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Thread: n/a cams with t25g - too much overlap for small turbo ?

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Posts: 1-10 of 13
2009-02-20 04:11:07
#1
n/a cams with t25g - too much overlap for small turbo ?
Hey guys, need some advice and a discussion

Stock W10 conversion and keeping the t25g for now.

I have a set of N/A cams from a stock highport - i think that's the stock cam we get here in australia. I also have a set of Tomei Poncams for the SR20DE (11.2/260 in/ex) lying about.

My question is will installing either of these cams actually be a downgrade because of the greater overlap intended for n/a engines. the little t25g can barely hold boost in the high RPM range as is so will the additional overlap be more of a bad thing rather than good?

has anyone used this combination?

thanks for any advice/information!
2009-02-20 05:57:23
#2
Originally Posted by Blitzsss
Hey guys, need some advice and a discussion

Stock W10 conversion and keeping the t25g for now.

I have a set of N/A cams from a stock highport - i think that's the stock cam we get here in australia. I also have a set of Tomei Poncams for the SR20DE (11.2/260 in/ex) lying about.

My question is will installing either of these cams actually be a downgrade because of the greater overlap intended for n/a engines. the little t25g can barely hold boost in the high RPM range as is so will the additional overlap be more of a bad thing rather than good?

has anyone used this combination?

thanks for any advice/information!


The poncam was created specifically for the t25.
2009-02-20 06:39:09
#3
somebody did the comparisson to stock 91-94 high port cams and one of the cams were more aggresive then the det cams. i think vadim did
2009-02-21 02:42:53
#4
the stock manifold is a divided design so you shouldn't have problems.
2009-02-21 13:49:12
#5
Originally Posted by se-riousclassic
The poncam was created specifically for the t25.


yes but there is a DET and DE version of the poncam

DET = 11.5, 256 in/ex

DE = 11.2, 260 in/ex apparently more overlap as an N/A engine needs, this is what im querying is going to be bad for a small turbo setup?


Originally Posted by jen36
somebody did the comparisson to stock 91-94 high port cams and one of the cams were more aggresive then the det cams. i think vadim did


thanks, yes these are the other options im thinking of installing, however as above being n/a cams i'd imagine they'd have greater overlap which is what im querying.

Originally Posted by Coheed
the stock manifold is a divided design so you shouldn't have problems.


sorry not following what you mean here.

from my very basic understanding N/A engines have/need more overlap, when both the exhaust and intake valves are open so that the gasses going out the exhaust help suck air in to the intake.

now in a turbo setup this is not good as boost in blown straight out. now with an already very small turbo that can barely hold 12psi flat this could actually make things worse.

can anyone please help ?
2009-02-21 14:09:12
#6
Originally Posted by Blitzsss
Hey guys, need some advice and a discussion

Stock W10 conversion and keeping the t25g for now.

I have a set of N/A cams from a stock highport - i think that's the stock cam we get here in australia. I also have a set of Tomei Poncams for the SR20DE (11.2/260 in/ex) lying about.

My question is will installing either of these cams actually be a downgrade because of the greater overlap intended for n/a engines. the little t25g can barely hold boost in the high RPM range as is so will the additional overlap be more of a bad thing rather than good?

has anyone used this combination?

thanks for any advice/information!


Just throw some cam gears on the Tomei's and tune them out. Shouldn't expect much more from a T25 in the higher RPM anyway.
2009-02-22 01:36:30
#7
yeah i don't want to go for that hassle or cost. i have these sets of cams lying around so if i can chuck them in and drive off with an improvement i'll do it - otherwise not spending the money or the time.

just don't know the theory side behind this wether it will make an improvement or not.

i guess i'll have to try it and see but i really though someone here could give me an answer about the overlap
2009-02-22 02:34:29
#8
Originally Posted by Blitzsss


sorry not following what you mean here.

from my very basic understanding N/A engines have/need more overlap, when both the exhaust and intake valves are open so that the gasses going out the exhaust help suck air in to the intake.

now in a turbo setup this is not good as boost in blown straight out. now with an already very small turbo that can barely hold 12psi flat this could actually make things worse.

can anyone please help ?



The main reason overlap isn't desireable in a boosted application is because of reversion. The excessive backpressure will cause a dillution of the intake charge while both valves are open (overlap). Many people run big cams on small turbos on the stock nissan manifolds without any issues. The manifold is cast, but has separate channels for each port. This reduces the likelyhood of reversion immensly.

If it were a log manifold, then I wouldn't do it.
2009-02-22 03:10:12
#9
thank you very much for clearing that up.

but no one can say wether its a good or bad thing ? will it cause boost to drop off even quicker as the t25g struggles.
2009-02-22 15:58:17
#10
not sure if this will help but this is what I had experiance.
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