!

P.S. it's a 93 fxdwg

 

On my '97 WG, I have: SE Heads, Andrews EV-27 cams, SE A/C w/ Heavy Breather, HD Adjustable Pushrods, Thunderheader, previously with CV-40 carb with different Jets and I did not have a problem with - hard starting, a rough idle, Yes - I run 93 gas mostly and it did not appear to run hot. I believe compression wound being ~ 9.5:1.

I have also since did the Diamond Cutting to my Cylinders and Heads. Also have changed to a, for now, S&S A/C and a Big PSI Carb. Have not gone to the tuner yet.

Hope this helps.

 

I have 10.1 compression ratio. I installed compression releases for starting. It takes a big load off the starter. If you go with 10.5, I would recommend compression releases or a tougher starter.

 

Glad you mentioned the starter...after I had work done on the bike, they did recommend going to a bigger starter as mine was having problems.

 

It is a bear to start without the releases. It bucks something fierce if they aren't engaged.

The benefit to having higher compression is that it gives you more consistent torque through the gear. Stock EVOs stop pulling at 3500 RPM - the higher compression will get you to 6000 easily.

As an after-effect, the exhaust "pops" better - not backfire . . . more like a top fuel racer.

 

I guess it depends on the gas in your area. I talked to a guy the other day that has a 10.5:1 Evo. He said his pings pretty bad on 93 octane, and he puts octane booster in it if he's going to run it hard. But, all the stations around here have that 10% Ethanol crap.

My Evo's 9.5:1, and I've had no issues. Maybe you could go in the middle and get some 10:1 pistons? I would probably get compression releases if you go anything over 9.5:1.

 

Thanks for all the info guys, I think I'm going to shoot for the middle about 9.5:1. I believe my stock flattops with my "head-quarter" ported and milled heads with the right gaskets will land me there or somewhere near.

 

What cam are you running and how many cc's are your heads?

 

what he said

 

I'm looking at the HQ 0029 cam, and I have not cc'd the heads yet. they are stock heads milled .080"
The cam is from head-quaters.com