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Club4AG Forum Topics » General Technical Discussions » High gear/ Low RPM pinging after 15%-25% throttle
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 High gear/ Low RPM pinging after 15%-25% throttle
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somerandomNinja
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368 posts [100%]
Miami FL
 High gear/ Low RPM pinging after 15%-25% throttle9:05 AM 12/2/2011 Reply  Edit

Hey guys, I just dropped in a freshly rebuilt 4age in my 86 last Friday and got it running about 3 days ago. I keep getting pinging in 3rd-5th gear at around 2000-4000 RPM when under load. Here is a list of specs of the motor and what I've tried to do to fix this issue.

87 MR2 4age (Big port head/7-Rib block with no oil squirters)
-.20 over bore 10.4:1 AE92 OEM pistons
-Shaved/resurfaced head and block
-.8mm TRD head gasket
-Head is completely stock other than some mild porting
-Mikuni 44's side drafts

The carbs were running rich before the rebuild and all I did after the rebuild was the pistons and head gasket and when I tuned them now they respond and run great. I don't know if it is leaning out because I don't have a wideband but it surely doesn't feel like it is. Can it be my high compression on the stock cams?

I am running 93 octane, and I have tried advancing the timing, retarding it, leaving the dizzy in the center. I am running just a MSD 6al and the idle doesn't change when I move the dizzy around. Im also using .32 gapped spark plugs, the spec for the AE92 since it is higher compression. I know it is originally .44, may this cause the pinging?

I have a MSD 8981 Timing curve computer I have to hook up, maybe this can fix it? I've used an MSD 6al alone before and never had this issue, can anyone please try and help me out? I'm stumped and have to get the car at tip top shape before an event on 12/18...

Thanks in advance.



somerandomNinja
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368 posts [100%]
Miami FL
  »9:07 AM 12/2/2011 Reply  Edit

Here's a pic of it, just in case you guys wanted to see it...
oldeskewltoy




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13133 posts [99%]
Portland

 Re: High gear/ Low RPM pinging after 15%-25% throttle (somerandomNinja) »8:05 PM 12/2/2011 Reply  Edit

Quote, originally posted by somerandomNinja »
Hey guys, I just dropped in a freshly rebuilt 4age in my 86 last Friday and got it running about 3 days ago. I keep getting pinging in 3rd-5th gear at around 2000-4000 RPM when under load. Here is a list of specs of the motor and what I've tried to do to fix this issue.

87 MR2 4age (Big port head/7-Rib block with no oil squirters)
-.20 over bore 10.4:1 AE92 OEM pistons
-Shaved/resurfaced head and block
-.8mm TRD head gasket
-Head is completely stock other than some mild porting
-Mikuni 44's side drafts

The carbs were running rich before the rebuild and all I did after the rebuild was the pistons and head gasket and when I tuned them now they respond and run great. I don't know if it is leaning out because I don't have a wideband but it surely doesn't feel like it is. Can it be my high compression on the stock cams?

It sure can... how much was cut from the head? How much was cut from the block? As it is now... 10.4 (likely correct for O/S pistons are slightly higher CR) + .5 (gasket) + .3 to .5 (or more?) on head, and another .2 to .3 on the deck. Guessing here, but you are likely running a static CR of around 11.2, or higher. I'm pretty sure the redtop bigport also uses the shorter duration stock cams. At the very least, I'd try a set of bluetop cams, they may lower the DCR enough, otherwise you might need a set of 256, or 264 cams to get the DCR acceptable

also what type of porting? Were the chambers fully deshrouded?


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somerandomNinja
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368 posts [100%]
Miami FL
  »9:46 PM 12/2/2011 Reply  Edit

The shaving of the head and block was minimal. It was just to seat the metal head gasket. The porting was done by a previous owner, he told me he just sanded enough to port match the manifold for the Mikuni's.

I have the big port head but originally red top lettering on the valve cover. Dont these have the same duration as the big port blue top?

tottacrolla
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343 posts [100%]
Oxford

  »3:20 AM 12/3/2011 Reply  Edit

You really need to measure the chamber volume.
I had a similar problem as yours and found out by measuring the volume of the chambers the head had been skimmed previously.
allencr


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4088 posts [93%]
Tallahassee FL
 Re: (tottacrolla) »4:59 PM 12/3/2011 Reply  Edit

You really need to know what the ignition timing is at.
Moving the dizzy with no affect means it's way out to me.
somerandomNinja
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368 posts [100%]
Miami FL
 Re: (allencr) »10:00 PM 12/3/2011 Reply  Edit

I haven't put it on a timing light just yet. Only person I can get one from lives an hour away so I'm going to try and borrow it from him tomorrow. Hopefully the timing is the issue...
jondee86
Reggae Rules...



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4374 posts [96%]
Wellington, NZ

 Re: (somerandomNinja) »11:53 PM 12/3/2011 Reply  Edit

What allencr said. As I understand it, the MSD6-AL is just a dumb igniter.
Takes a signal from the dissy and uses it to trigger the coil, but no internal advance
curve, So although your advance is fixed at whatever you set it at, you should still
see some change when you rotate the dissy.

Leaves the likely suspects as too much advance or a lean mixture. Might be worth
trying some gas with a higher octane rating.

Cheers... jondee86


jondee86 has been a member for 735,126 days.

Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

somerandomNinja
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368 posts [100%]
Miami FL
  »2:41 PM 12/5/2011 Reply  Edit

Ended up being the timing. I was at around 25 btdc at idle... Should have seen that when I put the motor in. I set it now to 10-15 btdc and it responds and pulls great. Only thing wrong was when I threw it in 4th at like 25mph and gave it like 75% throttle, it did like 2 or 3 pings then kept going fine. I would say that is from the cam duration if not, I'll play with the timing again if it doesn't go away. But thank you guys for the help and suggestions, hopefully other members read this and learn from it. Lol
jondee86
Reggae Rules...



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4374 posts [96%]
Wellington, NZ

 Re: (somerandomNinja) »8:07 PM 12/5/2011 Reply  Edit

Might be something you have to live with. If you don't have any advance curve,
timing will always be a compromise between too much at low rpm and not enough
at high rpm.

Cheers... jondee86


jondee86 has been a member for 735,126 days.

Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

allencr


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4088 posts [93%]
Tallahassee FL
 Re: (somerandomNinja) »10:15 PM 12/5/2011 Reply  Edit

Quote, originally posted by somerandomNinja »
...I threw it in 4th at like 25mph and gave it like 75% throttle, it did like 2 or 3 pings.... I would say that is from the cam duration...

I would not say it was 'from the cam duration', that really makes no sense concerning detonation.
It is entirely normal and is expected to happen when an engine is being abused.

accelerating 4th gear @ 25mph = abnormal abuse
Regular or premium octane?
If that's the only time it pings, I might bump it up a little more!



somerandomNinja
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368 posts [100%]
Miami FL
  »7:42 AM 12/6/2011 Reply  Edit

I'm using premium 93. Its the highest rating available at the pump.
 



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