North American Turbocoupe Organization



huge compression loss!!
TurboDrag2604 Offline
Member
#1
Can head porting and polishing and big valves (1.89I 1.60E) cause a huge compression loss, using a cometics gasket?
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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jangus Offline
Senior Member
#2
Taking too much material out of the chambers while porting and polishing plus going to a thicker head gasket can cause some compression loss, but it shouldn't be huge.
Did you just swap this head on? If so, having the cam timing off can show as a large compression loss.
88TC 5speed, 160,000+, ranger roller, Evergreen T3, cone filter, manual boost controller.
One parts car still waiting to be taken apart.
Waiting in the garage: SC throttle body, SC intercooler, graphic equalizer, among other things.
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TurboDrag2604 Offline
Member
#3
it is a multi layer steal gasket, but stock thickness.. and it is the stock head that was ported and polished. same size valves BO sales.. Just before i took the head off to be machined, the head gasket was blowed (was a stock head gasket) and the back two cylinders had 126 and the gasket was blowed between 1 and 2 and they had 80psi compression. now i have the head back it has 90psi on all cylinders. done a leak down test and it passed.

Side note (car has 80k miles) machine shop said the head was in perfect shape
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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natmac3 Offline
Administrator
#4
Did you have the same cam when you tested before?
1987tc
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TurboDrag2604 Offline
Member
#5
yess same cam and it is set at the same timing. i have even advanced it 4 degrees with the cam gear. I add oil and it does not come up any
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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Pete D Offline
Administrator
#6
I'd recheck cam timing if they all read 90psi
Pete Dunham


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TurboDrag2604 Offline
Member
#7
I have rechecked it and everything checks out.. the main reason i am concerned is because i just done the T5-C4 swap and got everything in. I back it out of the garage and go to pull out in low gear and it has no power, as in it is geared way too high. I even advanced the cam 4degrees and it doesnt change anything. but when it reaches 4k rpms (when the turbo kicks in) then it really takes off. Is low compression the reason for this?
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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jangus Offline
Senior Member
#8
How did you check cam timing? You're not relying on the stock timing pointers are you? I'd use the string method. I know when I changed out my factory head gasket for an 8993 I had a heck of a time getting the cam timing right. I was 1/2 a tooth advanced or retarded, never could get it to line back up exactly. Ended up just using the adjustable gear to get it back to zero. Then I could adjust my cam timing to suit my tastes from there.
You're "no power until 4K" sounds like cam timing is a tooth off. It'll run that way, but the low rpm response is just garbage.
88TC 5speed, 160,000+, ranger roller, Evergreen T3, cone filter, manual boost controller.
One parts car still waiting to be taken apart.
Waiting in the garage: SC throttle body, SC intercooler, graphic equalizer, among other things.
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TurboDrag2604 Offline
Member
#9
Thanks Jangus. I marked everything with a sharpie before pulling the head and then lined it all back up when it came back. I think it may be retarded 1/2 a notch or more since it has no power on bottom end. and when i say no power, i mean you can out run it on foot til 4k. But i am using my Adjustable cam gear to see if i can get it back. And hoping this is the reason for the low compression.

another side note, what is the lowest gear anyone has ran with a C4? I am looking at a 5.71 or so, just because it seems to be geared way too high!
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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TurboDrag2604 Offline
Member
#10
sorry pete, I didn't know these cars would run a notch out of time. i just thought by marking the spots that i could put it back right, but now that i think about it there is a good possibility its a notch or so out retarded. THANKS!
88 TC weekend warrior: Bo's 1.9, 255Lph walboro, Kirban AFPR, C4 tranny, Ported BIG VALVE head 1.89/1.60 Valves, Gillis Manual BV, Garret T3/T4 stg.III .63 A/R Turbo @25psi, 4.30 gear, Stinger FMIC, Toyo Drag Radials, 3' Exhaust. my horse likes to drink the racing fuel!(110 octane) PiMP going on soon!
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