Hello and specs

[dnseal]

Hello all, new to the site, but reviewed enough to see that there is a boat load of valuable data here, so thanks and a "atta boy" to you all. My son and I just bought two xs400's, a 1977 for him and a 1981 for myself. Plan was cafe build and that may occur later but for now simple mods and ride. Both bikes are in excellent condition, in fact they are so nice its why were not chopping (back seat area) them up to make true cafe's.

I was looking around and found two advertised hp ratings for these bikes... 38 and 27, depending on which site you look at. It's not really important how much power they make as these are not intended for hold on speeds, but it would be nice to know how much hp they (77 and 81) have. Being that compression is about 115 lbs on the 77 it's also not worth the investment of carb and exhaust upgrades yet. Which leads me to my next question; would a simple re-jetting kit and free flow exhaust make a notable difference on a 140lb compression engine? Of course the best thing to do is rebuild upper half and get compression back close to specs and then do mods later, but riding for a while learning bike and creating a plan is immediate and most productive plan.


Be Safe
Gil

 

[thielb]

DOHC is 38 and SOHC is 27. thats my understanding but i may be wrong. Pretty sure though

 

[Geologic]

The 27 hp is on the German specific xs400s and not the US models. The US models were rated at 36hp

 

[dnseal]

Ask a question and get two answers.... Smile. Anybody else so we can average out the numbers.... Smile.

Thanks
Gil

 

[markjs]

The DOHC is 46HP I read.

 

[dnseal]

OOOoops the average is getting skewed.. anybody else hear, see, have input?

 

[markjs]

Well I know the 36 and 46 numbers for the SOHC and DOHC to be accurate. The SOHC is on Wikipedia and sourced, but unfortunately I cannot remember nor find where I found the DOHC numbers in a repeat search but I remember distinctly it being reputable and everyone knows an SOHC is an older less optimal type of engine design (barring all other considerations other than maximum performance), and it stands to reason that there would be a significant increase in performance with a higher tech design. Those numbers at least for US models are accurate.

Some sources rate the SOHC at 38, but that could be a slightly different variable and a separate test by a different party. Actually just looked again and the 36 was at 8500RPM and the 38 at 8800RPM. The DOHC redlines at 10K.

 

[Lou Ranger]

One has to also consider whether a particular source is quoting a brake-horsepower rating or a net SAE hp - but that's for a separate thread ....

This site says the 392 sohc is 38 hp (http://www.motorcyclespecs.co.za/model/yamaha/yamaha_xs400%2077.htm)
This site says it is 27 hp (http://www.bikez.com/motorcycles/yamaha_xs_400_1981.php)
I suspect the first site is wrong (I have found that it makes lots of proofing errors)

I have also read somewhere that the dohc has nearly 50 hp, but I don't believe it. These sites seem to be wrong more than they are right: http://www.bikez.com/motorcycles/yamaha_xs_400_dohc_1984.php

 

[thielb]

yup just triple checked. DOHC is 46. i keep seeing both 27 and 38 for the SOHC though

 

[Scorpio1963]

I hope its the 38 hp.27 seems kinda low.

 

[Lou Ranger]

yup just triple checked. DOHC is 46. i keep seeing both 27 and 38 for the SOHC though

I can't believe that 3 sources would get it wrong. There's not way my Maxim has 46 hp and it runs perfectly.
I think sites like MCS simply put the new hp rating (38) onto their spec sheet when the new dohc came out and left all the other engine descriptions alone.
I think these guys have it right: http://www.ehow.com/list_7425541_yamaha-xs-400-maxim-specifications.html

 

[thielb]

http://www.bikez.com/motorcycles/yamaha_xs_400_dohc_1982.php
http://motoprofi.com/motospecspictures/yamaha/xs_400_dohc-1982.html
Does the seca have a different DOHC than the maxim?

 

[thielb]

your ehow link also said xs400L maxim. What about the xs400J maxim?

 

[Scorpio1963]

I believe they basicaly the same bike with the only difference be head light changes.

 

[dnseal]

OK guy's I found a 1982 copy of CYCLE and the attached pdf's shows a (their) comparison of four bikes, with the Yamaha being the only 400cc in the group. The Yamaha they are reviewing is the XS400SJ, which I am not familiar with. I guess its for the 1982 XS400 as the magazine was printed in January of 1982. Anyways the HP rating stated here is 29.72 @ 8500 RPM's.

I have a 1981 XS400SH and still don't know what the HP rating is. Odd that all us grown capable and intelligent guys can't find a true HP rating for these bikes. Then again, maybe I just did and don't like it.... smile.

Also interesting is how the little 400 held it's own in the 1/4 mile, even beating out the KZ440.


Gil

 

[Lou Ranger]

your ehow link also said xs400L maxim. What about the xs400J maxim?

The owner's manual for my Maxim denotes it as the XS400J
The corresponding Seca is the XS400RJ

The Service Manual covers 3 designations for each:

i) XS400J, XS400K & XS400L

ii) XS400RJ, XS400RK & XS400RL
​

 

[dnseal]

Has anybody ever dyno'd one of these bikes? Still curious about the HP rating as I find three different ratings each for three different years. I definitely don't like the sound of 27 HP, good this I have a Modified V-Max for my speed needs.

 

[willem]

In my official documentation my DOHC has 46 HP as well. But there are some restricted DOHC's out there too, as here in the Netherlands you could only ride a 33 HP bike when you were under 21.

 

[xschris]

I have seen 36 for the sohc 400 but that was pre 80 models with less restrictions, so the later might be more like 32-34 (at the crank not rear wheel) Until one gets on a dyno we will never know.

 

[xschris]

Here is a dyno sheet for a stock 82 xs650 with pods and free flow exhaust. It shows 42hp at rear wheel. So I could believe than a 80-82 sohc stock engine could make 30-32hp at rear wheel being 250+cc's smaller