Decking? Reliability? BHG?

carl johansson · 12-10-2011, 12:46 AM

So i know decking increases compression of the engine. I run an 88 7-MGE with maual trans in the 24 hours of Lemon series. I am wondering what decking would do for power - but much more importantly, does it reduce reliability and longevity of the engine, and particularly does it make a BHG more likely.

We have to keep these things on the track for 14- 24 hours, so far the engine has been stone cold reliable - with over 200 hours of racing, without ever opening up the motor. But if we go the decking of the head route - what are the reliability consequences?

Carl Johansson

87hilux7mge · 12-10-2011, 04:09 AM

i dont know if decking would net enough gains for the cost and PITA it can be with these motors. my head was decked about .004 IIRC... with the hks 1.2mm hg, .008 off my head and new stock type GE pistons, i calc'd out at 9.58:1 compression. so with alot of milling done to everything, i gained .38 points of compression. i doubt itll show much on a dyno.

biggest thing is if you mill too much out of everything, then you have to account for the belt slack and therefore (i think) retarded timing. (correct me if im wrong on that...) adjustable cam gears can fix that, and provide a power increase, or at least the ability to change where the power is and how long it pulls out.

anyway, in my opinion, it wouldnt be worth it to deck the block in pursuit of power. if it needs to be done, then do it, but otherwise, dont mess with it.

im also unsure of the effects on the quench area when decking on these motors...

as far as i know, reliability isnt affected as long as too much isnt taken out of the block. that more depends on how much care you take with rebuilding it and the carefulness of the machinist.

cre · 12-10-2011, 10:50 PM

It's not remotely worth it, IMHO. You get more issues with preignition/ping than any power gain. You'd have to go far more extreme than shaving will allow for in order to see anything significant (higher compression pistons and definitely not cast ones).

You have to take a LOT off of the head and block for it to affect timing to any significant degree.

carl johansson · 12-11-2011, 10:20 PM

Thanks guys,
thats kind of what i figured, just wanted to run it by the experts!
staying stock - can't argue with results so far!

Carl Johansson

87hilux7mge · 12-11-2011, 10:48 PM

just calc'd out how much i lost from the machining. had .008" taken from the head, .004" from the block and went to the 1.2mm HG. total loss is .017905" or about .45mm. so yea, i guess one could get away without the cam gears. considering .45mm would be about 1/2 to 1* of timing (?), and thats pretty small beans.

im getting them so i can fiddle with the lobe separation angle. after the super itll be an big advantage.

yep, stock is good. if it aint broke, you havent driven it hard enough yet!

cre · 12-11-2011, 11:52 PM

Of course you could always go with a shorter timing belt OR figure out how to mount another tensioner pulley on the OTHER side of the belt.

87hilux7mge · 12-12-2011, 12:44 AM

very true, i have not checked on a shorter belt possibility. i just wouldnt figure that a .45 mm shorter belt would be available, haha.

i was also considering trying to fab up a hydraulic belt tensioner compared to the spring and bolt setup. not sure if it would work but i dont see why not?

cre · 12-12-2011, 04:02 AM

It's just nowhere near worth the effort. You'd have to take of a couple millimeters before it was really justified and there's no reason to remove that much material.... On an block bad enough to merit that much material cut off you'd need to put on a thick MHG or add a shim and if going for higher compression using better pistons is far preferable to bringing the stock pistons past the deck during operation.

I may be off, but I believe that a different belt would require either smaller teeth (leaving less contact area) or smaller teeth and new gearset... The removal of one whole tooth would be too much and you'd end up just as far off in the opposite direction.

87hilux7mge · 12-12-2011, 09:00 AM

Yea, probably right. Just not worth the effort. However I still want to try the hydraulic tensioner idea tho.

12-10-2011, 12:46 AM	#1
carl johansson 3" Exhaust Join Date: Nov 2007 Location: Auberry, California Posts: 141	Decking? Reliability? BHG? So i know decking increases compression of the engine. I run an 88 7-MGE with maual trans in the 24 hours of Lemon series. I am wondering what decking would do for power - but much more importantly, does it reduce reliability and longevity of the engine, and particularly does it make a BHG more likely. We have to keep these things on the track for 14- 24 hours, so far the engine has been stone cold reliable - with over 200 hours of racing, without ever opening up the motor. But if we go the decking of the head route - what are the reliability consequences? Carl Johansson

12-10-2011, 04:09 AM	#2
87hilux7mge walbro fp Join Date: Dec 2010 Location: spokane Posts: 479	i dont know if decking would net enough gains for the cost and PITA it can be with these motors. my head was decked about .004 IIRC... with the hks 1.2mm hg, .008 off my head and new stock type GE pistons, i calc'd out at 9.58:1 compression. so with alot of milling done to everything, i gained .38 points of compression. i doubt itll show much on a dyno. biggest thing is if you mill too much out of everything, then you have to account for the belt slack and therefore (i think) retarded timing. (correct me if im wrong on that...) adjustable cam gears can fix that, and provide a power increase, or at least the ability to change where the power is and how long it pulls out. anyway, in my opinion, it wouldnt be worth it to deck the block in pursuit of power. if it needs to be done, then do it, but otherwise, dont mess with it. im also unsure of the effects on the quench area when decking on these motors... as far as i know, reliability isnt affected as long as too much isnt taken out of the block. that more depends on how much care you take with rebuilding it and the carefulness of the machinist. __________________ 87 toyota 4x4 single cab short box pickup, 7MGZE.

12-10-2011, 10:50 PM	#3
cre Toyota Racing Development Join Date: Apr 2007 Posts: 5,038	It's not remotely worth it, IMHO. You get more issues with preignition/ping than any power gain. You'd have to go far more extreme than shaving will allow for in order to see anything significant (higher compression pistons and definitely not cast ones). You have to take a LOT off of the head and block for it to affect timing to any significant degree. __________________ If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum. If you're so inclined I'm always more than happy to accept tips via PayPal. Tip Jar --->

12-11-2011, 10:48 PM	#5
87hilux7mge walbro fp Join Date: Dec 2010 Location: spokane Posts: 479	just calc'd out how much i lost from the machining. had .008" taken from the head, .004" from the block and went to the 1.2mm HG. total loss is .017905" or about .45mm. so yea, i guess one could get away without the cam gears. considering .45mm would be about 1/2 to 1* of timing (?), and thats pretty small beans. im getting them so i can fiddle with the lobe separation angle. after the super itll be an big advantage. yep, stock is good. if it aint broke, you havent driven it hard enough yet! __________________ 87 toyota 4x4 single cab short box pickup, 7MGZE.

12-11-2011, 11:52 PM	#6
cre Toyota Racing Development Join Date: Apr 2007 Posts: 5,038	Of course you could always go with a shorter timing belt OR figure out how to mount another tensioner pulley on the OTHER side of the belt. __________________ If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum. If you're so inclined I'm always more than happy to accept tips via PayPal. Tip Jar --->

12-11-2011, 10:20 PM	#4
carl johansson 3" Exhaust Join Date: Nov 2007 Location: Auberry, California Posts: 141	Thanks guys, thats kind of what i figured, just wanted to run it by the experts! staying stock - can't argue with results so far! Carl Johansson

12-12-2011, 12:44 AM	#7
87hilux7mge walbro fp Join Date: Dec 2010 Location: spokane Posts: 479	very true, i have not checked on a shorter belt possibility. i just wouldnt figure that a .45 mm shorter belt would be available, haha. i was also considering trying to fab up a hydraulic belt tensioner compared to the spring and bolt setup. not sure if it would work but i dont see why not? __________________ 87 toyota 4x4 single cab short box pickup, 7MGZE.

12-12-2011, 04:02 AM	#8
cre Toyota Racing Development Join Date: Apr 2007 Posts: 5,038	It's just nowhere near worth the effort. You'd have to take of a couple millimeters before it was really justified and there's no reason to remove that much material.... On an block bad enough to merit that much material cut off you'd need to put on a thick MHG or add a shim and if going for higher compression using better pistons is far preferable to bringing the stock pistons past the deck during operation. I may be off, but I believe that a different belt would require either smaller teeth (leaving less contact area) or smaller teeth and new gearset... The removal of one whole tooth would be too much and you'd end up just as far off in the opposite direction. __________________ If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum. If you're so inclined I'm always more than happy to accept tips via PayPal. Tip Jar --->

12-12-2011, 09:00 AM	#9
87hilux7mge walbro fp Join Date: Dec 2010 Location: spokane Posts: 479	Yea, probably right. Just not worth the effort. However I still want to try the hydraulic tensioner idea tho. __________________ 87 toyota 4x4 single cab short box pickup, 7MGZE.

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