Compression Question

[NitroTate]

Ok so I discussed this with the guys I rode with over memorial but wanted to throw the question out to everyone else.

 

Keep in mind shee runs great right now but I'm wondering if I'm actually causing a little decrease in performance due to the following or not?

 

I have 17cc domes in my Pro Design Cool head right now and I'm at 160psi in both cylinders @ elevation of 5280 (riding elevation is different).

 

I'm only running 91 octane pump gas normally. ($$$$) Am I missing out on some potential performance that I may be able to gain back by putting my 20cc domes back in?

 

Here's my mods and conditions:

 

Stock stroke

Medium port job with worm holes

stock carbs

dual K&N pods

T5's

170 mains

+4 timing

Avg 70 deg outside temp.

Riding Elevation: 7300'

 

Thanks guys

Tate

[Animalman294]

I'd be runnin' 50/50 if were me at 160psi. Better to spend a few extra dollars and be safe than have to spend a lot on tore up parts. Listen to the motor that right there and the plugs will tell you what you need to know.............

[jbooker82]

I would say your fine with 160 psi. Most builders shoot for 180 psi when running straight race fuel. You start getting over 180 psi that becomes excessive compression.

[C&B]

I'd be runnin' 50/50 if were me at 160psi. Better to spend a few extra dollars and be safe than have to spend a lot on tore up parts. Listen to the motor that right there and the plugs will tell you what you need to know.............

Same here. ran the same comp for years. Now Im 180 plus, so its straight race fuel (fuck). youll be good to go, no detonation which is good.

[okbeast]

Are you really running 170 mains in stock carbs? Surely that's a typo.

[blowit]

I do not see any concerns with 160psi comp with the +4 timing and 91oct fuel. I would not sweat it, especially with the avg ambient temp.

 

 

 

 

Brandon

[burleybanshee]

You should be fine run 91, every engine is slightly diffeent. I have 165 and she does not have an issue at 400 ft elevation. When i go to pismo, sea level and cooler climate, i run 50/50, just because of the compression rising, want to be safe.

 

You are not losing out for 160psi, if you are real high, well above 180, then the motor doesn't want to rev out as quickly, that would be the only performance disadvantage, but greater response out of the hole. trade off.

[gregrob]

Tate, would be worth pulling your head and checking your domes for any signs of detonation.

 

If there are no signs of detonation on the domes, and it doesnt try to back the domes out you will probably be fine on good old 91.

 

It's one of those border line things where it's better to be safe than sorry for me, but every bike is different and the only way to tell is to run it see what happens.

 

And yes that must be a typo, pretty sure he meant 270 jets... About right for our elevation, that's what I was running on my stock stroke with T-5's, etc...

[NitroTate]

Yep, sorry guys didn't notice the typo... it's definitely 270's.

 

And yeah I planned on pulling the domes this weekend to check em out. That's kind of why I threw the question out there so that while I have them apart I could change if it seems necessary.

 

Thanks for all the input guys!!

 

Glad to know I'm usually thinking on the same track as most others.... Kinda show's ya that you might just know somethin' :biggrin:

[FireHead]

I would say your fine with 160 psi. Most builders shoot for 180 psi when running straight race fuel. You start getting over 180 psi that becomes excessive compression.

Word. Just make sure you are checking your plugs for detonation errosion every once in awhile. :geek:

[NitroTate]

Word. Just make sure you are checking your plugs for detonation errosion every once in awhile. :geek:

 

 

Yeah I do, thanks, and I'm probably going to just try getting some race fuel now and then anyway and mix 50/50. I wish I could run race gas all the time :ermm: My bike really wakes up when I do...