Yamaha Heads

[nitrohydro2000]

Can someone tell me what the difference between 6g700, 6g701 (220) heads. Also does anyone know about how much compression a set of HT5250 heads would have? My highest hole now is around 120psi with 225 heads.

Thanks,
nitro

[mcyama]

That is a good question,I don't remember, hopefully someone will be able to tell both of us. I want too think that the 6g700 series were of a lower compression ratio. As for as the hydro tec heads give Wayne a call. So what 225 heads do you have 6k7 or 6r5? Hold up, something is coming back to me. The 6g700 heads are of equal CC's and the 6g701 cylinders 3 and 5 are of a greater CC. Sorry, let's wait to see what someone else may say. Thanks Martin

[nitrohydro2000]

Sorry, my heads are 6k700.

[mcyama]

K7, are good heads, just not as tight as g7. Thanks martin

[nitrohydro2000]

Thanks, I'm looking for a little better balance on compression than my k7's. I'm a little timid on the Hydrotec heads b/c too much compression means only good gas, 87 is high enough and hard to find with out at least 10% ethanol nowadays.

[Markus]

Thanks, I'm looking for a little better balance on compression than my k7's.

They are designed like that for a reason. Wayne, as well, always built Yamahas with lower compression on the bottom cylinders.

220 Special heads are supposed to be the best, followed by 225 Excel.

With 87 octane gas, you should run some loose heads, though.

[DoktorC]

With 87 octane gas, you should run some loose heads, though.

Ya...the 225/excel where designed to run on premium...

[nitrohydro2000]

Thanks everyone, Went with the 6K701's, will have to see how they work out.

Forgot to add the heads were HT-8250 Hydro-tec said they would be 155-160PSI. Too much for my fishin motor, might try it on my next powerhead, plan to build a horizontal later this year. Thanks Again!

[nitrohydro2000]

Installed the 6G701 Heads here are my compression numbers: 1-116, 2-116, 3-116, 4-119, 5-114, 6-117. I guess my question is it worth the money to cut the heads to get back to 130-135? The block is not worn out the exhaust ports are about .040 high. Thanks

[Markus]

Installed the 6G701 Heads here are my compression numbers: 1-116, 2-116, 3-116, 4-119, 5-114, 6-117. I guess my question is it worth the money to cut the heads to get back to 130-135? The block is not worn out the exhaust ports are about .040 high. Thanks

Definitely. Higher compression will give more power. Best value for money mod there is.

Go with lower compression on 5 and 6 than on the others, though.

[nitrohydro2000]

Who is the best to send the heads to? I have the setup a my fathers shop to cut them, but haven't cut any Yamaha heads, other than ATV's. Don't want to screw up a set or two learning, how much to cut (.050,.060,?)? If I do end up cutting a set, how much squash depth do I need to add back after cutting? I assume to lower the compression on the lower two cyl's you give them more squash depth, right? Also for a 15 to 20 PSI bump, how much would you increase jetting? I'm running 180's now. Might be easier to send them to someone that knows what they are doing.

[nitrohydro2000]

I have a set of HT-8200 hydro-tec heads a guy let me borrow. My compression is now around 140 haven't had time to check all cyl's, on a 25-30 lb. boost how much would you increase jet sizing? Is there a formula for this?

Thanks,
Nitro

[Mr. Demeanor]

I would just ask Hydrotech since its there head.

I have always been a little confused on how compression effects jetting. I dont understand how compression effects the amount of air an engine takes in?????

[nitrohydro2000]

Called Hydrotech they suggested 185's, I plan to drill mine and give it a run. Wish me luck.

Thanks,