Compression & Dome Size on Ported Cylinder

[Matt_E]

My new top-end is puzzling me somewhat.
It is a ported 61X big bore @ 84mm. I do not have the porting specs, but it certainly didn't look race-ported to me (but I didn't measure)
The bore is fresh.
I am using Wiseco pistons with 6 thou clearance and an ADA girdled head with 84mm 35cc domes, and B-pipe.
I am at 700ft elevation. I get 125 psi (maybe 130) cranking compression, even in both holes. Starter is just about new, spins the engine just fine.
The gauge, while cheap, works fine. For reference, I tested a stock 650SJ and get 150psi in both holes on that.

According to ADA charts, 35cc domes would yield approx 200psi on a stock 64x cylinder. So I am little off.:no:
I pulled the head after measuring the first time, because it seemed so low. The rings are good (brand-new) and so is the bore.
So my guess is that the porting is causing the low compression due to higher ports.
Now - the motor has very nice power. I really have no complaints, not even in the bottom end department. But, that could be because I don't know any better. The motor is new, so I don't know what it could be like with more compression.

I understand that cranking compression is not the only measure of compression.
To the builders: A cranking ratio would be more useful, right?
How is that calculated? I assume it's (volume above ports)/(dome volume)
Is that right?
What ratio am I shooting for to stay within premium pump gas range?
I run a stock ignition, advanced a couple degrees, dual 48 Full Spectrums with Dual Stage reeds in stock cages.
I do WOT runs.
I don't want to just slap smaller domes in there without fully understanding the consequences.
33cc domes seem to give me an additional 15psi, bringing me to 140.
Or 31cc domes, getting me to 160

 

[Big Kahuna]

if your compression is that low, then your port timing has been raised up some. When you raise the port, the area above the roof is reduced giving you less compression. If you had 38cc domes I would trade you my 33cc domes. I have 215 lbs at 250' above sealevel. (I think that is how far above sealevel we are, may be closer to 400')

 

[Matt_E]

I realize the effect that raised ports have on cranking (static) compression.
I am worried that my dynamic (running) compression is indeed much higher.
I wish not to blow this motor.
It doesn't feel like a race motor, not even at this low compression.

 

['Crockett]

My new top-end is puzzling me somewhat.
It is a ported 61X big bore @ 84mm. I do not have the porting specs, but it certainly didn't look race-ported to me (but I didn't measure)
The bore is fresh.
I am using Wiseco pistons with 6 thou clearance and an ADA girdled head with 84mm 35cc domes, and B-pipe.
I am at 700ft elevation. I get 125 psi (maybe 130) cranking compression, even in both holes. Starter is just about new, spins the engine just fine.
The gauge, while cheap, works fine. For reference, I tested a stock 650SJ and get 150psi in both holes on that.

According to ADA charts, 35cc domes would yield approx 200psi on a stock 64x cylinder. So I am little off.:no:
I pulled the head after measuring the first time, because it seemed so low. The rings are good (brand-new) and so is the bore.
So my guess is that the porting is causing the low compression due to higher ports.
Now - the motor has very nice power. I really have no complaints, not even in the bottom end department. But, that could be because I don't know any better. The motor is new, so I don't know what it could be like with more compression.

I understand that cranking compression is not the only measure of compression.
To the builders: A cranking ratio would be more useful, right?
How is that calculated? I assume it's (volume above ports)/(dome volume)
Is that right?
What ratio am I shooting for to stay within premium pump gas range?
I run a stock ignition, advanced a couple degrees, dual 48 Full Spectrums with Dual Stage reeds in stock cages.
I do WOT runs.
I don't want to just slap smaller domes in there without fully understanding the consequences.
33cc domes seem to give me an additional 15psi, bringing me to 140.
Or 31cc domes, getting me to 160

Matt,

You are using ADA's numbers for a 760 when what you actually have is a 61x 701 bored to 760. . . . . . I don't remember off hand, but if the 61x has a higher exhaust port than the 760 then that may be what it causing part of the confusion.

If 31cc gets you 160 I'd run them . . . . . 160 will give you more snap down low and is still well within what you can run with the crappy gas that's availible these days . . . . .

The domes in my ADA head I put on my SXR are about 10 lbs light over what ADA says they are supposed to be so that may be another part of the issue also . . . .

Add up different stock cylinder port height, altitude, port work and ADA's claimed numbers being slightly off and it would be easy to arrive at where you are . . . . . .

Compression is like jetting . . . . . it is, what it is . . . do what you have to do to get it where it needs to be . . . .

Just my $ .02 worth . . . . . good luck with it. . . .

:biggrin:

 

[Matt_E]

The X ports are lower, making for HIGHER compression than a true 760 cylinder.

 

[Mark44]

Matt what is your squish?

Mark44

 

[Big Kahuna]

Matt what is your squish?

Mark44

that also!

 

[Mark44]

I has an issue with low compression on my new ported motor with intended domes and to find out my squish was way off so I decked the cylinder.

Mark44

 

['Crockett]

The X ports are lower, making for HIGHER compression than a true 760 cylinder.

I knew the 61x ports were lower than the 62t 701, couldn't recall the stock height on the 760 . . . . sounds like the exhaust has been raised then.

Do you remember if the exhaust port "tab" in the sleeve had been removed or not ? If its been removed and the roof of the port was smooth and matched, then the port has been raised at least 1mm-1.5mm.

What does the squish band measure and are you running a stock base gasket thickness ? Any chance that cylinder has been re-sleeved ? . . . .


Jeez, there are so many possible varibles . . . .

I'm not claiming to be an expert, these are just the thoughts that are coming to mind based on the stuff I've built . . . . . . I'm just trying to get some thoughts on the table for you to consider, that's all . . . . .

 

[dbrutherford]

Compression ratio is:

volume at BDC / volume at TDC

On four strokes I was always told 11:1 for pump gas. Beats me though. I always went by 180 psi for 93 octane. But you already knew all that.

Good luck.

 

[Matt_E]

Do you remember if the exhaust port "tab" in the sleeve had been removed or not ? If its been removed and the roof of the port was smooth and matched, then the port has been raised at least 1mm-1.5mm.

This a 61X cylinder. As far as I know, it doesn't have the tabs you speak of.
Further, it has aftermarket bigbore sleeves installed - those don't have those tabs either.

What does the squish band measure and are you running a stock base gasket thickness ? Any chance that cylinder has been re-sleeved ? . . . .

Not sure on the squish, I'll measure tonight.
Base gasket is stock thickness.
Cylinder has been resleeved.

 

[yamanube]

This thread makes me a little more comfortable running 33cc domes on my BB. I have yet to install the flywheel and bendix but hope to get some compression numbers next week.....maybe?

 

[orangefinger]

Do you have any low end Power or does it come delayed ??

 

[Matt_E]

Pulls fine, it's not delayed. One of the better engines I've ran.

 

[STEELDOGG]

This a 61X cylinder. As far as I know, it doesn't have the tabs you speak of.
Further, it has aftermarket bigbore sleeves installed - those don't have those tabs either.



Not sure on the squish, I'll measure tonight.
Base gasket is stock thickness.
Cylinder has been resleeved.

Very similar to the cylinders I am running - X but with larger sleeves. I checked my compression yesterday @ 510 ft. I am getting 187-188ish on both holes with ADA 35cc domes. Don't know exactly what type of porting Tricky put on it, but i like it. With those compression numbers, you must have some wild azz porting. Good luck.

 

[SUPERTUNE]

Matt, All the replacement sleeves for the 61X that I have seen are 62t spec's, the exceptions are as follows: R&D dominator sleeves, northwest sleeves, and Randy's custom sleeves at Watcon. (la sleeve get you as they are a different part # but is the same sleeve as the 62t sleeves.
Quit being a hacker and spec out the port mapping, your squish clearances, then figure your port height in stroke distance, (need this to figure compression ratio) then pre-assemble engine @ TDC, grease piston and set head on and tighten up (don't need o-rings just seal with grease) Get a cheap BURET, filled with mineral spirts, and fill up combustion chamber and go up 3-4 threads up sparkplug hole. Read buret in cc's and calculate into displacement factor.

displacement + fill volume divided by fill volume = compression ratio!

 

[Matt_E]

Chuck,

I didn't port this one.
I bought it this way.

Squish is 45 thousandths.
Distance from top of exhaust port to deck is 1.470 inch, or 37.34mm. That seems to be a couple of mm higher than stock 61X porting, no?
I tried measuring the compressed volume, but I didn't have a pipette big enough. I used a 5ml syringe, but by the time I squirted four or five of those in, some had already leaked past the rings.
However, I did check dome volume in the following way: I put a spark plug all the way into the dome, turned it upside down, and filled the dome with water.
I got 40cc in to fill it all the way up, level with the actual sealing surface. I know that the piston crown occupies some of that space, so that number is probably no good.
I didn't have time or resources tonight to do a portmap, unfortunately.

 

[brentR]

185psi with 35cc domes @ 83mm bore here & lpw ported

 

[Matt_E]

^^ Those are 81mm domes.

 

[brentR]

i retract my stupidness