I have a 69 firebird and damaged a couple pistons (detonation). I was presuming I should replace all of them (weight matched and new material). I don't have the time to do it myself so i was wondering what you think I should expect a shop to charge me for labor [number of hours, I can assume a rate], I am wondering if they could do it with engine still in car too. What are your thoughts?
Also, i was wondering what type of pistons I should get; standard cast, hypereutectic, or forged. It is just a nice weekend driver, ~400hp, current pistons are cast, mildly driven.
I probably only have about 50 miles on the engine. I built the engine about 35 years ago and the car pretty much sat in my garage ever since. I would only start it and let it run every once in a while. I finally got work done on it so I could get it on the road and then i go ahead and wreck the engine; just beating the crap out of myself for it so frustrating.
With such low miles you should not have a ridge at the top of the cylinder and you should have very little cylinder wear. If you want to go to the inexpensive route you could probably have the two pistons replaced, motor in car, with the same piston that you rebuilt the motor with and you shouldn’t have any issues with piston to wall clearance. Although I would check the diameter of the two new pistons compare to the diameter of the pistons that are coming out.If you are replacing all eight pistons I would imagine they would want to pull the motor. Why did you have the detonation problem, what heads are you using and what’s your compression ratio. You may want to put in dished pistons. If you’re doing 8 new pistons I would used forged TRW’s.
Last edited by 1badindn; 03/31/2102:25 PM.
1967 Firebird 400 Convertible Montreux Blue/Light Blue top/Black interior. Maybe one of one color combination, if not, one of very few.
I have #48 heads with 1.65 ratio rockers. I believe it is ~10.75 - 1. The detonation i think is from a little bit of low octane fuel (1-2 gallons just to get me to the gas station) i put in because i was afraid to run out of fuel and i was really running it hard through the gears after i filled up with 93 so probably still some low octane crap stuff still in there. I am so pissed off at myself to say the least!
I probably should have all 8 replaced. If you recommend forged, would i then need to change rods too? I have a cast crank so you still think i should go with forged pistons?
I think you can use the same rotating assembly. I think the forged pistons will hold up better to detonation. You don’t want to do this again so I would get a good machine shop or yourself to figure out what your static compression ratio is, then figure away to get your CR down to a point you can run pump 93. Or replace the pistons and mix in race fuel, that gets expensive.
1967 Firebird 400 Convertible Montreux Blue/Light Blue top/Black interior. Maybe one of one color combination, if not, one of very few.
thanks again 1badindn! I am going to ask a mechanic who recently did some work for me on the car to see if he will do the work, he is an expert in Pontiacs, only works on classic cars anymore too, so I hope he will do the work and I am curious to hear what he recommends.
A little different but relevant story. I had issues with my newly rebuilt Sprint smoking. It appeared to all be coming from the #6 cylinder. Opted to pull the piston and see what was going on. Found out I didn’t rotate the oil rings correctly. Anyways the engine had 2000 miles on it. I deliberated replacing the rings, rescoring the walls, etc. finally decided WTH and simply rotated the rings and put it back together. Long story short, no issues smoking gone.
With your engine, unless something was damaged, should be ok with a simple replacement. My only concern would be balancing or lack there of.
The pistons are TRW forged, the rest of the bottom end is basically stock other than upgraded bearings and rings. I too run high compression heads and can tell you 93 octane is not enough. As stated above, mixing racing fuel is one option. If you drive a lot It can be an expensive hassle, I don’t drive a lot and love my high compression so that’s what I do.
I've met a few Pontiac owners here in BC and they use 94 and still running 10:75.1 ..one or two have stated they do use an octane boost additive. I've even heard of people running on 94 with 12.1. but when I read peoples post over the years its been you cant run on pump gas over 9:5.1 . I do have a 454 .030 over flat top with a 280H cam in my bird at the moment and its 10:5.1 and I run on chevrons 94 anyone have any thoughts on this issue? I am curious as I do intend to build a 400HO with #16 heads
I have built 13 of these engines in the past 10 years and can tell you theirs no way with today's gas i would run any Pontiac over 9:1 Today's gas is garbage. The last Pontiac engine i rebuilt was for a guy with a 1978 trans am. he wiped out all the bearings because he did not have the timing set properly. so the engine needed a another crank and set of rods. Mind you the 1978 engine compression was a lot lower than 10:5:1. but still all bearings were down to copper from detonation.
I guess I don't get all the issues with detonation. Most Pontiacs run well on 12-15 deg. BDTC. To get detonation, should be pretty obvious so I don't understand how you can drive a car that has excessive detonation and more that just an occasional pin going up hill or at full throttle acceleration. Or am I missing something here?
I have #48 heads with 1.65 ratio rockers. I believe it is ~10.75 - 1. The detonation i think is from a little bit of low octane fuel (1-2 gallons just to get me to the gas station) i put in because i was afraid to run out of fuel and i was really running it hard through the gears after i filled up with 93 so probably still some low octane crap stuff still in there. I am so pissed off at myself to say the least! ...
That doesn’t sound right to me. A gallon or 2 of regular in a full tank of premium? ???? you need to figure out the detonation problem or it may just happen again.
Good luck.
I'm a hobbyist. Not a professional. Don't be hatin'!
bigchief, So that engine detonated due to poor timing as you stated in your comment which had nothing to do with whichever fuel he was using.. Ive been reading more articles of people using pump gas 94 on high CR with some modifications like retard timing, different cam longer duration, keeping your fuel system clean, a richer fuel mix, avoid ethanol, clean up sharp edges in the combustion chamber, using additive cleaners to keep carbon buildup from happening.
Also ive seen quite a few article that say Pontiac over rated the CR by at least 0.5. which would bring the 400 HO down to 10.25:1 possibly even to 10:1.or lower yet
I read an article the other day in hotrod about running as much as 12:1 on pump gas compression comprehension
from what I found out so far is that it is very possible to run 10.75:1 on 94 octane .
I wont do a build until I am satisfied though.
There has to be some members here running high CR on pump gas
I don't know the quality of the gasoline you can get in Canada, but here in USA it is garbage and nothing like the gasoline of the 1980's. The ethanol blended into the gasoline is so random and is a nightmare to run a vehicle with a carburetor. i work on 1936-1990's classics and muscle cars everyday. i have worked on cars that the fuel bowl was almost completely corroded away from the modern gas. Back in the 1980's in the USA we used to be able to get 93 octane at sunoco that was sufficient to run a Pontiac with the factory compression ratio. I have been using CP
I don't know the quality of the gasoline you can get in Canada, but here in USA it is garbage and nothing like the gasoline of the 1980's. The ethanol blended into the gasoline is so random and is a nightmare to run a vehicle with a carburetor. i work on 1936-1990's classics and muscle cars everyday. i have worked on cars that the fuel bowl was almost completely corroded away from the modern gas. Back in the 1980's in the USA we used to be able to get 93 octane at sunoco that was sufficient to run a Pontiac with the factory compression ratio. I have been using CP pistons that have a 24cc dish in Pontiac 400's that drop the compression down enough to run today's pump gas. For the record i have not yet measured the combustion chamber in a pontiac cylinder head that was actually 72cc. they always have been a bit more. the only casting i would say that would be close is a #48 casting head. i have measured sets that have been under 72cc. Retarding timing to run a higher compression ratio is not a good idea, and the so called bigger cam will bleed off the cylinder pressure is also a myth. The static compression is what the engine truly is. always shoot for no more than 9.2:1 compression to be safe.
...and the so called bigger cam will bleed off the cylinder pressure is also a myth. The static compression is what the engine truly is. always shoot for no more than 9.2:1 compression to be safe.
????... I disagree. Not looking to get into a drawn out debate on this, but some cams can surely bleed off compression to some degree. Depends on the cam’s specs, installation, valve overlap,... I will qualify my statement by saying I am not a professional mechanic, and have not done any specific testing of different cams. My experience is only 4, total, engine builds.
I run 10.5 with no detonation, with Al heads. I use 94 octane...sometimes 93. Only when not do I get some “run on”. However, I think bigchief’s advise regarding staying around 9.2 is spot on. There’s not enough to gain, for a street machine, by pushing the limits on CR. You’re at the mercy of the fuel you can get, and your tuning ability.
Your results may vary.
Drive it like you stole it.
Best of luck.
I'm a hobbyist. Not a professional. Don't be hatin'!