Low compression repair options

[Redcardbob]

The only problem that I can see with this forum, is that it is so robust that it can sometimes be hard to find an answer to a specific question!

I have an '85 that I am slowly upgrading for 50/50 drag and street use. (By the way, I live in Denver at 5800ft) I've got the HD Clutch mod, Morleys Muscle Jet kit and airbox, a VooDoo pipe, the HD oil kit on the way, and I am considering adding a 40 shot of nitrous. As-is, I am running about 12sec in the 1/4 (again, altitude) but I know there should be more speed. I ran a compression test, and am only getting about 90 on all 4 cyls. After a squirt of oil into each hole, it goes up to about 115, which is still far short of the altitude-corrected spec I see.
Unless I'm wrong, I see at least a teardown, hone, and re-ring in my future.
So, here's my question: Is there a stock bore piston available that is higher compression? I could do a hone and replace pistons and rings a lot cheaper than a complete bore job, but would there be a worthwhile gain? Running race gas, or octane booster isn't a problem.
Of course, once I tear it down and look at the bores, I may not have the hone option.
Appreciate any thoughts on this. I've done a lot of SBC work in my life, but this v4 is all new to me!
Thanks!

 

[Radioguylogs]

Did you measure your compression with the throttle held wide open?

If not, that might be a reason why your readings are lower than expected.

 

[02GF74]

as I read it there are 2 separate questions:
1. measured engine compression ratio is low, can I get away with fixing it on the cheap (hone and new rings)?
2. my drag strip times are slow, can I get more power with high CR pistons?

According to Haynes manual, to get at the bores, for honing or rebore, requires taking the engine out. I've never done this but expect it is not a quick job so if you were to do it, then instead of just doing the hone/new rings in the hope that CR will improve, you may as well go for a rebore in case the CR does not improve, so would avoid having to remove engine again plus the associated costs in gaskets (not cheap).... and you end up killing goth birds with the same stone.

There are pistons for large capacities and possible higher CR (read up how far others have gone); first port of call would be Sean Morely.

 

[MaxMidnight]

Before tearing it down I'd try one of the oil/ engine cleaning treatments to make sure that the issue isn't gummed up rings.
Not sure if it is available in the US but have seen good results with Forte Oil Treatment

 

[Stephan]

As mentioned above, the obvious point is to make sure that the throttle is wide open when running the test and keep the engine turning over until the needle does not go any further round the dial. (Assuming analogue kit here!) It is possible to take the front cylinder head off (just!) and therefore see the front cylinder bores by removing the front engine mounts and the upper rear engine mount, slackening the lower rear engine mount then pivoting the engine backwards in the frame. Not easy and not much easier than taking the engine out, but getting the engine back in can be a nightmare. I would say that a valve regrind would be just as necessary as honing or replacing rings.

 

[maleko89]

Think about doing a leak down to make sure valves aren't losing compression. For compression test, engine needs to be warmed up.

 

[Redcardbob]

Thanks for your input everyone!
I did have the throttle wide open when I was testing, and all the cylinders were even at 90. I could do a leak down to verify what's going on, but with a couple squirts of oil in the cylinders, the compression jumped up to about 115, which if I remember right tells me I have a ring sealing problem.
Engine sat for about a year before I bought it, and I have been through the carbs, but I never thought about possible sticking rings. I think the chances of all four cylinders sticking would be slim, but it might be worth a try as Maxmidnight suggested before I tear it down.
One other possibility: has anybody ever seen a compression tester that was way off on its readings? This is a harbor freight unit...

 

[MaxMidnight]

Regardless of the accuracy of the tester there is still an issue with the rings/ bores if you can gain 25 p.s.i. by adding some oil into the bores.
That still wouldn't exclude a valve seat issue though, which a leak down test would highlight.

 

[Radioguylogs]

I use a $10 Harbor Fright gauge, and it is reasonably accurate, although it is possible your copy might be defective. Maybe you want to test it on something else, like the car or the lawnmower?

I wonder if your Vmax would be hard to start if the compression was only 90 psi.

 

[Parminio]

The only problem that I can see with this forum, is that it is so robust that it can sometimes be hard to find an answer to a specific question!

I have an '85 that I am slowly upgrading for 50/50 drag and street use. (By the way, I live in Denver at 5800ft) I've got the HD Clutch mod, Morleys Muscle Jet kit and airbox, a VooDoo pipe, the HD oil kit on the way, and I am considering adding a 40 shot of nitrous. As-is, I am running about 12sec in the 1/4 (again, altitude) but I know there should be more speed. I ran a compression test, and am only getting about 90 on all 4 cyls. After a squirt of oil into each hole, it goes up to about 115, which is still far short of the altitude-corrected spec I see.
Unless I'm wrong, I see at least a teardown, hone, and re-ring in my future.
So, here's my question: Is there a stock bore piston available that is higher compression? I could do a hone and replace pistons and rings a lot cheaper than a complete bore job, but would there be a worthwhile gain? Running race gas, or octane booster isn't a problem.
Of course, once I tear it down and look at the bores, I may not have the hone option.
Appreciate any thoughts on this. I've done a lot of SBC work in my life, but this v4 is all new to me!
Thanks!

If I were even going to entertain the thought of all of that, I'd just bite the bullet, call Sean Morley, or email him at [email protected], and talk to him about getting building you a 1500.

 

[one2dmax]

Remember that these are vacuum sensitive slides. You need to prop them open to ensure full airflow. I don't really think you're going to have much of a chance in number but if the cylinders are fairly even across them all then likely you have a good engine.

 

[sdt354]

Remember that these are vacuum sensitive slides. You need to prop them open to ensure full airflow. I don't really think you're going to have much of a chance in number but if the cylinders are fairly even across them all then likely you have a good engine.

Thanks fofr that Sean, it's easy to overlook, the slides that is.

 

[Redcardbob]

Remember that these are vacuum sensitive slides. You need to prop them open to ensure full airflow. I don't really think you're going to have much of a chance in number but if the cylinders are fairly even across them all then likely you have a good engine.

That's a great point that i overlooked!
If i get a bump by putting oil in the cyls, wouldn't that indicate a ring issue? 115 with a shot of oil seems low, even without propping the slides open.

 

[Redcardbob]

If I were even going to entertain the thought of all of that, I'd just bite the bullet, call Sean Morley, or email him at [email protected], and talk to him about getting building you a 1500.

If that pesky budget issue wasn't always a factor, i'd already be waiting that check!

 

[Redcardbob]

I use a $10 Harbor Fright gauge, and it is reasonably accurate, although it is possible your copy might be defective. Maybe you want to test it on something else, like the car or the lawnmower?

I wonder if your Vmax would be hard to start if the compression was only 90 psi.

Good point, I'll test it out on another engine. It fires up fast, and seems to run strong. No smoke, etc.

 

[Redcardbob]

Did you measure your compression with the throttle held wide open?

If not, that might be a reason why your readings are lower than expected.

I did, but i didn't prop the slides open as Sean suggested. That's next!

 

[maleko89]

I’ve never propped the slides. Just open throttle all the way, charged battery and warm engine is all that Yamaha manual talks about.

 

[Redcardbob]

I’ve never propped the slides. Just open throttle all the way, charged battery and warm engine is all that Yamaha manual talks about.

That's how i did it, but it's definitely worth a try Seans way before a teardown!

 

[maleko89]

You can but the readings won’t mean anything since Yamaha has a range of readings based on not doing that.

 

[Radioguylogs]

I’ve never propped the slides. Just open throttle all the way, charged battery and warm engine is all that Yamaha manual talks about.

Same here.