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10.5 CR with boost on 91 pump gas?

Posted: Thu Sep 13, 2012 2:12 pm
by MalcolmV8
Anyone have any good experience with octane and compression ratios in motors? This relates to my turbo'd civic I built. Stock it's 9.6:1 CR and runs pretty good. It was rated for 87 pump gas stock. I'm running 10 lbs of boost on it with 91 octane (premium in KC) and no issues. However I'd like to put forged rods and pistons in there so it never lets go as well as have the option of running a little more boost in the future if I want.

Ideally I would have liked to stick with the 9.6:1 I have now but in the aftermarket all I find is either 9.0:1 or 10.5:1 for my motor. I'd really like to avoid dropping the CR due to decreased mpg and throttle response. Especially on such a small motor. Maybe I'm over thinking this and 9.0:1 won't be a big deal at all.

10.5:1 sounds like a blast and I've heard from some others that have done similar that it works great. However I've also had a lot of people caution me saying I'll have to pull so much timing to run that 10.5:1 with 91 pump gas it'll be a turd and perform so much better if I went with 9.0:1 and added more boost and timing. Humm tuff call.

Any opinions or experience?

Re: 10.5 CR with boost on 91 pump gas?

Posted: Fri Sep 14, 2012 6:45 am
by cgrey8
I've got no experience with that. But if your options are 9 or 10.5 and all you plan for is a piston change, then it seems your options are to drop to 9. However you might be able to get some of that back by having the block decked to push the pistons out of the hole a tad and/or run a thinner head gasket. By how much this would help would need to be run through the calculations. I have no clue what the aftermarket options are for Honda. Is there any chance they have aftermarket cams? If so, that's another place to look for ways to move the dynamic compression around a bit. Although to know where to go, you need to know where you are at. Any chance you have access to your engine's cam specs and other pertinent compression ratio data? I use the DCR calculator posted on this site:

It has a number of various engine details that if you can figure them out will at least tell you where you are today...assuming you can get access to all the info it needs.

I think I've posted this up somewhere else, but I'll post these instructions up again:
DCR Usage_Step1.jpg
DCR Usage_Step2.jpg
DCR Usage_Step3.jpg
And here's a rather conservative rule of thumb about DCR:

General rule of thumb is 87 octane tops out at 7.9 DCR and that's in detonation-resistant engines that aren't boosted.
93 octane (Georgia's premium octane) can run up to about 8.5 DCR...again naturally aspirated detonation-resistant engines.
So I'm assuming 91 octane might get to 8.2 DCR?

If you have a slow flame front engine, little-to-no quench, an engine that just tends to detonate, or are planning to run higher boost levels, then you'll want to stay below the numbers.

So again, knowing where you are at now with the engine will likely be your best indicator as to what your options are.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Fri Sep 14, 2012 8:46 am
by MalcolmV8
Chris, there are a number of cams available but it gets more complex because the cams have two sets of profiles they can run and switch between with vtec. It's a pretty slick setup really in that you can keep the first cam profile mild like stock and get great drive ability and idle etc but when you get on it hard at a certain RPM and load it'll flip cam profiles and give you the performance you desire. That might actually be some for me to look into because if I have vtec kick in right around when boost kicks in I could drop my dynamic CR.

Finding specs though on this car has been somewhat of a challenge and me probably not been into Hondas like I am Fords probably doesn't help although I have to admit it's been a fun project that's fun to drive.

I'd like to avoid decking the block because of cam timing and from everything I read on the honda forums they say stick with the OEM head gasket. Apparently a lot of guys have issues with aftermarkets blowing out but like anything your mileage will vary.

I will see what I can find out as far as specs on the engine go.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Fri Sep 14, 2012 11:17 am
by MalcolmV8
Chris, I checked around and called a few places on cam options and it seems all of them have a more aggressive non vtec profile that while they claim is still very streetable it has a lopy idle. I'm not into that. I pretty much hate aggressive cams on a daily driver. I wished I could have found someone that made a stock profile on the non vtec part of the cam and then a really aggressive profile on vtec but not finding it with the little searching I did. Also their cams require stiffer springs and new valve guides etc. A lot more than I'm wanting to do for a daily driver with some pep to it.

So instead I'm thinking either just 9.0 and have fun or do the 10.5 pistons and use water/meth injection. After using it on my Cobra and the GT500 I just recently built I'm sold on it. It's super cheap to run and works fantastic. Cools in the intake charge, bumps octane like crazy and is cheap. It's hard to beat. My Cobra is running 20 lbs of boost on 91 pump octane with it and runs fantastic.

My reasons for really wanting to run the 10.5:1 is because of increased efficiency out of boost. I'll have better gas mileage while just cruising and also the throttle response will be so much better and it'll spool into boost much more responsively too. I think it'll make a really nice street car that way. I REALLY wish I could test drive a 9.0 and 10.5 combo on my car first and make a decision lol.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Fri Sep 14, 2012 3:43 pm
by cgrey8
I forgot all about the variable DCR capability of Hondas. And I too don't know how you would determine "effective" cam events at mildest to wildest.

But thanks to the variable characteristic, you might just get away with 10.5 SCR. And if installing meth injection is an option for you, then the 10.5 option is a no brainer over the 9.0...particularly if you can also retune to loosen the DCR even at cruising conditions in an effort to get the DCR lower at those lower load conditions if detonation becomes a problem. But if others are happy with the results, then don't hesitate.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Sat Sep 15, 2012 12:35 am
by MalcolmV8
Well I went back and forth on this a lot and just couldn't decide. I had some users report no improvements going to 10.5:1 in terms of low end power or throttle response and a lot of those for 10.5 had 93 or a different motor so it left a lot up in the air as to what I'd actually experience. I did notice a common theme of people complaining about tuning been harder and a lot less forging with the 10.5. Probably because getting timing just right is a lot more critical.
So finally I just made the safe choice and ordered 9.0:1. It's what the majority run and every high performance shop I called said 10.5:1 are for N/A builds, not boost. While I'm sure I could have made it work with water/meth injection I'm thinking I need to draw the line somewhere. This is how my daily driver's end up been 600 + whp monsters lol.

Going to pull the motor this weekend and tear down. New pistons and rods arrive Wednesday so hopefully Thursday it'll be in the machine shop. They said two days to bore it out, surface head and notch block and girdle for the aftermarket rods. We'll see how it goes. I'll keep you posted.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Sun Sep 16, 2012 9:12 am
by MalcolmV8
Well it looks like I'll be sending these pistons back and exchanging them. I was so caught up in compression ratio I opted for the stronger 2618 alloy not thinking. I need to send them back and get a 4032 alloy piston. Why?
The 4032 has a higher silicon content which has less thermal expansion allowing you to run tighter piston to wall clearances. This means no piston slap and less blow by.
The 2618 is no doubt stronger, 12% more tensile strength, more malleable and less likely to have catastrophic failure from cracks. However since they expand more than a 4032 alloy piston you need to run a larger piston to wall clearance which results in piston slap on cold start and more blow by.

The blow by I could live with but piston slap every cold start on a daily driver will not work for me. I run a 2618 alloy piston in my Cobra and I had the machine shop go in the middle of the range on piston to wall clearances to try and get a good compromise between performance and no piston slap. For the first few thousand miles it seemed like it worked too as I had no piston slap on cold start up but once the motor broke in and loosened up you do hear it on cold start. I'm just thankful it's very quite and hard to hear but it's still there and annoying. I can't tell you how many times I get the "oh dang your motor's got a tick" on cold start lol.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Mon Sep 17, 2012 9:21 am
by cgrey8
The machinist told me the new technology in forged pistons allows for forged pistons to be used with tighter clearance. He too was saying bore clearances in the .003-.004" range was typical with these new forged pistons. In fact, he said they make pistons so the machinists don't have to custom size the bore to the piston like they used to. Now you bore the block to whatever even size (i.e. .030" over) and you buy the piston for the size bore where the piston mfg has turned the piston to the right diameter for that standard size bore.

Although I swear the looseness of my pistons when I was doing the deck height measurements seemed far greater than what I considered a .003-.004" clearance to be. I didn't think to actually use the calipers on the block and pistons just to see what the actual diameters were. So I unfortunately expect I'll have a fair amount of piston slap until the engine warms up.

Re: 10.5 CR with boost on 91 pump gas?

Posted: Mon Sep 17, 2012 9:48 am
by MalcolmV8
Ah that's a bummer. As you say piston slap is not a requirement these days but it does depend on your application. My Cobra's engine for example is running so much boost and produces so much heat that piston expands a lot. In fact the reason for my rebuild was I got the original pistons so hot they expanded to much and scuffed themselves and the cylinder walls a tad. Granted it took a lot of abuse that day and was doing 0-160 and back to 0 back to back runs. After approximately 10 back to back runs I had a break in runs and when I fired it backup I had a ticking noise.

So that's when I went with the 2618 alloy and looser piston to wall clearance and never had a problem since. In this instance some minor piston slap at start up is a reality of this setup.

You are right though in that the tolerances are so much better now days. When the machine shop bored my cobra's block and did the work I asked them if the pistons where numbered and for a specific cylinder and he said no. He measured all the pistons to be sure and they are close enough that he only bored one size on all cylinders. He did however tell me not to mix the rods and pistons as he had carefully balanced each pair. He also gave me a spec chart of all the weights and balances so I could reproduce that engine or parts of it in the future should I ever need to.

For this civic motor though I don't want piston slap at all. I know it's just a picky thing and personal preference but man to me it sounds like a broken motor when it starts up. Like a car with bad lifers when you fire it up. It drives me nuts.