[Hondaromper]

I've been lurking around for a couple months now and after reading just about everything I could find about porting the stock grom head I've seen no mention about recutting the valve/seat and blending the seat/throat/bowl.

I haven't been inside a grom head yet but I've been in over 60 holes (dont judge) and had a couple of 8v vw heads that were done professionally, and every single one of them needed work between the back of the valve and the stem guide. For the most part the valve seats are just pressed in at the factory, pressure tested and sent out the door, which usually leaves an appreciable ridge where the valve seat meets the head. Has honda addressed this issue for the grom? Ive "ported" 2 crf70 heads and they both needed the seats blended into the throat and tidied up behind the stem guide.

Also, why no mention of 3 or 5 angle grinds or radius cutting the exhaust valves?

 

[Justin V]

No shops near me will get valves. I resorted to my own porting and cleaned up the bowl throat and tried to remove the least amount of material. Opened up the port to about 25mm to suit my throttle.

 

[Hondaromper]

How badly was the seat protruding into the throat? Did you have any other mods you were performing at the time? Were the gains worthwhile?

I'm just surprised I haven't seen any mention of this on these forums yet.. In the small block world the single biggest gains to be had aside from a properly flow tested "port job" (raised port floors etc.) is having the angle grind done on the valves and seats and then blending the seat into the throat of the port.

It would be great to find someone with the tooling to cut the stock valves and seats. Seeing as my old machinist charged me $10 a hole to cut them both, its a very cost effective way of seeing gains across the entire rev range

 

[Justin V]

I don't recall the protrusion. It was just an abrupt or poorly angled seam.

I did a bit backwards and pulled my cam and reinstalled it with stock (sold aftermarket cam). I ran a few auto tune rides with a mismatched inlet and throttle that probably didn't help. I could hit 58mph. Used to struggle into the wind to go 45 and topped at 55 with a hindle and panel filter.

I put a different cam in, but also did a bit more so it won't be that great of a comparison. There's a couple shots of the port work on my build.

 

[Hondaromper]

Thanks for your input Justin V, I will definitely be looking into your build!!

Based on the 2 crf70 heads that I did there was definitely an appreciable ridge on the backside of the seat (I'd guess about to 1/16th of an inch on the worse of the two), so theres definitely room for improvement without altering the shape or size of the port. The one of those heads was ran with a stock camshaft and the gains were definitely worthwhile imo.

I had an 89 vw fox (1.8l 8v) that I did a basic street job on, 3 angle grind on a small valve head with a bowl blend and very conservative gasket matching. With a mild camshaft that engine would pull hard to 7k rpm, which is 1k rpm higher than it would rev stock with the same camshaft. No loss in low end power whatsoever, throttle response and fuel economy also improved significantly.

 

[Whorider]

I ported my head. My head had a void on the backside of the exhaust bowl. I didn’t fill the void, just blended as best I could. I had my valves seats reground to 5 angle. Stock valve seat grind on mine was garbage.

 

[Whorider]

a little port porn

 

[Hondaromper]

Nice work whorider! Do you have a flow bench? Also, do you have the dimensions used for the 5 angle grind? Whats your setup and how did it work out for you?

I believe you when you say your seats were shit. Can't say ive ever seen anyone get gains from lapping their valves alone until i came on this forum, so it would appear to me that honda left a lot on the table when it came to finishing the head on the grom.

I wish I had the time to build a flow bench and do some experimenting with cleaning up and altering the short turn radius (flatten the port floor leading into the short turn radius creating a low pressure area) among other things. But for now I'm going to have to depend on the kindness of strangers and just see whats working for others.

At this time my plan is to do as much as possible with what I have before I start digging deep into my pockets (xmas is coming). So I'll definitely be conservatively porting the stock head and intake and probably just run an efie and see how it all works out. I'd love to get a cam and reflash from either DHM or CJR but I'm not positive that either one of those tunes will be able to accurately factor in the changes from the head work.. PCV with an auto tune is getting a little beyond my price point for a tuner for now, but seems to be my best option.

 

[Ninelives17]

Most people seem to recommend an Aracer setup over the PCV setup. Might want to look into it when you get to that point.

 

[Whorider]

I went with a CJR reflash, it keeps up with the ported head just fine. I had a machine shop do the valve seat work. I don’t have a flow bench, I’ve just cleaned the port bowls and flattened the short sides along with port matching.

 

[Whorider]

The tune Cameron had closest to my build was as follows.
Koso 170
Koso camshaft
Yoshimura exhaust
Chimera intake
I have a milder Kitaco type 1 cam and Takegawa STD exhaust. I’m happy with the performance I’m getting. No hard HP data or Gps speed data. It does roll on second gear wheelies geared one tooth down on the counter shaft sprocket. Which is all i wanted. 😎

 

[Hondaromper]

What rpm does your kitaco cam and ported head breath to? Good to hear that your reflash worked out! It definitely seems like the way to go.

 

[Whorider]

I don’t have a tachometer, it’s definitely a mid range build though. I don’t spend much time wound out. It will pull a decent grade keeping up with freeway traffic. I should get a tach, lame it’s not stock.

 

[Hondaromper]

Good to know, thanks! If you ever found out what the angles were that were used to cut the valve seats that could be some valuable info for some of us!!

 

[Hondaromper]

I got good news and bad news. The good news is I've gone through my 2018 groms head and reinstalled it reusing the stock head gasket with great success! The bad news is I also installed a dhm reflashed ecu with a tb cam and a $120 amazon exhaust, so i can't truthfully say how much I gained from the head work alone..

The intake port shape was decent, notably the floor. The valve seat alignment wasn't too bad, however the casting of the head seemed to flair outwards (parallel to the short turn radius) where it met the seat, which left an appreciable void. I tried my best to blend the seat in but the void was too deep and I was at my limit for blending the seat, so I left it as is for now. I'll either need to add material behind the seat to blend in the throat or have larger valves put in it to fix the issue, but that will be for another day. For the rest of the intake port I merely tidied up some of the casting and removed a small amount of material from the roof of the port to match it up with the insulator gasket, doing my best to maintain the shape of the port, thats it.

Exhaust side needed some serious work between the seat and the short turn radius but otherwise wasnt too bad. Once again I cleaned up some of the bad casting and tried not to alter the shape of the port.

The intake manifold on the other hand was very nicely finished, so I left it alone.

Exhaust is a high mounted amazon header with a 2" straight thru shorty muffler. Running stock airfilter with "airbox mod" and the stock inlet pipe.

It warmed up to 4c yesterday so I took it for a little shakedown run and was surprised to say the least! Bottom end power was down marginally, but I only rode it for about 10km so the ecu still has some learning to do. But that top end! Starts to pick up solidly around 6k and is making good power by 7k. 4th gear feels like it has more snort than 3rd on a stock bike, it actually feels adequate now!! I tried to keep the revs below 9k, but I couldn't help it! Full tuck with 14/34 gearing on flat ground with very little (if any) wind i was able to hit an indicated 126kmh before i backed off on the throttle. Much better than my previous record of 106kmh drafting my wife's car! Now I can cruise comfortably at an indicated 106kmh sitting upright! It will be interesting to see what kind of gains an upgraded intake will make now..

Now I know that the cam and reflash played a huge part in this outcome, but for anyone who's wondering whether or not to "port" their stock head on a more or less stock bike I'd say go for it! This is a mass produced head that isn't made to high standards and there is a lot to be gained from simply cleaning up the casting flaws left by the factory. A couple of hours, some valve lapping compound and a dremel is all you need! Just remember to be conservative! And don't touch your bloody valve seats with the dremel!!

 

[Ninelives17]

The DHM hi-comp piston will get back all your bottom end and more!

 

[Hondaromper]

The DHM hi-comp piston will get back all your bottom end and more!

I've considered it, but my last daily ride was a wr250r with a 290cc BBK that was running 13.2:1 compression, and getting decent fuel where I live is a pain. I basically couldn't ride farther than 50km from my house without carrying extra fuel with me! So the idea of running 87 octane is very appealing.

I might consider a 143cc takegawa cylinder in the future. They claim that their E-stage kit runs on 87 just fine, and it shouldn't need an oil cooler either! But for now I'm just gonna cross my fingers and hope I can start riding sooner than later!! 2 weeks ago it was -49c here lol

 

[Whorider]

I run my KOSO 170cc build on 87 octane with no detonation issues. That TB can is a pretty aggressive cam. I’d be curious how the bottom end felt with the stock cam.

 

[Hondaromper]

I run my KOSO 170cc build on 87 octane with no detonation issues. That TB can is a pretty aggressive cam. I’d be curious how the bottom end felt with the stock cam.

What compression ratio is your 170 and are you running an oil cooler?

If i get the time this summer I'd like to throw the stock cam and ecu back in it just to see how much of a difference the headwork made. Any other head that I've done similar work to always picked up a decent amount of power across the entire rev range and also notably better throttle response and fuel mileage..

I wonder if anyone makes an adjustable cam gear for the grom?🤔

 

[shush319]

The DHM hi-comp piston will get back all your bottom end and more!

Man, I've been holding off installing mine, but now I may have to. Been sitting in the closet for probably half a year now. Hope it holds up to a high revving build......