99 head swap and compression
 

Hi all, I'm currently rebuilding a 99 head and intake to fit on my 1.8 mk1 (94) as I know that there are gains to be made from the head as it flows much freer than the stock head/intake. After this I will be making a choice as to whether I should stay NA or go turbo (for drift).

My question is, what does a 99 head swap do to the compression figures? Does it raise or lower compression of the engine? (assuming stock head gasket is used)

Also would like to hear from anyone whos turbo'd with a 99 head swap, was it worth while?

Welcome aboard seal. You should've taken the time to read the FAQs and do a search. You would've found the answers to all your questions.

FWIW, a 99 head swap is common and it sure means much better figures all around.

Now; please post your location below your avatar.

Thanks Rafta. I've done a lot of searching on this on many different miata forums but have never found the answer, theres a lot of talk of 99 head swaps, and a lot of separate talk about compression ratios but I have never found a straighforward answer of the compression ratio of a 99 head on a 94, 1.8 engine. (which is what i'm hoping to find out)... unless my search skills are that bad!

true, i dunno if ive ever heard that either. Is the compression difference due to piston differences in the motor or less quench area due to head design? Is the head gasket the same thickness? The answer to these questions would help find the answer.

Its in the Pistons, they're slightly less concave than the earlier year models. if the gasket is any different, its probably only oil/water passages, doubt it matters even a minuscule amount if they changed the gasket size. I have a compression dropping head gasket for my Audi, but that bitch is like half an inch thick (not really) 2-3 mm iirc, so i dont think they would change compression with a gasket if they were already changing pistion, or, even if they weren't.

its too bad you missed the part where he suggested you PUT YOUR MOTHERFUCKING LOCATION IN YOUR GOD DAMNED PROFILE. Follow the fucking rules...please. :)

As for compression ratio...it supposedly stays unchanged but only patsmx5 knows the truth since he measure that shit before headwork.

Hustler did that swap just recently. Why don't you add your location details under your screen name, then ask him about it. He won't help you if you are in violation of standard protocol.

I didn't cc the head and measure the volume before and after. But I know what dimensions I changed, and I did some quick math a while back and figured that it reduced the static compression ratio from 9.5:1 to at least 8.5:1, maybe as low as 8:1.

+1 on all the advice given. The 94 vs. 99' head should have the same volume combustion chambers. So compression should stay the same. The pistons were changed in 99' to increase the compression. Turbo FTW.

I guess it is a valid question. Has someone ever cc'ed a 99 vs 94-97 to see if it alters compression, assuming the same bottom end? Me thinks it would be a negligible difference, if any, when strictly considering compression.

well, considering the pistons are different ratios, we'd have to measure to really know. I wonder if I'm even lower than 8.5:1 with the addition of my head?

I've seen both heads before, they "look" the same on the combustion chambers. 99' head has a slightly better angle of attack (not good, just less shitty). Diff spec cam w/ solid lifters on the 99', but other than that, they appear highly identical. If there is a difference, it's very small and largely insignificant.

No, I'm talking about what he is talking about (I think) that if he takes off his 94-97 head and puts on a 99, will the compression ratio be any different on the same bottom end The only difference will be if the head itself is a higher or lower volume in each cylinder which will alter compression ratio. Thus, my question if anyone has actually cc'ed a 94-97 and a 99 head. I'm guessing negliglble difference unless someone works the cylinder side somewhat, which will lower compression or mill the head, which will increase it. But, stock-for-stock, are they the same cc's?

EDIT: Ahh, Pat ninja'ed ahead of me. I figured they would be close.

correct me if im wrong but didn't they jump to a hemispherical design on 99+? so i've got a hemi? haha

99-00 head swap does make a power difference. went and drove a 95 with similiar mods as mine and it just seemed to lag behind. and my VICs isnt even working! haha

All miata heads are "hemi". VICS are normally open when not hooked up, so you won't see any more power hooking them up such that they stay closed until activated. Mine's unhooked so it just stays open all the time. Seems to work better on my setup with them open all the time.

Ah, i failed, i thought i was cool for a minute tho.

Stealth97 said he saw a 10ft-lb or so increase when he had VICS working, but he has a nice n/a build. What im stumped on is Artie's stock 99 manifold with butterflies in but not working and having more mid-range torque than the gutted 99 manifold. What caused it to have more tq if the butterflies were sitting wide-open vs not even there?

Because they hacked the shit out of that manifold on the gutted on. Look at the pics. They got that american in 'em that says "if 3 is good, 10 is better!", so they cut all kinds of metal out. It gets technical, but long and skinny runners helps low end, short and fat runners help top end. But they took skinny and fat to the extreme. They basically tuned the intake for 14K RPMs. And lost some low end because of it.

My motor makes more power from 4k+ with the VICS open. Whether it makes a difference below 4k, dunno, but it's not noticeable.

ah ok, gotcha. i figure ill just keep VICS in and maybe play with it post-boost. if i get all power happy ill just go for a BEGi style intake manifold.

I don't see the point in owning a miata if it doesn't have a 99 head.

Yep, absolutely right, although the point you make about cc'ing should also be valid and tell us the answer right? I'm guessing many people have cc'd a 94 bottom end so we'd need someone to tell us what the cc value is of a 99 head and we should be able to calculate the compression ratio with that right? (assuming we use a standard head gasket).

Anyway, the reason why i wanted to know is, that i wanted to find out if the 99 head swap makes the engine better for NA (by increasing compression) or better for turbo (by decreasing compression).

and apologies for the location; now added :)

Neither of the reasons above. It's better for both NA AND turbo applications because it FLOWS better. That's it in a nutshell.